IANAL, but I think you can trademark letters, and that similarly to colors, given the letter is known to correspond to your business in a particular market segment, and with the caveat that the trademark would only apply in that market segment.
Theoretically a space/" " is just another character, so maybe. But practically speaking, it would probably be legally filed as "space", and the company's trademark would be on either them naming their company "space", or on them stylizing it as a " ". That's my best guess, at least.
> our awareness of the galactic bodies itself is somehow influencing them, as in the double-slit experiment
In the double slit experiment, the awareness is not influencing the outcome. The act of measuring is. Pretty sure the act of measuring the galactic bodies has no impact on them in any meaningful way.
No, but there is a cool blending of the two concepts when light is bent by large gravity wells so that it actually shows the same star multiple times. When we observe which copied star "produced" a certain photon, it technically collapses the quantum possibilities backwards in time as that photon was emitted potentially billions of years ago.
In this case it's light bending around a gravity well which produces multiple "copies" of the star, but the light from each copy arrives at different points in time.
So they could capture the light and attempt to capture the "same" light again later to try to verify or change the result they received.
I did some more reading about it after posting, and the original experiment was intended to test if light "chooses" to be a wave OR a particle in a way they could affect, and that it could only be one or the other at a time. The truth seems to be more that it acts as both at the same time and whatever sensor equipment you use to pick it up is what it acts like.
I'm not sure about that. I suppose it really depends on your interpretation of the results, but I didn't think the cosmic interferometer experiment convinced physicists of retrocausality, by-and-large.
Functionally it means we can trace a single photon back to the source which emanated it, lensed or unlensed. That said, if quantum effects are not bound by time or space technically the photon ALWAYS came from an individual source and our clarifying which one it came from just collapses the wave from superposition of where the photon could go to where it did go.
As I understand the double slit experiment, this is a fundamental property of light as a photon exhibits wave-particle duality. If so, retrocausality in this case would just mean the fundamental wave function can be collapsed into actuality without time or space being involved.
> Why would distance from earth influence the speed of expansion of a distant galaxy?
It wouldn't - but it may influence how we measure distance. If we're using the wrong distance measurements then we're calculating the speed of expansion incorrectly.
At any point in space, a celestial object at distance d will tend to appear to move away from you faster than an object at a distance less than d. The only thing special about the Earth here is that it happens to be where we live.
no it's because space itself is expanding which causes things more distant from each other to be moving away from each other faster, the wikipedia article about hubbles law has a graphic illustrating the priciple with bread https://en.wikipedia.org/wiki/Hubble%27s_law
Wikipedia is not a proper source and I’ve no idea why people stopped considering this and now use it as such. The graphic may be fine, but provide a real source when pushing back against another commenter.
To be fair to Wikipedia, it’s come on a long way since its early days when anyone could make whatever edits they wanted. The most popular entries on the site are very well vetted, though not perfect. Just my 2 cents.
It's distance between any two points, however the distance measuring techniques to which we have access can only be performed by astronomers on Earth. Hence, one of those points will be Earth.
Right. A common analogy is the dough for a loaf of raisin bread. Consider the raisins on the surface of the dough. Pick one raisin to be your point of view (analogous to Earth, in this case). If the dough rises and expands from (say) 50 cm to 100 cm in diameter, another raisin adjacent the "Earth" raisin won't move very much, but the distance to a raisin at the diametrically opposed point will increase from about 78 cm to about 157 cm. The distant raisin will thus appear to be moving at a higher velocity than the adjacent raisin.
I feel like that analogy should be included anytime someone mentions that things further away from us are moving away faster than things not as far away.
My intuition of 'things far away moving away faster' causes me to conclude that things far away are therefore accelerating, because the more they move away the faster they move away, but my intuition is probably wrong as it doesn't line up with the raisin bread analogy.
This observation is the reason we think the universe is expanding.
As an analogy, consider the 2 dimensional surface on the surface of a balloon. As you inflate the balloon, the distance between any 2 points increases, and it increases more the farther away the points are from each other.
> Does that suggest that our awareness of the galactic bodies itself is somehow influencing them, as in the double-slit experiment?
"Awareness" is not a thing, not even in the double slit experiment. The term 'measurement' refers to a specific kind of interaction that bridges quantum systems with classical systems, although I believe a good case could be made that these waveforms never actually fully collapse.
Likewise, if there is anything special about Earth's position in the greater cosmos, it would be a trick of perspective or perception - unless there are any completely disruptive new discoveries about the nature of reality. However, my money would be on the fact that the universe is simply not as uniform as we thought.
Galaxies themselves aren't expanding, they're gravitationally bound. Galaxies are moving apart from one another, however.
The issue is that not all galaxies are moving away from us. The ones that are closer to us have a lot of peculiar velocity [1]. This means they can be moving toward us or moving tangentially to us or any other direction. If we want to characterize the expansion of the universe as a whole, we need to account for this in our models. It turns out to be a lot more complicated than we previously thought.
