This sort of brushes on it but for a long time there was hope at resolving the Hubble Tension by saying that Hubble telescope’s measurements were incorrect, because that would be the most simple explanation. This was not the case, so if anything, the mystery deepens. I don’t know for certain but I believe Hubble’s estimation has been widely accepted for a while though, because we’ve been using the 13.8 billion cosmological age estimate ever since I started brushing up my layman's understanding of the subject.
> we’ve been using the 13.8 billion cosmological age estimate ever since I started brushing up my layman's understanding of the subject.
I remembered the age of the universe as as 13.7 billion years, but I wasn't sure why that was.
Well, the initial WMAP results in 2003 supported an age of 13.7 billion years. Later results nudged this upwards to 13.8 billion years. Of course, all the results have error bars.
If a computer chugs along doing { counter++; } at 1 clock cycle per clock cycle for 13.7 billion clock cycles, it will think 13.7 billion clock cycles have passed when counter is 13.7 billion.
On the other hand, if a computer chugs along at one clock cycle per clock cycle for 1 clock cycle, and reads &counter and sees 13.7 billion, it will think 13.7 billion clock cycles have passed.
Either way it's perfectly capable of introspecting it's source code and logically stepping back until the memory location was 0 to see how many clock cycles would have been required to reach it's current state, but that sort of reasoning is completely devoid of meaning without both perfect knowledge of what the true start state was, and a guarantee that no external influences have occurred.
Here in reality, we know neither the our start state nor our isolation level, but the hubris of many is too string to not at least try finding some logical step-back functions and iterating them until they don't know how to go any further, then proudly proclaiming that "The Start". (after all - how could it not be The Start, look, I can iterate the inverse of my step-backward function from then to now and it matches! QED!)
I agree with you 100% on your assessment of what can be known, but I think I disagree nearly as strongly on your assessment of the humanity that springs from attempting to know it.
To exist in the state you describe - with neither start state nor isolation level measurable in any tamper-proof way - and to yet still dedicate one's life to observing and pondering the complexity of the resulting cosmos is, to my eye, laudable and beautiful.
Where you see hubris, I see humility. Everyone who attempts to expand the corpus of human understanding of cosmology knows that the endgame is somewhere short of perfection. And yet they are inspired to carry on. It seems to me that matters of state, economics, medicine, technology, and many other fields will benefit from a similar disposition.
I think the hubris comes in when folks assume that because that answer is presented by Science/Observation rather than Religion/Philosophy, it's somehow guaranteed to be "more accurate", when in reality both are guesses with equally unknowable error bars.
No, we measure the frequency of vibrations of _light_, not of a type of atom. Specifically, it is light emitted by cesium atoms that are transitioning from one specific energy state to another specific energy state. Although this is arbitrary, it is highly reproducible and would give precisely the same measured lengths of time at any point since the big bang.
That assumes that fundamental laws of physics did not change (will not change). This is what we believe and have no evidence otherwise. This is important since we rely on measuring atomic transitions of cesium atoms which itself were formed/forming billions of years after the big bang itself.
The laws of physics invariance under time is a core to our understanding. It would be very disrupting if we found otherwise.
Right, but most deviations one can think of (like, changes over time to physical "constants") would have an observable effect, so ancient galaxies would look much more different from modern galaxies and spectra wouldn't look the same other than a red shift, which moves all the lines in a uniform way.
Fun fact: the Oklo reactor, a naturally occurring nuclear reactor that was active more than a billion years ago, was used to test if physical constants were the same in ancient times.
And we can split hairs and conclude that for "the last two billion years, on this planet, in this galaxy, the physics affecting nuclear decay have not changed"
It's great to know that say dating using carbon-14 decay is still useful over those time ranges on planet earth (I don't know if that is something we care about given that fossils don't tend to contain much carbon, but coal and oil deposits are around 400 million years old).
I don't want to imply that this is too small of a sample size, but I will imply that nuclear decay, and the movement of galaxies across the universe might be unrelated. Don't know. Not sure how we'd measure that. Supernova observations would tell us about nuclear fusion and it's limits. Does it tell us about nuclear fission? I don't know.
Time in this context is just an arbitrary measurement. Like extrapolating the calendar back to the Big Bang which is when space/time began, another way to think about time.
Unfortunately we can’t observe fundamental things like “what are the rules of physics and time at the beginning of the universe?”. We look for clues and make large assumptions, but given that the universe experienced a 10^78 factor expansion during the Big Bang, assuming that actually happened, then why would it make sense to assume that the rules of the universe today are the rules for the very early beginning of the universe? A strand of DNA would become 10 light years. Given that relativity redefined our understanding of basic physics but only applies as we approach the speed of light, it would stand to reason that the rules of physics would be different from our current models based on today’s observations when the matter of the universe is packed much more tightly together.
Entirely reasonable assumptions! Our models match surprisingly well though... The CMB has a blackbody spectrum that aligns with predictions, we see galaxies more or less when and where we'd expect them, stellar populations look like what we'd expect for a universe made of hot hydrogen, and more! It's not quite perfect, but modern physics explains stuff really quite well even billions of years ago!
Modern physics is guided by those observations, they can't be then used as an argument for its veracity.
Let's say I walk into a room and observe someone writing a tally mark on a chalk board once every second. I count 4x10^17 tally marks. I might assume that 4x10^17 seconds ago that same person entered the room and started tallying. I might even observe for the next 4x10^17 seconds they continue to tally. Heck I might even see a recorder going that when I play back at what I assume to be 1x speed, has chalk scratches at regular intervals for 4x10^17 seconds. I still don't have any actual evidence that they started those 8x10^17 seconds ago.
The inflationary epoch where it expanded by 10^78 in volume happened in the first 10^-32 seconds. The furthest galaxy we can see (fairly poorly) is 300M years after the Big Bang. It's likely if time or the rules were different, 300M years was enough for things to mostly die down to steady state. And as you say, they match more or less but those errors could easily hide remnants of when things were different. Of course, these are all numbers that assume the Big Bang theory is correct which is difficult to impossible to falsify since we can't possibly observe or test anything from that long ago. We'll have to wait to see if refinements to our model that clear up contradictions change what we think about the beginning of the universe and other boundary conditions.
Well for one, if the laws of physics are not time invariant (i.e. the laws of physics are not the same at all points in time) then energy can be created or destroyed [1]. So that would be quite a shocker.
Yes, but my understanding is that dark energy doesn't play by the same rules, it's an exception. I certainly can't explain why but also it may not be known exactly why, given dark energy is an unexplained phenomenon.
The party line is that energy is not conserved at cosmological scales. However, it's more of a semantic question: We can tell you exactly by how much it gets violated (that's basically the first Friedmann equation), and if you prefer, you can attribute the missing energy to the gravitational field. A lot of physicists don't like that approach as it isn't possible to write down a corresponding stress-energy tensor, ie gravitational energy cannot be properly localized.
It's really not. If there is nonzero energy, and our current models don't allow for energy to be created, either we need new models that do allow for energy to be created (time-variant physics, for instance), or we need the existing models to propose a way it was initialized. Without that there's a gaping hole in the model the size of all of the energy in the universe.
Certainly you can't 1) have a model where energy is constant 2) believe there is nonzero energy in the universe and 3) dismiss any model where energy can change as bogus out of hand because you believe it should be constant, without counter-proposing how it even got here.
Nobody argues it wouldn't be nice. Physics argues we can't ever know. It's a rather significant difference and if this gets proven wrong, you'll find that physicists are the ones partying the hardest.
The difficulty is we don't know what happened in the first planck second after the big bang, let alone before the big bang (if that's meaningful).
We haven't unified quantum mechanics and relativity. Hence we can't be certain that singularities exist, or that the universe started off in a singularity.
I'm aware. What I'm saying is if that's how much you don't know, you'd better not make any claims like "X can't be accurate because it doesn't match what I think I do know".
Saying "X is true because otherwise, it would be quite a shocker" isn't really a proof.
It's almost an appeal to common sense. (which, admittedly, is often the best argument we have). IMHO there's often a bit of over-confidence among scientists about the universe being 13 billion years old and what happened during the Big Bang, if it just relies on such a common sense argument.
I know it's unscientific to suggest maybe laws of physics are not time invariant across such scales, because until we have a time machine we can't test this theory, but then flipping the argument (that laws of physics are definitely time invariant) is also technically unscientific -- we're basically assuming this without strong evidence.
This goes back to the old debate on the problem of induction in science, (see David Hume, Karl Popper, etc.) and I think it isn't emphasized enough in modern discussions that, perhaps, there's a small chance that these foundational concepts in physics could be invalid.
I did not present proof. They asked what it would mean and a consequence of physical laws not being time invariant would be that energy does not need to be conserved.
To the extent that relates to epistemology that would be more like anti-proof I guess? It does not prove that the laws are time invariant, rather it raises the bar to demonstrate that the laws are not time invariant because that means energy need not be conserved. You can not get one without the other without attacking even deeper fundamentals of modern scientific models. So you must either demonstrate the linked claims, which are pretty foundational themselves, or you must overturn basically everything; both of which demand very robust evidence.
I have had the same thought that primordial reality had the same timestream as us but it was much longer, like the first "year" of reality was far longer than one year today, just the thinking "could time have been shorter or longer, why not"
I'll ignore your analogy with a CPU, which I don't think is apposite.
Whether there's a 'system clock', a sort of reference clock that can tell you how fast time is passing, so you can calibrate other clocks against it, seems to be the same category error. As far as I can see, either time is all there, all at once; or one second lasts exactly one second. (I've perpetrated the same category error there, because a second doesn't 'last' for some period of time; it just is).
[Edit] That's not very clear. I mean: if you are measuring a distance, you use a calibrated ruler. If you doubt the calibration of your ruler, you might recalibrate for precision. But you don't have a clear idea of what one foot is, it doesn't make any sense to ask whether your one-foot ruler has grown or shrunk; how would you tell if the purported 'fact' is true or false?
And if some fact about the Universe means that it has grown or shrunk, how would you tell? If you can't in principle tell whether a 'fact' is true or false, it follows that the 'fact' isn't a fact, because it has no effect on anything.
You're certainly welcome to completely ignore the question at hand, it's just surprising that you'd use so many characters to do so.
The question is if there's a different sort of thing beyond our idea of time. We have our concept of N caesium oscillations is the base reference for everything, and the duration of all action is derived from it. Could there be more depth to the rate at which things occur than that, especially on cosmic scales? Could other processes, which operate at different base clock levels, be interacting with the universe we observe in ways we don't yet understand? Could those processes have clock levels that vary over history with respect to our caesium definition?
You can claim not, that's its precisely that shallow. But neither of us can provide evidence either way, and your belief is simply much less interesting to me.
Could those processes, for instance, have configured the initial state of the entire universe in 6 of their days, but 13.8B-6,000 of our call years? I see no reason why not.
> You're certainly welcome to completely ignore the question at hand
What I ignored was an analogy. Not an argument from analogy; and not a question. Just a rhetorical device, which I considered irrelevant, and didn't feel like arguing about.
> the base reference for everything
Caesium oscillations are a proxy for the passage of time. I can imagine a universe in which the rate of Caesium oscillations might vary, or be influenced, making atomic clocks unreliable. I don't know how one might measure the passage of time directly; I suspect it's impossible, and can only be done with proxies.
I don't see why you couldn't have more than one timescale operating and interacting. It's an interesting speculation. But William of Ockham advised against multiplying hypotheses; and I can't see what multiple interacting timescales might explain, that can't be explained without them.
Basically it means that we can't regard the Hubble result as a mismeasurement and the age of the universe seems to be different depending on how you measure it.
From the article:
"The bottom line is that the so-called Hubble Tension between what happens in the nearby Universe compared to the early Universe’s expansion remains a nagging puzzle for cosmologists. There may be something woven into the fabric of space that we don’t yet understand."
It's amazing that barely a 100 years ago The Great Debate in astronomy was weather the Milky Way was the extent of the universe or things like Andromeda were their own 'island universes'. In the 1920s, Edwin Hubble showed that Andromeda was far outside the Milky Way by measuring Cepheid variable stars. These are the same stars that we are measuring today in this debate.
https://en.wikipedia.org/wiki/Great_Debate_(astronomy)
And not much longer before that the discussion was how long would the sun last - 5,000 years or so was the estimate if it was a big ball of burning gas (source: A scientific American article I read, wish I could find it again, hoping someone here knows)
Before we know better it can be just that spacetime was expanding at a different rate (we still would need at least one another Planck that operates in roughly same range to confirm this).
Hubble wavelenght range - 0.1 to 0.8 μm
Webb wavelenght range - 0.6 to 28.3 μm
Planck wavelenght range - 330 to 10000 μm
My understanding is that Planck was observing photons that have happened much more earlier.
I'm reading their comment as a joke about how software engineers tend to overestimate their own expertise on things like physics and are not actually anywhere close to experts.
Software engineers presenting weird pseudo science as serious physics is one way this manifests.
It's astounding to me that space is so empty that in the billions of light years from here to the edge of the universe, there's orders of magnitude less total interference than what you get from a small cloud.
It's also astounding that celestial objects emit enough photons that thousands per second travel within the arc that goes the distance from the star to somewhere inside the radius of your pupil.
And if this wasn't the case, then we'd never even be aware of any of this.
Also that the light and gravitational waves from colliding neutron stars can travel 100 million light years and arrive at earth within a second of each other. That's a mind-bogglingly small amount of drag.
> While objects cannot move faster than light, this limitation only applies with respect to local reference frames and does not limit the recession rates of cosmologically distant objects
Given that v is velocity in the opposite direction, and c is the constant reference frame speed of light; do we account for velocity in determining whether light traveling at c towards earth will ever reach us?
v - c < 0 if v>c
v + c > c if v>0
Are tachyons FTL, is there entanglement FTL?
How far away in light years does a mirror in space need to be in order to see dinosaurs that existed say 100 million years ago?
> do we account for velocity in determining whether light traveling at c towards earth will ever reach us?
