What about 2nd law of thermodynamics in cyclic universe model?

[Jarek Duda]

Not everyone likes the idea of universe ​​created from a point singularity, so recently grows in popularity cyclic model - that our universe will finally collapse, use obtained momentum to bounce (so-called Big Bounce) and become the new Big Bang.
One might criticize that we "know" that universe expansion is accelerating. But it is believed to be pushed away by "dark energy", so accordingly to energy conservation, this strength should decrease with R^3 ... while attracting gravity weakens like 1/R^2 and so should finally win - leading to collapse.

But it seems there is a problem with the second law of thermodynamics here - on one hand entropy is said to be always increasing into the future, on the other Big Bangs should intuitively 'reset the situation' - start new entropy growth from minimum.
I wanted to collect the possible approaches to this problem and discuss them - here is a schematic picture of the basic ones (to be expanded):

The age of thermal death means that there are nearly no changes, because practically everything is in thermodynamical equilibrium, most of stars have extinguished.

1) The second law is sacred - succeeding Big Bangs have larger and larger entropy,
2) It is possible to break 2nd law, but only during the Great Bounce,
3) It is possible to break 2nd law in singularities like black holes - the universe may be already in thermal death, while the entropy slowly "evaporate" with black holes (I think I've heard such concept in Penrose lecture in Cracow),
4) The second law of thermodynamics is not fundamental, but effective one - physics is fundamentally time/CPT symmetric. So Big Bounce is not only single Big Bang, but from time/CPT symmetry perspective, there is also second BB-like beginning of universe reason-result chain in reverse time direction. The opposite evolutions would finally meet in the extremely long central thermal death age, which would probably destroy any low-entropic artifacts.

Personally,
I see 1) as a total nonsense - thermal death is near possible entropy maximum (like lg(N)).
Also 3) doesn't seem reasonable - hypothetical Hawking radiation is kind of thermal radiation - definitely not ordering energy (decreasing entropy), but rather equilibrating degrees of freedom - leading to thermalization of universe.
2) sounds worth considering - physics doesn't like discontinuities, but Big Bounce is kind of special - crushes everything, resetting the system.
And 4) is the most reasonable, but requires accepting that thermodynamical time arrow is not fundamental principle, but statistical effect of e.g. low entropic BB-like situation: where/when everything is localized in small region. To see that 2nd law can work in both time directions, there is nice thought experiment: http://www.sciforums.com/showthread.php?t=111570

Assuming our universe will eventually collapse, which thermodynamical scenario seems most reasonable? Why?
Perhaps above list requires expansion?
Did universe started in a point, or maybe something ends - something begins (like tonight)?

 

[Jarek Duda]

Why moved??
Here are Wikipedia articles:
http://en.wikipedia.org/wiki/Big_Bounce
http://en.wikipedia.org/wiki/Cyclic_model
There are serious people working on it, like Penrose ... but there is some conflict with 2nd law I'm asking about? (stated also in these articles)

You should clearly state scientific correctness for censorship on this forum.

 

[origin]

You should clearly state scientific correctness for censorship on this forum.

This is not censorship, your idea is not in the mainstream so it was moved to the alternative theories. Your idea is an alternative to the the generally accepted theory. Relax.

 

[Jarek Duda]

Not mine, but the cited Cyclic universe Wikipedia article says that it was proposed by Einstein in 1930s
So every post not supporting "the generally accepted" inflation theory, automatically goes to "On the fringe"?

ps. Penrose claims that there is even seen experimental evidence of something before Big Bang: http://physicsworld.com/cws/article/news/44388
This article supports what I have written about what I think I've heard on Penrose lecture - scenario 3) :
"Central to Penrose's theory is the idea that in the very distant future the universe will in one sense become very similar to how it was at the Big Bang. He says that at these points the shape, or geometry, of the universe was and will be very smooth, in contrast to its current very jagged form. This continuity of shape, he maintains, will allow a transition from the end of the current aeon, when the universe will have expanded to become infinitely large, to the start of the next, when it once again becomes infinitesimally small and explodes outwards from the next big bang. Crucially, he says, the entropy at this transition stage will be extremely low, because black holes, which destroy all information that they suck in, evaporate as the universe expands and in so doing remove entropy from the universe. "

 

[AlphaNumeric]

You are wrong about the behaviour of dark energy. It is possible for the rate of expansion to accelerate if particular combinations of the cosmological constant and matter content are positive. I believe it is covered on Wikipedia in the page about the FRW metric. If you think your conclusion is right why do you think cosmologists think the universe has an ever increasing expansion rate? Doesn't it seem odd to you how cosmologists don't reach that very simple conclusion of your? Didn't you look into it?

