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Old 09-15-2019, 11:27 AM
Ynnad is offline
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Heat Death of the Universe


One of the more widely accepted conjectures on the ultimate fate of the universe is “heat death.” In short, the heat death of the universe is a consequence of the inevitable increase in entropy of the entire universe which results in the universe become less dense and colder.

For the purposes of this discussion, let us ignore the conjectures that some sort of quantum fluctuation will eventually create a new universe or revitalize this universe.

Let us consider a potentially immortal magic observer of our universe. (This ought to get some SDMB people stirred up, invoking magic in a physics discussion.) This magic observer does not interact with the universe in any way and does not affect the thermodynamics of the universe. This magic observer perceives space and time in a way similar to humans. Would the magic observer ever observe the final heat death of the universe or only observe the universe getting even colder and even less dense? In other words, will the heat death of the universe (if that is the ultimate fate of the universe) ever actually occur or will the universe just get very boring?

I suppose we could use math instead of magic in this discussion (the word asymptote comes to mind) but it is too early in morning for math.
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Old 09-15-2019, 11:41 AM
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The answers are "we don't know" and "in light of the heat death ending, the beginning of the universe is impossible".

By "impossible", I mean we have not yet discovered the principles of physics that allow the universe, a clearly complex system, to come from a very simple system ("absolute nothingness"). This appears to be a violation of thermodynamics.
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Old 09-15-2019, 11:47 AM
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Originally Posted by Ynnad View Post
One of the more widely accepted conjectures on the ultimate fate of the universe is “heat death.” In short, the heat death of the universe is a consequence of the inevitable increase in entropy of the entire universe which results in the universe become less dense and colder.

For the purposes of this discussion, let us ignore the conjectures that some sort of quantum fluctuation will eventually create a new universe or revitalize this universe.

Let us consider a potentially immortal magic observer of our universe. (This ought to get some SDMB people stirred up, invoking magic in a physics discussion.) This magic observer does not interact with the universe in any way and does not affect the thermodynamics of the universe. This magic observer perceives space and time in a way similar to humans. Would the magic observer ever observe the final heat death of the universe or only observe the universe getting even colder and even less dense? In other words, will the heat death of the universe (if that is the ultimate fate of the universe) ever actually occur or will the universe just get very boring?

I suppose we could use math instead of magic in this discussion (the word asymptote comes to mind) but it is too early in morning for math.
I think that "heat death" is an asymptotic state. The universe gets closer and closer to the ultimate state, but never reaches it.
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Old 09-15-2019, 11:54 AM
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And if you try to apply some reasonable standard criterion to draw the line of when "heat death" occurs, you inevitably find that that line was already crossed far in the past.
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Old 09-15-2019, 12:05 PM
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Observation, magic or otherwise, is irrelevant. Of course, in the far future we are discussing everybody is long dead, all the stars have burned out or exploded, etc. Wait long enough and even black holes evaporate.

As for this cooling business, the idea is that there is some cosmological constant causing the universe to expand. Any two different points will expand exponentially away from each other, so there will be nothing to observe or interact with-- indeed "very boring". All you are left with are isolated particles and thermal radiation at 10-30 degrees or so, so you do get your heat death, in this case at an extremely small positive temperature.

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Originally Posted by SamuelA View Post
The answers are "we don't know" and "in light of the heat death ending, the beginning of the universe is impossible".

By "impossible", I mean we have not yet discovered the principles of physics that allow the universe, a clearly complex system, to come from a very simple system ("absolute nothingness"). This appears to be a violation of thermodynamics.
Impossible? Violation? Once you are at thermal equilibrium at some nonzero temperature, the random fluctuations the OP wanted to gloss over will, eventually, create any state you like. This is a classic thermodynamical principle.

Last edited by DPRK; 09-15-2019 at 12:09 PM.
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Old 09-15-2019, 12:20 PM
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Impossible? Violation? Once you are at thermal equilibrium at some nonzero temperature, the random fluctuations the OP wanted to gloss over will, eventually, create any state you like. This is a classic thermodynamical principle.
Uh, what? How does that create space-time and a singularity of enormous mass and an entire universe?

By entropy hypotheses, everything winds down, yet the creation of the universe is obviously a winding up. It is not clear yet (obviously it's possible, but not by any known rule) why this happened.

