This might be obvious to most, but at first it wasn't clear to me why in 150 billion years the vast majority of the known universe will be unreachable. The answer is in the acceleration of expansion (which the blogpost mentioned but I didn't connect the dots on at first); eventually the rate of expansion ("velocity") will greater than the speed of light.
Parsing this, I’d like to match it up with the Hubble horizon, at least for your stated condition that the velocity of expansion is greater than the speed of light: That already holds for us relative to points across the Hubble horizon. Their “past-emitted” light is still reaching us, meanwhile their “presently emitted” light never will.
I need to check the proper definitions but that’s the gist. Kinda erie!
The speed of light (in vacuum) is constant for accelerated observers too. :-)
The dipole anisotropy of the Cosmic Microwave Background will continue to be a good tool to see the constancy of the speed of light for all observers into the practically infinite future, assuming the infinite future looks a lot like de Sitter space (which is essentially what the standard cosmology predicts).
At that point in time (assuming time has any meaning in that reference frame,) everything will be moving away from everything else faster than any force, even information, could possibly travel between them. There won't be computers. There won't even be atoms to make matter to make computers with.
I'm not precisely sure what you're referring to as "the change" here. The rate of expansion is increasing continuously though, if that's what you're asking.
As far as the measurability of the speed of light, I don't think anything changes in principle. It should remain measurable and constant. However, it gets a lot more complicated in practice, since it's pretty unclear who or what would actually be doing the measuring and how they might go about it.
Actually it might. If expansion accelerates indefinitely, which is one of the theories, then eventually expansion will be stronger than the binding force of atoms and then sub-atomic particles. This is called the "Big Rip".
We have lots of observations of spiral galaxies at ever-increasing redshifts, and they don't look very different at different times -- closer galaxies are not collapsing more slowly than the distant ones.
We also have good views of the Lyman-alpha forest through galaxy clusters at different redshifts, and the distribution of the spectral lines are another line of evidence that members of galaxy clusters closer to us have smaller radial components in their peculiar velocities than those at higher redshift/smaller angle/lower brightness.
To get to a big rip you have to impose a further time derivative and figure out how to keep it small now and make it big later. AFAIK nobody who has tried this has avoided significant complexity to match the observational data we have today, except by pushing the Big Rip so enormously far into the future that we don't have enough data to constrain that future. The wikipedia article refers to WMAP9, which ended 2013 (~ 9 years after WMAP turned on in 2003). There is more recent data from Planck and other observatories, and they make it harder and harder to justify a Big Rip for the reasons half-admitted in the "Observed universe" section of that wikipedia article.
Moreover, MESSENGER and other spacecraft sent to parts of our solar system have given us a lot of data about the metric of our solar system, and it is non-expanding; orbiting clocks support this more locally. Likewise, there are plenty of sensitive terrestrial experiments that highly constrain any expansion term in plausible Standard Model + gravity Lagrangians.
It can't be ruled out but it has been effectively pushed back so far that it's the least of our worries about "what happens hundreds of trillions of years in the future" : if one accepts there there is a Big Rip field, there's no compelling reason why it should have significant effects on matter in the local group that soon, and several arguments about why it shouldn't.
Distant regions of the universe already are receding from us faster than light. Due to the accelerating rate of expansion, that horizon beyond which this is true is shrinking inwards towards us, so that in 150 bn years even galaxies outside our local group will be beyond that horizon.
It's a reasonable concern. I think we'll probably survive indefinitely, after all if Mars can be colonised it's hard to imagine we could screw up Earth so badly it's less habitable than that.
However, the planetary environment could become so hostile that it takes so much effort to maintain civilization that our capabilities become severely limited. So my concern isn't so much us dying out, as we become doomed to a fairly marginal existence, incapable of interesting achievements such as interstellar exploration.
Fear of individual/personal death is somewhat mitigated because we know other beings will survive us. Now imagine if all the life forms and stars destiny is an eternal and permanent death, all of them, just like forever. Feels as staring at an abyss.
If anyone begins to feel trapped in a cage, please don't just yet. For instance, astronomers currently estimate on the order of 10¹² planets in Milky Way alone, of which up to 11 billion could be Earth-sized and orbiting Sun-like stars [1]. So there's still plenty to explore/colonize, especially if you include the few tens of neighboring galaxies.
Even the closest galaxies are so far away, its unlikely we can reach them even with a warp drive.
However, it's possible that the accelerated expansion might prove an aberration that only makes the galaxies seem as if they are "quickly" moving beyond our reach. In which case, what a relief! LOL
so a star A at the distance 13.7B ly from us is running away with "c". Until you believe that today is an unique moment in time, we can expect that in 100M years a star B at the distance 13.8B ly will be also moving with "c" away from us.