As far as I know this is not necessary because the speed of light is constant regardless of the velocity of both the source and the observer (this is Einstein's special relativity: https://en.m.wikipedia.org/wiki/Special_relativity)
>> While objects cannot move faster than light, this limitation only applies with respect to local reference frames and does not limit the recession rates of cosmologically distant objects
Then the length traveled changes for two photons emitted when they cross the starting line at different velocities:
const d = distance = 1
# d_start = 0
# d_finish = d
c: Velocity # of a photon in a vacuum
v1: Velocity # of photon emission source 1
v2: Velocity # of photon emission source 2
v1 + c != v2 + c
distance / (v1 + c) ?= distance / (v2 + c)
((v1 + c)*t) - ((v2+c)*t) ?!= 0
Should (v + c) be prematurely reduced to just (c), with a confirmed universal expansion rate?
Tachyons aren't a thing. Tachyons are sci-fi nonsense.
Nothing in the universe can travel faster than the speed of light. This does not hold for the universe itself. It can and does expand faster than the speed of light, using specific reference frames (i.e., big enough).
So, space can increase FTL. Particles do not travel faster than light tho, that is nonsense.
Well, we have (virtual) particles that can travel backwards in time, without breaking causality. There's no proof that Tachyons exist, they are purely hypothetical, but they are not outright nonsense.
Take a star in a region of the universe that recedes from us at 3c. In what sense is the star not traveling faster than the speed of light relative to us?
If the universe is expanding faster than the speed of light, how are particles not traveling faster than the speed of light indeed with zero acceleration relative to the expansion?
If the Copenhagen interpretation is correct, particle states are correlated after a photonic beam splitter; if you measure entangled photons after a beam splitter, their states are still linked.
If virtual particle states are entangled with particle states in black holes or through ER=EPR bridges, is there effectively FTL?
> do we account for velocity in determining whether light traveling at c towards earth will ever reach us?
No, because the speed of light is constant for all observers. From our frame of reference on earth, light from distant receding galaxies is always moving towards us at exactly the speed of light, c. Those galaxies also observe the light moving away from them at exactly c.
That seems contradictory and unintuitive, that two observers moving away from each other both measure light moving c relative to themselves, but that’s reality.
It’s another measurement that changes: if c is always constant, then it must be the passage of time and the distance travelled that we observe differently.
> That seems contradictory and unintuitive, that two observers moving away from each other both measure light moving c relative to themselves, but that’s reality.
Light is a wave. Photon is a complex thing (Hopfion?), but it's a wave, so it waves something, a medium. Speed of wave propagation in a medium is constant. IMHO, it's intuitive.
Gravitational waves existing doesn't confirm the existence of a medium any more than discovering light behaves like a wave confirms the existence of the aether.
If you want to posit that light has a medium, you need to redo 100 years of physics, so start showing your work. You mention elsewhere "doing a calculation", and that's just not nearly good enough. You want to overturn perfectly working physics, you NEED to show up with receipts. That's table stakes.
> How far away in light years does a mirror in space need to be in order to see dinosaurs that existed say 100 million years ago?
A mirror in a vacuum 1ly away will return the photonic signal from t=0 at t=2 light years, with diffraction (due to matter in [solid, liquid, gas, plasma, and superfluid/superconductor] phases describable with superfluid quantum gravity (e.g. Fedi's with Bernoulli pressure))
# m = meters
# s = f(decay_rate_of_cesium_atom)
# c = const x: meters/second
Time = TimeInLightYears
t_tx = t_transmitted: Time = 0
t_rx = t_received: Time
d = distance: Time
# d == d_ab == d_ba
# t_tx, d, t_rx,
test_data = [
[0, 0, 0.0],
[0, 2, 1],
[0, 1, 0.5],
[-1, 0, None],
[-100e6, 0, None], # dinosaurs
]
@pytest.mark.parametrized('t_tx, d, t_rx', test_data)
def test_test_data(ttx, d, trx'):
assert trx == ttx + (2*d)
m = scattering_matrix: # FluidDiffractionTensorMatrix
assert is_YangBaxterMatrix(m)
A water droplet [in space] reflects enough [photonic,] information to recover a modulated signal and also a curvilinear transformation of Escher looking into a crystal ball, with c the speed of light as a consideration only at cosmological distances.
How large of water droplet, a regular or irregular spheroid or reflective and/or lensing matter configuration is necessary to reflect a sufficient amount of photonic information to recover information from cosmological information medium, in terms of constructor theory?
Almost all claims any serious scientist makes concern the observable universe.
Also, these are very smart people. You'd be surprised what we can work out with a few very reasonable assumptions and the observations we made. The dismissiveness really isn't warranted.
So you're saying the scientific media is where the "dumb people" are, not the scientists themselves. While I agree, it doesn't really change the point that these sorts of headlines/articles are heliocentric in nature and mischaracterize our understanding of the universe.
> Almost all claims any serious scientist makes concern the observable universe.
One last thought on this.. I understand these sorts of understood meanings work fine when speaking with professional peers, as they'd understand the assumption. But if you continue to do this when speaking to people outside your field, you will not be saying what you mean to say and will be teaching people incorrect ideas.
A little background based on a few articles about this plus my recollection of PBS Space Time videos on this: There are at least two ways to try to figure out the rate of expansion of the universe (which is called the Hubble Constant).
• From variations in the cosmic microwave background (CMB) which are the result of certain conditions in the early universe it is possible to figure out what the expansion rate should be now.
• From looking at very distant galaxies and noting how far away they are and how fast they are receding from us the expansion rate can be calculated.
Theory says that these should give the same expansion rate. When the rates were first found using those two methods they gave different results, but the error bars on both were large enough to overlap. People expected then that further refinement of both methods to decrease the error bars would converge to some common value.
That did not happen. Refinement of the CMB measurements got to 67 +/- 0.5, and refinement of the galaxy distance/speed method got to 73 +/- 1. Those do not overlap.
This non-overlap between the possible ranges given by the two methods is called the Hubble tension, and it is one of the most irksome problems in cosmology.
Possible explanations include:
• Some sort of error in how we measure the variations in the CMB.
• Some sort of error in the distant galaxy distance or speed measurements, which until the James Webb telescope were almost entirely Hubble telescope measurements.
• We're missing something in our understanding of the physics.
These new results add a bunch of data from the James Webb telescope, which observes in different wavelengths than Hubble. These results fit with the Hubble measurements.
They do not resolve the Hubble tension. What they do is remove most doubt that the distant galaxy results involve some sort of Hubble measurement error. I believe cosmologists are pretty confident of the CMB measurements, and so this will be interpreted as telling us that the Hubble tension is not just a problem with our measurements. There is either physics that we got wrong or physics we need to discover.
This, together with the endless philosophising around dark energy, dark matter and whatnot paints a pretty strong arrow towards our models having some flaws when it comes to their large-scale application. I hope to live long enough to see where we made our mistake and get a better model.
Almost every model we build of more trivial things based on observation always turns out to be not really right. I cannot imagine why one of the universe that has had multiple version updates in the last 100 years to not also be grossly mistaken. I also don't expect the full model to be simple or beautiful. We may be thinking wishfully based on massive extrapolation and cutting corners to suit our narrow view into the world.
I think the truth is likely that we live in a very complex universe that can be approximated by these sweeping laws in general, but is very "messy" close up. Consider that while the universe roughly obeys probabilities, it also has will and intent (to a degree we can debate in another conversation).
Hopefully we can agree that at least part of the universe "chooses" outcomes, rather than them occurring randomly. You could assume that "choosing" is what random outcomes feel like but I don't think that's productive.
Given that we have evidence of physical systems evolving their states by conscious choice, that begs the question "Why would large, complex physical systems have a governing force that doesn't exist in smaller physical systems?" Occam's Razor suggests that consciousness and will drive the evolution of all physical systems, and our elegant equations are merely statistical approximations following the law of large numbers given a population with a natural variance. Of course we can debate what it means for a low information system to be conscious and have will, but I think that's a more meaningful debate than trying to pinpoint the exact moment that a system becomes complex enough for the magical emergence fairy to sprinkle consciousness dust on inert matter.
We know for sure that there are issues with our current theories. Our two best theories, general relativity and quantum mechanics, are not compatible with each other.
I mean effectively we are regardless. The cosmological event horizon is a thing.
Also, I forget off the top of my head, but there's some oddities depending on your chosen frame of reference when looking @ hawking radiation (skip to part about relativity of the vacuum) that I think could apply to the cosmic event horizon as well
"Humason assembled spectra of the nebulae and I attempted to estimate distances." So wrote Hubble of his colleague Milton Humason in 1935 by which time spectra had been obtained for over 150 nebulae. Hubble was a stern warner of using the Doppler effect for galaxies and argued against the recessional velocity interpretation of redshift, convincing Robert Millikan, 1923 recipient of the Nobel Prize for Physics and director of physics at the California Insitute of Technology, that the redshift interpretation as an expanison of the universe was probably wrong, the year before both of their deaths in 1953.
Hubble ended his book Observational Approach to Cosmology[+] with the statement:..."if the recession factor is dropped, if redshifts are not primarily velocity-shifts, the picure is simple and plausible. There is no evidence of expansion and no restriction of time-scale, no trace of spatial curvature, and no limitation of spatial dimensions. Moreover, there is no problem of internebular material. The observable region is thoroughly homogeneous; it is too small a sample to indicate the nature of the universe at large. The univers might even be an expanding model, provide the rate of expansion, which pure theory does not specify, in inappreciable. For that matter, the universe might even be contracting."
Could it be that nearby galaxies are akin to small angle approximations in spacetime trajectory, but as you get really far away (e.g. CMB) the perspective distortion increases hyperbolically? I notice that if you normalize the Hubble data by their Lorentz factor you get back a constant expansion rate: https://www.desmos.com/calculator/llhnja1ocb
There's a 'fringe' theory that the CMB is not the echo of the big bang but rather the redshifted black body radiation of intergalactic dust (apparently the numbers are about right). Although the primary proponent of this theory is trying to justify some sort of cyclic universe model with it, if my understanding is correct it would still be compatible with a 'standard' big bang that has the "longer" (aka galactic expansion) timeframe.
And in any case physics still needs to explain what happened to that blackbody radiation.
> According to the standard big bang theory the CMB should look like black body radiation.
Yes, that's my point.
>As for smearing, IIRC black body curve has the central limit property
Surely that can only be true for a very specific distribution of temperatures? To go to an extreme, if you combine the spectrum of two objects of different temperatures you do not get anything like a black body curve.
I agree with you; you can't build up a perfect blackbody spectrum by superposing different near-blackbody spectra, and that led to the cosmological thermalization problem. Furthermore, baryonic intergalactic dust isn't like dark matter: we see redshifted spectral features of components of such dusts imprinted on all sorts of backgrounds from quasars to the CMB. The dusts tend to be greybodies. (I'm ignoring the warm-hot intergalactic medium here).
The CMB has narrow spectral lines imposed on it by galaxies, dust clouds, and similar objects. Those lines can be used to measure the CMB effective temperature at an object's redshift. A specific example of how this works is linked at the bottom of this comment. If a similar highly dusty object is found in the foreground of a bright quasar (merely nearby quasars were used in the case below), we could get an even tighter measurement of the CMB effective temperature at the dust, and perhaps look in new ways at how the CMB deviated (at the dusty object) very slightly from a blackbody spectrum. This amounts to a new line of evidence in the study of the expansion history.
More generally, constraining the evolution of the CMB temperature in the matter-dominated era (T_cmb > 4 K, z > ~ 0.4) will prove fatal to ideas like some in this thread (which is already in trouble because of other spectroscopy e.g. the Lyman-alpha forest and Gunn-Peterson trough).
I don't really understand the summary of the "'fringe' theory" at the top of the thread (and I can't identify which specific theory throwawaymaths means). I guess the idea could be that some unknown mechanism might homogenize the emissive components of baryonic intergalactic dust but not circumgalactic medium spectral features. I suppose one could try to investigate a non-adiabatic expansion to explain those features found in the CMB in a nonstandard way. Non-adiabatic components in the expansion have been examined by various theoretical teams in the context of quantum cosmology and/or "fifth force" dark energy, although mostly at much larger lookback times than the the standard surface of last scattering.
I don't understand what was meant by '"longer" (aka galactic expansion) timeframe'.
Finally, "echo of the big bang" isn't really helpful in understanding the origin of the CMB photons, why they're now filling space with a thermal Planck spectrum, and why we detect small variations in that spectrum in narrow views along different directions, but like "fabric of spacetime" I guess we're kinda stuck with the expression.
- --
(Open Access) Riechers, D.A., Weiss, A., Walter, F. et al. Microwave background temperature at a redshift of 6.34 from H2O absorption. Nature 602, 58–62 (2022). https://doi.org/10.1038/s41586-021-04294-5
I am not a physicist, but iirc the idea of the alternative explanation of the CMB is that if you time evolve backwards intergalactic dust over expansion, you reach a point where the dust gets really dense and effectively opaque.
So this isn't summing all of the dust you go through, but rather the redshift all the way back to an era when the universe was super dusty (with the expected thermal signature of that era)
Maybe you can dig up a reference, because you're on the verge of telling the conventional cosmology in a time-reversed way.
Under time reversal eventually the very first stars disintegrate into mostly atomic hydrogen, which compresses and heats adiabatically. Eventually it becomes hot enough that the hydrogen ionizes. Ignoring processes which alter photon number, the hot nuclei and electrons form a dense fog.
As with a fog here on earth, where incoming light can get bounced in a random direction off a fog droplet ("Mie scattering"), photons scattering off these hot charged particles are scattered in a random direction ("Compton scattering" at high energies, "Thomson scattering" at low energies). When we densify the fog, the free-streaming length of light decreases, increasing the fog's opacity.
Under the normal direction of time, therefore, the CMB is when de-ionization enormously shrinks the analogue of the fog droplet size ("Thomson cross-section"), and expansion increases the distance between the fog droplet analogues. The free-streaming length can become effectively infinite, like clear night air after a fog dissipates. The spectrum of the light at that point encodes the effective temperature of the scattering medium. In an expanding universe, that spectrum will lose energy (i.e., redshift).
The electrically neutral mostly-hydrogen then takes three principal forms: collapsing clouds of gas, which eventually form the first low-metallicity stars; cold dusts of neutral atoms at various sparser densities; and a warm-to-hot sparse intergalactic medium. The latter two is where we should find the so-called "missing baryons", the large fraction of atoms not found in stars and galaxies.
Backlighting by active galactic nuclei and UV-hot stars ruins the idea that a microwave-bright diffuse dust of electrically neutral atoms or molecules could generate the CMB with its spectral features. You'd have to keep the stars from heating the dust elements, while keeping the dust dense enough to generate the CMB photon-density. How do you preserve that density during expansion?