Last edited by SamuelA; 09-15-2019 at 12:21 PM.
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Old 09-15-2019, 12:34 PM
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... there is some cosmological constant causing the universe to expand. ... All you are left with are isolated particles and thermal radiation at 10-30 degrees or so ... .
This all sounds very plausible. But wouldn't this thermal radiation imply the continued existence of time and space?
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Old 09-15-2019, 12:37 PM
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This all sounds very plausible. But wouldn't this thermal radiation imply the continued existence of time and space?
"Heat Death" doesn't mean that the universe stops existing, just that there is no usable energy (no temperature difference with which a heat engine can be powered).
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Old 09-15-2019, 12:42 PM
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This all sounds very plausible. But wouldn't this thermal radiation imply the continued existence of time and space?
Yes, this is assuming a de Sitter cosmology. Time and space have not been destroyed, but everything is in a very boring sort of heat death.
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Old 09-15-2019, 12:58 PM
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"Heat Death" doesn't mean that the universe stops existing, just that there is no usable energy (no temperature difference with which a heat engine can be powered).
So you are saying that heat death is not the end of the universe, it is just the end of work in the universe. Sort of like my workplace on a Friday afternoon.

I can go along with that.

So, maybe due to the continued expansion of the universe we actually do get to the "heat death" of the universe in a finite amount of time but this may not be the end of the universe.

Do I even know what I am talking about?
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Old 09-15-2019, 01:16 PM
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So you are saying that heat death is not the end of the universe, it is just the end of work in the universe. Sort of like my workplace on a Friday afternoon.

I can go along with that.

So, maybe due to the continued expansion of the universe we actually do get to the "heat death" of the universe in a finite amount of time but this may not be the end of the universe.

Do I even know what I am talking about?
I do not believe enough physics is known to say with any certainty what the ultimate fate of the universe is after a ludicrous amount of time. For instance, large enough thermal fluctuations imply something interesting will happen again, so the heat death is not the end. On the other hand, highly theoretical considerations like string theory might imply that the de Sitter vacuum itself is not stable and will decay into something else. Anyway, this is all on such ludicrous time scales that it is possible that effects and considerations nobody has thought about will change the picture, so nobody can really say what happens after the end.
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Old 09-15-2019, 03:10 PM
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So would it be plausible to say that in a non-expanding universe heat death would only be approached asymptotically but in an expanding universe it would eventually occur in a finite amount of time.
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Old 09-15-2019, 03:25 PM
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And if you try to apply some reasonable standard criterion to draw the line of when "heat death" occurs, you inevitably find that that line was already crossed far in the past.
Really? Not a standard like "no more fusion," or "black holes radiate themselves to death," or even "matter breaks down and decay into its constituent components (gluons and such)"?

I mean, I can get that we can only estimate these timelines, but to say "it’s either already here, or we don’t have a clue, couldn’t possibly define it accord any reasonable standard" seems to go a bit too far in the other direction.
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Old 09-15-2019, 05:30 PM
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I would say heat death is not relevant in current cosmological models. Expansion in these models is such that we cannot ever exchange energy with the majority of the observable universe (as it currently is - obviously we are currently receiving energy from all of the observable universe's past and always will do).

For heat death to be relevant in a comsological model, I believe the model must have extend infinitely in to the conformal future (NB conformal time is not the same as comsological time).
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Old 09-16-2019, 06:40 AM
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Penrose's Conformal Cyclic Cosmology


The final heat death stage (is "entropy death" more descriptive?) isn't reached after quadrillions of years, or even quintillions. It takes longer than that for massive black holes to evaporate, and then googols of more years for protons to decay (if they decay at all).

One of the stranger hypotheses (Penrose's Conformal Cyclic Cosmology) links the "heat death" to the "big bang" of a new cycle. (I hope one of the experts will correct any errors in the following. I give myself at best a 50-50 chance of even getting the basics correct.)

At first it seems absurd that an entropy death should resemble a big bang. The former occupies a huge amount of space, is very dilute, very cold and has very high entropy. The latter is very small and dense, very hot, and has very low entropy. How could opposites be the same thing from different perspectives?

One key is that almost all the mass-energy of the earliest universe was in the form of kinetic energy. What mass there was (in the form of Higgs bosons?) was such a tiny portion of the total mass-energy as to be effectively zero. Similarly, the very late universe — assuming protons do eventually decay — would have very little of its mass-energy in the form of massive particles.

And massive particles are required for clocks and rulers! In the very early or very late universe, there were almost no massive particles, so no clocks or rulers. Without clocks or rulers a universe with 10 meter diameter is identical to a 20-meter scaled-up universe in which the (non-existent) rulers would be twice as big, and the (non-existent) clocks would be twice as slow. Of course the scaling factor needed to equate the entropy-death universe with the universe a quectosecond after the big bang would not be two: it would many many googols!

IIRC Penrose now claims that details of the cosmic background radiation may provide a glimpse of the previous cycle, and hence evidence for his model.

I think many would be laughed at if they proposed "Conformal Cyclic Cosmology," but even his detractors admit that Roger Penrose is no dummy. What do physicists think of it? How off-base was my explanation?

Last edited by septimus; 09-16-2019 at 06:44 AM.
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Old 09-16-2019, 08:51 AM
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I remember reading about Penrose's model, I think the idea is that the final state, 'heat death'(?), is conformlly equivalent to the initial state, 'big bang', so when it reaches its final state the Universe undergoes a phase/scale change and starts over. I also believe though that the central idea of the Universe undergoing a phase change due to conformal equivalence is seen as a bit dodgy.