The crisis in cosmology (aka the Hubble tension [2]) is that our two means of characterizing the expansion of the universe, models of the cosmic microwave background (CMB) and measurements on the cosmic distance ladder using standard candles (Cepheid variables [3] for up-close measurements, Type Ia supernovae [4] for more distant measurements) disagree with one another, and that disagreement is getting worse, not converging.
Not an astrophysicist but something like MOND makes sense to me. It seems like it's much more probable that our model is slightly broken vs. there being a massive amount of... well, mass that we can't find no matter how hard we look.
Is there something about the idea that's really damning and nonsensical that a lay person may not notice?
Basically, the center of mass in the cluster, and the center of the matter we can see, are way far apart from one another, in a way that ends up being super clear to observe. We can infer the center of mass by the surveying the gravitational lensing we observe, and we can observe the center of the baryonic matter by looking at the x-ray sources. The offset in distance here ends up being pretty clear evidence that most of the mass exists as stuff we can't see (in any wavelength, only via the lensing.)
There are two problems with MOND. One is that it exchanges fine arbitrary parameters of dark matter with the arbitrary parameters of MOND. There isn’t a theory the explains MOND or the parameters. It is just as much of bodge as dark matter.
Second is that MOND doesn’t explain all the observations. There are galaxies that have too much or too little gravity. Either MOND varies exactly as we neeed it, or need MOND and dark matter. That isn’t any simpler. It might work if there was theory that explained smaller amount of dark matter and MOND could handle the rest.
This is a bit of an indirect answer, but I watched this not too long ago and it made it seem clear that MOND doesn't work so well, if not in general at least in the case discussed, which seems pretty damning regardless: https://youtu.be/HlNSvrYygRc?si=6on0pD9_jsed9Dlm
I've also heard the bullet cluster pointed out as a strong counterpoint to MOND--I'll let you read the wikipedia page rather than attempting to explain this, poorly: https://en.wikipedia.org/wiki/Bullet_Cluster
As a layman who watches tons of astronomy and physics videos, my general sense is that dark matter is more or less simply a placeholder, such that everything else around it is pretty consistent, but it just represents this unknown matter that we can clearly observe interacting _gravitationally_ with galaxies and other objects in the universe, but otherwise doesn't seem to interact. Whereas my impression of MOND is that it's a set of heuristics that requires a lot of tweaking to work, whereas GR is quite simple and very strongly experimentally verified. But, again, layman
TLDW: there's a lot of evidence on why the phenomena we watch is mostly favored to be described as 'something' that is conveniently called dark matter as opposed to a revision of gravity.
Listen, in general I agree with you, but it is a theory that was built only on observations without any theoretical foundation, and modifying the theory parameters every couple of years whenever a new observation was proving it wrong. I am sorry, but at this point MOND is on the same level with the flat earth theory and maybe it should be renamed as the conspiracy theory of gravity.
I get it, but the problem is that your response assumes knowledge that most readers don't have.
Maybe to those with that knowledge, your reaction is obvious and needn't be spelled out beyond a snarky dismissal. But to the rest of us, it's not obvious at all and the dismissal is unhelpful.
If you know more than others, that's great, but please share it in a way that the rest of us can learn from. If you don't want to do that or don't have time, that's fine, but in that case please don't post like you did in the GP. Among other undesirable side-effects, it acidifies the thread.
I have a theory in my head for years now. It is probably wrong, but here it comes:
In an infinite universe the total amount of gravity that affect us in one point in space is defined by the event horizon if we assume that gravity travels with light speed. Every atom in the universe has a very small influence on us. But this event horizon expands with light speed all the time. I wonder if this could lead to very small but permanent increasing gravitational pull from all directions at once. In other words, and increasing inflation.
Sort of like a pair of ice climbers, where one climbing up and securing themselves allows the other climber to safely climb higher. Eventually we can't see the ice climbers anymore, but that doesn't mean they aren't still helping each other climb higher.
"Gravitational force" doesn't work for describing the dynamics of the universe as a whole.
What would result in a contracting universe is a large enough density of matter (about a factor of 20 larger than the actual density in our universe if we just look at ordinary visible matter). But this does not mean "increasing gravitational force". As I have pointed out in other posts upthread, the "gravitational force" on a given piece of matter due to the rest of the matter in the universe (if we leave out local influences, like our solar system or galaxy for us here on Earth) is zero. This is true regardless of the current state of expansion or contraction.
The force of gravity is reduced by the inverse square of the distance (newton's law). Thus as space expands, and matter red shift away from us, the force of gravity reduce over time. The maximum force of gravity was just after the big bang.
All of this is wrong. Newtonian gravity does not work for describing the universe as a whole. As I pointed out in response to the GP upthread, the "force of gravity" on us due to the overall matter distribution of the universe is zero.
