I’ve read many times that arrow of time can be explained from extremely low entropy of the Universe at the Big Bang (http://preposterousuniverse.com/eternitytohere/faq.html). The argument goes that initially the Universe was very smooth. As time passed, matter gathered together in clusters due to gravitation, until eventually we got stars, galaxies and super massive black holes.

There is a notion of entropy, related to gravitation. In classical statistical physics without taking into account gravity the gas spreads evenly in the volume. If gravity is taken into account than the most probable state of the system is when all gas forms a large black hole. However I don’t see how Hawking evaporation is taken into this picture. From this I conclude that somewhere there must be a textbook-level theory of entropy that takes into account gravity, Hawking radiation and expansion of the Universe.

In general, we may consider a toy model – the Universe, consisting entirely of black holes of different sizes and velocities, see how it evolves and get the most probable state (or states).


If the Universe contained only black holes with a certain mass and velocity distribution, how would it evolve over time? Is it enough to know the mass/velocity distribution to predict the general evolution of such Universe? Is it possible to predict the spectrum of background radiation in such a Universe at any moment of time?

  • $\begingroup$ Have humans evolved in the midst of BHs to predict those..? Actually, who is gonna predict those numbers? $\endgroup$ – Waffle's Crazy Peanut May 24 '13 at 15:34
  • $\begingroup$ Hi Oleg, and welcome to physics.SE. I edited your question to clarify the grammar. Please feel free to revert my edit if this is what what you meant to ask. $\endgroup$ – Colin McFaul May 24 '13 at 17:54
  • $\begingroup$ You should provide more context for your question. Do you basically mean a N-body problem involving black holes? $\endgroup$ – Brandon Enright May 24 '13 at 18:07
  • $\begingroup$ Hi Brandon, I mean the situation when N is so large that it is more convenient to consider black holes from the statistical point rather than as an instance of N-body problem. $\endgroup$ – Oleg S. May 24 '13 at 22:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.