Cosmological Models Predicting Heat Death of the Universe

[Cosmo Novice]

I have read that some cosmological models predict a heat death of the universe. Eventually all matter (beginning with superclusters, clusters, then galaxies, then stars) will lose all energy and separate (due to expansion which will eventually occur even on a galactically local level) so that all energy in U becomes diffuse.

At this point there are no more matter/energy interactions as everything (I think I read something about point particles) is so spread out it is in its own OU.

Can someone more knowledgeable please expand on my statements above?

Also at this point would it be reasonable to say that t=0 again as effectively the second law of thermodynamics becomes irrelevant when change is impossible as all particles exist entirely in their own OU?

 

[Matterwave]

The heat death model is, as far as I know, sort of just the extreme extension of the 2nd law of thermodynamics. Entropy always increases, and so, eventually you will reach a point where no order can be found. That's about all I know about this...

 

[Chalnoth]

I have read that some cosmological models predict a heat death of the universe. Eventually all matter (beginning with superclusters, clusters, then galaxies, then stars) will lose all energy and separate (due to expansion which will eventually occur even on a galactically local level) so that all energy in U becomes diffuse.

At this point there are no more matter/energy interactions as everything (I think I read something about point particles) is so spread out it is in its own OU.

Can someone more knowledgeable please expand on my statements above?

As long as our universe continues to expand, yes, this is the case. You can read a bit more on it here:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe

Also at this point would it be reasonable to say that t=0 again as effectively the second law of thermodynamics becomes irrelevant when change is impossible as all particles exist entirely in their own OU?

No. T=0 is just an arbitrary point in time, and we expect the laws of physics were working just fine then. There was stuff going on both before and after. Our main problem is that we aren't yet sure as to what the effective laws of physics were at that time.

 

[Cosmo Novice]

As long as our universe continues to expand, yes, this is the case. You can read a bit more on it here:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe


No. T=0 is just an arbitrary point in time, and we expect the laws of physics were working just fine then. There was stuff going on both before and after. Our main problem is that we aren't yet sure as to what the effective laws of physics were at that time.

Ok so t=0, I understand this is arbitrary but would it be arbitrary in time or space/time?

So if t=0 is effectively the absolute beginning of U in standard cosmo models, and disregarding the lack of theory pre-plancke time then is the environment at t=0 the same as t=heat death?

It is hard for me to try to verbalise the point I am making but I think you get it.

 

[Chalnoth]

So if t=0 is effectively the absolute beginning of U in standard cosmo models,

The thing to bear in mind here is that for the cosmological models that have a beginning, those models become nonsensical before you go that far back, indicating that new physics is required to describe things before you even get to that point.

 

[Cosmo Novice]

The thing to bear in mind here is that for the cosmological models that have a beginning, those models become nonsensical before you go that far back, indicating that new physics is required to describe things before you even get to that point.

Ok so at a certain point, before we reach t=0 then GR breaks down. I think I read this required a unification of QM and GR theory?

If known science breaks down and becomes nonsenical then how is it proven/theorised that t=0 actually existed? Is it possible that t=one Plancke unit was actually the beginning, but then would that not actually be t=0?

I guess what I am asking now is do cosmological models require t=0 or is this just a methodology applied by physicists to apply a beginning to time which in truth may or may not have happened (I am in no way questioning current scientific thinking just looking to expand my meagre knowledge.)

I guess another way to ask the question of heat death is does t=finite?

Thanks for the previous posts

 

[Chalnoth]

Ok so at a certain point, before we reach t=0 then GR breaks down. I think I read this required a unification of QM and GR theory?

That or some matter/energy component of the universe that existed at that time but is no longer significant.

If known science breaks down and becomes nonsenical then how is it proven/theorised that t=0 actually existed? Is it possible that t=one Plancke unit was actually the beginning, but then would that not actually be t=0?

Generally, we don't. In essence, there are some known limitations to how much we know about the early universe. The "t=0" time in our cosmological models is known to be artificial. When you see people talking about such things in popular talks/books/whatever, bear in mind that often times language gets a little bit sloppy when trying to explain things to a person that hasn't studied the subject in detail for many years.

So when you hear a cosmologist talk about "the big bang" or "t=0", don't think of an actual beginning, but instead, "some arbitrary time when our observable universe was very, very small". We do have some very limited information about what may or may not have happened before that time, but so far much of it is just speculation.

I guess what I am asking now is do cosmological models require t=0 or is this just a methodology applied by physicists to apply a beginning to time which in truth may or may not have happened (I am in no way questioning current scientific thinking just looking to expand my meagre knowledge.)