At the top of the thread you said "the primary proponent of this theory is trying to justify some sort of cyclic model", which doesn't escape this point. (It also didn't lead me to a reference).
However, if instead whoever you are struggling to remember is the proponent of an eternal and static cosmology -- with no expansion, ever -- some of these problems with the idea that the CMB can be the product of cold dust could be overcome. The idea might be that we still have a speed of light lookback, with more distant galaxies being older. The older galaxies being redder could be some sort of dust that is very thick in the distant past, thick enough to completely shroud whole galaxies like a lampshade, turning hot thermal Planck spectra into cold thermal Planck spectra. Then you have to (a) get rid of the dust over time without sending more or hotter photons in our direction, and (b) add spectral features to the cold thermal Planck spectra before it gets to us. I don't see how either could be done without very different atomic and/or gravitational physics than we have in our solar system. Adding in supernovae -- whose light-curves we see redshifted as per the article at the top -- makes this idea even harder, because then you need a lampshade-dust that down-converts photon energies in even more ways.
I'm not aware of any published or even serious attempt to do this, although I didn't look too deeply into the literature beyond confirming that this wasn't something proposed by Jayant Narlikar.
> How do you preserve that density during expansion
You don't. We aren't in a particularly dusty universe right now. It's also not completely clear. There is dust. This dust is dilute. If our universe is expanding, at some point that dust was denser.
Now run the numbers from known estimates of how thinly dusty out universe is now. Given the rate of expansion, You might get a somewhat dusty universe at some time after the big bang, no?
I don't know how to say this in any plainer English. Your comments are full of lots of words that are muddling the point.
> I'm not aware of any published or even serious attempt to do this
That's too bad. I would want to see it done, if only to confirm that the CMB can't be intergalactic dust from a denser era
Im not a physicist. I am not selling a theory that needs to answer to a simple challenge. Cosmologists are, it is their responsibility. Moreover, if I did, it would be a huge waste of time because dollars to doughnuts I get gatekept and no one believes me, and, if anything it could make challenging the CMB worse because this model gets filed away as "that one crackpot throwawaymaths idea don't listen to it".
Back when I was a hard scientist it was MY responsibility to show that I had done all the controls correctly. And trust me that was hours, weeks of grueling manual labor. A pity if physicists aren't held to the same standard.
True; people with any sort of expertise will be less generous with their attention if it's clear that you won't make an effort to understand what it is you're challenging.
Additionally, is it really you that's mounting a challenge? Quoting you: "There's a 'fringe' theory ... the primary proponent ... if my understanding is correct it would still be compatible with ..." <https://news.ycombinator.com/item?id=39674424>.
So,
> crackpot throwawaymaths idea
it is not really your idea, is it?
I see you as taking the position that you're entitled to require me to make efforts to assist you. I also note that what I did volunteer upthread, you rejected as "muddling the point". Fine. I think we've evaporated each other's conversational good will. Bye bye.
What you were doing was muddling the point. As best I can tell (and it was difficult to understand what you were saying), you were talking about summing blackbody radiation over time and distance. That's not what this model is.
> if you combine the spectrum of two objects of different temperatures you do not get anything like a black body curve.
Also true for two normally distributed variables and yet the normal distribution has the central limit property.
Presuming the hypothesis, do you suspect that you'd be looking at exactly two grains of dust, one of which has a crazy high temperature and the other has a crazy low temperature in any given pixel of the CMB?
I can’t believe it. First they take Pluto from us, next they’re going to tell me the static on my tv set isn’t the remnants of the Big Bang, but just hot dust? My childhood is crumbling!
Your comment made me realize how some younger people have no idea of, or at least no first-hand experience of static on TV and radio since it is all digital nowadays.
My great grandparents had a black and white TV when I was a toddler, and I'm 36!
The amount of change millennials and the generations before and after us have seen is boggling.
I was out in country Victoria, Australia a few months back and the internet was TERRIBLE. I'm talking, JPG's loading line by line terrible. And this was on 'alleged' 4G.
I felt pretty nostalgic for all of 2-3 minutes before I started pulling my hair out. I feel nostalgic about it again now.
You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.
Just a fringe speculation without any physical basis: maybe the Big Bang emits a 4-dimensional spherical shell of matter at c?
With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”;
diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection;
and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric;
and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion,
with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
While its probably totally wrong, I think your tone and general attitude to people throwing around imaginative and creative ideas is extremely unproductive and frankly rude. If you want to stifle creative discussions, this is exactly how you'd do it.
I found the post to be very stimulating regardless of its validity. And it's not like they're trying to spread it as gospel. There was a lot of disclaimer.
If you want creativity, take an art class. This is cosmology, ostensibly a science. Pulling wild ideas out of thin air and basically ad-libbing a "theory" is not.
So we have to put everyone who dares to imagine down about it?
I see value in sharing a wild idea purely for the inspiration it may give others to think about things from a unique perspective. A lot of great science started as a strange idea.
What's the value in """daring to imagine""" about cosmology? This is nothing more than reality based fan fiction.
>A lot of great science started as a strange idea.
Disagree. A lot of great science started with "This weird math I'm doing is the only way to fit this data", which is an extremely different thing.
Do you not think physicists and cosmologists and their nerdy friends have thought of wild ideas? Do you not think undergrad students spend drunken friday nights theorycrafting universes?
It's pretty disrespectful to "why don't you just" a profession without showing that you at least did your homework first, or that you have looked at whoever definitely asked that question before you.
There are people in this post reinventing Tired light and Aether based theories for crying out loud, theories with fairly conclusive reasons we abandoned them.
When you just ass-pull theorycrafting without any of the fundamentals, all you do is talk in circles. It's no better than all the dumb "the brain is like a computer" arguments by analogy in neuroscience posts.
It's not even wrong.
We live in a world where the people who had to cheat off me in high school biology angrily insist that the vaccine doesn't work. Do we really benefit from playing such anti-scientific games?
Thank you. It's good to know I'm not the only one who feels disrespected from people on the sidelines who clearly never opened a cosmology textbook.
It's one thing to ask curious questions, but "why don't you just do the thing that was ruled out 100 years ago" and expecting a detailed rebuttal is a bit rich. It's like asking "why don't they cure cancer by cutting it out of the body". Or "why don't you just apply basic set theory to P=NP".
It's always the same on these HN threads about cosmology. Even worse with dark matter, probably because you can understand rotating curves with high school physics. No one ever invents or roots for fringe theories on baryonic acoustic oscillations in the angular power spectrum of the cosmic microwave background.
I'm not nearly as bothered by people typing out what they themselves admit are essentially physics fantasies (especially when prefaced with "I'm not a physicist" or the like), as by people who try to convince readers with even less exposure to physics that some alternative to good theories is clearly better but that the powers that be of theoretical physics are somehow suppressing it, and then clearly have little understanding about how the good theories work. There's a lot of that, especially in DM threads.
I think we observed some of the galaxies moving away from us at rates faster than speed of light, multiple times faster, which wouldn't work with objects drifting away if we hold by that speed of light is max speed matter can move at.
That's physically impossible. You can't observe something that's beyond your "light cone", as any galaxy "moving" (it's not really moving, it's the space that's getting expanding) faster than light would be. What you're referring to is the fact that we can confidently predict that galaxies at the edge of the observable universe, which we currently see moving away from us really fast, but as they were in the distant past due to their light taking billions of years to arrive at us, are currently, if we could actually see them where they are right now (again: we can't), "moving" faster than light away from us.
Once a galaxy has moved beyond or "light cone", it's lost forever: you won't see it again even if you try moving towards it at light speed for all eternity.
The Hubble sphere (the place where recession velocities hit the speed of light) is not the same as the particle horizon (our past lightcone at current cosmological time, the boundary of the observable universe) or the cosmic event horizon (our past lightcone at infinite cosmological time, the boundary of the asymptotically observable universe).
Observations indicate that the expansion of the universe is accelerating, and the Hubble constant is thought to be decreasing. Thus, sources of light outside the Hubble horizon but inside the cosmological event horizon can eventually reach us. A fairly counter-intuitive result is that photons we observe from the first ~5 billion years of the universe come from regions that are, and always have been, receding from us at superluminal speeds.
This is correct: For example, at time of emission of the light we receive today, GN-z11 had a recession velocity above 4c. A redshift of 1090 (which is the approximate redshift of the cosmic microwave background) corresponds to a recession velocity on the oder of 60c.
The speed of light, or C, is the max speed information can move through our 3d space.
Having objects moving away from us at a speed greater than C, isn't weird. The observable universe is a 3d subspace of a higher dimensional object. A good analogy is a balloon, where there's a 2d subspace on a 3d object that's being inflated. Even if you can only move at a certain velocity, the balloon can inflate such that the 2d surface expands faster than the max velocity we prescribe for it, and would grow faster at the beginning even if we pumped a constant amount of air into said balloon.
Great analogy for gravity too! because you could create dimples in the balloon (gravity wells) which would curve a straight trajectory, while being unnoticed to an observer on the 2d subspace.
I've wondered: Do we know that the balloon has always been inflating at the same rate? Do we know if the dimples on the balloon expand as fast as the rest of it? Do the areas around the dimples expand faster?
I'll get to your questions below. If you want any of the preliminaries or the non-tl;dr answers explained a bit more simply, say so in a reply and I'll do my best in response.
Your questions (if you pardon the expression) poke holes in the balloon analogy. The latex or other stretchy balloon material is denser and under less tension around dimples. Local experiments by a (spatially) 2d observer could determine these features experimentally, and is likely to determine that there is a shear force that is not confined to its 2d "world". The 2d observer could in principle also do geometry and discover the amount of large-scale positive spatial curvature of its "world"; when we do that at cosmological scales we find flat or even slightly negative spatial curvature. The 2d observer could also discover the gravitation of our world: put a drop of water somewhere on the surface of an inflated ballon, and that drop will tend to roll downwards. We haven't found anything like that.
Humanity has looked for forces that give even the slightest evidence in favour of extra spatial dimensions, but there is no experimental evidence that favours having more than our familiar three. We've also looked at many many ways in which space could be some sort of medium comparable to the balloon latex, and practically none of them has survived contact with experiment (and those that survive are mostly hard to analogize with stretchy latex, even when entropic forces -- those are why you can scrunch or inflate a balloon and when you release the scrunching-pressure or internal air pressure the balloon relaxes to pretty much its original shape -- are relevant gravitationally).
> Do we know if dimples on the balloon expand as fast as the rest of it
tl;dr: yes: the material of galaxy clusters collapses gravitationally; galaxy clusters expand away from each other.
The scale of cosmology is such that galaxies are considered so small that you can treat the entire collection as a set of fluids or a dust that dilutes with the expansion of the universe. The part of any given galaxy that's mostly protons is a mere "dust mote" that floats in free-fall. And the entire dust expands, we don't capture local gravitational collapse.
However, physical cosmologists can also take gravitational collapse into account, for instance to study structure formation ("why are there filaments of galaxies"?). Typically we would take the cosmological expanding spacetime and embed within it "vacuoles" which are collapsing spacetimes, i.e., where the dusts tend to concentrate to a single point over cosmological times. These would typically represent a galaxy cluster. We have some mathematical techniques to figure out what happens at a "junction" between the collapsing spacetime and the expanding spacetime, and the junction is usually at the point where the influence of the collapsing mass is very small. This approach accords well with a lot of observations of how radiation leaves galaxy clusters, and how matter might fall into galaxy clusters from "the great beyond" represented by the expanding matter fluids.
It also lets us use much more complicated models of matter ("enriched chemistry" is the jargon) within the vacuole while ignoring it in the mostly-diffuse-hydrogen extragalactic space, which is useful for figuring out how galaxies assemble and how their first stars ignite.
> Do we know if the dimples on the ballon expand as fast as the rest of it?
> Do the areas around the dimples expand faster?
tl;dr: (1) yes, we know, and are improving accuracy and precision (2) space expands between clusters of galaxies, and matter out there dilutes away; matter within clusters of galaxies tends to concentrate into stars, black holes, and the like, so the behaviour is really opposite.
The "areas around the dimples" are analogous to the expanding cosmological spacetime. The dimples themselves are analogous to a gravitationally b...
I am not a physicist or cosmologist so i won't pretend to understand all of your answers but this has given me a great starting point to learn more and i greatly appreciate you taking the time to write it. I had a pretty theory that we were seeing galaxies accelerate away from each other due to the rate of expansion being greater outside of gravity wells. I will reread your answer a couple of times until i get off there is any support for it but my initial reading suggests it's up there with light needing a medium to travel.
Yes, the "drifting" component you speak of is called the peculiar velocity. It's responsible for the "finger of god" effect if we don't account for it (galaxies appear to be aligned along the line of sight of the observer) and becomes negligible at large enough distances. I guess technically it's hard to differentiate because it's usually random, but it enters the error analysis and is accounted for.
Of course the rate of expansion is not constant; That's the whole deal with dark energy which is understood to drive accelerated expansion of space due to its negative pressure (when you make something with negative pressure larger in volume you have to do positive work on it or give it energy and that energy is dark energy).
But the Hubble tension is something else: It's about measurements of H0 (the current value of the Hubble parameter) using two different methods (early universe and CMB OR recent universe and supernovae, etc.) that do not agree. The disagreement is about two estimations of the current rate of expansion called H0; Everyone agrees H(t) has been increasing (=accelerating expansion); But accelerating toward which recession velocity is the question.
> (abstract) ... However, a persistent difference, called the Hubble Tension, is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang. The NASA/ESA/CSA James Webb Space Telescope has confirmed that the Hubble Space Telescope’s keen eye was _right all along_ (emphasis mine), erasing any lingering doubt about Hubble’s measurements.
This feels like bad reporting to me. A better title would be "Webb confirms Hubble's measurement of Universe’s expansion rate", but that would probably be less exciting...
> Some sort of error in how we measure the variations in the CMB.
IIRC, the prediction that comes from the CMB depends on the lambda cold dark matter model, and there are plenty of independent reasons to be skeptical of that model already. If I were betting which one's most likely to be wrong, that's where I'd put my money.
Dr.Becky goes over this in a video [0] from a year ago about the divergent results obtained by the two main ways we measure the rate of expansion.
