Equilibrium at Universe (possible?)

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    Equilibrium Universe
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Discussion Overview

The discussion revolves around the concept of equilibrium in the universe, particularly in relation to the Friedmann-Robertson-Walker (FRW) metric and the implications of cosmic expansion on thermal properties. Participants explore the definitions and interpretations of equilibrium, large-scale homogeneity, and the slow expansion argument in cosmology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how the universe can be at equilibrium given that the FRW metric lacks a time-like Killing Vector.
  • Another participant suggests that the term "equilibrium" may be misapplied and proposes that "large-scale homogeneity" might be a more accurate description.
  • Several participants reference a document discussing thermal energy distributions and the nature of equilibrium, noting that it does not imply a static state but rather a system well-described by thermal distributions.
  • A participant elaborates on the slow expansion argument, explaining that in the early universe, local heat exchange occurred much faster than the expansion rate, allowing for local thermal equilibrium to be approximated despite cosmic expansion.
  • It is noted that while the universe cools over longer time scales due to expansion, local thermal equilibrium states can be maintained at gradually decreasing temperatures as long as the thermalization time scale remains shorter than the expansion time scale.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the term "equilibrium" in the context of the universe, with some advocating for a redefinition while others maintain the original terminology. The discussion remains unresolved regarding the precise nature of equilibrium in cosmological terms.

Contextual Notes

There are limitations in the discussion regarding the definitions of equilibrium and homogeneity, as well as the assumptions underlying the slow expansion argument. The implications of these concepts on the thermal properties of the universe are not fully resolved.

ChrisVer
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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
 
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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..
 
ChrisVer said:
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.
 
ChrisVer said:
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.
 

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