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

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Discussion Overview

The discussion explores how the laws of thermodynamics might change in a hypothetical contracting universe, contrasting with the current understanding of an expanding universe. Participants consider implications for entropy, temperature, and the nature of thermodynamic laws in such a scenario.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that entropy will still increase or remain the same in a contracting universe, but they note a lack of explanations or mathematical backing for these claims.
  • Others propose that the second law of thermodynamics might be reversed, or that local decreases in entropy could occur.
  • There is a question about whether an increase in temperature of the universe constitutes an increase in entropy, especially as the universe approaches a "big crunch." Some argue that entropy must decrease since it was low at the beginning of the "big bang."
  • One participant mentions that in an idealized closed Friedmann-Robertson-Walker (FRW) model, the concept of entropy is meaningless due to the deterministic nature of the model, which maintains zero entropy at all times.
  • Another participant emphasizes the need for specific references to mainstream models to ground the discussion, questioning the validity of speculative claims without such references.
  • Some participants express skepticism about the feasibility of experimental verification of these hypotheses, while others argue that non-realistic models can still aid in building intuition.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how the laws of thermodynamics would change in a contracting universe. Multiple competing views are presented, and the discussion remains unresolved.

Contextual Notes

Participants note limitations in the discussion, including the lack of specific models or references to support claims, which may hinder the ability to answer questions about the thermodynamic implications of a contracting universe.

KurtLudwig
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TL;DR
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?
 
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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.
 
KurtLudwig said:
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.
 
KurtLudwig said:
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.
 
Thank you for your answers.
 
anuttarasammyak said:
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.
 

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