Does this account for all the extra space being created - from what I have understood from "always right" youtube videos - parts of the universe are moving away from us with speed greater than the speed of light (or more precise the space between the points is increasing at rate higher than C, no actual movement is being done)
> Does this account for all the extra space being created
As I have pointed out upthread, the GP's "theory" is not correct, so it doesn't account for anything.
"Expansion of space" is just a consequence of the overall spacetime geometry of the universe, which is due to its overall average matter distribution (and to dark energy, which is what is causing the expansion to accelerate).
I don't know what theory of gravity you are trying to use, but it isn't the correct one.
In our actual model of the universe, using the correct theory of gravity, the "total amount of gravity" affecting us (or any point) from the rest of the universe (i.e., once we factor out local influences like our solar system and our galaxy) is zero. That is because the average matter distribution in the universe is the same in all directions from us, so the "gravity" from it cancels out. The average matter distribution in the universe affects its overall rate of expansion over time, but this is not the same as the kind of "gravity" you are thinking of.
Also, while our universe does have a cosmological horizon (due to accelerating expansion), this horizon does not work the way your hypothetical "event horizon" does.
In short, your "theory" is not even wrong, because it doesn't even start from a correct underlying theory of gravity.
> That is because the average matter distribution in the universe is the same in all directions from us, so the "gravity" from it cancels out.
Can't gravity have an infinitesimally small effect, which means this matter distribution has to be perfectly balanced for its gravity to cancel out to zero at some point?
Only in speculations that we have no way to test now or in the foreseeable future. We are many, many orders of magnitude away from being able to probe spacetime on such scales.
> this matter distribution has to be perfectly balanced for its gravity to cancel out to zero at some point?
In principle, there are of course effects due to the matter distribution not being perfectly spherically symmetric about us. But except for the obvious effects that are due to nearby obvious objects--like the Earth, the Sun, our galaxy--in practice the gravitational effects of the rest of the universe on us are negligible.
> while our universe does have a cosmological horizon (due to accelerating expansion), this horizon does not work the way your hypothetical "event horizon" does.
I think this statement is really the crux of the counter-argument. Your statement that matter has average uniform density in all directions around us is obviously only correct in sufficiently large frames of reference; there are galactic voids, and galactic-super-strands, uniformity really only exists within the "mathematically and hypothetically infinite" frame.
You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does; and/or possibly, expand on how large the frame would have to be to achieve reasonable uniformity.
> Your statement that matter has average uniform density in all directions around us is obviously only correct in sufficiently large frames of reference
Yes, agreed. But the distance to our cosmological horizon is large enough that the averaging assumption is fine on that scale.
> You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does
Fair enough. But first I'll reference an excellent paper by Davis & Lineweaver (2003) [1]. You can find much more detail there than I'm going to give here.
Some simple facts about our cosmological horizon are:
(1) It is currently receding from us very slowly (in terms of proper distance); and asymptotically it will be at a constant distance from us forever (in more technical language, our universe approaches de Sitter spacetime asymptotically, as all other matter and energy becomes negligible compared to dark energy; and in de Sitter spacetime the cosmological horizon is at a constant distance forever). It certainly is not receding from us at the speed of light.
(It is true that the horizon is a lightlike surface--but that does not mean it's receding from us at the speed of light. In fact, counterintuitively, to the extent it is "moving", locally, in any direction, that direction is towards us, not away from us! But due to the curvature of spacetime, its proper distance from us will asymptotically be constant.)
(2) It is not a boundary between things that can affect us gravitationally and things that can't. To the extent there is such a boundary in GR, it's our past light cone (which is shown in the diagrams in Figure 1 of the paper I referenced). But one also has to consider the other caveats I gave in my earlier post.
(3) The cosmological horizon is a boundary in spacetime, not space. It is a boundary between the region of spacetime that will be able to causally affect us into the infinite future, and the region that won't. Particular objects, as they recede from us, will move beyond the cosmological horizon and will then no longer be able to causally affect us. But events that occurred in those objects before they moved beyond the horizon will still be able to causally affect us--though of course it will take time for those effects to propagate to us. For example, the light we see from distant objects is a causal effect, but we see the objects not as they are "now" but as they were when the light was emitted. It's quite possible for a distant galaxy whose light we are seeing now to be beyond our event horizon "now", but they weren't when they emitted the light.
The spacetime diagrams in Figure 1 of the paper I referenced can be very helpful in making all this clearer.
> expand on how large the frame would have to be to achieve reasonable uniformity.
A few billion light years is certainly large enough. Our cosmological horizon is quite a bit further away than that (about 16 billion light years, according to the paper I referenced).
Sure, and whenever Warren Buffet gives a speech to 1000 college students, the average wealth of every person in the auditorium briefly jumps to millionaire-level, until Buffet leaves the room.