Cosmic Microwave Background Vs. Supernovae. As the accuracy of each method has improved, the end results have diverged.
Unfortunatelly, title of article on HN contradicts content of article, so many HN readers skip article because of "confirmed" (in other words: nothing new).
IMHO, a Tired Light theory will better explain facts, but it will require paradigm shift, so there will be a lot of resistance before revolution.
I have my own theory, which is not disproved yet: gravitational background noise slow down light a bit. Gravitation affects whole stream of photons in uniform way, not individual photons. Moreover, it doesn't change direction of photons, so no blur or scattering. My napking maths, which I did few months ago, tells that gravitational delaying should case effect of same magnitude as in red shift, (I did calculation for one frequency only, for proper calculation I need to know the temperature of the noise).
I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again.
How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.
However, wouldn't we expect to see the same observation even without any acceleration? Imagine there are some objects in space all moving in random directions and speeds, relative to Earth. After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed!
In short, even if galaxies weren't accelerating, we would still see that the further away a galaxy is, the faster it recedes.
You're right: the "more distant galaxies are moving away faster" point is just Hubble's original observation of an expanding universe. It's not an argument for cosmic acceleration. (If you see people making that claim, they're probably either speaking carelessly or not experts themselves.)
The conclusion that the expansion is accelerating was a quite recent result: 1990s, I believe. It's based on careful measurements of supernova explosions of a type with computable intrinsic brightness in increasingly distant galaxies, and the exact pattern seen in their apparent redshifts vs. apparent brightness. It was a shocking discovery when it came out, with two separate teams announcing the result pretty much neck and neck. There's also independent and compatible evidence for acceleration from the exact pattern of variations in the temperature of the cosmic microwave background seen at different points in the sky.
Their light is more red shifted the farther away they are. I'm no expert on this, but I believe in a constant-speed scenario they would have equal red shift no matter the distance
The assumption made here is that relative velocity is the only method that would redshift light. Gravitational redshift is a thing and our model of gravity is incomplete.
> The assumption made here is that relative velocity is the only method that would redshift light.
Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime.
> Gravitational redshift is a thing
Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift.
> our model of gravity is incomplete
In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.
I'm not sure what you mean by "phantoms in the data". The distribution of stress-energy is a free parameter in GR; it has to be inferred from observations.
The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory.
"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.
>"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties.
It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.
What evidence do you have that what they are doing now doesn't work, and does the all the evidence of how they work support your hypothesis?
Be detailed, because your comment just has some motivational speaker nonsense but no depth. For example, in the last 20 years cosmology has:
+ Refined its model of stellar formation based on observational data of the number of planets found observationally, and used this to validate and invalidate several model adjustments.
+ Observed galaxies that appear not to have dark matter, and by their existence and behavior validate some theories of dark matter, and validated others, which predict such galactic behavior. (e.g. some theories attempting to update gravity).
+ Run simulations of stellar and glactic formation that predicted structures in the universe that were later observed.
Everywhere they look they are finding things the models don't explain well, and refining the models - that is literally using the data to improve the models.
If you think you can come up with something better, then do it - all you gotta do is make up some mumbo jumbo and write down any old equation. It probably should:
- provide the same results as were observed when the plugging in the experimental parameters of existing experiments.
- explain "wierd stuff" in the data that existing models couldn't.
- predict future observations of the known phenomena with the same or better accuracy as the old model
- predict currently unobserved and unpredicted phenomena
Go ahead and take a stab real quick - I'm sure you can do it. I mean Gallieo did it, so did Newton and Einstein. Next up is willis936.
But in this case you and your friend are not right about the process. There was a claim that "the models aren't being updated based on the new data" which is categorically false. It's not that they pointed out problems in the process, it's that they flat out lied about things - such dishonesty doesn't help solve any problems you imagine you see, its just trolling.
> we haven't made any new discoveries in cosmology in the past two decades
Is that true?
At least for my hobbyist understanding of the progress of cosmology, quite a lot seems to have happened in the past two decades. Confirmation of the Higgs Boson at CERN [0] kept me up all night to watch the press conference; I found it extremely exciting. (Maybe you count this strictly as observational particle physics and not cosmology, but I might appeal for it to be allowed in the context of your critique).
And what of TFA? Isn't what we're reading now a new discovery in cosmology?
What about the rush of exoplanet discoveries?
What about the dramatically different galactic properties now observed in increasingly strange corners of the observable universe (including some which perhaps give insight into some of the properties of "dark matter" or whatever it ends up being)?
Yes, it's true. Mainstream science refuses to accept anything radically new because of huge baggage. Nobody wants to look stupid, then relearn, recalculate, republish, reteach everything, or lose their tenures, grants, etc. It's why science advances in small incremental steps. AFAIK, there is a team of scientists secretly working on radically new set of theories (I got contact but cannot join because of war).
That notion is ridiculous. Finding something radically new is every scientist's dream. Look at how Einstein is perceived, who arguably found one of the most radically new theories. Nobody thought he looked stupid or lost tenure. No one goes into science hoping to simply confirm what everybody already thought was true.
The true reason we have made little progress in the past 20 or so years (and a 20 year slouch is historically nothing unusual) is that pretty much all data we collected in that time frame confirmed the standard model. It's the one big dilemma cosmology has. The standard model (LambdaCDM) works unreasonably well. Our problems with it are largely theoretical. New data is also hard to come by. Look at how long it took to plan, build and launch Euclid, cosmology's big hope of finding new physics. The hubble tension from the OP's article is already the most interesting discovery since 1998 when evidence for dark energy was first seen.
And trust me, all scientists know that all models are wrong. This isn't some unique insight that is beholden to amateur scientists on the sidelines.
As I understand it, if the expansion was constant, farther away stuff would still be more red shifted. Stuff twice as far away appears to be moving twice as fast. It helps me to imagine the expansion of a metal cookie sheet, where the two edges are moving apart faster relative to each other compared to the speed that they're moving away from the center.
The surprising bit is that the far away stuff seems to be even more red shifted than that, so we're not just expanding, but the rate of expansion seems to be accelerating.
If your idea was the case there would always be new things from very far away heading towards us, this is not the case. If the universe is flat and infinite there would be no end in supply of new galaxies with all velocities and you would always have the same mix as the initial mix of velocities. That’s not what we see.
I'm so glad "the infinite universe" as an idea is finally falling off. It works great in the Hitchhikers Guide, I love the floopy mattresses and planets that grow screwdrivers but nothing real is infinite.
I didn't even realize how many people held that belief til that article about how the universe isn't as big as we thought
> After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed!
Sure, but to argue that this explains what we observe today, you would need to show that as of today it has been “long enough,” which is its own can of worms to open.
You might say “obviously it has been long enough for full sorting, because we observe a fully sorted data set of speed correlated with distance.” But that would be begging the question.
It's because the movement is ascribed to the expansion of space itself, not the individual galaxies. We don't have any reason to believe galaxies are moving in random directions at random speeds (not at scales that explain the redshifts we call the expansion of the universe).
In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-moving galaxies close to us that just started really far away. Objects would be entering our local universe from outside it, and that simply doesn't happen.
> In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us
Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post.
I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case.
Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!
> I'm picturing more of an explosion in empty space.
No, that's not what the big bang is.
> this would imply a "center" to the explosion, which I've also heard is not the case
That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.
As I understand it, before the big bang the whole observable universe was contained in a small sphere and then it started to expand metrically. Is this interpretation correct?
Another thing: suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to? To an infinite void? Is it correct to say that stars radiate energy to the infinite then?
It's just an interpretation. Your interpretation is similar to the Big Bang model of visible Universe expansion. If you can convince us that your model is better than other models, then we will use your model, but nobody can prove than a model is correct, unless we will find a hidden recorder somewhere which was turned on for few dozens of billion years.
Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium (photon is a wave, so it waves something).
That sounds suspiciously like postulating the 'ether'. Surely what a photon 'waves' is the electromagnetic field, which is not a medium, and which fills the whole of spacetime. There is no 'end of the medium'.
"Field" means 3d array of numbers. Spacetime means 4d array of numbers. You are talking about mathematical model of Universe, while I'm talking about physics. Mountain is not just an excitement in a height field. If photon is not waving something, then it's not a wave. Physicists prove that photon is a wave.
No, it's not, it's our best current model's description of the actual physical reality of our universe.
> nobody can prove than a model is correct
That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data. For example:
> Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium
For the scenario that was posed, a laser beam that never hits anything, none of your statements here are true. The first is ruled out by the scenario; the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole), and there is no "end of the medium" (again, models in which there would be an "end of the medium", i.e., where the universe stopped containing matter and started being just vacuum, are known not to correctly model our data).
I have described what actually happens in my own response to the GP upthread.
> (photon is a wave, so it waves something).
Light is an electromagnetic wave; what "waves" is the electromagnetic field. (If you use a "photon" model, you are using the quantum electromagnetic field as opposed to its classical approximation.) There doesn't have to be any other "medium"; the electromagnetic field is present everywhere.
> No, it's not, it's our best current model's description of the actual physical reality of our universe.
OK, it's our best model, but it doesn't invalidate other models, less complete or less popular, it compete with them.
> That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data.
Yep. The article is about the Huble Tension, which invalidates Big Bang model. We still use it.
> the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole)
The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.
I assume that the only infinite thing in infinite Universe is Universe itselft. All other things are finite. Thus, a photon has finite life, like any other wave.
> there is no "end of the medium"
The right-hand rule in EM suggests that we are in north hemisphere of something, so south hemisphere will have symmetrical rule, unless you believe that God chose right-hand rule for the whole infinite universe. If we are in a sphere, then that sphere rotates and have a boundary.
> what "waves" is the electromagnetic field.
"Field" is an array of numbers. You are mixing model and reality.
One model can't invalidate other models. Only data can invalidate a model.
What other models do you have in mind?
> the Huble Tension, which invalidates Big Bang model
No, it doesn't. It means we have more work to do, to figure out why two calculations of the Hubble constant, by different routes, give different answers.
Invalidating the Big Bang model would be finding evidence that there was no Big Bang at all. The Hubble tension is nothing of the sort.
> The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.
None of these are issues at all. The model accounts for them all in a perfectly self-consistent fashion.
Also, your nomenclature is biased: for example, the "FTL speeds" you refer to are coordinate speeds, which have no physical meaning. "FTL" in General Relativity means "moving outside the light cones", and that does not happen.
> The right-hand rule in EM suggests that we are in north hemisphere of something
The right-hand rule is a human convention. It tells us nothing about physics.
> You are mixing model and reality.
No, you are incorrectly assuming that the word "field" can only refer to the model. That's not the case. Physicists commonly use the word "field" to refer to both the mathematical object in the model and the actual physical thing that is being modeled. Light is "waves of the electromagnetic field" in the latter sense.
> before the big bang the whole observable universe was contained in a small sphere and then it started to expand
We have no evidence of any time when the universe was not expanding. At the earliest times we have evidence of, the universe was already expanding (extremely rapidly--much, much, much more rapidly than it is now). At those times, our observable universe was indeed contained in a very small volume.
> suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to?
Since the universe is spatially infinite in our best current model, the laser beam will just keep on going forever.
> To an infinite void?
According to our best current model, no, the laser beam will never stop passing by matter, of approximately the same average density as the matter we can see.
> Is it correct to say that stars radiate energy to the infinite then?
Yes, as long as you recognize that "the infinite" never becomes a "void".
>According to our best current model, no, the laser beam will never stop passing by matter, of approximately the same average density as the matter we can see.
I didn't understand this part, why would the laser beam would never stop passing by matter? Because of the metric expansion of the universe? Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?
> why would the laser beam would never stop passing by matter?
Because there is matter everywhere in our universe. It never stops.
> Because of the metric expansion of the universe?
It is true that, in order for there to be matter everywhere in our universe, it must be either expanding or contracting. (There is an edge case, the Einstein static universe, but it is unstable against small perturbations, like a pencil balanced on its point, so it is not a viable option.)
> Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?
If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor.
But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusion is that the expansion is accelerating.
> galaxies further away are moving faster than that
No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now.
Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.
> How do we know that galaxies are accelerating away from us and not moving at constant speed?
More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing).
> People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.
That observation tells us that the universe is expanding, but by itself it does not tell us whether the expansion is accelerating or decelerating or neither. So you are correct that that observation alone is not sufficient to show that the expansion is accelerating.
What we look at to see how the expansion rate changes with time is a comparison of three pieces of observed data, galaxy by galaxy: redshift, brightness, and angular size. The relationship between these three quantities is what cosmologists use to construct a model of the expansion history of the universe, which in turn tells us things like what I said above, that the expansion has been accelerating for the last few billion years but before that it was decelerating.
Meaning, why did the expansion change from decelerating to accelerating a few billion years ago? Because that was when the density of matter, which had dominated the dynamics until then, became smaller than the density of dark energy, which has dominated the dynamics since then. The dark energy density doe not change with time, but the density of matter decreases as the universe expands.
How is is that dark energy density does not change with time? Surely the total amount of dark energy has to be constant (energy can't be created or destroyed, and all that), and then as the universe expands, that's then the same amount of energy over a larger volume, right?
I think the story is that dark energy is indeed created, in the new emptiness resulting from the expansion of space.
<mumble> I believe it's supposed to be spacetime that expands, not 'space'. But it's beyond me to explain what that even means; as I understand it, spacetime refers to the whole Universe, across all of time. To 'expand' means 'to become larger over time'. But if the thing that's expanding includes time itself, then I'm bewildered.
It is in fact the metric expansion of space. The spatial part of the Robertson-Walker spacetime metric expands equally along the time axes of a family of freely-falling future-directed worldlines (we can call them "Eulerian observers", and individual clusters of galaxies are good approximations).
That is, in the past, galaxy clusters are relatively close together, and in the future they are relatively very far apart.
If you need an image, think of a vase of cut flowers, with the stems tightly bound together at the bottom of the vase, and the flowers loosely separated at the top. Time is in the direction away from the vase's bottom. A super thin slice through a stem represents a snapshot of a galaxy cluster at a particular time in its existence.
Dark energy may be the energy of vacuum itself, that's why it's constant. And no, energy conservation does not apply in this case. There is a good blog article on precisely this question by Sean Carroll: https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
> How is is that dark energy density does not change with time?
Because that's how a cosmological constant works.