 

[Jarek Duda]

First of all, it is not my conclusion - in opposite to many physicists, I'm not completely sure this is the only possible option.
I just wanted to discuss if/how eventual future collapse of our universe can be coped with thermodynamics.

Secondly, cosmologists believe that expansion rate is increasing, because basing on observed brightness of distant supernovas as standard candles, they evaluated the distance and it was larger than expected for constant growth (recent Nobel prize).
There are reasons to doubt certainty of this conclusion (like understanding supernovas), but would current accelerated expansion already ensure that this acceleration will last forever? (to call not being completely sure of it a fringe science...)

Dark energy is said to be e.g. 74% of universe mass - dividing it by current universe volume, we get its average mass/energy density, untrue?
So while universe expansion, this density should decrease, untrue?
If dark energy (in opposite to gravity) pushes everything away, shouldn't decrease of its density reduce this effect?

FRW metric assumes constant cosmological constant (density), but its current "generally accepted" interpretation is density of some kind of energy.
In this case we have the most fundamental principle of physics - energy conservation - it conserves not density, but the total amount.
Is there a single reasonable non-historical argument that it is density ('cosmological constant') what should be maintained, not total energy like in standard physics???
Imagine a bent sheet of rubber - is the unbending force constant, or maybe it reduces while it became flat?

 

[AlphaNumeric]

Secondly, cosmologists believe that expansion rate is increasing, because basing on observed brightness of distant supernovas as standard candles, they evaluated the distance and it was larger than expected for constant growth (recent Nobel prize).
There are reasons to doubt certainty of this conclusion (like understanding supernovas), but would current accelerated expansion already ensure that this acceleration will last forever? (to call not being completely sure of it a fringe science...)

The question of whether or not the evidence points to the expansion of the universe and its acceleration is different from your claim the expansion does like 1/r^3 compared to gravity, which is a statement about the theoretical models.

Dark energy is said to be e.g. 74% of universe mass - dividing it by current universe volume, we get its average mass/energy density, untrue?
So while universe expansion, this density should decrease, untrue?
If dark energy (in opposite to gravity) pushes everything away, shouldn't decrease of its density reduce this effect?

FRW metric assumes constant cosmological constant (density), but its current "generally accepted" interpretation is density of some kind of energy.
In this case we have the most fundamental principle of physics - energy conservation - it conserves not density, but the total amount.
Is there a single reasonable non-historical argument that it is density ('cosmological constant') what should be maintained, not total energy like in standard physics???
Imagine a bent sheet of rubber - is the unbending force constant, or maybe it reduces while it became flat?

I would suggest you look up some cosmology, specifically the FRW metric and it's implications. The energy density is per unit volume and if its a constant then it's a constant. This doesn't necessarily violate energy conservation, because the FRW metric has energy conservation.

This can be seen by considering the EFEs, which can include the cosmological constant as the constant of integration when you construct them, ie $$G_{ab} \to G_{ab} + \Lambda g_{ab}$$. The field equations are then constructed from the Einstein-Hilbert action, $$\mathcal{L} = \sqrt{|g|}(R + \Lambda)$$. Local energy conservation is satisfied by any metric satisfying the EFEs because the GR version of the conservation laws are $$G^{ab}_{\qquad;b} = 0 = T^{ab}_{\qquad;b}$$. Since adding $$\Lambda g^{ab}$$ to $$G^{ab}$$ doesn't alter this because by definition of the Levi-Civita connection $$g^{ab}_{\qquad;b}=0$$ and $$\Lambda$$ is constant.

A general rule of thumb when doing anything to do with energy conservation in GR, especially when you're talking about large regions, is not to trust your intuition. Mass and energy in GR are very sticky concepts, trying to use Newtonian or gas like interpretations is asking to go down the wrong path.

 

[Jarek Duda]

Yes, in terms of Einstein's cosmological constant you are indeed right - mathematical term carrying energy and negative pressure can make covariant derivative vanish.
But GRT is macroscopic theory - it should somehow be a result of what is going on in the microscopic physics it averages (like stress-energy tensor)... how to construct cosmological constant from there?
The negative pressure and the fact that increase of volume of "constant energy density" seems to not increase the total energy (as you say), makes this hypothetical term extremely different from what we know in standard physics.

