I How would the laws of thermodynamics change in a contracting universe?

[KurtLudwig]

TL;DR Summary

How would the laws of thermodynamics change in a contracting universe?On the internet, I get different answers: Most say that entropy will still increase or stay the same. Some say the second law will be reversed. Some say that there will be a local decrease in entropy.
The universe as a whole will increase in temperature. Is that considered an increase in entropy?

How would the laws of thermodynamics change in a contracting universe?
(Of course we live in an accelerating and expanding universe. So the questions are hypothetical.)
On the internet, I get different answers: Most say that entropy will still increase or stay the same, but no explanations or mathematics were given. Some say the second law will be reversed. Some say that there will be local decreases in entropy.
The universe as a whole will increase in temperature. Is that considered an increase in entropy?
But as the universe continues toward the "big crunch", entropy need to decease, since at the beginning of the "big bang" entropy was low?
I believe that since the universe is getting smaller, at least the definition of disorder will need to be modified.
Is it all semantics? I wonder what the members of physics forums think?

 

[anuttarasammyak]

In physics hypothethis should be proved/denied by observation or experiment. I don't think we have a chance of experiment on this issue though I would like to.

 

[PeterDonis]

On the internet, I get different answers

You need to give specific references. Particularly for a topic like this, we need to have some kind of mainstream reference as a basis for discussion. If there are no such references, that means the topic is too speculative to be usefully discussed here.

 

[PeterDonis]

The universe as a whole will increase in temperature. Is that considered an increase in entropy?
But as the universe continues toward the "big crunch", entropy need to decease, since at the beginning of the "big bang" entropy was low?

As far as an idealized closed FRW model that expands and then contracts is concerned, the question of "entropy" is meaningless, because the model is an exact deterministic solution, whose "entropy" is zero at all times since there is no uncertainty about its state.

Of course what you are trying to envision is a more realistic model in which the FRW solution is just an approximation. But that is where we need actual references as a basis for discussion: what kind of approximation is being used? Does it allow for the "Big Bang" and "Big Crunch" ends of the universe to have different properties? (Note that the idealized FRW model does not; they have to be the same.) Without an actual reference as a basis for discussion, questions like there are not even answerable.

 

[KurtLudwig]

Thank you for your answers.

 

[pines-demon]

In physics hypothethis should be proved/denied by observation or experiment. I don't think we have a chance of experiment on this issue though I would like to.

I think that this is too reductive. Sure it is good to attach hypothesis to observation but sometimes not-so-real models can help build an intuition. What I think this question is missing is a proper framework. Using model "insert specific model name here" what happens with "insert variable here" when "variable here" decreases.