This is just a bit of idle wondering, another little bit of amateur cosmology from someone who should probably know better. The question I’m asking myself today is, is the second law of thermodynamics connected to the expansion of the universe?
The second law of thermodynamics — the law of entropy — is a fascinating thing. It’s the law that makes the past different from the future; it’s the law that predicts an effective end to the Universe, yet it’s the law that makes life possible. It’s also a deeply mysterious law. It took well over a century for true meaning of entropy to be understood (in fact arguments on the subject still rage today), and we still don’t understand, on a cosmological level, exactly why it was so low in the past.
One of the things that’s often said about entropy is that it means “disorder”. This post is about that idea. It’s worth discussing for two reasons: firstly, it’s wrong. It’s close to the truth, in the same sort of way that a spectrum is close to a rainbow, but not the same. Secondly, the real truth is much more interesting.
Let me prefix this by saying that the early universe was a very boring place. There was a lot of energy, (at one point too much to even allow the formation of matter) and everything was very ordered with no interesting features. The proof of this comes from the recently measured cosmic microwave background radiation which found that at the point it was created (about 370 000 years after the big bang). The differences in temperature between any points in the universe were around one part in 100 000 (i.e. if it was 100 000K then the max temperature difference would be 1K). A rough idea of the second law of thermodynamics would also make this clear that it is required – if everything gets less ordered with time (entropy increases) then the start of the universe must have been a very well ordered place.