There are alternate models where there is "dark energy" (as in, stress-energy that causes accelerated expansion) whose density does change with time (for example, a "Big Rip" model in which the dark energy density increases with time), but such models do not match our best current data.
One thing that is not often mentioned is that this effect only applies outside the local supergroup; within the supergroup gravity overrides the expansion of spacetime and holds us together (for now!)
How do we know that galaxies are accelerating away from us and not moving at constant speed?
There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift.
It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confirming the red shift calculations with a different method. That means that if the red shift was caused by any other effect, say the light "degrading" after millions of years of travelling the void, all the calculations would be invalid.
I've asked about this many times and the answers are in the line of "we don't know any other reason for the light to red shift" and "the current theoretical frame is consistent", even if there isn't any other measure to be consistent with.
There was a prediction (expansion is related to Big Bang) that the far away galaxies, being younger, would have a different composition. This prediction seems to be failing, but advances in instrumental could give us a more precise answer in the future.
“Degrading” sounds very intuitive to me. Can the frequency of the light waves simply slow down over a very long distance/time? Or maybe the speed of light simply slows down over an unimaginably long distance? We don’t have any model to describe such behavior, but everyday objects around us all slow down one way or another, what makes light so different?
IIRC, this was one of the explanations proposed when the existence of a red shift was first noted: that the light is somehow slowly losing its energy over very long distances, becoming “redder” as it did so. It ultimately lost out to the dark energy / space-time expansion theory, although I do not recall why. Presumably there was some observation that precluded “degrading” light from being the sole explanation.
There are several challenges for "tired light", or indeed any theory that's an alternative to expansion of the universe.
The theory has to explain why the light gets redshifted, but does not get blurred, and the spectral lines do not get broadened. This severely restricts the type of interactions possible. Also the theory has to explain the consistency between redshifts within our own galaxy, to that of far-away galaxies.
My theory is that redshifting caused by gravitational background noise. I did calculation somewhere on HN or Youtube few months ago and numbers are of same magnitude.
Tired light is tricky to be possible because light doesn’t experience time or distance since it travels at c. Light itself from its frame of reference is emitted and received simultaneously. That’s not to say other perspectives can have different views though!
>It's impossible to triangulate those huge distances
Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of time.
For triangulation to work you need to move, not your target. Triangulation is only used for objects within about 1000 parsecs, where we can triangulate using the movement of the Earth along its orbit.
The American astronomer, Halton Arp, had a theory he called "intrinsic redshift". My limited understanding is that he saw evidence of "very close" and "very far away" structures that are connected to each other in space, which makes no sense. He theorized that redshift may also be indicative of the age of a galaxy, rather than only indicating velocity.
The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.
Degrading light sounds like an interesting idea to me, you could call it the “cosmological damper” if you will.
Thought experiment: imagine you have light particles in a perfectly circular orbit around a black hole. Does the light ever fall into the black hole or does it orbit for eternity?
There a whole set of theories, called "Tired light" theories, which tries to explain Cosmological Red Shift by degradation of photons with time. Buy they require whole set of different cosmological principles: a medium for light propagation is required to dump lost energy into, no Big Bang. But even with a tired light theory, galaxies are accelerating toward attractors, see https://arxiv.org/pdf/1702.02483v1.pdfhttps://www.youtube.com/watch?v=NpV0GQo3P0c
> Hawking radiation would interfere with the eternal orbit
For tractability Hawking radiation tends to be studied using a non-interacting scalar field (often massless), rather than the fields of the Standard Model (which are very much interacting, and have a variety of masses).
Hawking started that in his "Black hole explosions?" (1974) which (if you're not allergic to sci-hub) you can read at <https://sci-hub.se/https://www.nature.com/articles/248030a0>: "consider (for simplicity) a massless Hermitean scalar field \phi which obeys the covariant wave equation ... in an asymptotically flat space time containing a star which collapses to produce a black hole.". In the short paper, he's very much not considering photons or pions, and really doesn't need to in order to make his point.
The thermal spectrum of a real black hole will be in all the Standard Model fields, and except very late in the eventual evaporation will be dominated by very low energy photons originating at a fair distance outside the event horizon.
What do you mean by "eternal orbit"?
Real black holes do not extend to the infinite past. If a black hole ultimately Hawking-evaporates, in finite time there's nothing left to orbit. Late in the evaporation, there will also be a lot of very high energy gamma rays and heavier particles excited in their respective quantum fields of the Standard Model. These will all tend to participate in high-energy multi-particle interactions rather more complicatedly than the end result of Hawking's 1974 description.
Many black hole solutions of the Einstein Field Equations of General Relativity admit a photon sphere or equivalent(s) where light can orbit the black hole, but not usually stably, and in physically reasonable black hole solutions there is no stable circular orbit for light at all, so no eternal orbit. (The orbit can be quite long on human scales though, before the light eventually leaves the area around the black hole, or spirals into the black hole).
You're not going to get a simple answer because the answer is quite complex.
Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference.
It's quite fascinating, but I can't really dump out an entire book into a comment.
> It's quite fascinating, but I can't really dump out an entire book into a comment.
Speaking of which, do you happen to know a good book which illuminates this whole story step by step? I have yet to find one which doesn't start with "Let us assume an FLRW metric…"
Funny, because that article says exactly the same.
"Short" distances are calculated by parallax, taking Earth's orbit diameter as the base.
From there, it's a extrapolation of the spectrum of known types of objects. Not triangulation. Moreover, IIRC individual stars can't be discerned in other galaxies, even the nearest ones, except maybe supernovas.
So at the distances that red shift manifests, no triangulation, only red shift.
-Galaxies are not accelerating, space is expanding.
> Imagine there are some objects in space all moving in random directions and speeds, relative to Earth...
-No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood.
I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is).
They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity.
There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.
What does "space is expanding" mean? That the distance between objects is increasing? How can you tell the difference between "space expanding" and "objects moving in a non-expanding space"? Is there any way to tell the difference or is it just that all objects are moving away at the exact speeds that satisfies the "space expanding" explanation and nothing else?
But then, I'm back to what does "space is expanding" mean? What is doing the expansion?
space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.
Spacetime is 4D array of points. It's like a movie file, but with 3D frames instead of 2D frames (pictures). Expansion of space means that coordinates of points in a frame changed to move away from us, to match movie (model) with reality.
An array of 4D points implies some sort of construct upon which the points exist. As far as I know, the that was the concept of "the ether" and that fell out of fashion long ago.
So again, how can you tell the difference between space expanding or two things actually just moving away from each other (to keep it to a very simple example of two bodies).
things are moving away from other things the faster the farther away they are, uniformly across the universe. everything is moving away from everything, not just two particular points.
is there a difference if there isn't a difference?
Imagine a deflated baloon with two dots drawn close to each other. Now inflate it; the dots didn't move but the plane that they were drawn/positioned on did.
I think it has to do with the observation that no matter in which directinn we look, galaxies are moving away. Its not like we look towards one direction and say, "aha, that must be the center of the universe". Its the same story in ALL directions - we wee galaxies moving away from us at increasing velocity.
If we can travel faster than light does it blow up the theory that the universe expands? Because if we can travel faster than light the universe is theoretically be infinite if I can go to a point pas the furthest reaches of stars/matter.
If we can travel faster than light, that would blow up all the theories that say that we cannot. And FTL travel would imply time travel (if in your reference frame, something is traveling faster than light, there's another reference frame where it is going backwards in time). A lot of science fiction just sweeps this under the table and pretends that we have Newtonian absolute time to go along with the FTL travel.
If we can do FTL it may just prove that light speed can be achieved by brute force, or an alternative method. No laws needs to be broken and new laws can be found. Like quantum physics and classical physics.
The extra energy comes from the increase in gravitational potential energy between objects. The accelerating increase in this potential energy implies the total energy of the universe is increasing. I’m not talking about harvesting dark energy directly.
Heh, I had to go off and read some stuff to get my head around what "negative" potential energy would even mean (high school physics was decades ago).
So... the negative is just a convention to represent work done against the "field", and positive is work done by the field? ie more of a vector than actually being negative energy? I think I get that bit now.
So now I'm wondering if that still applies to an expanding universe vs eg a rocket leaving earth. If things aren't moving further apart by work (ie force x distance) being done against their gravitational fields so much as space time expanding, is there an increase in potential energy? And then if objects are moving apart faster than escape velocity, could that still be seen as increasing potential energy?
We haven't detected dark matter or dark energy outside of their visible effects on the larger universe. Maybe we can't interact with it with baryonic matter. A dark energy engine made out of dark matter would be invisible.
two black holes in highly elliptical orbit, so they they nearly collide when they come together. at the center of mass is a cloud of iron atoms. When the black holes nearly collide tidal forces across iron nuclei rip them apart. These ripped apart atoms can then be re-harvested and used as fuel in a fusion engine. The energy loss from ripping the atoms apart is then recovered by the black holes by taking advantage of dark energy when they go apart, which increases the distance between them, increasing the gravitational potential energy.
I could definitely be wrong, but I don't see how dark energy would enter this scenario. DE doesn't seem to play a role in gravitationally bound systems like two black holes orbiting each other.
dark energy increases the distance between objects, adding to the system's energy by increasing the gravitational potential energy. Without the contribution from dark energy, after several cycles the pull from the iron atoms would cause the black hole orbit to decay.
given that a telescope conceived while Bill Clinton was president in the 1990's got named so as to complete the name "Webb Hubble", let's please please please call the next telescope "Chelsea".
Even though the title is copied from the article, we should change the title to "New data indicates the Webb and Hubble telescopes agree on the universe's expansion rate, but not with the cosmic microwave background measurement based expansion rate"
Even the article seems confusing at first. The new measurements don't seem to be shedding light on anything or removing any confusion. They just confirm that hubble measurements already made. The Hubble tension remains as confounding a problem as ever.
Can someone help me understand - point me toward some reading or ELI5 - what is the universe expanding into? (Or probably, why is that question not formulated well?)
The universe is not expanding into anything. It is infinite. However, new space is being created in the voids between objects. It isn't super intuitive or particularly easy to grasp, things are just getting farther apart.
One of the quantities described by Einstein's equations of General Relativity is the metric tensor—a matrix of matrices—that describes how "far" things are in space and time given the stuff in the local environment. One of the things that the equation tells us is that objects that are not gravitationally bound will tend to get farther away from each other as time passes. We call this the expansion of space. As far as we can tell, there isn't anything "outside" the universe is expanding "into"; distances just become larger somehow.
This article is titled "Webb and Hubble confirm Universe's expansion rate", however I don't see the expansion rate actually listed there, or in the comments. I see a mention of age, but not the rate. Does anyone know the rate? I clicked through to the paper itself[1], but wasn't able to interpret it from the details I could see
Stupid question: how do we know that the universe is strictly exponentially expanding, and not both expanding and contracting in perpetuity like a sin wave? And could such an idea have anything to do with the Hubble tension?
The "big crunch" wikipedia says that it's an idea about the end of the universe, where expansion will reverse and everything will collapse back in on itself.
That is not what I'm asking. I'm asking if expansion and contraction could fluctuate in perpetuity, like stress waves through a block of jello.
We believe that the universe is accurately described by the Friedman-Lemaitre-Robertson-Walker metric, for which we have good reasons, and which only allows a few trajectories of the scale factor. I'm not aware of any model with a sine-like scale factor, but there are people considering cyclical cosmologies. But they would still be a series of big bangs and big crunches.
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[ 5.0 ms ] story [ 310 ms ] threadI remembered the age of the universe as as 13.7 billion years, but I wasn't sure why that was.
Well, the initial WMAP results in 2003 supported an age of 13.7 billion years. Later results nudged this upwards to 13.8 billion years. Of course, all the results have error bars.
Not to mention the underlying philosophical assumptions, such that the "rate of time" has been constant across all of... time.
Aka: How do we know a "year" 13 Billion "years" ago bears any resemblance to one now? What would it mean for it not to?
If a computer chugs along doing { counter++; } at 1 clock cycle per clock cycle for 13.7 billion clock cycles, it will think 13.7 billion clock cycles have passed when counter is 13.7 billion.
On the other hand, if a computer chugs along at one clock cycle per clock cycle for 1 clock cycle, and reads &counter and sees 13.7 billion, it will think 13.7 billion clock cycles have passed.
Either way it's perfectly capable of introspecting it's source code and logically stepping back until the memory location was 0 to see how many clock cycles would have been required to reach it's current state, but that sort of reasoning is completely devoid of meaning without both perfect knowledge of what the true start state was, and a guarantee that no external influences have occurred.
Here in reality, we know neither the our start state nor our isolation level, but the hubris of many is too string to not at least try finding some logical step-back functions and iterating them until they don't know how to go any further, then proudly proclaiming that "The Start". (after all - how could it not be The Start, look, I can iterate the inverse of my step-backward function from then to now and it matches! QED!)
To exist in the state you describe - with neither start state nor isolation level measurable in any tamper-proof way - and to yet still dedicate one's life to observing and pondering the complexity of the resulting cosmos is, to my eye, laudable and beautiful.
Where you see hubris, I see humility. Everyone who attempts to expand the corpus of human understanding of cosmology knows that the endgame is somewhere short of perfection. And yet they are inspired to carry on. It seems to me that matters of state, economics, medicine, technology, and many other fields will benefit from a similar disposition.
For a hypothetical entity inside the game, nothing would have changed.
https://www.vogonswiki.com/index.php/List_of_CPU_speed_sensi...
We measure time in vibrations of a cesium isotope IIRC
The laws of physics invariance under time is a core to our understanding. It would be very disrupting if we found otherwise.
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...
It's great to know that say dating using carbon-14 decay is still useful over those time ranges on planet earth (I don't know if that is something we care about given that fossils don't tend to contain much carbon, but coal and oil deposits are around 400 million years old).
I don't want to imply that this is too small of a sample size, but I will imply that nuclear decay, and the movement of galaxies across the universe might be unrelated. Don't know. Not sure how we'd measure that. Supernova observations would tell us about nuclear fusion and it's limits. Does it tell us about nuclear fission? I don't know.
Let's say I walk into a room and observe someone writing a tally mark on a chalk board once every second. I count 4x10^17 tally marks. I might assume that 4x10^17 seconds ago that same person entered the room and started tallying. I might even observe for the next 4x10^17 seconds they continue to tally. Heck I might even see a recorder going that when I play back at what I assume to be 1x speed, has chalk scratches at regular intervals for 4x10^17 seconds. I still don't have any actual evidence that they started those 8x10^17 seconds ago.