So maybe this simplest allowed additional term is indeed "the biggest Einstein's mistake" and we should search for a better explanation of highly probable current accelerated expansion.
The question is if we can be sure that the real answer also brings force to accelerate expansion to infinity?
Or maybe in some billions/trillions years gravity will finally win?

For example we observe noise of electromagnetic field (CMB) - photons bouncing from everything and so pushing everything away.
This force alone is definitely not sufficient for current acceleration, but there are also fields corresponding to other interactions (weak/strong/gravitational) - thermodynamics suggests that their degrees of freedom should also contain some thermal noise ... energy density which cannot be directly observed ("dark").
Some of these degrees of freedom may require active galaxy region to be filled with energy - this density would grow near galaxies - like what is expected for dark mass.

Why dark energy/mass cannot be just thermal noise of interaction fields?
What do you think dark energy is microscopically?

 

[AlphaNumeric]

But GRT is macroscopic theory - it should somehow be a result of what is going on in the microscopic physics it averages ... how to construct cosmological constant from there?
The negative pressure and the fact that increase of volume of "constant energy density" seems to not increase the total energy (as you say), makes this hypothetical term extremely different from what we know in standard physics.

Hence why it is called 'dark energy', we're still working on it.

So maybe this simplest allowed additional term is indeed "the biggest Einstein's mistake" and we should search for a better explanation of highly probable current accelerated expansion.

And people are and it may well be something else. The point of my correction was that your claim the acceleration term should drop off like 1/r^3 so gravity always wins is not necessarily universally true. I made no statement about whether or not the cosmological constant is really the way to go about thing, I instead highlighted your "I only consider collapse scenarios" was flawed.

The question is if we can be sure that the real answer also brings force to accelerate expansion to infinity?
Or maybe in some billions/trillions years gravity will finally win?

Who knows, that doesn't negate my point, which was the negation of your summary logic gravity always wins.

For example we observe noise of electromagnetic field (CMB) - photons bouncing from everything and so pushing everything away.

Pushing matter about in space-time is quite different from the expansion of space-time. Besides, the light pressure isn't strong enough to do such a thing and in an isotropic and homogeneous universe there's no prefer preferred direction to be pushed.

Some of these degrees of freedom may require active galaxy region to be filled with energy - this density would grow near galaxies - like what is expected for dark mass.

That's a lot of supposition you have there. Got anything other than just pulling lots of maybes out?

Why dark energy/mass cannot be just thermal noise of interaction fields?

Define 'interaction fields' and their thermal noise and then show it's possible for them to explain the observations. Until then why should anyone stop considering other things too?

What do you think dark energy is microscopically?

I don't know.

 

[Jarek Duda]

I don't know why you have interpreted that "I only consider collapse scenarios"?
Personally I just don't think we know the ultimate answer and so we should be open minded - in opposite to the person moving it to "on the fringe" (prometheus? bringing fire ... of censorship), I think both basic scenarios are still "probable"/worth considering.
So for example I think we should understand well thermodynamics of collapse eventuality, especially it alone is extremely interesting and educative problem (started by Einstein in 1930s).

About hypothetical eventuality for dark energy to be not some extremely exotic entity, but just thermal noise ...
The "pushing away of everything" by electromagnetic CMB noise (tiny waves) can be seen as in intuitive picture about universe expansion: universe as balloon with painted stars - its "internal pushing away" corresponds to force giving it tendency to expand.

By "interaction fields" I've meant fields responsible for interactions, like SU(2) for weak interaction, SU(3) for strong interaction (chromodynamics) - like EM field, they have degrees of freedoms (a lot of them), so equipartition property of thermodynamics suggests they should be filled with thermal noise/energy ("tiny waves"), do you disagree?
Ok, the question is if there was a possibility to fill these degrees of freedom - it could be partially done while Big Bang, more recent supernovas, stars (local "dark matter") and there is extremely low but nonzero interaction with e.g. EM field, so through billions of years they could be also partially thermalized with CMB radiation.
I don't know how to estimate this energy in credible way - the density of thermodynamical degrees of freedom of these fields. Generally the number of quantum degrees of freedom grows with the depth of potential well and Higgs potential is relatively deep in comparison to EM.
Maybe you have seen such calculations or know how to make them?

And generally, if you cannot ultimately exclude that universe will finally collapse, we can return to the thread - how do you think such hypothetical collapse would look from thermodynamical perspective?