2003年10月21日
Here's an interesting question ----
What would happen if there were enough matter in the universe that the expansion of the universe reversed (and resulted in a "big crunch")? Would entropy still increase?
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This is a great question. It actually leads to some really deep, cutting edge issues, but there are a few things to say at the elementary level.
First--- yes, entropy always increases, but no, in an expanding universe, the system does not get closer to equilbrium all the time.
This seems odd.
The reason it does not get to equilibrium is that it is thinning out all the time. The particle collisions become infrequent. At the same time, expansion draws it out of equilibrium (for example, adiabatic expansion of matter and radiation, if they are not coupled by microscopic collisions, will draw their temperatures apart, because the equations of state differ.)
This explains how the universe started in a nearly equilibrium state (we still see the blackbody radiation spectrum) yet is far out of equilibrium today.
If there is a big crunch (which by the way seems very unlikely now), then the collisions kick in as things come crashing together. Things become very unstable due to gravity-- there is a tendency to form lots of black holes. In contrast to the big bang plasma, which is nearly uniform, the crunch plasma is a mess. The extra structure has a lot of entropy due to gravity.
Indeed, the highest entropy state for any quantity of energy is a black hole. That is another story.
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Craig J. Hogan
Divisional Dean of Natural Sciences
Professor of Astronomy
Professor of Physics
College of Arts and Sciences
University of Washington
posted by Biochemie on 2:34 上午
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