Sean at Cosmic Variance has a very interesting post on Boltzmann and entropy. Given that entropy is generally increasing, why was the universe ever in a low-entropy state? One idea proposed by Boltzmann himself is that we are living in a small low-entropy fluctuation in a much larger universe. (According to statistical mechanics, entropy can decrease, but is just very unlikely to do so.) If we take Boltzmann’s idea seriously, then we would expect to be living in the most likely fluctuation compatible with our existence, which does not seem to be the case. Sean has much more on this.