[1] https://en.m.wikipedia.org/wiki/Noether%27s_theorem
Certainly you can't 1) have a model where energy is constant 2) believe there is nonzero energy in the universe and 3) dismiss any model where energy can change as bogus out of hand because you believe it should be constant, without counter-proposing how it even got here.
We have that, it's the big bang.
The argument being:
You can't propose physics ever changed, that'd mean energy could have been created at some point!
Energy was created at some point, wouldn't it be nice to know when?
Everything created at the Big Bang exactly as it is now, stop asking questions.
The difficulty is we don't know what happened in the first planck second after the big bang, let alone before the big bang (if that's meaningful).
We haven't unified quantum mechanics and relativity. Hence we can't be certain that singularities exist, or that the universe started off in a singularity.
It's almost an appeal to common sense. (which, admittedly, is often the best argument we have). IMHO there's often a bit of over-confidence among scientists about the universe being 13 billion years old and what happened during the Big Bang, if it just relies on such a common sense argument.
I know it's unscientific to suggest maybe laws of physics are not time invariant across such scales, because until we have a time machine we can't test this theory, but then flipping the argument (that laws of physics are definitely time invariant) is also technically unscientific -- we're basically assuming this without strong evidence.
This goes back to the old debate on the problem of induction in science, (see David Hume, Karl Popper, etc.) and I think it isn't emphasized enough in modern discussions that, perhaps, there's a small chance that these foundational concepts in physics could be invalid.
To the extent that relates to epistemology that would be more like anti-proof I guess? It does not prove that the laws are time invariant, rather it raises the bar to demonstrate that the laws are not time invariant because that means energy need not be conserved. You can not get one without the other without attacking even deeper fundamentals of modern scientific models. So you must either demonstrate the linked claims, which are pretty foundational themselves, or you must overturn basically everything; both of which demand very robust evidence.
This talk about the "rate of time" doesn't make any sense to me. A second takes one second, and always has.
Isn't this like asking whether the length of a metre might have changed over time? Or the mass of a kilogramme? It looks to me like a category error.
The root of the question is whether there exists an external "system clock", and what that would even mean.
I'll ignore your analogy with a CPU, which I don't think is apposite.
Whether there's a 'system clock', a sort of reference clock that can tell you how fast time is passing, so you can calibrate other clocks against it, seems to be the same category error. As far as I can see, either time is all there, all at once; or one second lasts exactly one second. (I've perpetrated the same category error there, because a second doesn't 'last' for some period of time; it just is).
[Edit] That's not very clear. I mean: if you are measuring a distance, you use a calibrated ruler. If you doubt the calibration of your ruler, you might recalibrate for precision. But you don't have a clear idea of what one foot is, it doesn't make any sense to ask whether your one-foot ruler has grown or shrunk; how would you tell if the purported 'fact' is true or false?
And if some fact about the Universe means that it has grown or shrunk, how would you tell? If you can't in principle tell whether a 'fact' is true or false, it follows that the 'fact' isn't a fact, because it has no effect on anything.
See Russells Teapot.
The question is if there's a different sort of thing beyond our idea of time. We have our concept of N caesium oscillations is the base reference for everything, and the duration of all action is derived from it. Could there be more depth to the rate at which things occur than that, especially on cosmic scales? Could other processes, which operate at different base clock levels, be interacting with the universe we observe in ways we don't yet understand? Could those processes have clock levels that vary over history with respect to our caesium definition?
You can claim not, that's its precisely that shallow. But neither of us can provide evidence either way, and your belief is simply much less interesting to me.
What I ignored was an analogy. Not an argument from analogy; and not a question. Just a rhetorical device, which I considered irrelevant, and didn't feel like arguing about.
> the base reference for everything
Caesium oscillations are a proxy for the passage of time. I can imagine a universe in which the rate of Caesium oscillations might vary, or be influenced, making atomic clocks unreliable. I don't know how one might measure the passage of time directly; I suspect it's impossible, and can only be done with proxies.
I don't see why you couldn't have more than one timescale operating and interacting. It's an interesting speculation. But William of Ockham advised against multiplying hypotheses; and I can't see what multiple interacting timescales might explain, that can't be explained without them.
https://en.wikipedia.org/wiki/Hubble%27s_law#Determining_the... (caption: "Value of the Hubble constant in (km/s)/Mpc, including measurement uncertainty, for recent surveys[54]")
From the article:
"The bottom line is that the so-called Hubble Tension between what happens in the nearby Universe compared to the early Universe’s expansion remains a nagging puzzle for cosmologists. There may be something woven into the fabric of space that we don’t yet understand."
Before we know better it can be just that spacetime was expanding at a different rate (we still would need at least one another Planck that operates in roughly same range to confirm this).
Hubble wavelenght range - 0.1 to 0.8 μm Webb wavelenght range - 0.6 to 28.3 μm Planck wavelenght range - 330 to 10000 μm
My understanding is that Planck was observing photons that have happened much more earlier.
That's quite a strong claim. I'm skeptical. Sources?
> Have physicists looked into this?
They shelved it right next to "God Made The Universe" in the "Unfalsifiable Propositions" section, under the title "Grad Students Made The Universe."
Software engineers presenting weird pseudo science as serious physics is one way this manifests.
I could be wrong.
It's also astounding that celestial objects emit enough photons that thousands per second travel within the arc that goes the distance from the star to somewhere inside the radius of your pupil.
And if this wasn't the case, then we'd never even be aware of any of this.
Expansion of the universe: https://en.wikipedia.org/wiki/Expansion_of_the_universe :
> While objects cannot move faster than light, this limitation only applies with respect to local reference frames and does not limit the recession rates of cosmologically distant objects
Given that v is velocity in the opposite direction, and c is the constant reference frame speed of light; do we account for velocity in determining whether light traveling at c towards earth will ever reach us?
Are tachyons FTL, is there entanglement FTL?How far away in light years does a mirror in space need to be in order to see dinosaurs that existed say 100 million years ago?
As far as I know this is not necessary because the speed of light is constant regardless of the velocity of both the source and the observer (this is Einstein's special relativity: https://en.m.wikipedia.org/wiki/Special_relativity)
>> While objects cannot move faster than light, this limitation only applies with respect to local reference frames and does not limit the recession rates of cosmologically distant objects
Then the length traveled changes for two photons emitted when they cross the starting line at different velocities:
Should (v + c) be prematurely reduced to just (c), with a confirmed universal expansion rate?Nothing in the universe can travel faster than the speed of light. This does not hold for the universe itself. It can and does expand faster than the speed of light, using specific reference frames (i.e., big enough).
So, space can increase FTL. Particles do not travel faster than light tho, that is nonsense.
> from https://news.ycombinator.com/item?id=35877402#35886041 : "EM Wave Polarization Transductions" Lt. Col. T.E Bearden (1999) :
>> Physical observation (via the transverse photon interaction) is the process given by applying the operator ∂/∂t to (L^3)t, yielding an L3 output
[and "time-polarized photons"]
If the Copenhagen interpretation is correct, particle states are correlated after a photonic beam splitter; if you measure entangled photons after a beam splitter, their states are still linked.
If virtual particle states are entangled with particle states in black holes or through ER=EPR bridges, is there effectively FTL?
There is FTL within dielectric antennae.
No, because the speed of light is constant for all observers. From our frame of reference on earth, light from distant receding galaxies is always moving towards us at exactly the speed of light, c. Those galaxies also observe the light moving away from them at exactly c.
That seems contradictory and unintuitive, that two observers moving away from each other both measure light moving c relative to themselves, but that’s reality.
It’s another measurement that changes: if c is always constant, then it must be the passage of time and the distance travelled that we observe differently.
Light is a wave. Photon is a complex thing (Hopfion?), but it's a wave, so it waves something, a medium. Speed of wave propagation in a medium is constant. IMHO, it's intuitive.
Which was unobserved in the michleson-morely experiment and other followup experiments because....?
If you want to posit that light has a medium, you need to redo 100 years of physics, so start showing your work. You mention elsewhere "doing a calculation", and that's just not nearly good enough. You want to overturn perfectly working physics, you NEED to show up with receipts. That's table stakes.
A mirror in a vacuum 1ly away will return the photonic signal from t=0 at t=2 light years, with diffraction (due to matter in [solid, liquid, gas, plasma, and superfluid/superconductor] phases describable with superfluid quantum gravity (e.g. Fedi's with Bernoulli pressure))
A water droplet [in space] reflects enough [photonic,] information to recover a modulated signal and also a curvilinear transformation of Escher looking into a crystal ball, with c the speed of light as a consideration only at cosmological distances.How large of water droplet, a regular or irregular spheroid or reflective and/or lensing matter configuration is necessary to reflect a sufficient amount of photonic information to recover information from cosmological information medium, in terms of constructor theory?
Inverse scattering transform: https://en.wikipedia.org/wiki/Inverse_scattering_transform
R-matrix > R-matrix method in quantum mechanics: https://en.m.wikipedia.org/wiki/R-matrix :
> [now generalized for photonic diffraction]
Quantum inverse scattering method > Procedure: https://en.wikipedia.org/wiki/Quantum_inverse_scattering_met... :
> 1. Take an R-matrix which solves the Yang–Baxter equation.
> 2. Take a representation of an algebra T_R satisfying the RTT relations. [[clarification needed]]
> 3. Find the spectrum of the generating function t(u) of the centre of T_R
> 4. Find correlators
Also, these are very smart people. You'd be surprised what we can work out with a few very reasonable assumptions and the observations we made. The dismissiveness really isn't warranted.
IE. I'm criticizing pop-science, not science.
One last thought on this.. I understand these sorts of understood meanings work fine when speaking with professional peers, as they'd understand the assumption. But if you continue to do this when speaking to people outside your field, you will not be saying what you mean to say and will be teaching people incorrect ideas.
• From variations in the cosmic microwave background (CMB) which are the result of certain conditions in the early universe it is possible to figure out what the expansion rate should be now.
• From looking at very distant galaxies and noting how far away they are and how fast they are receding from us the expansion rate can be calculated.
Theory says that these should give the same expansion rate. When the rates were first found using those two methods they gave different results, but the error bars on both were large enough to overlap. People expected then that further refinement of both methods to decrease the error bars would converge to some common value.
That did not happen. Refinement of the CMB measurements got to 67 +/- 0.5, and refinement of the galaxy distance/speed method got to 73 +/- 1. Those do not overlap.
This non-overlap between the possible ranges given by the two methods is called the Hubble tension, and it is one of the most irksome problems in cosmology.
Possible explanations include:
• Some sort of error in how we measure the variations in the CMB.
• Some sort of error in the distant galaxy distance or speed measurements, which until the James Webb telescope were almost entirely Hubble telescope measurements.
• We're missing something in our understanding of the physics.
These new results add a bunch of data from the James Webb telescope, which observes in different wavelengths than Hubble. These results fit with the Hubble measurements.
They do not resolve the Hubble tension. What they do is remove most doubt that the distant galaxy results involve some sort of Hubble measurement error. I believe cosmologists are pretty confident of the CMB measurements, and so this will be interpreted as telling us that the Hubble tension is not just a problem with our measurements. There is either physics that we got wrong or physics we need to discover.
I guess if humans have will and are part of the universe, then the universe has it too, but I don't think that's what you meant.
Given that we have evidence of physical systems evolving their states by conscious choice, that begs the question "Why would large, complex physical systems have a governing force that doesn't exist in smaller physical systems?" Occam's Razor suggests that consciousness and will drive the evolution of all physical systems, and our elegant equations are merely statistical approximations following the law of large numbers given a population with a natural variance. Of course we can debate what it means for a low information system to be conscious and have will, but I think that's a more meaningful debate than trying to pinpoint the exact moment that a system becomes complex enough for the magical emergence fairy to sprinkle consciousness dust on inert matter.
https://en.wikipedia.org/wiki/General_relativity#Relationshi...
< atomic and > planetary have very important applications!
But I'd argue not nearly as many as "Here are all the formula that govern a falling apple."
Also, I forget off the top of my head, but there's some oddities depending on your chosen frame of reference when looking @ hawking radiation (skip to part about relativity of the vacuum) that I think could apply to the cosmic event horizon as well
https://youtu.be/isezfMo8kWQ?si=5m_L6JtZ7p7Ls6xH
Hubble ended his book Observational Approach to Cosmology[+] with the statement:..."if the recession factor is dropped, if redshifts are not primarily velocity-shifts, the picure is simple and plausible. There is no evidence of expansion and no restriction of time-scale, no trace of spatial curvature, and no limitation of spatial dimensions. Moreover, there is no problem of internebular material. The observable region is thoroughly homogeneous; it is too small a sample to indicate the nature of the universe at large. The univers might even be an expanding model, provide the rate of expansion, which pure theory does not specify, in inappreciable. For that matter, the universe might even be contracting."
[+] https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf
source: https://plasmauniverse.info/people/contributors.html
Redshifts can be way higher than 1 that in your scaling is a magic value.
https://en.wikipedia.org/wiki/List_of_the_most_distant_astro...
And in any case physics still needs to explain what happened to that blackbody radiation.
Wouldn't that smear the spectrum so that it no longer so matches the black body radiation of a single object of a set temperature?
As for smearing, IIRC black body curve has the central limit property
Yes, that's my point.
>As for smearing, IIRC black body curve has the central limit property
Surely that can only be true for a very specific distribution of temperatures? To go to an extreme, if you combine the spectrum of two objects of different temperatures you do not get anything like a black body curve.
The CMB has narrow spectral lines imposed on it by galaxies, dust clouds, and similar objects. Those lines can be used to measure the CMB effective temperature at an object's redshift. A specific example of how this works is linked at the bottom of this comment. If a similar highly dusty object is found in the foreground of a bright quasar (merely nearby quasars were used in the case below), we could get an even tighter measurement of the CMB effective temperature at the dust, and perhaps look in new ways at how the CMB deviated (at the dusty object) very slightly from a blackbody spectrum. This amounts to a new line of evidence in the study of the expansion history.
More generally, constraining the evolution of the CMB temperature in the matter-dominated era (T_cmb > 4 K, z > ~ 0.4) will prove fatal to ideas like some in this thread (which is already in trouble because of other spectroscopy e.g. the Lyman-alpha forest and Gunn-Peterson trough).