I dont know why but my mind enters an endless loop when thinking about the vastness of space and where that space exists. Almost as if it reaches the end of human imagination. I can imagine the multiverse, but i cant imagine the “nothingness” in which they exist. I cant imagine where everything came from and where it goes. It makes no sense. I understand the explanation of physics and time, the big bang, expansion, but where the heck does it all exist in? The void of what? What are they expanding into? I understand matter existing in the universe, but where is the universe expanding in? What are the laws of that “in”? That void exists in something but what does the something exist in? It’s almost as weird as if nothing ever existed yet it exists.
The picture I've been given is living on the surface of a balloon. Draw a few dots on the surface, blow in the balloon. Every points expands away from every other point.
Of course with a balloon there is always an outside, but the point is that it isn't expanding like if it were pushing against an external wall.
It's expanding everywhere. It's expanding in the space between your fingers. If you look there, it's not pushing against any outside. There is just more space per space, the longer you let it age.
> It's expanding in the space between your fingers
That's not the case (at least not yet [0].) Anything that's bound together by local forces such as gravity or electromagnetism isn't affected by the universe's expansion. You have to get to the scale of the space between galaxies (and even then, galaxies which aren't orbiting/colliding with one another) before the expansion has any effect.
[0] One of the hypotheses (of many: https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...) of the future of the expanding universe predicts that the expansion rate will increase indefinitely, do the point where the expansion energy (dark energy) actually does become enough to expand the space between stars, planets, and even matter (and eventually the atoms in your body), but we don't really have any proof of this. This theory is known as the "big rip": https://en.wikipedia.org/wiki/Big_Rip
My understanding is that it does expand on small scales (such as between your fingers), but at that scale the expansion is so weak/slow that it is immediately compensated by the local attractive forces.
Sure. You could model it as being there being an extremely weak expansion force happening between your fingers, but the force not being enough to overcome any other local forces (gravity, molecular bonds, etc)… said force is not strong enough to overcome planetary orbits either, or orbits of stars around the galactic center, or the orbits of nearby galaxies around each other… but once you start getting into intergalactic scales where the force attracting galaxies together is very weak, the expansion force starts to actually make a difference.
But the point is the space between your fingers isn’t literally expanding. It’s not like the universe is just expanding everywhere uniformly, which is what tux3 seemed to imply. The distance between your fingers is staying the same, and so is everything else nearby.
My point though is that yes space is literally expanding everywhere uniformly, it’s just expanding “through your fingers” (i.e. your fingers don’t expand along with it) because the local forces keep your fingers (and the planet and the galaxy) together.
This is roughly similar to how planets don’t collapse due to gravity, because the stronger electromagnetic repulsion makes them stable bodies, but this doesn’t mean that the gravitational field still isn’t there throughout the planet.
> It’s not like the universe is just expanding everywhere uniformly, ...
I thought this is exactly what most modern physicists believe.
The wavelength of photons travelling in space is supposed to scale perfectly with the expansion of the universe, like an ink dot on a balloon getting wider as the balloon is being inflated.
This is why we have the Cosmic Microwave Background consisting of photons that would have been maybe in the UV range when emitted by excited atoms back when the universe became "transparent".
This is hard to communicate so I’m sure I’ve been misleading.
“Space” is expanding uniformly, yes. But not the matter that exists in space, nor is the distance between local matter increasing.
It’s not like the entire universe is undergoing a magnification of scale where everything is enlarging including each individual atom.
So it’s misleading to say “the space between your fingers is expanding”, because it might give the impression that the distance between your fingers will actually be larger as time passes. This is not the case.
If “space is expanding”, but the matter inside this space is unaffected by said expansion, then you’re left with a very subtle phenomena which is easy to misunderstand.
True. I think that the best way to explain this to someone who does not know about the phenomenon is to start with the phrase "existing space continuously creates new space" and everything should follow from this fact.
I actually find your "it is and it isn't" to be much more confusing than "it is but the other forces recompact matter faster than the new space emerges".
So then all our parts are "falling" together constantly through this expanding space? I say falling, but I understand it is other local forces besides gravity.
I struggle to wrap my mind around what that means for concepts I learned long ago in college physics... what is an inertial frame of reference then? What about the speed of light and relativistic effects?
I suppose the answer is back to the original statement that the effect is really only noticable at cosmological scales. Locally, it's well below any margin of error?
I've seen this tied up with one idea where space-time itself is ripped apart, leaving nothing that can expand and instead "everything" once against occupying the same space less than an atom (if it can even be quantified), reduxing a Big Bang.
Otherwise we increasingly approximate a transition to "nothing" into infinity.
I understand what you mean and I can easily get this feeling when looking at the night sky on a clear night. I have to quickly abort the loop because it's not very pleasant.
