# Equilibrium at Universe (possible?)

• ChrisVer

#### ChrisVer

Gold Member
How can Universe be at equilibrium when the FRW metric has no time-like Killing Vector?

Request: Please, move this into Cosmology thread... :( Mistakenly I posted it in HEP

Last edited:

How can you write an ordered post if the universe is at equilibrium?

Do you mean the large-scale homogeneity instead of equilibrium?

http://www.damtp.cam.ac.uk/user/db275/Cosmology/Chapter3.pdf
page:5 as you scroll down, the subnote with number 7...
I am trying to pursue a little further and understanding better what they meant in it... In fact I am looking for a better explanation on the slow expansion-argument as well..

http://www.damtp.cam.ac.uk/user/db275/Cosmology/Chapter3.pdf
page:5 as you scroll down, the subnote with number 7...
I am trying to pursue a little further and understanding better what they meant in it... In fact I am looking for a better explanation on the slow expansion-argument as well..
It's not an equilibrium in the strict sense of a final state of a system that no longer changes. What it essentially means is that the system is well-described by a thermal energy distribution.

I am looking for a better explanation on the slow expansion-argument as well..

Sorry to necropost but I'm just running across this thread. The slow expansion argument is that, in the early universe, the time scale for local heat exchange, which is what drives the system to local thermal equilibrium, was much shorter than the time scale of expansion; in other words, on the time scale for achieving local thermal equilibrium, the universe could be approximated as not expanding. Of course over longer time scales, the expansion does affect the thermal properties, because it causes the universe to cool; but as long as the thermalization time scale is much shorter than the expansion time scale, the cooling can be thought of as just a succession of local thermal equilibrium states at gradually decreasing temperature, because local heat exchange happens fast enough to keep adjusting to the expansion.