I don't really understand the summary of the "'fringe' theory" at the top of the thread (and I can't identify which specific theory throwawaymaths means). I guess the idea could be that some unknown mechanism might homogenize the emissive components of baryonic intergalactic dust but not circumgalactic medium spectral features. I suppose one could try to investigate a non-adiabatic expansion to explain those features found in the CMB in a nonstandard way. Non-adiabatic components in the expansion have been examined by various theoretical teams in the context of quantum cosmology and/or "fifth force" dark energy, although mostly at much larger lookback times than the the standard surface of last scattering.
I don't understand what was meant by '"longer" (aka galactic expansion) timeframe'.
Finally, "echo of the big bang" isn't really helpful in understanding the origin of the CMB photons, why they're now filling space with a thermal Planck spectrum, and why we detect small variations in that spectrum in narrow views along different directions, but like "fabric of spacetime" I guess we're kinda stuck with the expression.
- --
(Open Access) Riechers, D.A., Weiss, A., Walter, F. et al. Microwave background temperature at a redshift of 6.34 from H2O absorption. Nature 602, 58–62 (2022). https://doi.org/10.1038/s41586-021-04294-5
So this isn't summing all of the dust you go through, but rather the redshift all the way back to an era when the universe was super dusty (with the expected thermal signature of that era)
Under time reversal eventually the very first stars disintegrate into mostly atomic hydrogen, which compresses and heats adiabatically. Eventually it becomes hot enough that the hydrogen ionizes. Ignoring processes which alter photon number, the hot nuclei and electrons form a dense fog.
As with a fog here on earth, where incoming light can get bounced in a random direction off a fog droplet ("Mie scattering"), photons scattering off these hot charged particles are scattered in a random direction ("Compton scattering" at high energies, "Thomson scattering" at low energies). When we densify the fog, the free-streaming length of light decreases, increasing the fog's opacity.
Under the normal direction of time, therefore, the CMB is when de-ionization enormously shrinks the analogue of the fog droplet size ("Thomson cross-section"), and expansion increases the distance between the fog droplet analogues. The free-streaming length can become effectively infinite, like clear night air after a fog dissipates. The spectrum of the light at that point encodes the effective temperature of the scattering medium. In an expanding universe, that spectrum will lose energy (i.e., redshift).
The electrically neutral mostly-hydrogen then takes three principal forms: collapsing clouds of gas, which eventually form the first low-metallicity stars; cold dusts of neutral atoms at various sparser densities; and a warm-to-hot sparse intergalactic medium. The latter two is where we should find the so-called "missing baryons", the large fraction of atoms not found in stars and galaxies.
Backlighting by active galactic nuclei and UV-hot stars ruins the idea that a microwave-bright diffuse dust of electrically neutral atoms or molecules could generate the CMB with its spectral features. You'd have to keep the stars from heating the dust elements, while keeping the dust dense enough to generate the CMB photon-density. How do you preserve that density during expansion?
At the top of the thread you said "the primary proponent of this theory is trying to justify some sort of cyclic model", which doesn't escape this point. (It also didn't lead me to a reference).
However, if instead whoever you are struggling to remember is the proponent of an eternal and static cosmology -- with no expansion, ever -- some of these problems with the idea that the CMB can be the product of cold dust could be overcome. The idea might be that we still have a speed of light lookback, with more distant galaxies being older. The older galaxies being redder could be some sort of dust that is very thick in the distant past, thick enough to completely shroud whole galaxies like a lampshade, turning hot thermal Planck spectra into cold thermal Planck spectra. Then you have to (a) get rid of the dust over time without sending more or hotter photons in our direction, and (b) add spectral features to the cold thermal Planck spectra before it gets to us. I don't see how either could be done without very different atomic and/or gravitational physics than we have in our solar system. Adding in supernovae -- whose light-curves we see redshifted as per the article at the top -- makes this idea even harder, because then you need a lampshade-dust that down-converts photon energies in even more ways.
I'm not aware of any published or even serious attempt to do this, although I didn't look too deeply into the literature beyond confirming that this wasn't something proposed by Jayant Narlikar.
You don't. We aren't in a particularly dusty universe right now. It's also not completely clear. There is dust. This dust is dilute. If our universe is expanding, at some point that dust was denser.
Now run the numbers from known estimates of how thinly dusty out universe is now. Given the rate of expansion, You might get a somewhat dusty universe at some time after the big bang, no?
I don't know how to say this in any plainer English. Your comments are full of lots of words that are muddling the point.
> I'm not aware of any published or even serious attempt to do this
That's too bad. I would want to see it done, if only to confirm that the CMB can't be intergalactic dust from a denser era
Why don't you do that, and share the results here?
> I would want to see it done
Why don't you do it yourself?
Back when I was a hard scientist it was MY responsibility to show that I had done all the controls correctly. And trust me that was hours, weeks of grueling manual labor. A pity if physicists aren't held to the same standard.
Imagine this exchange.
Reviewer: Did you run this control?
Scientist: No, you run it.
Good luck publishing.
Additionally, is it really you that's mounting a challenge? Quoting you: "There's a 'fringe' theory ... the primary proponent ... if my understanding is correct it would still be compatible with ..." <https://news.ycombinator.com/item?id=39674424>.
So,
> crackpot throwawaymaths idea
it is not really your idea, is it?
I see you as taking the position that you're entitled to require me to make efforts to assist you. I also note that what I did volunteer upthread, you rejected as "muddling the point". Fine. I think we've evaporated each other's conversational good will. Bye bye.
Also true for two normally distributed variables and yet the normal distribution has the central limit property.
Presuming the hypothesis, do you suspect that you'd be looking at exactly two grains of dust, one of which has a crazy high temperature and the other has a crazy low temperature in any given pixel of the CMB?
The amount of change millennials and the generations before and after us have seen is boggling.
I was out in country Victoria, Australia a few months back and the internet was TERRIBLE. I'm talking, JPG's loading line by line terrible. And this was on 'alleged' 4G.
I felt pretty nostalgic for all of 2-3 minutes before I started pulling my hair out. I feel nostalgic about it again now.
Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?
With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”;
diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection;
and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric;
and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion,
with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
>A lot of great science started as a strange idea.
Disagree. A lot of great science started with "This weird math I'm doing is the only way to fit this data", which is an extremely different thing.
Do you not think physicists and cosmologists and their nerdy friends have thought of wild ideas? Do you not think undergrad students spend drunken friday nights theorycrafting universes?
It's pretty disrespectful to "why don't you just" a profession without showing that you at least did your homework first, or that you have looked at whoever definitely asked that question before you.
There are people in this post reinventing Tired light and Aether based theories for crying out loud, theories with fairly conclusive reasons we abandoned them.
When you just ass-pull theorycrafting without any of the fundamentals, all you do is talk in circles. It's no better than all the dumb "the brain is like a computer" arguments by analogy in neuroscience posts.
It's not even wrong.
We live in a world where the people who had to cheat off me in high school biology angrily insist that the vaccine doesn't work. Do we really benefit from playing such anti-scientific games?
It's one thing to ask curious questions, but "why don't you just do the thing that was ruled out 100 years ago" and expecting a detailed rebuttal is a bit rich. It's like asking "why don't they cure cancer by cutting it out of the body". Or "why don't you just apply basic set theory to P=NP".
It's always the same on these HN threads about cosmology. Even worse with dark matter, probably because you can understand rotating curves with high school physics. No one ever invents or roots for fringe theories on baryonic acoustic oscillations in the angular power spectrum of the cosmic microwave background.
BAOs are clearly the sucker-marks made when the great and mighty Kroll <https://tardis.fandom.com/wiki/Kroll> seized the walls of the universe!
https://insidetheperimeter.ca/wp-content/uploads/2021/05/Bar...
I'm not nearly as bothered by people typing out what they themselves admit are essentially physics fantasies (especially when prefaced with "I'm not a physicist" or the like), as by people who try to convince readers with even less exposure to physics that some alternative to good theories is clearly better but that the powers that be of theoretical physics are somehow suppressing it, and then clearly have little understanding about how the good theories work. There's a lot of that, especially in DM threads.
Once a galaxy has moved beyond or "light cone", it's lost forever: you won't see it again even if you try moving towards it at light speed for all eternity.
Cf the last paragraph of https://en.wikipedia.org/wiki/Hubble_volume
Observations indicate that the expansion of the universe is accelerating, and the Hubble constant is thought to be decreasing. Thus, sources of light outside the Hubble horizon but inside the cosmological event horizon can eventually reach us. A fairly counter-intuitive result is that photons we observe from the first ~5 billion years of the universe come from regions that are, and always have been, receding from us at superluminal speeds.
The next episode, "How Far Beyond Earth Could Humanity Spread?" [2] might also be interesting.
[1] https://www.youtube.com/watch?v=eVoh27gJgME
[2] https://www.youtube.com/watch?v=R-eIUxwcQo4
Having objects moving away from us at a speed greater than C, isn't weird. The observable universe is a 3d subspace of a higher dimensional object. A good analogy is a balloon, where there's a 2d subspace on a 3d object that's being inflated. Even if you can only move at a certain velocity, the balloon can inflate such that the 2d surface expands faster than the max velocity we prescribe for it, and would grow faster at the beginning even if we pumped a constant amount of air into said balloon.
Great analogy for gravity too! because you could create dimples in the balloon (gravity wells) which would curve a straight trajectory, while being unnoticed to an observer on the 2d subspace.
Your questions (if you pardon the expression) poke holes in the balloon analogy. The latex or other stretchy balloon material is denser and under less tension around dimples. Local experiments by a (spatially) 2d observer could determine these features experimentally, and is likely to determine that there is a shear force that is not confined to its 2d "world". The 2d observer could in principle also do geometry and discover the amount of large-scale positive spatial curvature of its "world"; when we do that at cosmological scales we find flat or even slightly negative spatial curvature. The 2d observer could also discover the gravitation of our world: put a drop of water somewhere on the surface of an inflated ballon, and that drop will tend to roll downwards. We haven't found anything like that.
Humanity has looked for forces that give even the slightest evidence in favour of extra spatial dimensions, but there is no experimental evidence that favours having more than our familiar three. We've also looked at many many ways in which space could be some sort of medium comparable to the balloon latex, and practically none of them has survived contact with experiment (and those that survive are mostly hard to analogize with stretchy latex, even when entropic forces -- those are why you can scrunch or inflate a balloon and when you release the scrunching-pressure or internal air pressure the balloon relaxes to pretty much its original shape -- are relevant gravitationally).
> Do we know if dimples on the balloon expand as fast as the rest of it
tl;dr: yes: the material of galaxy clusters collapses gravitationally; galaxy clusters expand away from each other.
The scale of cosmology is such that galaxies are considered so small that you can treat the entire collection as a set of fluids or a dust that dilutes with the expansion of the universe. The part of any given galaxy that's mostly protons is a mere "dust mote" that floats in free-fall. And the entire dust expands, we don't capture local gravitational collapse.
However, physical cosmologists can also take gravitational collapse into account, for instance to study structure formation ("why are there filaments of galaxies"?). Typically we would take the cosmological expanding spacetime and embed within it "vacuoles" which are collapsing spacetimes, i.e., where the dusts tend to concentrate to a single point over cosmological times. These would typically represent a galaxy cluster. We have some mathematical techniques to figure out what happens at a "junction" between the collapsing spacetime and the expanding spacetime, and the junction is usually at the point where the influence of the collapsing mass is very small. This approach accords well with a lot of observations of how radiation leaves galaxy clusters, and how matter might fall into galaxy clusters from "the great beyond" represented by the expanding matter fluids. It also lets us use much more complicated models of matter ("enriched chemistry" is the jargon) within the vacuole while ignoring it in the mostly-diffuse-hydrogen extragalactic space, which is useful for figuring out how galaxies assemble and how their first stars ignite.
> Do we know if the dimples on the ballon expand as fast as the rest of it?
> Do the areas around the dimples expand faster?
tl;dr: (1) yes, we know, and are improving accuracy and precision (2) space expands between clusters of galaxies, and matter out there dilutes away; matter within clusters of galaxies tends to concentrate into stars, black holes, and the like, so the behaviour is really opposite.
The "areas around the dimples" are analogous to the expanding cosmological spacetime. The dimples themselves are analogous to a gravitationally b...
But the Hubble tension is something else: It's about measurements of H0 (the current value of the Hubble parameter) using two different methods (early universe and CMB OR recent universe and supernovae, etc.) that do not agree. The disagreement is about two estimations of the current rate of expansion called H0; Everyone agrees H(t) has been increasing (=accelerating expansion); But accelerating toward which recession velocity is the question.
Thank you! They should have clarified this up front in the title or the abstract. The original article makes it sound like it has been resolved:
> (title) Webb & Hubble confirm Universe’s expansion rate
> (abstract) ... However, a persistent difference, called the Hubble Tension, is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang. The NASA/ESA/CSA James Webb Space Telescope has confirmed that the Hubble Space Telescope’s keen eye was _right all along_ (emphasis mine), erasing any lingering doubt about Hubble’s measurements.
This feels like bad reporting to me. A better title would be "Webb confirms Hubble's measurement of Universe’s expansion rate", but that would probably be less exciting...
IIRC, the prediction that comes from the CMB depends on the lambda cold dark matter model, and there are plenty of independent reasons to be skeptical of that model already. If I were betting which one's most likely to be wrong, that's where I'd put my money.
[0] 'theJWST just made the "Crisis in Cosmology" WORSE' https://www.youtube.com/watch?v=hps-HfpL1vc&t=858s
IMHO, a Tired Light theory will better explain facts, but it will require paradigm shift, so there will be a lot of resistance before revolution.
I think the main point of resistance is the incompatibility with observations. All Tired light models have been falsified: https://en.wikipedia.org/wiki/Tired_light#Specific_falsified...
If you're aware of a model that can fit some or all of our observations, please share it!
How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.
However, wouldn't we expect to see the same observation even without any acceleration? Imagine there are some objects in space all moving in random directions and speeds, relative to Earth. After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed!
In short, even if galaxies weren't accelerating, we would still see that the further away a galaxy is, the faster it recedes.