If we allow for accelerated expansion of the Universe, then it means that the star B is closer to us then the star A (with the star A moving away faster than "c" in 100M years from today - i.e. accelerated expansion) - i.e. today the star B is closer to us than 13.7B ly and runs away slower then "c", and thus in the next 100M years the star B would be expected to make a journey longer than 100M ly - from some point today closer to us than 13.7B ly to the point at 13.8B ly in 100M years - i.e. it would need to run away faster then "c" during those 100M years which is clearly a contradiction.
That leads to conclusion that whatever speed a star runs away with today, it will be running away with the same speed tomorrow - that means decreasing Hubble constant - which is normal giving the same energy being spread in a bigger and bigger space. And when it comes to observing the observable Universe - the star A will always run away with "c", and whatever stars B are closer to us then the star A ( and thus running away slower than "c" today), those stars B will never run away faster than "c" - thus the light from them will always come to us.
Note: the stars which are farther away moving faster isn't acceleration of expansion. Acceleration of expansion would be if the same stars were running away faster tomorrow than today - which would clearly lead to the contradiction pointed above. Hubble constant decreasing with time is the only way to avoid that contradiction. That also probably explains different Hubble constant values we currently have - as they are obtained using signals from different points in the Universe history.
That humans exist a mere 14 billion years into a universe that will exist for many quintillions of years is so exceedingly improbable that we may very well be the first sentient life. This is one of my favorite explanations for the Fermi Paradox.
My other favorite is that there is no Paradox. The signs of alien civilizations are everywhere,
but humans cannot perceive or understand it
Aren't most of those quintillions spent during the heat death when there no real available energy and everything is nothing more than a barely held together soup of matter?
It's definitely interesting, but I don't get how this kind of thing doesn't make other people feel a kind of intense sadness.
It's like the intersection of the insignifigance and smallness of my own individual existence + the immense scale of time to underscore that fact + the fact that even all the things that are spectacular and powerful also meet the same kinda end as me.
Something about the 58 billion trillion trillion trillion trillion years last black hole thing just bums me out.
The visible light era of the Universe is relatively short compared to what comes after. People think of this as a long dark night. To some extent that's true.
Another perspective is that this will be the Golden Age of the Universe [1].
I do understand what you mean. I have a personal philosophy that deals directly with existential dread. I may not explain it well here, but I'll give it a go: In the face of the yawning void both physical and metaphysical, we humans have the preternatural ability to construct meaning where none exists. We are smaller than mites, floating on dust, vulnerable and doomed; and yet, here we are, defying that nothingness; writing, loving, making things against odds so unlikely we have a difficult time comprehending it. We have the nearly divine ability to pick something arbitrarily and declaring "This matters. This is important, because I say it is." and therefore it really is important and meaningful. Until we decide it's not. This ability to construct meaning is extremely powerful, and will serve us as we march into the trillionth eon at the end of time.
I believe that spacefaring life is relatively rare in our neighbourhood to the point where it's entirely possible we're the only one.
As for signs of alien civilization being everywhere, here I have to respectfully disagree. I'll summarize why.
I firmly believe the likely future of humanity is not on planets but in orbitals. Planets are a great way of storing mass. They're a poor way of creating living area. I think the estimate is that 1% of the mass of Mercury could fully encompass the Sun in a Dyson Swarm of orbitals with no material stronger than stainless steel. I believe humanity will be capable of this within 1,000 years and that's super-conservative.
Objects in space that absorb Solar radiation heat up. The only way of releasing that is by radiating it away. That radiation is of a wavelength solely determined by the temperature of the object. For any reasonable temperatures, that's in the infrared spectrum.
So a Dyson Swarm will have a very particular spectrum. They'll be IR lanterns. Sure one star we may not see out of a whole galaxy but the barrier between doing this to one star to the whole galaxy is only a matter of a relatively short amount of time (cosmologically speaking). One million years for the Milky Way (out of 10B+ years).
There are a bunch of assumptions in this and each would be their own topic. But as a solution to the Fermi Paradox, it only takes one.
Recycling waste heat is a common objection but that just reduces your emissions, it doesn't eliminate them. If it did, that would break thermodynamics.
This argument is strengthened by this article: over time more mass is becoming unreachable. For truly long-lived civilizations it makes the most sense to grab as much mass as you can and sequester it. Again, it only takes one.
Materials absorb some quantity of IR radiation. This will heat them up. What you're talking about is essentially a variation of recycling heat. It's imperfect so there'll be some IR signature.
>> The signs of alien civilizations are everywhere, but humans cannot perceive or understand it
> As for signs of alien civilization being everywhere, here I have to respectfully disagree.
This is all speculation, of course, but that's all we have. This particular answer to the Fermi Paradox goes like this:
Human reasoning and interests are entirely circumscribed by our perception. We are capable only of dimly imagining that other kinds of perception and reasoning exist, but not imagining what or how that might be. Our fictional aliens are (necessarily) only ourselves in disguise.
If other strategies or sentient processes are possible, we will be unable to recognize them as such*. It's not necessarily that alien civilizations are "smarter" or more "advanced", but so different that we wouldn't even recognize them as sentient.
