Why do so many people claim cyclic models brake down due to entropy?

  • Thread starter lukesfn
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To my intuition, I find more easy to believe that the universe is infinitely old and the big bag was just one of an infinite number of similar events in a larger universe, rather then the universe had a beginning.

However, after watching many documentaries, and reading a lot of popular science, and doing a little more research into the basic principles, I find again and again, people talking about how the cyclic models always break down because entropy can only increase, and that the 2nd law of thermodynamics makes a heat death inevitable.

This was totally against my intuition, so I had to better my understanding of entropy. After understanding entropy much better and the second law, I could not find any reason why breaks the infinite cyclic universe at all, it just prevents anybody from observing the infinite cycle.

But why do so many physicists and commentators talk about cyclic models being incomparable with entropy then? Am I missing something or do many physicists not understand entropy properly. It seems physicality even try to come up with new cyclic models that destroy the observer, or that push entropy out beyond a horozion, but I can't see why this is necessary or why entropy applies at all.

Why don't all these people look at the Poincaré recurrence theorem which could be interpreted as suggesting the universe actually has to be cyclic, but instead quote the second law of thermodynamics, which actually doesn't apply in this case.

This is driving me nuts.

Here are some reasons why entropy shouldn't really be applied so strongly to the evolution of the universe.
1) Entropy requires and observer who knows something about the state of the closed system. Obviously this is not possible for the universe because there is horizon that limits knowledge of the entire system. Also, wouldn't the uncertainty principal prevent entropy from being known?
2) Entropy was created to explain how much energy is available for an observer to do useful work with. It isn't so useful for describing the evolution of a system with no observer.
3) Lets say, hypothetically, that we have an observer who has enough knowledge of the universe to calculate its entropy, then sure for that observer, entropy can only increase, but that entropy is only relative to there point of view, so quite meaningless for the over all evolution of the universe. Entropy in this case relates to how much the observer can be aware of the state of the universe. As the universe evolves, the observer will lose access to more and more information and can't get it back due to the uncertain principal. This observer would see a new big bang as an increase in entropy.
4) If there is an observer who knows everything and can is always aware of the full state of the universe, then it is that observer who is braking the 2nd law, not the universe its self. Such an observer could alternative calculate entropy as being constant. As I see it, constant entropy is the only meaningful value of entropy for the universe.

Basically, it seems to me that many people make the mistake and say that 2nd law prevents an infinity old universe, but that is a massive misunderstanding, it only prevents an infinitely old universe from being observed through out its life time by a single entity.

There is not even any reason to require an observer to be destroyed in a new big bang or crunch. Maybe an observer can go into hibernation and conserve its energy through a heat death, and wait for a new big bang, peak its head out, and notice some more usable energy, however, the every time it peaks is head out to take a look, it bleeds energy. Eventually, it will find its self waiting for new big bang that takes too long to arrive, and it will not survive.

To the answer to the question of "why did the universe start in a low entropy state?" should surly be "Wow, that's amazing, do you have evidence that the universe started in a low entropy state? No? You don't? Then why are you asking why something is a certain way, when there is no evidence that it actually is that way?"

If there is a flaw in my reasoning, or something I don't know, please somebody let me know.

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I liked your post. You have identified several reasons that entropy might not be welldefined or 2nd law might not be applicable at some point in certain types of universe histories.

It's good to be skeptical, when you've thought about it and have reasons to be.

But I don't get the impression that nowadays so many people reject cyclic models, or at least reject such models on 2nd Law grounds. That kind of objection may have begun to dwindle and go out of fashion.

If you do a search of the Quantum Cosmology literature, and limit it to recent papers, you will find the most highly cited ones are typically using bounce models. That kind is increasingly accepted, a growing number of people are doing research in it. Observational tests are being devised. The researchers know all about the 2nd Law, of course, but it doesn't prevent a growing interest in that type of model. I'll get a links to searches ranked by number of citations. The top 20 will be mostly Loop cosmology, a type that predicts a bounce (replacing both the crunch and the big bang singularity).

This may be slow:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2008&FORMAT=WWW&SEQUENCE=citecount%28d%29 [Broken]

If it is slow, try this:

The idea of listing like this is not to read the articles but to notice how much of the recent work is bounce.
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  • #3
I liked your post.

You have identified several reasons that entropy might not be welldefined or 2nd law might not be applicable at some point in certain types of universe histories.
I think I want to be much more general then that.

I'm really trying to make the point that applying entropy to the evolution of universe not applicable in most cases and that framing arguments about the evolution of the universe around entropy is mostly unhelpful.

The concept of entropy and the 2nd law appear to have been created to help work out how much energy can be obtained for a closed system to do useful work, and that is the area in which the 2nd law is applicable. I don't think that the universe however is trying to do any useful work, so does it make any sense at all the 2nd law? Its not applicable.

Very often when I hear or read something about the beginning/evolution/fate of the universe, I hear somebody talking as if entropy is the overriding dominating factor in the evolution of the universe.

But I don't get the impression that so many people reject cyclic models. Or that they reject such models on 2nd Law grounds. That kind of objection may have begun to go out of fashion.
Perhaps it is because I have been following popular sources and they are all many years behind the latest research, or quoting research long dis-proven, miss-quoting entirely, or just dumping things down to the point where they are meaningless

I'd love to be pointed towards some resent articles on the topic, however, I'm not sure I have the patients for anything that quickly falls into too much mathematical jargon .

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