Doesn't sound like the paper's (the one you linked) authors don't necessarily agree with your summary. From the discussion on page 16:
These results seemingly worsen the recently established tension between the inferred value of H0 from early and late
times Universe probes, which has been argued to potentially be the sign of new cosmological physics (see for example
Refs. [78–83]). This might appear to be in contradiction with the possibility, explored for example in Refs. [74, 84–
93], that local gravitational physics could alleviate the Hubble tension. Amongst these, a class of models achieve a
lowering of H0 under the assumption that we live in an underdense region, whose inner expansion rate is on average
larger than the background one. Some results in the literature, see for example refs. [94–96], seem to corroborate
the latter assumption finding evidence of local voids which averaged on spheres of r ≳ 100Mpc have density contrasts
of δ ≤ −0.1, unexpected within the ΛCDM model. Computing the average density contrast of a sphere centered
in Laniakea with radius r ≈ 110 Mpc (i.e. the average distance of the boundary of the ellipsoid from the center)
using the CF4 reconstruction we found δ ∼ −0.06, within the prediction of the concordance model (see for example
Fig. 6 of Ref. [55]). However, this sphere is not centered in the Milky Way, which might explain why the result
differs from the aforementioned ones. Indeed, overdensities such as Laniakea are surrounded by voids (from which
they have collected matter), and therefore any sufficiently spherical average will include these under-dense regions.
On the other hand, Refs. [55, 79, 97] also found no evidence of any large void or overdensity, thus disfavoring a local
resolution of the Hubble tension. Our analysis corroborates these results, suggesting instead that the tension is likely
to be (slightly) worsened by Laniakea’s backreaction. An important caveat, however, is that our analysis does not
exclude the possibility that large voids in the annular region between 110 − 400 Mpc outside Laniakea could balance
and overcome the backreaction from Laniakea, like a rather picturesque Matryoshka doll. Alternative modelling
choices accounting for the impact of these voids are therefore required to fully understand the impact our cosmic
environment’s gravitational backreaction, which will be the focus of forthcoming studies.
I do disagree with nightmare, but it's all in perspective. More like an infinitely deep well of incomprehensible potential to me. Deep water is scary but a well is life giving. It's a little bit of both
While we're all throwing out our whacky idea laundry in front of people I presume actually know what they're talking about:
I've wondered for many years if our universe isn't like a supersaturated (SS) solution (1), where mass/energy and the fabric of space-time itself can eventually combine and settle into a balanced but very precarious state, like an SS solution comprised of Mass-Energy-Space-Time [MEST] as a single "fabric" (which perhaps was the state of the universe prior to the big bang, and will be again after the universe' heat death).
Say this large swath of peaceful-yet-highly unstable MEST fabric is perturbed slightly (like dropping a seed crystal in a SS solution) - then matter, energy space and time precipitate outward in a rapidly evolving chain reaction. The horizon of this chain reaction would be a place where matter and energy particles are continually being generated (liberated) from the serene METS fabric, which generates a gravitational pull in all directions.
So, all of the mass in our universe is actually being pulled outward, and because this precipitation horizon is expanding (growing in surface area), it generates an increasing amount of matter and hence an increasing amount of gravity that ultimately pulls matter within this horizon outward - at an accelerated rate. Perhaps the big bang was really, instead, a 'big fizz' - chain reaction of precipitated matter and energy from an initial supersaturated MEST fabric.
Or maybe I'm just full of silly nonsense. Either way, would love to hear an actual astrophysicist 's take on that idea. Maybe gently tear it to shreds:)
This article starts from an infinity of axioms that all have to be true for that single statement of the title to have even a small chance of making sense.
102 comments
[ 0.16 ms ] story [ 145 ms ] threadTheoretically a space/" " is just another character, so maybe. But practically speaking, it would probably be legally filed as "space", and the company's trademark would be on either them naming their company "space", or on them stylizing it as a " ". That's my best guess, at least.
Does that suggest that our awareness of the galactic bodies itself is somehow influencing them, as in the double-slit experiment?
In the double slit experiment, the awareness is not influencing the outcome. The act of measuring is. Pretty sure the act of measuring the galactic bodies has no impact on them in any meaningful way.
There is no way to verify that. Big parts of quantum physics is more or less pseudoscience.
1. https://www.nature.com/articles/s41550-021-01450-9
So they could capture the light and attempt to capture the "same" light again later to try to verify or change the result they received.
I did some more reading about it after posting, and the original experiment was intended to test if light "chooses" to be a wave OR a particle in a way they could affect, and that it could only be one or the other at a time. The truth seems to be more that it acts as both at the same time and whatever sensor equipment you use to pick it up is what it acts like.
[citation needed]
As I understand the double slit experiment, this is a fundamental property of light as a photon exhibits wave-particle duality. If so, retrocausality in this case would just mean the fundamental wave function can be collapsed into actuality without time or space being involved.
Am I far off base?
It wouldn't - but it may influence how we measure distance. If we're using the wrong distance measurements then we're calculating the speed of expansion incorrectly.
This place isn't debate club and nobody's getting a prize for winning an Internet argument.
My intuition of 'things far away moving away faster' causes me to conclude that things far away are therefore accelerating, because the more they move away the faster they move away, but my intuition is probably wrong as it doesn't line up with the raisin bread analogy.