We humans will no doubt, assuming our current state of knowledge endures, create Dyson swarms all over the local cluster, but that is a strategic inevitability given the best of our current understanding; an understanding that, again, is circumscribed by our bodies and minds and perceptions, which in turn are the result of specific evolutionary pressures. There is no reason to expect that these specific pressures uniquely equip us to grasp the entirety of this vastly complicated cosmology in which we find ourselves.
"Assuming our current state of knowledge endures" is doing a lot of heavy lifting, though. How likely is it that 1,000 or 10,000 years of research and increasingly sophisticated understanding of ourselves and our Universe would lead to Dyson as being our best option? Personally, I think unlikely, but who knows?
There are too many mysteries and unknowns for us humans to be too certain about anything cosmological or metaphysical. Way, way too many unknown unknowns.
* Unless aliens are close enough "like us" to be recognizable, but this is exactly my argument: this answer to the Fermi Paradox is that our perception and reasoning ability gives us such a thin slice of understanding the Universe that we have less chance of grasping "alien civilization" than grasshoppers have of understanding how bats "hop". Our dominance of Earth gives us a kind of provincial self-confidence that we understand everything important, but really, that is unlikely.
Let's assume as true (because we're speculating) that we are not among the first, and that some beasts already have situated themselves in the best possible position to take advantage of the future. Why, then, cannot we perceive them?
Also for most of those 14 bn years heavy elements weren't abundant enough for many rocky planets to form. It took several generations of stars and cycles of supernovas to end up with the heavy element rich environment we find ourselves in. Our star's siblings might be the first generation with a rich enough planetary environment and abundance of heavy elements to support life and technological civilizations.
I don’t think human science is remotely advanced enough to make one million year predictions, let alone 150 billion. We still have the giant dark matter fudge factor that eludes experimental verification, we’re still inventing new particles to make quantum math work, we still can’t come up with any theory that explains all our observations.
Extreme predictions require extreme precision of data and model, and it seems that our models have already hit their limits on much more pedestrian problems. Like Newton’s physics, they are a function that fits reality well only within a certain range of values.
This is not an anti-science sentiment, by the way, just maintaining scope and margins of error.
We're talking about the expansion of the Universe here. This expansion has 14 billion years of evidence in the observable Universe to support it.
Experimental evidence has shown this expansion is increasing not decreasing.
This isn't to say that the expansion won't slow, stop or even reverse but that claim requires a testable theory. The Universe is expanding based on all our understanding and all the experimental evidence we have to date is not an equivalent position to "well, we really don't know what will happen in a billion years".
Evidence doesn’t show anything, it supports things. Theories are replaced all the time by new theories that are supported by all the same evidence, and even more. Since this has been happening to theories on the fate of the universe on average once per generation for the past 100 years, and each successive theory has also had experimental support and scientific consensus, I’d say evidence supports a prediction of at least one more change, wouldn’t you?
One wonders if it is some fundamental assumption that limits further understanding of the Universe. Is it our intuitive relationship with numbers, time, and physical space that is limiting? We all grow up with societally imposed relationships between numbers, time, and physical space, but are these learned or do they actually reflect objective reality. The frameworks derived from these fundamental assumptions are excellent for making predictions and are testable within those frameworks.
> One wonders if it is some fundamental assumption
One could even go further...
Russell once compared the movement of the planets around the sun with people carrying torchs and walking around a mountain during the night. Because it is dark, we don't see that people are carrying those torchs and we are wondering why those lights move in such strange ways. Then then sun rises (Einstein presents his relativity theory) and we understand.
We hope such an explanation also exists for the entire universe and its physical laws. But could it be that the explanation is beyond the capabilities of the human brains? I am not talking about complexity or problem size. That could be solved with computers. Maybe there is an elegant, obvious and compact explanation why the universe behaves like it does, but it so much beyond what we consider as logical that if an alien had written it down in a book, we would not even realize what the book is about. Like trying to explain to a dog the concepts of "yesterday" and "tomorrow". Is there a reason to assume that our ape brains are able to understand everything?
One interesting question of this is if science changes once the other galaxy groups disappear. By that I mean if a human was to redo all previous experiments have we lost any data that would result in us building a different model of the universe than what we've built with the other galaxy groups present. Sure, we will have records of evidence from the past, but records of evidence from experiments that can no longer be repeated is no longer reliable data. This is ignoring the whole development of humanity and science in the intervening 150 billion years which may change what options are available to use then.
I'm also not sure if this is the most up to date model of the universe. I've recently dived deep into quantum field theory and theoretical physics (well as deep as one can without understanding a field of spinors) and I've watched a number of lectures given over the last decade that include alternate takes on expansion. I could be completely misunderstanding what was being spoken about, but it appears one possible model of reality is a sort of bubble in hyperexpanding space that gives rise to a big bang event that eventually leads to the bubble becoming hyper expanding space itself that can give rise to further bubbles.