In general, there are very good arguments that according to our cosmological models, there had to be a beginning to our region of space-time. Here is a rough sketch of two of them:

1. Entropy considerations. A universe expanding into the past would require a continuous decrease in entropy into the past. This would mean that the entropy in the distant past was infinitely smaller than the entropy today, which means infinite fine tuning: such a universe is strongly, strongly disfavored by simple probability estimates.
2. General Relativity very generically predicts singularities. Basically, if you have an expanding universe with any matter or radiation in it at all, there is necessarily a singularity in the finite past.

People generally try to solve this issue by using one of two approaches. One approach is to examine quantum gravity. We know that GR can't be entirely correct, after all, so maybe when a universe gets really dense, quantum gravity behaves sufficiently differently that it resolves the singularity problem and gives a nice, continuous universe. The main research in this area surrounds Loop Quantum Cosmology, where the picture of the universe is that the current expansion phase resulted from a "bounce" of a previous contraction. Marcus here on these forums is a big fan of this view. I am extremely skeptical.

The second approach is to imagine that GR is mostly correct in the early universe, but we can't just extrapolate back that far because there was some event in the finite past that started it all off, such as a quantum vacuum fluctuation. The picture here is of each region of the universe being birthed from a rare random event in a previous universe, random events that are rare but common enough if there is a small but positive cosmological constant.

One thing I'd like to point out, however, is that precisely when this event occurred, whether a bounce or a quantum vacuum fluctuation or something else, is as yet unknown.

 

[Cosmo Novice]

That or some matter/energy component of the universe that existed at that time but is no longer significant.


Generally, we don't. In essence, there are some known limitations to how much we know about the early universe. The "t=0" time in our cosmological models is known to be artificial. When you see people talking about such things in popular talks/books/whatever, bear in mind that often times language gets a little bit sloppy when trying to explain things to a person that hasn't studied the subject in detail for many years.

So when you hear a cosmologist talk about "the big bang" or "t=0", don't think of an actual beginning, but instead, "some arbitrary time when our observable universe was very, very small". We do have some very limited information about what may or may not have happened before that time, but so far much of it is just speculation.


In general, there are very good arguments that according to our cosmological models, there had to be a beginning to our region of space-time. Here is a rough sketch of two of them:

1. Entropy considerations. A universe expanding into the past would require a continuous decrease in entropy into the past. This would mean that the entropy in the distant past was infinitely smaller than the entropy today, which means infinite fine tuning: such a universe is strongly, strongly disfavored by simple probability estimates.
2. General Relativity very generically predicts singularities. Basically, if you have an expanding universe with any matter or radiation in it at all, there is necessarily a singularity in the finite past.

People generally try to solve this issue by using one of two approaches. One approach is to examine quantum gravity. We know that GR can't be entirely correct, after all, so maybe when a universe gets really dense, quantum gravity behaves sufficiently differently that it resolves the singularity problem and gives a nice, continuous universe. The main research in this area surrounds Loop Quantum Cosmology, where the picture of the universe is that the current expansion phase resulted from a "bounce" of a previous contraction. Marcus here on these forums is a big fan of this view. I am extremely skeptical.

The second approach is to imagine that GR is mostly correct in the early universe, but we can't just extrapolate back that far because there was some event in the finite past that started it all off, such as a quantum vacuum fluctuation. The picture here is of each region of the universe being birthed from a rare random event in a previous universe, random events that are rare but common enough if there is a small but positive cosmological constant.

One thing I'd like to point out, however, is that precisely when this event occurred, whether a bounce or a quantum vacuum fluctuation or something else, is as yet unknown.

Thanks for clearing this up. So essentially when physicists talk about t=0 they are talking about an arbitrary time where GR becomes the standard - and any time prior to Plancke time is essentially a mystery in that there are no current physical laws which can be applied.

I have heard reference to a "bounce" before but think this was in reference to Cyclic Universe theories (which I believe current accelerating expansion models refute) and so I think this is different to Loop Quantum Cosmology.

I am going to do a little more research into the above - thanks very much for the post.

 

[Chalnoth]

Thanks for clearing this up. So essentially when physicists talk about t=0 they are talking about an arbitrary time where GR becomes the standard - and any time prior to Plancke time is essentially a mystery in that there are no current physical laws which can be applied.

Well, it's not a complete mystery. But it can get horridly complicated trying to explain what we do and do not know, and in any case there is a lot that we don't know. It is very true that we can't tell the story of what happened around that time in full. But it is also true that we do have a good amount of evidence surrounding what was going on at that time. The evidence just isn't good enough to tell the full story, at least not yet.

I have heard reference to a "bounce" before but think this was in reference to Cyclic Universe theories (which I believe current accelerating expansion models refute) and so I think this is different to Loop Quantum Cosmology.

Yes, there have been multiple attempts to talk about a bouncing universe. Loop Quantum Cosmology is the most serious of these.