As an analogy, consider the 2 dimensional surface on the surface of a balloon. As you inflate the balloon, the distance between any 2 points increases, and it increases more the farther away the points are from each other.
"Awareness" is not a thing, not even in the double slit experiment. The term 'measurement' refers to a specific kind of interaction that bridges quantum systems with classical systems, although I believe a good case could be made that these waveforms never actually fully collapse.
Likewise, if there is anything special about Earth's position in the greater cosmos, it would be a trick of perspective or perception - unless there are any completely disruptive new discoveries about the nature of reality. However, my money would be on the fact that the universe is simply not as uniform as we thought.
The issue is that not all galaxies are moving away from us. The ones that are closer to us have a lot of peculiar velocity [1]. This means they can be moving toward us or moving tangentially to us or any other direction. If we want to characterize the expansion of the universe as a whole, we need to account for this in our models. It turns out to be a lot more complicated than we previously thought.
The crisis in cosmology (aka the Hubble tension [2]) is that our two means of characterizing the expansion of the universe, models of the cosmic microwave background (CMB) and measurements on the cosmic distance ladder using standard candles (Cepheid variables [3] for up-close measurements, Type Ia supernovae [4] for more distant measurements) disagree with one another, and that disagreement is getting worse, not converging.
[1] https://en.wikipedia.org/wiki/Peculiar_velocity#Cosmology
[2] https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_tension
[3] https://en.wikipedia.org/wiki/Cepheid_variable
[4] https://en.wikipedia.org/wiki/Type_Ia_supernova
Am I right?
Is there something about the idea that's really damning and nonsensical that a lay person may not notice?
Basically, the center of mass in the cluster, and the center of the matter we can see, are way far apart from one another, in a way that ends up being super clear to observe. We can infer the center of mass by the surveying the gravitational lensing we observe, and we can observe the center of the baryonic matter by looking at the x-ray sources. The offset in distance here ends up being pretty clear evidence that most of the mass exists as stuff we can't see (in any wavelength, only via the lensing.)
Second is that MOND doesn’t explain all the observations. There are galaxies that have too much or too little gravity. Either MOND varies exactly as we neeed it, or need MOND and dark matter. That isn’t any simpler. It might work if there was theory that explained smaller amount of dark matter and MOND could handle the rest.
I've also heard the bullet cluster pointed out as a strong counterpoint to MOND--I'll let you read the wikipedia page rather than attempting to explain this, poorly: https://en.wikipedia.org/wiki/Bullet_Cluster
As a layman who watches tons of astronomy and physics videos, my general sense is that dark matter is more or less simply a placeholder, such that everything else around it is pretty consistent, but it just represents this unknown matter that we can clearly observe interacting _gravitationally_ with galaxies and other objects in the universe, but otherwise doesn't seem to interact. Whereas my impression of MOND is that it's a set of heuristics that requires a lot of tweaking to work, whereas GR is quite simple and very strongly experimentally verified. But, again, layman
TLDW: there's a lot of evidence on why the phenomena we watch is mostly favored to be described as 'something' that is conveniently called dark matter as opposed to a revision of gravity.
https://news.ycombinator.com/newsguidelines.html
Maybe to those with that knowledge, your reaction is obvious and needn't be spelled out beyond a snarky dismissal. But to the rest of us, it's not obvious at all and the dismissal is unhelpful.
If you know more than others, that's great, but please share it in a way that the rest of us can learn from. If you don't want to do that or don't have time, that's fine, but in that case please don't post like you did in the GP. Among other undesirable side-effects, it acidifies the thread.
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...
I have a theory in my head for years now. It is probably wrong, but here it comes:
In an infinite universe the total amount of gravity that affect us in one point in space is defined by the event horizon if we assume that gravity travels with light speed. Every atom in the universe has a very small influence on us. But this event horizon expands with light speed all the time. I wonder if this could lead to very small but permanent increasing gravitational pull from all directions at once. In other words, and increasing inflation.
What would result in a contracting universe is a large enough density of matter (about a factor of 20 larger than the actual density in our universe if we just look at ordinary visible matter). But this does not mean "increasing gravitational force". As I have pointed out in other posts upthread, the "gravitational force" on a given piece of matter due to the rest of the matter in the universe (if we leave out local influences, like our solar system or galaxy for us here on Earth) is zero. This is true regardless of the current state of expansion or contraction.
[0] https://en.m.wikipedia.org/wiki/Mach%27s_principle
As I have pointed out upthread, the GP's "theory" is not correct, so it doesn't account for anything.
"Expansion of space" is just a consequence of the overall spacetime geometry of the universe, which is due to its overall average matter distribution (and to dark energy, which is what is causing the expansion to accelerate).
In our actual model of the universe, using the correct theory of gravity, the "total amount of gravity" affecting us (or any point) from the rest of the universe (i.e., once we factor out local influences like our solar system and our galaxy) is zero. That is because the average matter distribution in the universe is the same in all directions from us, so the "gravity" from it cancels out. The average matter distribution in the universe affects its overall rate of expansion over time, but this is not the same as the kind of "gravity" you are thinking of.