I recommend the whole thing if you aren't familiar with Boltzmann brains and boxes, but if you want to see the issue with the current model presented in this article go to 41 minute mark and to see a possible alternate model go to about 44:50.
The galaxies of the local group won't recede from us.
Telescopes as recently as the early 20th century could not resolve the sky well enough to determine which nebulae are in the Milky Way, are satellites of the Milky Way, are in the local group, or are outside it. This lack of observation allowed the Shapeley-Curtis https://en.wikipedia.org/wiki/Great_Debate_(astronomy) to last as long as it did -- it ended with clear observations of light curves within M31 and other galaxies in the local group. Those observations will remain available for a lonnnnnnnnnnng time after there are no other galaxy groups in the view of anything in the local group.
Moreover, the relic fields (cosmic microwave background and cosmic neutrino background) will be detectable (at least in principle) as they grow ever colder in the far future, so at least the radiation and dark energy sectors can be rediscovered in the distant future when there are no highly redshifted galaxies left to see.
We are lucky to have clear views of redshifted, dimmed, smaller-on-the-sky spiral galaxies at all sorts of orientations to us (including face on). Those certainly help us to understand galactic evolution, and also provide us lots of interesting objects (gas and dust clouds, active nuclei, stars, supernovae) "in the past". In the far future, the local group will have fewer spirals, fewer stars, almost no low-metallicity stars, and so on, and recovering those from observation of what's left in the distant future will be more difficult. But around the garganguan black holes left in the local group in the very far future there will still be plenty of dust and gas and cooling stellar remnants; the occasional star will probably form, but not often. Still, the volume will be large enough far into the future that "not often" simply invites wider searches of the sky. Supernovae don't happen often, but thanks to all-sky surveys like http://www.astronomy.ohio-state.edu/~assassin/index.shtml we see A LOT more of them than could have been hoped for a hundred years ago. Far future astronomers might have a super-hyper-ultra ASAS-SN type programme to look for the earliest signs of star formation in what's left of the local group.
Of course as you note, even better would be if the far future astronomers can recover the records of earlier astronomers, rather than having to rediscover everything from scratch using the "records" provided by nature at astronomical scales.
The far far future where one cannot see distant galaxies from the local group is very very very very very much earlier than the epoch of fluctuations into Boltzmann brains, vacuum decay, etc. etc., none of which is a really reliable prediction of well-tested theory. (In fact, they are diagnostic of theoretical deficiencies: how do we prevent objects that can happen according to a theory from happening according to that theory, given that we do not see those objects? This recurs a lot at the edges of theoretical physics: in General Relativity it's the basis for developing the https://en.wikipedia.org/wiki/Energy_condition (s) for instance).
* spacetime itself literally “creates itself out of nothingness”. We say it “stretches”, but don't picture a string of cheese “thinning out” but rather something of constant density being continuously expanded.
* local groups (of galaxies) are gravitationally bound i.e they won’t scatter around and they attract each other with gravity so they will stay together even though more spacetime is created inside of them.
* in the empty “space” between local groups something (dark matter? Dark energy? ) causes this steady continuous creation of new spacetime
* this creation of spacetime is itself accelerating, but this is not speed, so distant local groups are not “moving away from us” but are being “displaced away from us”. Hence the displacement rate is allowed to be faster than the speed of light (since it’s not a speed)
* every single object in the universe moves at exactly and precisely the speed of light c through spacetime. Objects at rest (a completely stationary astronaut stranded in the empty space between local groups) is also moving at the speed of light in spacetime: he is moving only through time.
* A photon is moving only through space and has no time
Sources: The Elegant Universe, Brian Greene, ScienceClic YouTube, and my memory of these
It makes sense if one pictures the aether as a finite substance over an infinite surface area of and in of itself. Like a limited amount of paint that gets smooshed and pushed around. Intuitively I'd postulate that the aether is an incompressible fluid at 'neutral' - a fundamental formless form with directionality. Then when the aether interacts with itself in opposing currents it creates an inertia against itself causing it to evaporate into matter all the way into planet cores, rock, water, air, then outer space. It basically becomes a pressure gradient at different critical boundaries. Makes sense to me. Can simply a lot of Science most likely, just import the electron as a monopole that builds up into hydrogen and so on. Aether splits into E- and E+ and clumps until 1836 (proton/2 918 aether e+ e-).
Can be radically simplified with Torus, Aether, and Hyperboloid fluidic models around Vortices.
Yeah, I know, it's hard to make the leap. It required a total inversion for me but it all comes together in contemplation. I think there's a close equivalence to aether and bose-einstein condensate.
> local groups (of galaxies) [have] more spacetime [...] created inside of them
No. For all practical purposes (FAPP), galaxy clusters are not expanding. Swiss cheese cosmologies where Lemaître-Tolman-Bondi (LTB) solution vacuoles are embeded within the usual Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological solution generate observables that closely match luminosity-redshift and angular diamater-redshift relations for active galactic nuclei (SN Ia supernovae) extremely well.