This is not directly related to the idea of conformal time which I mentioned above though. A conformal mapping is one which preserves angles, but not necessarily distances, for example some maps of the World use conformal projections. In relativity it is often useful to conformally map a spacetime to flat Minkwoski space and this was also Penrose's idea. This is particularly useful in cosmology and conformal time is comsological time mapped conformally to Minkwoski space.
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Old 09-16-2019, 08:58 AM
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Just remember that infinity is a verrrry long time. So even if a quantum fluctuation large enough to cause a new big bang is massively unlikely, it is not impossible and WILL happen.
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Old 09-16-2019, 11:50 AM
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The heat death hypothesis implies a closed system. If the universe is infinite, by definition, it cannot be a closed system.
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Old 09-16-2019, 12:24 PM
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The heat death hypothesis implies a closed system. If the universe is infinite, by definition, it cannot be a closed system.
If the universe is expanding, though, then you will observe a sort of event horizon a finite distance away.
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Old 09-18-2019, 12:55 AM
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The heat death of the universe is an interesting topic and I've had a few questions as well but I've been reluctant to start a new thread for several reasons, one of them being the fact that probably no one really knows the ultimate answers to such fundamental questions, or there are more than one version, or to comprehend any of these answers one needs to speak Further Mathematics, and so on.

One question is about time. I hear that billions of billions of billions of billions of billions of years away from now time will have become meaningless. I have always wondered about the nature of time and I think if I understand what causes time to become meaningless, then I may be able to figure out what gives meaning to time. I for one tend to equate time with change and progression. If nothing happens in the distant future of the universe, then time no longer plays any role in the fabric of the universe.

Another question is about spacetime. I wonder how essential time is to the existence of the universe because it is possible that the entire system of coordinates should collapse in the absence of time. It is very likely that if time does not matter anymore, nothing else will, which translates into the actual death of the universe.

The final question is about space. Since the universe is expanding at an accelerating rate, billions of billions of billions of billions of billions of years from now everything in the universe will have thinned out to such an extent that one can hardly say that anything exists at all, which I think should render space meaningless as well - space itself may fade away like a fleeting waft of bad smell.
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Old 09-18-2019, 08:43 AM
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Here is a pretty good lecture on the heat death of the universe. It starts at the end of the Age of Stars when the last red dwarf flickers out, some 100-trillion years from now, so we're talking really deep time.
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Old 09-18-2019, 09:12 AM
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I am not a physicist so this is my armchair understanding from a couple of semesters of freshman level physics and reading a few books on cosmology. The way I understand it is that protons and neutrons are not stable when we start considering really long time periods, maybe something of the magnitude of TREE(3) years or some crazy number like that. Eventually all the neutrons decay into photons / electromagnetic radiation. The protons decay into positrons and radiation. The positrons and electrons destroy each other leading to more radiation. At this point all the particles would be gone, and we would be left with radiation that continues to slowly cool to almost absolute zero. Will this happen in the far distant future? I have no idea, but on the other hand I don’t see any reason to doubt such an outcome either. I suppose it could be possible that space is stretched out to such a large extent by that time that maybe some of the electrons and positrons simply never encounter each other, but that probably doesn’t help things much.
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Old 09-18-2019, 05:06 PM
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The longest timescales you'll ever have to consider will be the evaporation of supermassive black holes, which will take about a thousand googol seconds (I was really excited when I did that calculation; it's so rare that one encounters a googol of anything). But a googol is still far, far short of TREE(3).
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Old 09-18-2019, 07:42 PM
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I was under the impression that the universe just got very boring.

In a book I read on teh subject, the implication was that if the universe continues to accelerate that by 10^100+ years from now, there will only be 1 particle per physical area that is something like 10^80 larger than the entire current universe.

So it wouldn't die, it'd just be boring as hell. THis assumes protons do not decay.

The book was 'the five ages of the universe'
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Old 09-19-2019, 09:17 AM
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The way I understand it is that protons and neutrons are not stable when we start considering really long time periods[snip]
Nobody's sure. All that can be said for certainty is that if they decay they'll last somewhere around 1030 years. Like I said, we're talking deep time here -- deep enough to make a geologist gasp. The PBS lecture I linked to above covers both the protons are stable and protons aren't stable scenarios.
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Old 09-19-2019, 03:22 PM
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So far as I know, all of the proposed Grand Unified Theories (models which would unify the Strong and Weak Forces, with or without gravity) predict that protons are unstable, though they might make different predictions about their lifespan. So folks would be surprised if it turns out they don't decay at all. The figures you'll usually see bandied about are the experimental limits on proton lifespan: We've never verifiably seen a decay, and if it were less than that, with the experiments we've run, we would have seen it. The actual lifespan could well be far greater than that.
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