Has an infinitely bouncing universe really been discredited?

[MrQG]

I've read some arguments, which claim that an infinitely bouncing universe is not possible. For the sake of argument, let's ignore the fact that our universe is expanding at an ever increasing rate. Instead, I want to focus on whether or not an infinite bouncing universe would violate the Second Law of Thermodynamics. Essentially, the argument boils down to each cycle having an increase in entropy or cosmic radiation determined by previous cycles. However, if each cycle ends in a "big crunch" thereby collapsing back into a singularity, then would not all physical laws break down including the Second Law? Wouldn't this effectively "erase" all information from past cycles?

 

[marcus]

There is a recognized expert in general relativity and cosmology who might give some support. His name is Thanu Padmanabhan. He has published a number of papers which look at gravity and cosmology from a thermodynamic point of view (including a few for nonspecialist audience).

He claims that the entropy is observer dependent. It takes an observer to define what is macrostate and what is microstate. Suppose that there is no one who is watching from outside the universe and also no observer from before the bounce who survives to watch after the bounce. Who would be able to see a violation? Who would be able to build a perpetual motion machine and defy the Law? It becomes operationally difficult to say what things mean.

I can't answer your question. I just have the feeling that it is not a simple straightforward application of the second law. Maybe someone else here can answer more confidently.

Roger Penrose might say it was an open-and-shut case, but not everyone would agree.

I don't recall which paper by Padmanabhan I saw that about observer-dependent. Here are a couple you might glance at:
http://arxiv.org/abs/0911.5004
http://arxiv.org/abs/0910.0839

 

[agnifire]

I've read some arguments, which claim that an infinitely bouncing universe is not possible. For the sake of argument, let's ignore the fact that our universe is expanding at an ever increasing rate. Instead, I want to focus on whether or not an infinite bouncing universe would violate the Second Law of Thermodynamics. Essentially, the argument boils down to each cycle having an increase in entropy or cosmic radiation determined by previous cycles. However, if each cycle ends in a "big crunch" thereby collapsing back into a singularity, then would not all physical laws break down including the Second Law? Wouldn't this effectively "erase" all information from past cycles?

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You can have an infinitely bouncing universe but not an infinitely repeating one. In order for it to repeat information from the previous universe must be available. As information is entropy, entropy must increase. If there is no build up of entropy each bounce is unique and is the first bounce.

Ask yourself, if you had an infinitely repeating universe would you know how many times it repeated?

 

[MrQG]

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You can have an infinitely bouncing universe but not an infinitely repeating one. In order for it to repeat information from the previous universe must be available. As information is entropy, entropy must increase. If there is no build up of entropy each bounce is unique and is the first bounce.

Ask yourself, if you had an infinitely repeating universe would you know how many times it repeated?

I'm not necessarily worried about "repeating", but Robert J. Spitzer argues that we can get an idea of how many past cycles there were (assuming there were any) by noting the ratio between the amount of cosmic background radiation versus starlight.

 

[agnifire]

A finite number of cycles may be possible but if we can know the number of cycles that the universe has had then each cycle has added some information. If information is entropy and there were, an infinite number of cycles, would that mean an infinite amount of entropy has been added?

If each cycle is identical to the first one then each cycle is the first one.

 

[MrQG]

A finite number of cycles may be possible but if we can know the number of cycles that the universe has had then each cycle has added some information. If information is entropy and there were, an infinite number of cycles, would that mean an infinite amount of entropy has been added?

If each cycle is identical to the first one then each cycle is the first one.

Well that is my question... would "information" be preserved from cycle to cycle? If each cycle is separated by a singularity, then it seems that all information would be erased. I don't think this entails each cycle being identical, since a singularity is a point of lawlessness and literally anything could arise from it.

 

[MrQG]

By the way... Thank you Marcus for the links!

 

[agnifire]

Well that is my question... would "information" be preserved from cycle to cycle? If each cycle is separated by a singularity, then it seems that all information would be erased. I don't think this entails each cycle being identical, since a singularity is a point of lawlessness and literally anything could arise from it.

Which would mean that we cannot know the number of cycles. The number of cycles is information ergo it can not come from lawlessness.

 

[agnifire]

If no information can go from one cycle to the next then there can be no evidence from a previous cycle that there has even been one repetition. If that is the case then while the proposition may be true it is meaningless.

 

[MrQG]

If no information can go from one cycle to the next then there can be no evidence from a previous cycle that there has even been one repetition. If that is the case then while the proposition may be true it is meaningless.

Right. So technically, there could have been an infinitely bouncing universe. It cannot be disproved by using the Second Law... right?

 

[agnifire]

Right. I agree there could be an infinitely bouncing universe and it cannot be disproved by using the Second Law.

Honestly, I only object to the idea of a bouncing universe because of the possibility that there may be a repeating one. I have had a great life but I am such a complainer that I hope to never live this life again.

 

[MrQG]

Thanks for your responses agnifire. Do you suppose that there is anything that would prevent the universe from collapsing completely into a singularity, like a build up of pressure due to the build up of energy released from starlight or something else? If this is the case, then certainly it seems there couldn't be an infinitely bouncing universe.