LTB is a contracting spacetime -- there simply is no expansion within the vacuole, mathematically. Adding in expansion gives you something other than LTB, which leads to a different redshift for AGNs and SNs in the same cluster when correcting for peculiar motion within the cluster. The evidence favours LTB, and the physical interpretation is that the galaxy clusters and the gas surrounding them are condensing into a point over cosmologically long periods, while the galaxy clusters separate according to the surrounding expanding FLRW solution (which captures the Lyman-alpha forest, and the CMB redshift, among other observables).
Moreover, at the scale of the solar system, there is simply no measurable metric expansion. FAPP the solar system matches an LTB solution.
Since LTB solutions are asymptotically flat, we can nest them hierarchically, using an Israel-Darmois thin shell junction to capture how radiation and other matter moves between the nested LTB solutions and between the "outer" LTB solution for the vacuole in general and the FLRW solution. This works reasonably well, and there have been good numerical approaches since the mid-1990s (Musgrave, 1996).
> every single object in the universe moves exactly and precisely at the speed of light through spacetime
No. In General Relativity velocity vectors are ambiguous except at a coincident point. At a point occupied by a massless particle or wave (a photon or classical light, for instance) and one occupied by a massive particle, one can clearly and unambiguously demonstrate that the former is faster.
In a Lorentzian manifold, like our universe, there is a clear distinction between lightlike and timelike geodesics; light couples to one but not the other, and anything massive couples only to timelike (and not lightlike) geodesics. This is an inevitable consequence of the 1+3 dimensional geometry: https://en.wikipedia.org/wiki/Causal_structure#Tangent_vecto...
Light (and other massless waves or particles, those with "null" mass) simply do not have access to the same tangent vectors as massive objects, and vice-versa. This is extremely well supported by experiment and observation: https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi...
> a photon is moving only through space and has no time
A classical light wave is either at a point in spacetime or it is not. Quantum mechanical photons do not much differ.
"Movement through spacetime" is because you have chosen to split up the whole spacetime into spaces oriented by time, turning a worldline/worldtube into a set of time-ordered points. But your choice of splitting is not the right splitting any more than anyone elses. I can choose to split the universe into spaces oriented along the path of a photon emitted from a distant galaxy to my CCD detector. Or from the lightbulb across the room to my eyeball. It was obviously still in the distant galaxy, or in the filament, in my idea of the past, and a bit less past it was in flight between the two (if you believe in physical realism). That is, applying our favoured splitting to a system does not change the underlying worldtube, only how we label any given part of it.
One can carve a lightlike geodesic up however you ...
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[ 0.20 ms ] story [ 134 ms ] threadWhat will that do to the speed of light as measured on Earth?
And will this affect, say, the correct operation of computers?
The dipole anisotropy of the Cosmic Microwave Background will continue to be a good tool to see the constancy of the speed of light for all observers into the practically infinite future, assuming the infinite future looks a lot like de Sitter space (which is essentially what the standard cosmology predicts).
http://www.astro.ucla.edu/~wright/CMB-dipole-history.html
The far future of the universe will still have generally covariant physics, as does its far past, sharpening up your second sentence.
As far as the measurability of the speed of light, I don't think anything changes in principle. It should remain measurable and constant. However, it gets a lot more complicated in practice, since it's pretty unclear who or what would actually be doing the measuring and how they might go about it.
https://en.wikipedia.org/wiki/Big_Rip
We also have good views of the Lyman-alpha forest through galaxy clusters at different redshifts, and the distribution of the spectral lines are another line of evidence that members of galaxy clusters closer to us have smaller radial components in their peculiar velocities than those at higher redshift/smaller angle/lower brightness.
To get to a big rip you have to impose a further time derivative and figure out how to keep it small now and make it big later. AFAIK nobody who has tried this has avoided significant complexity to match the observational data we have today, except by pushing the Big Rip so enormously far into the future that we don't have enough data to constrain that future. The wikipedia article refers to WMAP9, which ended 2013 (~ 9 years after WMAP turned on in 2003). There is more recent data from Planck and other observatories, and they make it harder and harder to justify a Big Rip for the reasons half-admitted in the "Observed universe" section of that wikipedia article.
Moreover, MESSENGER and other spacecraft sent to parts of our solar system have given us a lot of data about the metric of our solar system, and it is non-expanding; orbiting clocks support this more locally. Likewise, there are plenty of sensitive terrestrial experiments that highly constrain any expansion term in plausible Standard Model + gravity Lagrangians.
It can't be ruled out but it has been effectively pushed back so far that it's the least of our worries about "what happens hundreds of trillions of years in the future" : if one accepts there there is a Big Rip field, there's no compelling reason why it should have significant effects on matter in the local group that soon, and several arguments about why it shouldn't.
You'll find a bit more substance (and higher difficulty) at https://en.wikipedia.org/wiki/Quintessence_(physics)#Tracker...