Also, while our universe does have a cosmological horizon (due to accelerating expansion), this horizon does not work the way your hypothetical "event horizon" does.
In short, your "theory" is not even wrong, because it doesn't even start from a correct underlying theory of gravity.
Can't gravity have an infinitesimally small effect, which means this matter distribution has to be perfectly balanced for its gravity to cancel out to zero at some point?
Only in speculations that we have no way to test now or in the foreseeable future. We are many, many orders of magnitude away from being able to probe spacetime on such scales.
In principle, there are of course effects due to the matter distribution not being perfectly spherically symmetric about us. But except for the obvious effects that are due to nearby obvious objects--like the Earth, the Sun, our galaxy--in practice the gravitational effects of the rest of the universe on us are negligible.
I think this statement is really the crux of the counter-argument. Your statement that matter has average uniform density in all directions around us is obviously only correct in sufficiently large frames of reference; there are galactic voids, and galactic-super-strands, uniformity really only exists within the "mathematically and hypothetically infinite" frame.
You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does; and/or possibly, expand on how large the frame would have to be to achieve reasonable uniformity.
Yes, agreed. But the distance to our cosmological horizon is large enough that the averaging assumption is fine on that scale.
> You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does
Fair enough. But first I'll reference an excellent paper by Davis & Lineweaver (2003) [1]. You can find much more detail there than I'm going to give here.
Some simple facts about our cosmological horizon are:
(1) It is currently receding from us very slowly (in terms of proper distance); and asymptotically it will be at a constant distance from us forever (in more technical language, our universe approaches de Sitter spacetime asymptotically, as all other matter and energy becomes negligible compared to dark energy; and in de Sitter spacetime the cosmological horizon is at a constant distance forever). It certainly is not receding from us at the speed of light.
(It is true that the horizon is a lightlike surface--but that does not mean it's receding from us at the speed of light. In fact, counterintuitively, to the extent it is "moving", locally, in any direction, that direction is towards us, not away from us! But due to the curvature of spacetime, its proper distance from us will asymptotically be constant.)
(2) It is not a boundary between things that can affect us gravitationally and things that can't. To the extent there is such a boundary in GR, it's our past light cone (which is shown in the diagrams in Figure 1 of the paper I referenced). But one also has to consider the other caveats I gave in my earlier post.
(3) The cosmological horizon is a boundary in spacetime, not space. It is a boundary between the region of spacetime that will be able to causally affect us into the infinite future, and the region that won't. Particular objects, as they recede from us, will move beyond the cosmological horizon and will then no longer be able to causally affect us. But events that occurred in those objects before they moved beyond the horizon will still be able to causally affect us--though of course it will take time for those effects to propagate to us. For example, the light we see from distant objects is a causal effect, but we see the objects not as they are "now" but as they were when the light was emitted. It's quite possible for a distant galaxy whose light we are seeing now to be beyond our event horizon "now", but they weren't when they emitted the light.
The spacetime diagrams in Figure 1 of the paper I referenced can be very helpful in making all this clearer.
> expand on how large the frame would have to be to achieve reasonable uniformity.
A few billion light years is certainly large enough. Our cosmological horizon is quite a bit further away than that (about 16 billion light years, according to the paper I referenced).
[1] https://arxiv.org/abs/astro-ph/0310808
The picture I've been given is living on the surface of a balloon. Draw a few dots on the surface, blow in the balloon. Every points expands away from every other point.
Of course with a balloon there is always an outside, but the point is that it isn't expanding like if it were pushing against an external wall.
It's expanding everywhere. It's expanding in the space between your fingers. If you look there, it's not pushing against any outside. There is just more space per space, the longer you let it age.
That's not the case (at least not yet [0].) Anything that's bound together by local forces such as gravity or electromagnetism isn't affected by the universe's expansion. You have to get to the scale of the space between galaxies (and even then, galaxies which aren't orbiting/colliding with one another) before the expansion has any effect.
[0] One of the hypotheses (of many: https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...) of the future of the expanding universe predicts that the expansion rate will increase indefinitely, do the point where the expansion energy (dark energy) actually does become enough to expand the space between stars, planets, and even matter (and eventually the atoms in your body), but we don't really have any proof of this. This theory is known as the "big rip": https://en.wikipedia.org/wiki/Big_Rip
But the point is the space between your fingers isn’t literally expanding. It’s not like the universe is just expanding everywhere uniformly, which is what tux3 seemed to imply. The distance between your fingers is staying the same, and so is everything else nearby.
This is roughly similar to how planets don’t collapse due to gravity, because the stronger electromagnetic repulsion makes them stable bodies, but this doesn’t mean that the gravitational field still isn’t there throughout the planet.
I thought this is exactly what most modern physicists believe.
The wavelength of photons travelling in space is supposed to scale perfectly with the expansion of the universe, like an ink dot on a balloon getting wider as the balloon is being inflated.