 

[cesiumfrog]

There is a recognized expert in general relativity and cosmology who might give some support. His name is Thanu Padmanabhan. He has published a number of papers which look at gravity and cosmology from a thermodynamic point of view (including a few for nonspecialist audience).

He claims that the entropy is observer dependent. It takes an observer to define what is macrostate and what is microstate. Suppose that there is no one who is watching from outside the universe and also no observer from before the bounce who survives to watch after the bounce. Who would be able to see a violation? Who would be able to build a perpetual motion machine and defy the Law? It becomes operationally difficult to say what things mean.

That's very interesting. There's obviously some truth to entropy being observer dependent, but to say it accounts for the problem of improbable state at the (worse: each) start of the universe is surprising.

 

[marcus]

That's very interesting. There's obviously some truth to entropy being observer dependent, but to say it accounts for the problem of improbable state at the (worse: each) start of the universe is surprising.

I should have put a paragraph break in there. HE says entropy is observer dependent. But he does not apply that idea to bounce cosmology.

What follows is my continuation. It was MY suggestion that the second law is not being broken if no one can observe a violation.

BTW I wouldn't call the start of expansion an "improbable" state. It happens naturally in LQC models, both the analytical versions and the numerical simulations. Intuitively all structure melts---geometry and matter become indistinguishable and uniform.
The LQC also initiates inflation naturally.

What you call "improbable" turns out to be inevitable---using this QG model. All I am pointing out is that it is not against the law.

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As I recall this is your field of research (or one of them.) Didn't you say you took part in the Sydney GR conference in 2007? If so (I could be mis-remembering) then you know the interesting and unintuitive fact that the uniform gravitational field is the lowest entropy state.
It is just the opposite from, say, a gas. When the gas is all spread out, that's high entropy. It naturally wants to spread out. But when the gravitational field is uniform and even it means things haven't clumped yet, and they want to clump---so even and level is low entropy.

Intuitively, in a bounce, the crunch destroys clumps---so you start with a clean slate.
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Ashtekar (a familiar name ) has a paper about inflation in LQG. It might interest you. I'll get the link.
http://arxiv.org/abs/0912.4093
Loop quantum cosmology and slow roll inflation
Abhay Ashtekar, David Sloan
"In loop quantum cosmology the big bang is replaced by a quantum bounce which is followed by a robust phase of super-inflation. We show that this phase has an unforeseen implication: in presence of suitable inflationary potentials it funnels all dynamical trajectories to conditions which virtually guarantee a slow roll inflation with more than 68 e-foldings, without any input from the pre-big bang regime. This is in striking contrast to the situation in general relativity where it has been argued that the a priori probability of obtaining a slow roll inflation with N e-foldings is suppressed by a factor Exp(-3N)."

 

[MrQG]

According to Robert J. Spitzer there are three reasons why an infinitely bouncing universe is not plausible. The first reason he calls "the radiation paradox". This involves the star light versus cosmic radiation ratio. He calls the second the "entropy paradox". Here he mentions that entropy builds up from each cycle and so if there were infinitely many bounces "before" our universe there should be an infinite (or at least maximal) amount of entropy. Yet, our universe began with a very tiny amount of entropy.

Finally, he makes mention of the "increase in cyclic expansion". He argues that an increase in radiation in each cycle will result in an increase in pressure thereby prolonging the "duration" of each cycle. This, he says, prevents an infinite regress. What do you all think of these arguments?

It seems to me that these arguments could only hold if the universe never actually ended and/or began in a singularity, since a singularity would render the Second Law inoperable. There would have to be some sort "pressure" which always overcame the gravitational force before it reached sub Planck scales.

 

[marcus]

You might want to give the source, or some background Robert J. Spitzer S.J. is author of New Proofs for the Existence of God
and also Five Pillars of the Spiritual Life: A Practical Guide to Prayer for Active People

I follow what you say about the entropy argument and the cosmic singularity.

Spitzer's radiation arguments do not appear to me to make sense. Starlight or any other kind of light cannot accumulate in the way he suggests. At least in the bounce cosmoloogy models that I see being researched today.
Above a certain temperature light begins to convert to matter and antimatter. Any light from a previous universe has already disappeared in the crunch well before re-expansion begins.

There are various bounce models. Maybe he is thinking of some kind that does not involve a "big crunch". One could call it a "restrained" bounce---more of a dip or curtsey than a collapse. I haven't seen much about that kind. It is not studied in the Quantum Cosmology contexts that I'm familiar with. May have been more on researchers' minds 20 years ago.

My wife is calling, have to go. Maybe someone else can give a more thorough answer.

 

[agnifire]

That's very interesting. There's obviously some truth to entropy being observer dependent, but to say it accounts for the problem of improbable state at the (worse: each) start of the universe is surprising.

Entropy is only a problem if you are talking about this universe. A bouncing universe can pass along information as long as you don't mind the universe having a uniform tempture.