Hurry up Elon, we're running out of time!
And most of what is still visible is also completely unreachable.
https://news.ycombinator.com/item?id=26732996
However, the planetary environment could become so hostile that it takes so much effort to maintain civilization that our capabilities become severely limited. So my concern isn't so much us dying out, as we become doomed to a fairly marginal existence, incapable of interesting achievements such as interstellar exploration.
[1] https://en.wikipedia.org/wiki/Milky_Way#Contents
However, it's possible that the accelerated expansion might prove an aberration that only makes the galaxies seem as if they are "quickly" moving beyond our reach. In which case, what a relief! LOL
If we allow for accelerated expansion of the Universe, then it means that the star B is closer to us then the star A (with the star A moving away faster than "c" in 100M years from today - i.e. accelerated expansion) - i.e. today the star B is closer to us than 13.7B ly and runs away slower then "c", and thus in the next 100M years the star B would be expected to make a journey longer than 100M ly - from some point today closer to us than 13.7B ly to the point at 13.8B ly in 100M years - i.e. it would need to run away faster then "c" during those 100M years which is clearly a contradiction.
That leads to conclusion that whatever speed a star runs away with today, it will be running away with the same speed tomorrow - that means decreasing Hubble constant - which is normal giving the same energy being spread in a bigger and bigger space. And when it comes to observing the observable Universe - the star A will always run away with "c", and whatever stars B are closer to us then the star A ( and thus running away slower than "c" today), those stars B will never run away faster than "c" - thus the light from them will always come to us.
Note: the stars which are farther away moving faster isn't acceleration of expansion. Acceleration of expansion would be if the same stars were running away faster tomorrow than today - which would clearly lead to the contradiction pointed above. Hubble constant decreasing with time is the only way to avoid that contradiction. That also probably explains different Hubble constant values we currently have - as they are obtained using signals from different points in the Universe history.
My other favorite is that there is no Paradox. The signs of alien civilizations are everywhere, but humans cannot perceive or understand it
Enjoy!
[1]: https://www.youtube.com/playlist?list=PLIIOUpOge0LvHsTP5fm8o...
TIMELAPSE OF THE FUTURE: A Journey to the End of Time (4K)
https://www.youtube.com/watch?v=uD4izuDMUQA
It's like the intersection of the insignifigance and smallness of my own individual existence + the immense scale of time to underscore that fact + the fact that even all the things that are spectacular and powerful also meet the same kinda end as me.
Something about the 58 billion trillion trillion trillion trillion years last black hole thing just bums me out.
Another perspective is that this will be the Golden Age of the Universe [1].
[1]: https://www.youtube.com/watch?v=Qam5BkXIEhQ
As for signs of alien civilization being everywhere, here I have to respectfully disagree. I'll summarize why.
I firmly believe the likely future of humanity is not on planets but in orbitals. Planets are a great way of storing mass. They're a poor way of creating living area. I think the estimate is that 1% of the mass of Mercury could fully encompass the Sun in a Dyson Swarm of orbitals with no material stronger than stainless steel. I believe humanity will be capable of this within 1,000 years and that's super-conservative.
Objects in space that absorb Solar radiation heat up. The only way of releasing that is by radiating it away. That radiation is of a wavelength solely determined by the temperature of the object. For any reasonable temperatures, that's in the infrared spectrum.
So a Dyson Swarm will have a very particular spectrum. They'll be IR lanterns. Sure one star we may not see out of a whole galaxy but the barrier between doing this to one star to the whole galaxy is only a matter of a relatively short amount of time (cosmologically speaking). One million years for the Milky Way (out of 10B+ years).
There are a bunch of assumptions in this and each would be their own topic. But as a solution to the Fermi Paradox, it only takes one.
Recycling waste heat is a common objection but that just reduces your emissions, it doesn't eliminate them. If it did, that would break thermodynamics.
This argument is strengthened by this article: over time more mass is becoming unreachable. For truly long-lived civilizations it makes the most sense to grab as much mass as you can and sequester it. Again, it only takes one.
> As for signs of alien civilization being everywhere, here I have to respectfully disagree.
This is all speculation, of course, but that's all we have. This particular answer to the Fermi Paradox goes like this:
Human reasoning and interests are entirely circumscribed by our perception. We are capable only of dimly imagining that other kinds of perception and reasoning exist, but not imagining what or how that might be. Our fictional aliens are (necessarily) only ourselves in disguise.
If other strategies or sentient processes are possible, we will be unable to recognize them as such*. It's not necessarily that alien civilizations are "smarter" or more "advanced", but so different that we wouldn't even recognize them as sentient.
We humans will no doubt, assuming our current state of knowledge endures, create Dyson swarms all over the local cluster, but that is a strategic inevitability given the best of our current understanding; an understanding that, again, is circumscribed by our bodies and minds and perceptions, which in turn are the result of specific evolutionary pressures. There is no reason to expect that these specific pressures uniquely equip us to grasp the entirety of this vastly complicated cosmology in which we find ourselves.