This is why we have the Cosmic Microwave Background consisting of photons that would have been maybe in the UV range when emitted by excited atoms back when the universe became "transparent".
“Space” is expanding uniformly, yes. But not the matter that exists in space, nor is the distance between local matter increasing.
It’s not like the entire universe is undergoing a magnification of scale where everything is enlarging including each individual atom.
So it’s misleading to say “the space between your fingers is expanding”, because it might give the impression that the distance between your fingers will actually be larger as time passes. This is not the case.
If “space is expanding”, but the matter inside this space is unaffected by said expansion, then you’re left with a very subtle phenomena which is easy to misunderstand.
True. I think that the best way to explain this to someone who does not know about the phenomenon is to start with the phrase "existing space continuously creates new space" and everything should follow from this fact.
I struggle to wrap my mind around what that means for concepts I learned long ago in college physics... what is an inertial frame of reference then? What about the speed of light and relativistic effects?
I suppose the answer is back to the original statement that the effect is really only noticable at cosmological scales. Locally, it's well below any margin of error?
Otherwise we increasingly approximate a transition to "nothing" into infinity.
Change 'where' to 'whether' and everything will fall into place.
https://arxiv.org/pdf/2311.00215.pdf
MOND hasn’t been having the rosiest of time lately. https://academic.oup.com/mnras/article/527/3/4573/7342478?lo...
https://youtu.be/-kTe0xRAU1w
These results seemingly worsen the recently established tension between the inferred value of H0 from early and late times Universe probes, which has been argued to potentially be the sign of new cosmological physics (see for example Refs. [78–83]). This might appear to be in contradiction with the possibility, explored for example in Refs. [74, 84– 93], that local gravitational physics could alleviate the Hubble tension. Amongst these, a class of models achieve a lowering of H0 under the assumption that we live in an underdense region, whose inner expansion rate is on average larger than the background one. Some results in the literature, see for example refs. [94–96], seem to corroborate the latter assumption finding evidence of local voids which averaged on spheres of r ≳ 100Mpc have density contrasts of δ ≤ −0.1, unexpected within the ΛCDM model. Computing the average density contrast of a sphere centered in Laniakea with radius r ≈ 110 Mpc (i.e. the average distance of the boundary of the ellipsoid from the center) using the CF4 reconstruction we found δ ∼ −0.06, within the prediction of the concordance model (see for example Fig. 6 of Ref. [55]). However, this sphere is not centered in the Milky Way, which might explain why the result differs from the aforementioned ones. Indeed, overdensities such as Laniakea are surrounded by voids (from which they have collected matter), and therefore any sufficiently spherical average will include these under-dense regions. On the other hand, Refs. [55, 79, 97] also found no evidence of any large void or overdensity, thus disfavoring a local resolution of the Hubble tension. Our analysis corroborates these results, suggesting instead that the tension is likely to be (slightly) worsened by Laniakea’s backreaction. An important caveat, however, is that our analysis does not exclude the possibility that large voids in the annular region between 110 − 400 Mpc outside Laniakea could balance and overcome the backreaction from Laniakea, like a rather picturesque Matryoshka doll. Alternative modelling choices accounting for the impact of these voids are therefore required to fully understand the impact our cosmic environment’s gravitational backreaction, which will be the focus of forthcoming studies.
I've wondered for many years if our universe isn't like a supersaturated (SS) solution (1), where mass/energy and the fabric of space-time itself can eventually combine and settle into a balanced but very precarious state, like an SS solution comprised of Mass-Energy-Space-Time [MEST] as a single "fabric" (which perhaps was the state of the universe prior to the big bang, and will be again after the universe' heat death).
Say this large swath of peaceful-yet-highly unstable MEST fabric is perturbed slightly (like dropping a seed crystal in a SS solution) - then matter, energy space and time precipitate outward in a rapidly evolving chain reaction. The horizon of this chain reaction would be a place where matter and energy particles are continually being generated (liberated) from the serene METS fabric, which generates a gravitational pull in all directions.
So, all of the mass in our universe is actually being pulled outward, and because this precipitation horizon is expanding (growing in surface area), it generates an increasing amount of matter and hence an increasing amount of gravity that ultimately pulls matter within this horizon outward - at an accelerated rate. Perhaps the big bang was really, instead, a 'big fizz' - chain reaction of precipitated matter and energy from an initial supersaturated MEST fabric.
Or maybe I'm just full of silly nonsense. Either way, would love to hear an actual astrophysicist 's take on that idea. Maybe gently tear it to shreds:)
1. https://youtu.be/G8nHu-IOpTg?feature=shared
The degenerate matter state is like a partial order of time. https://en.wikipedia.org/wiki/Causal_sets
It is a purely speculative article.
https://www.youtube.com/watch?v=-kTe0xRAU1w
Seems more concerns about the issue of cosmological constant and its disagreement with observation and current model.