The conclusion that the expansion is accelerating was a quite recent result: 1990s, I believe. It's based on careful measurements of supernova explosions of a type with computable intrinsic brightness in increasingly distant galaxies, and the exact pattern seen in their apparent redshifts vs. apparent brightness. It was a shocking discovery when it came out, with two separate teams announcing the result pretty much neck and neck. There's also independent and compatible evidence for acceleration from the exact pattern of variations in the temperature of the cosmic microwave background seen at different points in the sky.
Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime.
> Gravitational redshift is a thing
Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift.
> our model of gravity is incomplete
In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.
Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?
The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory.
"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.
It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.
How do you propose they do it differently?
What evidence do you have that what they are doing now doesn't work, and does the all the evidence of how they work support your hypothesis?
Be detailed, because your comment just has some motivational speaker nonsense but no depth. For example, in the last 20 years cosmology has:
+ Refined its model of stellar formation based on observational data of the number of planets found observationally, and used this to validate and invalidate several model adjustments.
+ Observed galaxies that appear not to have dark matter, and by their existence and behavior validate some theories of dark matter, and validated others, which predict such galactic behavior. (e.g. some theories attempting to update gravity).
+ Run simulations of stellar and glactic formation that predicted structures in the universe that were later observed.
Everywhere they look they are finding things the models don't explain well, and refining the models - that is literally using the data to improve the models.
If you think you can come up with something better, then do it - all you gotta do is make up some mumbo jumbo and write down any old equation. It probably should:
- provide the same results as were observed when the plugging in the experimental parameters of existing experiments.
- explain "wierd stuff" in the data that existing models couldn't.
- predict future observations of the known phenomena with the same or better accuracy as the old model
- predict currently unobserved and unpredicted phenomena
Go ahead and take a stab real quick - I'm sure you can do it. I mean Gallieo did it, so did Newton and Einstein. Next up is willis936.
If one does commit themselves to their passion they should strive to push the field further forward, not stagnate it.
It can be done.
https://arxiv.org/abs/2402.19459
Is that true?
At least for my hobbyist understanding of the progress of cosmology, quite a lot seems to have happened in the past two decades. Confirmation of the Higgs Boson at CERN [0] kept me up all night to watch the press conference; I found it extremely exciting. (Maybe you count this strictly as observational particle physics and not cosmology, but I might appeal for it to be allowed in the context of your critique).
And what of TFA? Isn't what we're reading now a new discovery in cosmology?
What about the rush of exoplanet discoveries?
What about the dramatically different galactic properties now observed in increasingly strange corners of the observable universe (including some which perhaps give insight into some of the properties of "dark matter" or whatever it ends up being)?
0: https://home.web.cern.ch/news/news/physics/new-results-indic...
And trust me, all scientists know that all models are wrong. This isn't some unique insight that is beholden to amateur scientists on the sidelines.
The surprising bit is that the far away stuff seems to be even more red shifted than that, so we're not just expanding, but the rate of expansion seems to be accelerating.
I didn't even realize how many people held that belief til that article about how the universe isn't as big as we thought
Sure, but to argue that this explains what we observe today, you would need to show that as of today it has been “long enough,” which is its own can of worms to open.
You might say “obviously it has been long enough for full sorting, because we observe a fully sorted data set of speed correlated with distance.” But that would be begging the question.
In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-moving galaxies close to us that just started really far away. Objects would be entering our local universe from outside it, and that simply doesn't happen.
Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post.
I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case.
Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!
No, that's not what the big bang is.
> this would imply a "center" to the explosion, which I've also heard is not the case
That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.
Another thing: suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to? To an infinite void? Is it correct to say that stars radiate energy to the infinite then?
Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium (photon is a wave, so it waves something).
That sounds suspiciously like postulating the 'ether'. Surely what a photon 'waves' is the electromagnetic field, which is not a medium, and which fills the whole of spacetime. There is no 'end of the medium'.
No, you’ve simply hit the limits of needing/wanting to understand something in terms of something else similar or more familiar.
Makes the same point on a related matter: https://youtu.be/Q1lL-hXO27Q
No, it's not, it's our best current model's description of the actual physical reality of our universe.
> nobody can prove than a model is correct
That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data. For example:
> Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium
For the scenario that was posed, a laser beam that never hits anything, none of your statements here are true. The first is ruled out by the scenario; the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole), and there is no "end of the medium" (again, models in which there would be an "end of the medium", i.e., where the universe stopped containing matter and started being just vacuum, are known not to correctly model our data).
I have described what actually happens in my own response to the GP upthread.
> (photon is a wave, so it waves something).
Light is an electromagnetic wave; what "waves" is the electromagnetic field. (If you use a "photon" model, you are using the quantum electromagnetic field as opposed to its classical approximation.) There doesn't have to be any other "medium"; the electromagnetic field is present everywhere.
OK, it's our best model, but it doesn't invalidate other models, less complete or less popular, it compete with them.
> That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data.
Yep. The article is about the Huble Tension, which invalidates Big Bang model. We still use it.
> the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole)
The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.
I assume that the only infinite thing in infinite Universe is Universe itselft. All other things are finite. Thus, a photon has finite life, like any other wave.
> there is no "end of the medium"
The right-hand rule in EM suggests that we are in north hemisphere of something, so south hemisphere will have symmetrical rule, unless you believe that God chose right-hand rule for the whole infinite universe. If we are in a sphere, then that sphere rotates and have a boundary.
> what "waves" is the electromagnetic field.
"Field" is an array of numbers. You are mixing model and reality.
One model can't invalidate other models. Only data can invalidate a model.
What other models do you have in mind?
> the Huble Tension, which invalidates Big Bang model
No, it doesn't. It means we have more work to do, to figure out why two calculations of the Hubble constant, by different routes, give different answers.
Invalidating the Big Bang model would be finding evidence that there was no Big Bang at all. The Hubble tension is nothing of the sort.
> The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.
None of these are issues at all. The model accounts for them all in a perfectly self-consistent fashion.
Also, your nomenclature is biased: for example, the "FTL speeds" you refer to are coordinate speeds, which have no physical meaning. "FTL" in General Relativity means "moving outside the light cones", and that does not happen.
> The right-hand rule in EM suggests that we are in north hemisphere of something
The right-hand rule is a human convention. It tells us nothing about physics.
> You are mixing model and reality.
No, you are incorrectly assuming that the word "field" can only refer to the model. That's not the case. Physicists commonly use the word "field" to refer to both the mathematical object in the model and the actual physical thing that is being modeled. Light is "waves of the electromagnetic field" in the latter sense.
We have no evidence of any time when the universe was not expanding. At the earliest times we have evidence of, the universe was already expanding (extremely rapidly--much, much, much more rapidly than it is now). At those times, our observable universe was indeed contained in a very small volume.
> suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to?
Since the universe is spatially infinite in our best current model, the laser beam will just keep on going forever.
> To an infinite void?
According to our best current model, no, the laser beam will never stop passing by matter, of approximately the same average density as the matter we can see.
> Is it correct to say that stars radiate energy to the infinite then?
Yes, as long as you recognize that "the infinite" never becomes a "void".
I didn't understand this part, why would the laser beam would never stop passing by matter? Because of the metric expansion of the universe? Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?
Because there is matter everywhere in our universe. It never stops.
> Because of the metric expansion of the universe?
It is true that, in order for there to be matter everywhere in our universe, it must be either expanding or contracting. (There is an edge case, the Einstein static universe, but it is unstable against small perturbations, like a pencil balanced on its point, so it is not a viable option.)
> Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?
No. Such a model cannot match our observed data.
If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor.
But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusion is that the expansion is accelerating.
No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now.
Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.
More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing).
> People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.
That observation tells us that the universe is expanding, but by itself it does not tell us whether the expansion is accelerating or decelerating or neither. So you are correct that that observation alone is not sufficient to show that the expansion is accelerating.
What we look at to see how the expansion rate changes with time is a comparison of three pieces of observed data, galaxy by galaxy: redshift, brightness, and angular size. The relationship between these three quantities is what cosmologists use to construct a model of the expansion history of the universe, which in turn tells us things like what I said above, that the expansion has been accelerating for the last few billion years but before that it was decelerating.
I think the story is that dark energy is indeed created, in the new emptiness resulting from the expansion of space.
<mumble> I believe it's supposed to be spacetime that expands, not 'space'. But it's beyond me to explain what that even means; as I understand it, spacetime refers to the whole Universe, across all of time. To 'expand' means 'to become larger over time'. But if the thing that's expanding includes time itself, then I'm bewildered.
That is, in the past, galaxy clusters are relatively close together, and in the future they are relatively very far apart.
If you need an image, think of a vase of cut flowers, with the stems tightly bound together at the bottom of the vase, and the flowers loosely separated at the top. Time is in the direction away from the vase's bottom. A super thin slice through a stem represents a snapshot of a galaxy cluster at a particular time in its existence.
https://s3-eu-west-1.amazonaws.com/images.linnlive.com/de6e1...
<https://media.istockphoto.com/id/578833902/vector/expansion-...> : time increases from the left to the right.
Because that's how a cosmological constant works.
There are alternate models where there is "dark energy" (as in, stress-energy that causes accelerated expansion) whose density does change with time (for example, a "Big Rip" model in which the dark energy density increases with time), but such models do not match our best current data.
< 47k years: radiation-dominated era
< 9.8G years: matter-dominated era
> 9.8G years: dark-energy-dominated era
There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift.
It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confirming the red shift calculations with a different method. That means that if the red shift was caused by any other effect, say the light "degrading" after millions of years of travelling the void, all the calculations would be invalid.
I've asked about this many times and the answers are in the line of "we don't know any other reason for the light to red shift" and "the current theoretical frame is consistent", even if there isn't any other measure to be consistent with.
There was a prediction (expansion is related to Big Bang) that the far away galaxies, being younger, would have a different composition. This prediction seems to be failing, but advances in instrumental could give us a more precise answer in the future.
The theory has to explain why the light gets redshifted, but does not get blurred, and the spectral lines do not get broadened. This severely restricts the type of interactions possible. Also the theory has to explain the consistency between redshifts within our own galaxy, to that of far-away galaxies.
Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of time.
The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.
If they could, Hawking radiation would interfere with the eternal orbit.
Yes, at high energies and with electrons or atomic nuclei nearby.
https://en.wikipedia.org/wiki/Two-photon_physics
> Hawking radiation would interfere with the eternal orbit
For tractability Hawking radiation tends to be studied using a non-interacting scalar field (often massless), rather than the fields of the Standard Model (which are very much interacting, and have a variety of masses).
Hawking started that in his "Black hole explosions?" (1974) which (if you're not allergic to sci-hub) you can read at <https://sci-hub.se/https://www.nature.com/articles/248030a0>: "consider (for simplicity) a massless Hermitean scalar field \phi which obeys the covariant wave equation ... in an asymptotically flat space time containing a star which collapses to produce a black hole.". In the short paper, he's very much not considering photons or pions, and really doesn't need to in order to make his point.
The thermal spectrum of a real black hole will be in all the Standard Model fields, and except very late in the eventual evaporation will be dominated by very low energy photons originating at a fair distance outside the event horizon.
What do you mean by "eternal orbit"?
Real black holes do not extend to the infinite past. If a black hole ultimately Hawking-evaporates, in finite time there's nothing left to orbit. Late in the evaporation, there will also be a lot of very high energy gamma rays and heavier particles excited in their respective quantum fields of the Standard Model. These will all tend to participate in high-energy multi-particle interactions rather more complicatedly than the end result of Hawking's 1974 description.
As a first step, you could can check out <https://en.wikipedia.org/wiki/Photon_sphere> and <https://en.wikipedia.org/wiki/Innermost_stable_circular_orbi...>
Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference.
It's quite fascinating, but I can't really dump out an entire book into a comment.
Speaking of which, do you happen to know a good book which illuminates this whole story step by step? I have yet to find one which doesn't start with "Let us assume an FLRW metric…"
https://en.wikipedia.org/wiki/G%C3%B6del_metric
There are versions of this which do provide another reason for red shift.
Personally I keep this in mind as a means to free my thinking from a single narrative.
https://en.wikipedia.org/wiki/Cosmic_distance_ladder
"Short" distances are calculated by parallax, taking Earth's orbit diameter as the base.
From there, it's a extrapolation of the spectrum of known types of objects. Not triangulation. Moreover, IIRC individual stars can't be discerned in other galaxies, even the nearest ones, except maybe supernovas.
So at the distances that red shift manifests, no triangulation, only red shift.
-Galaxies are not accelerating, space is expanding.
> Imagine there are some objects in space all moving in random directions and speeds, relative to Earth...
-No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood.
I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is).
They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity.
There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.
But then, I'm back to what does "space is expanding" mean? What is doing the expansion?
And secondly, what is the mechanism for "space expanding"? What is "space" in this context? What actually is expanding?
So again, how can you tell the difference between space expanding or two things actually just moving away from each other (to keep it to a very simple example of two bodies).
Space moving would be like balls placed on a bed sheet and the sheet expanding. The galaxies aren't moving- the sheet is.
What's actually happening is more like galaxies moving around on a sheet that's being pulled further at all sides.
is there a difference if there isn't a difference?
https://bigthink.com/starts-with-a-bang/universe-speed-of-li...
So... the negative is just a convention to represent work done against the "field", and positive is work done by the field? ie more of a vector than actually being negative energy? I think I get that bit now.
So now I'm wondering if that still applies to an expanding universe vs eg a rocket leaving earth. If things aren't moving further apart by work (ie force x distance) being done against their gravitational fields so much as space time expanding, is there an increase in potential energy? And then if objects are moving apart faster than escape velocity, could that still be seen as increasing potential energy?
I think I'm confusing myself further...
We haven't detected dark matter or dark energy outside of their visible effects on the larger universe. Maybe we can't interact with it with baryonic matter. A dark energy engine made out of dark matter would be invisible.
https://pagesix.com/wp-content/uploads/sites/3/2020/02/chels...
https://www.wnd.com/wp-content/uploads/2015/10/Webb-Hubbell_...
(Chelsea Clinton's resemblance to Clinton family friend Webb Hubble has prompted speculation...)
The title should be "Hubble Tension almost certainly not caused by measurement error."
[1]: https://iopscience.iop.org/article/10.3847/2041-8213/ad1ddd
https://youtu.be/TGpjGVNVYEg?t=397
It's beyond my depth to explain why he's wrong.
That is not what I'm asking. I'm asking if expansion and contraction could fluctuate in perpetuity, like stress waves through a block of jello.