"Assuming our current state of knowledge endures" is doing a lot of heavy lifting, though. How likely is it that 1,000 or 10,000 years of research and increasingly sophisticated understanding of ourselves and our Universe would lead to Dyson as being our best option? Personally, I think unlikely, but who knows?
There are too many mysteries and unknowns for us humans to be too certain about anything cosmological or metaphysical. Way, way too many unknown unknowns.
* Unless aliens are close enough "like us" to be recognizable, but this is exactly my argument: this answer to the Fermi Paradox is that our perception and reasoning ability gives us such a thin slice of understanding the Universe that we have less chance of grasping "alien civilization" than grasshoppers have of understanding how bats "hop". Our dominance of Earth gives us a kind of provincial self-confidence that we understand everything important, but really, that is unlikely.
Let's assume as true (because we're speculating) that we are not among the first, and that some beasts already have situated themselves in the best possible position to take advantage of the future. Why, then, cannot we perceive them?
Extreme predictions require extreme precision of data and model, and it seems that our models have already hit their limits on much more pedestrian problems. Like Newton’s physics, they are a function that fits reality well only within a certain range of values.
This is not an anti-science sentiment, by the way, just maintaining scope and margins of error.
Experimental evidence has shown this expansion is increasing not decreasing.
This isn't to say that the expansion won't slow, stop or even reverse but that claim requires a testable theory. The Universe is expanding based on all our understanding and all the experimental evidence we have to date is not an equivalent position to "well, we really don't know what will happen in a billion years".
Amid other information, this page lists major changes to cosmological theory: https://en.wikipedia.org/wiki/Timeline_of_cosmological_theor...
One could even go further...
Russell once compared the movement of the planets around the sun with people carrying torchs and walking around a mountain during the night. Because it is dark, we don't see that people are carrying those torchs and we are wondering why those lights move in such strange ways. Then then sun rises (Einstein presents his relativity theory) and we understand.
We hope such an explanation also exists for the entire universe and its physical laws. But could it be that the explanation is beyond the capabilities of the human brains? I am not talking about complexity or problem size. That could be solved with computers. Maybe there is an elegant, obvious and compact explanation why the universe behaves like it does, but it so much beyond what we consider as logical that if an alien had written it down in a book, we would not even realize what the book is about. Like trying to explain to a dog the concepts of "yesterday" and "tomorrow". Is there a reason to assume that our ape brains are able to understand everything?
I'm also not sure if this is the most up to date model of the universe. I've recently dived deep into quantum field theory and theoretical physics (well as deep as one can without understanding a field of spinors) and I've watched a number of lectures given over the last decade that include alternate takes on expansion. I could be completely misunderstanding what was being spoken about, but it appears one possible model of reality is a sort of bubble in hyperexpanding space that gives rise to a big bang event that eventually leads to the bubble becoming hyper expanding space itself that can give rise to further bubbles.
Here is one talk.
https://www.youtube.com/watch?v=jhnKBKZvb_U
I recommend the whole thing if you aren't familiar with Boltzmann brains and boxes, but if you want to see the issue with the current model presented in this article go to 41 minute mark and to see a possible alternate model go to about 44:50.
Telescopes as recently as the early 20th century could not resolve the sky well enough to determine which nebulae are in the Milky Way, are satellites of the Milky Way, are in the local group, or are outside it. This lack of observation allowed the Shapeley-Curtis https://en.wikipedia.org/wiki/Great_Debate_(astronomy) to last as long as it did -- it ended with clear observations of light curves within M31 and other galaxies in the local group. Those observations will remain available for a lonnnnnnnnnnng time after there are no other galaxy groups in the view of anything in the local group.
Moreover, the relic fields (cosmic microwave background and cosmic neutrino background) will be detectable (at least in principle) as they grow ever colder in the far future, so at least the radiation and dark energy sectors can be rediscovered in the distant future when there are no highly redshifted galaxies left to see.
We are lucky to have clear views of redshifted, dimmed, smaller-on-the-sky spiral galaxies at all sorts of orientations to us (including face on). Those certainly help us to understand galactic evolution, and also provide us lots of interesting objects (gas and dust clouds, active nuclei, stars, supernovae) "in the past". In the far future, the local group will have fewer spirals, fewer stars, almost no low-metallicity stars, and so on, and recovering those from observation of what's left in the distant future will be more difficult. But around the garganguan black holes left in the local group in the very far future there will still be plenty of dust and gas and cooling stellar remnants; the occasional star will probably form, but not often. Still, the volume will be large enough far into the future that "not often" simply invites wider searches of the sky. Supernovae don't happen often, but thanks to all-sky surveys like http://www.astronomy.ohio-state.edu/~assassin/index.shtml we see A LOT more of them than could have been hoped for a hundred years ago. Far future astronomers might have a super-hyper-ultra ASAS-SN type programme to look for the earliest signs of star formation in what's left of the local group.
