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Back in high school, I asked my teacher gave us a quick explanation of relativity. Specifically, he told us what $E=mc^2$ meant. He explained that, at least as far as we needed to be concerned, matter is simply condensed energy. This, to me, was amazing.

Obviously, it's not exactly that simple. Condensation isn't something that just happens with energy. However, it does mean that the forces which act on matter also act on energy.

Now, I am aware that gravity is a relatively weak force. Years after taking physics in both high school and college (though never at any real "scholarly level", I'm afraid), I remember asking my teacher something that made him smile and admit "I cannot answer that, and that is why I am a high school physics teacher and not an applied physicist. Questions like that are going to get you very far in this field."

That question was this: If gravity acts on all matter and all energy in the universe, wouldn't the universe eventually condense into pockets of energy and matter, after entropy causes the "heat death" of the universe?

Edit: better wording

Why wouldn't the same coalesce nice of matter and energy happen after the heat death of the universe?

If not, why?

Please note that I don't necessarily believe that the high-entropic state of the universe in billions and billions of years will coalesce into matter. I just want to know why entropy would continue.

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    $\begingroup$ matter is simply condensed energy this is a very lousy explanation. $\endgroup$ – jinawee Feb 2 '14 at 15:44
  • $\begingroup$ @jinawee, care to offer a less lousy explanation? $\endgroup$ – Pranav Hosangadi Feb 2 '14 at 15:50
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    $\begingroup$ It is a simplified explanation that works to explain the concept of relativity to the general population of students. For the purposes of most non-physics students' academics, it is more than adequate. $\endgroup$ – b4ux1t3 Feb 2 '14 at 15:53
  • $\begingroup$ @b4ux1t3: The universe has condensed in pockets of matter: that's what stars are. You ask after entropy slowly but inevitably worked its way through all matter but I don't know what you mean by this. Can you make clearer what this means. $\endgroup$ – John Rennie Feb 2 '14 at 15:54
  • $\begingroup$ Ah, yes, sorry. I typed that sentence in a hurry. Give me a moment, I'll fix it. $\endgroup$ – b4ux1t3 Feb 2 '14 at 15:55
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Three eventual cases, depending on how much stuff (of all kinds) vs. inial expansion fills the universe.
1) Too much stuff. Big Bang expansion slows, stops, reverses into the Big Crunch.
2) Just the right amount of stuff. Universal expansion asymptotically slows to a halt. A steady state universe is unstable to any perturbation, like a pencil standing on its sharpened point.
3) Not enough stuff vs. total expansion. The universe expands forever into heat death.

http://en.wikipedia.org/wiki/Accelerating_universe
http://www.preposterousuniverse.com/blog/2013/11/16/why-does-dark-energy-make-the-universe-accelerate/
http://www.eso.org/~bleibund/papers/EPN/epn.html

Observation so far votes for (3).

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  • $\begingroup$ That eso paper was illuminating. From what I'm piecing together from these, basically the distances between matter and energy would be so vast that gravity would be negligible at best? Is that a good approximation of understanding? $\endgroup$ – b4ux1t3 Feb 2 '14 at 16:09

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