# ? about oscillating models of the Universe and Entropy

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## Main Question or Discussion Point

I know that early oscillating models of the universe fail due to the second law of thermodynamics. One thing that I am unclear about is since as far as i know the laws of physics break down in a singularity can the second law of thermodynamics break down also?
When I see comments to the effect of a singularity is something we cant understand,describe,comprehend , the laws of physics but by the way the second law of thermodynamics still applies I am a little unsure why that law still applies.

I know I am missing something here and can someone explain this in layman terms

## Answers and Replies

PeterDonis
Mentor
2019 Award
I know that early oscillating models of the universe fail due to the second law of thermodynamics.
Can you give a reference for where you found this statement?

kimbyd
Science Advisor
Gold Member
I know that early oscillating models of the universe fail due to the second law of thermodynamics. One thing that I am unclear about is since as far as i know the laws of physics break down in a singularity can the second law of thermodynamics break down also?
When I see comments to the effect of a singularity is something we cant understand,describe,comprehend , the laws of physics but by the way the second law of thermodynamics still applies I am a little unsure why that law still applies.

I know I am missing something here and can someone explain this in layman terms
In the modern bounce models, there is no singularity and the laws of physics don't break down.

I know that early oscillating models of the universe fail due to the second law of thermodynamics. One thing that I am unclear about is since as far as i know the laws of physics break down in a singularity can the second law of thermodynamics break down also?
When I see comments to the effect of a singularity is something we cant understand,describe,comprehend , the laws of physics but by the way the second law of thermodynamics still applies I am a little unsure why that law still applies.

I know I am missing something here and can someone explain this in layman terms
From the Arxiv paper referenced in your first link:

One of the oldest questions in theoretical cosmology is whether an infinitely oscillatory universe which avoids an initial singularity can be consistently constructed. As realized by Friedmann and especially by Tolman one principal obstacle is the second law of thermodynamics which dictates that the entropy increases from cycle to cycle. If the cycles thereby become longer, extrapolation into the past will lead back to an initial singularity again, thus removing the motivation to consider an oscillatory universe in the first place. This led to the abandonment of the oscillatory universe by the majority of workers.​

Chronos
Science Advisor
Gold Member
Given we do not know [or even how to know] the initial conditions necessary to induce a 'Big Bang', assumptions regarding the initial 'vacuum state' / entropy/ whatever of the embryonic universe are unfounded. The expected result of applying unfounded assumptions to model building is nonsense in, nonsense out.

Not all bounce models are oscillating models. Some consider a one time bounce so the history of the universe looks like an hourglass. There are a number of possible solution I have heard to the entropy question:
1 the entropy gets reset at the bounce
2 the maximum entropy can be infinite and so never goes to maximum
3 the entropy has a a low point at the bounce, growing in both directions on either side, this is known as a Janus universe
4 entropy is observer dependant and as no observer can go through the bounce you basically get the same as in option 1

But of course no one knows, so statements about what is ruled out or not seems speculative to me. Nothing wrong with speculation , without we wouldn't get anywhere . but important not to treat it as fact.