Cosmic Horizon Problem

I am confused by the horizon problem in cosmology. I understand that there wasn't enough time for points widely-separated at decoupling to have been in contact, so the issue is to explain why the CMB is almost isotropic. What I don't understand is why the universe wasn't in thermal equilibrium at decoupling. The universe started from a singularity, when all parts of the universe, presumably, were in contact with all other parts, and hence in thermal equilibrium. So I guess my question amounts to asking why the universe got out of thermal equilibrium between t=0 and decoupling?
- Mike Shapiro (age 69)
west end nc 27376

That's a nice question, whose answer requires shedding a little false intuition about the early universe. You're assuming that very shortly after the Big Bang, since everything we see was so close together it all must have had a chance to equilibrate. However, the time was also very short, so it didn't have that chance in a non-inflationary picture. Trying to go back to the singularity itself doesn't work for two reasons. First, we have no description of the singularity. Second, hardly anyone believes that there really was a singularity. It appears in the non-quantized gravitational equations, but non-quantized anything seems inconsistent with the world we see. So it looks like you either need a way (inflation) for already equilibrated regions to spread out faster than possible in a simpler picture or some principle requiring that early on everything be already in equilibrium, e.g. due to some long past history (as in ekpyrotic models).

Mike W.

(published on 08/20/2015)