Of course as you note, even better would be if the far future astronomers can recover the records of earlier astronomers, rather than having to rediscover everything from scratch using the "records" provided by nature at astronomical scales.
The far far future where one cannot see distant galaxies from the local group is very very very very very much earlier than the epoch of fluctuations into Boltzmann brains, vacuum decay, etc. etc., none of which is a really reliable prediction of well-tested theory. (In fact, they are diagnostic of theoretical deficiencies: how do we prevent objects that can happen according to a theory from happening according to that theory, given that we do not see those objects? This recurs a lot at the edges of theoretical physics: in General Relativity it's the basis for developing the https://en.wikipedia.org/wiki/Energy_condition (s) for instance).
* spacetime itself literally “creates itself out of nothingness”. We say it “stretches”, but don't picture a string of cheese “thinning out” but rather something of constant density being continuously expanded.
* local groups (of galaxies) are gravitationally bound i.e they won’t scatter around and they attract each other with gravity so they will stay together even though more spacetime is created inside of them.
* in the empty “space” between local groups something (dark matter? Dark energy? ) causes this steady continuous creation of new spacetime
* this creation of spacetime is itself accelerating, but this is not speed, so distant local groups are not “moving away from us” but are being “displaced away from us”. Hence the displacement rate is allowed to be faster than the speed of light (since it’s not a speed)
* every single object in the universe moves at exactly and precisely the speed of light c through spacetime. Objects at rest (a completely stationary astronaut stranded in the empty space between local groups) is also moving at the speed of light in spacetime: he is moving only through time.
* A photon is moving only through space and has no time
Sources: The Elegant Universe, Brian Greene, ScienceClic YouTube, and my memory of these
Can be radically simplified with Torus, Aether, and Hyperboloid fluidic models around Vortices.
Also https://en.wikipedia.org/wiki/Gabriel%27s_Horn for the finite paint paradox.
No. For all practical purposes (FAPP), galaxy clusters are not expanding. Swiss cheese cosmologies where Lemaître-Tolman-Bondi (LTB) solution vacuoles are embeded within the usual Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological solution generate observables that closely match luminosity-redshift and angular diamater-redshift relations for active galactic nuclei (SN Ia supernovae) extremely well.
LTB is a contracting spacetime -- there simply is no expansion within the vacuole, mathematically. Adding in expansion gives you something other than LTB, which leads to a different redshift for AGNs and SNs in the same cluster when correcting for peculiar motion within the cluster. The evidence favours LTB, and the physical interpretation is that the galaxy clusters and the gas surrounding them are condensing into a point over cosmologically long periods, while the galaxy clusters separate according to the surrounding expanding FLRW solution (which captures the Lyman-alpha forest, and the CMB redshift, among other observables).
Moreover, at the scale of the solar system, there is simply no measurable metric expansion. FAPP the solar system matches an LTB solution.
Since LTB solutions are asymptotically flat, we can nest them hierarchically, using an Israel-Darmois thin shell junction to capture how radiation and other matter moves between the nested LTB solutions and between the "outer" LTB solution for the vacuole in general and the FLRW solution. This works reasonably well, and there have been good numerical approaches since the mid-1990s (Musgrave, 1996).
> every single object in the universe moves exactly and precisely at the speed of light through spacetime
No. In General Relativity velocity vectors are ambiguous except at a coincident point. At a point occupied by a massless particle or wave (a photon or classical light, for instance) and one occupied by a massive particle, one can clearly and unambiguously demonstrate that the former is faster.
In a Lorentzian manifold, like our universe, there is a clear distinction between lightlike and timelike geodesics; light couples to one but not the other, and anything massive couples only to timelike (and not lightlike) geodesics. This is an inevitable consequence of the 1+3 dimensional geometry: https://en.wikipedia.org/wiki/Causal_structure#Tangent_vecto...
Light (and other massless waves or particles, those with "null" mass) simply do not have access to the same tangent vectors as massive objects, and vice-versa. This is extremely well supported by experiment and observation: https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi...
> a photon is moving only through space and has no time
A classical light wave is either at a point in spacetime or it is not. Quantum mechanical photons do not much differ.
"Movement through spacetime" is because you have chosen to split up the whole spacetime into spaces oriented by time, turning a worldline/worldtube into a set of time-ordered points. But your choice of splitting is not the right splitting any more than anyone elses. I can choose to split the universe into spaces oriented along the path of a photon emitted from a distant galaxy to my CCD detector. Or from the lightbulb across the room to my eyeball. It was obviously still in the distant galaxy, or in the filament, in my idea of the past, and a bit less past it was in flight between the two (if you believe in physical realism). That is, applying our favoured splitting to a system does not change the underlying worldtube, only how we label any given part of it.
One can carve a lightlike geodesic up however you ...
For The rest I am not advanced enough to say !