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So, I am reading Introduction to Modern Cosmology by Andrew Liddle and have just learned that the universe is cooling as it is expanding. Now, I am mathematician knowing very little about physics, but there is a problem in my mind:

There is absolute zero. If the universe is cooling, somewhere in the distant future it will achieve absolute zero. Now, no thermodynamic processes are possible at absolute zero, meaning that literally everything will freeze (heat from the stars will be not delivered anywhere and they would cool out and freeze unable to heat anything). Nothing would move, as there is no work possible at absolute zero. So, everything will collapse; in other words, there will be an inevitable "Big Crunch". I know that this is an unlikely possibility, but do not know where I am mistaken.

Thanks for any help.

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    $\begingroup$ I fail to see the logical connection between a freezing universe and a spacetime big crunch $\endgroup$
    – user65081
    Jun 25, 2016 at 15:41
  • $\begingroup$ There is actually a theory called the Big Crunch....sort of the reverse of the Big Bang $\endgroup$
    – auden
    Jun 25, 2016 at 16:39
  • $\begingroup$ I'm sorry, the Big Crunch isn't what you were talking about...ignore the above comment. $\endgroup$
    – auden
    Jun 25, 2016 at 16:56
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    $\begingroup$ "If the universe is cooling, somewhere in the distant future it will achieve absolute zero". If the coffee in my cup is cooling, must it eventually achieve (or even come close to) absolute zero? $\endgroup$
    – WillO
    Jun 25, 2016 at 17:02
  • $\begingroup$ -1 for "there is absolute zero". No, there isn't. We even have a law against that: the third law of thermodynamics explicitly rules it out. More importantly, though, we can't know what happens at ultra-low temperatures until we have measured it. Nobody has measured it and none of us ever will, so the deep future of a heat death universe is, principally, unknown. If the book doesn't say that, it's time to use it as a door stop. $\endgroup$
    – CuriousOne
    Jun 25, 2016 at 18:04

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The heat death of the universe is the idea you are describing (this idea is also known as the Big Freeze). The problem with this idea is for it to work the cosmological constant has to be zero...and it isn't zero. It's very tiny, but it isn't zero. The other problem with your idea is the belief that because it all "freezes", so to speak, it'll all collapse in on itself. This is not necessarily the case. If you read about it at this website and look at the "Controversies" and "Current Status" sections, you'll be able to read about the problems with your theory. You can also read more about the cosmological constant here (especially relevant to this is the section entitled "Positive Value". One other website that describes the heat death of the universe is this one (the first couple of paragraphs describe what it is).

Hope this helps!

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  • $\begingroup$ So, any universe (in general relativity, with or without the cosmological constant) that is isotropic and homogeneous like ours, if the space is flat will a expand to infinity. Yes, a big freeze, but no collapse. The Big Crunch only happens if it a recollaspes, which means it is a positive curvature space. We know we are very close to flat, but with enough uncertainty it could still be open or closed. If closed, then the Big Crunch $\endgroup$
    – Bob Bee
    Jun 25, 2016 at 21:01
  • $\begingroup$ The cosmological constant is what controls whether or not we'll end up in either the big crunch or the big freeze. Otherwise, yeah. $\endgroup$
    – auden
    Jun 25, 2016 at 22:27
  • $\begingroup$ No. It's all the components of the energy density. The cosmological constant is 68%. The three, that, matter and radiation are within 2% of it being flat. The best measurements are that the cosmological constant has not changed. It is the three added up that count. Since now it seems flat, and thus a heat death, but the 2% the other way could make it a Big Crunch. It could be from matter, radiation or the cosmological constant. Matter uncertainty is bigger than cosmological constant unc. $\endgroup$
    – Bob Bee
    Jun 26, 2016 at 3:51

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