The second law of thermodynamics demands that the entropy of any closed macroscopic system should never decrease. The laws of physics naturally drive systems towards states of increasing disorder and thermal equilibrium, as we all know. Yet these basic precepts, when applied to the Universe as a whole, meet with an apparent paradox. The Universe, cosmologists believe on strong evidence, existed some 13 billion years ago in a hot, dense phase of remarkable uniformity; that is, in a homogeneous condition akin to thermal equilibrium. Since that time it has expanded and cooled, and the matter and energy within has condensed into a rich variety of ordered forms ranging from stars to living and thinking human beings. Now, evidently, the Universe is very much out of thermal equilibrium; its entropy has apparently decreased.
What's going on? In particular, if the Universe was once in equilibrium, how has it got so far away from it? The consensus seems to be that the resolution of this paradox has everything to do with gravity, and possibly with the peculiar nature of black holes, which carry enormous amounts of entropy and exist in abundance now, but didn't in the early Universe. Is this true? Perhaps. But it also seems that the applicability of notions such as entropy, thermodynamics and equilibrium becomes anything but clear when gravity enters the picture; consequently, this is an area where good science can be done merely by efforts to clarify fundamental issues, which David Wallace of Oxford University has recently tried to do (http://arxiv.org/abs/0907.0659; 2009). The argument that gravity is the key, Wallace suggests, seems to be correct, although persisting confusions often plague discussions of the matter.
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