How big do we need for comsological principle?

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Discussion Overview

The discussion revolves around the scale at which the universe can be considered homogeneous and isotropic, specifically in terms of Mega Parsecs (Mpc). Participants explore the boundaries of this principle, its establishment, and the relevance of recent surveys like the Sloan Digital Sky Survey.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • Some participants question the specific Mega Parsecs scale at which the universe is considered homogeneous and isotropic, noting that it is not the case on smaller scales.
  • One participant references a diagram from Ned Wright's site, suggesting a gradual transition to homogeneity as the scale increases.
  • Another participant estimates that the relevant scale is roughly 80 Mpc.
  • A later reply expresses gratitude for the information, indicating its usefulness for a recent cosmology exam.

Areas of Agreement / Disagreement

Participants generally agree that the universe is not homogeneous or isotropic on small scales, but there is no consensus on the exact Mega Parsecs scale or the historical context of its establishment.

Contextual Notes

The discussion does not clarify the assumptions behind the estimates or the specific definitions of homogeneity and isotropy being used, nor does it resolve the historical timeline of when these estimates were established.

skydivephil
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When we say the universe is homogenous and isotropic . How many Mega PArsecs are we talking about? Preusmably we all agree its not homogenous or isotropic on small scales. So where is the border and how when was it established? Is the current estimate, whatever it is, as recent as Sloan Digital Sky survey or much older than that?
 
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How many Mega PArsecs are we talking about?
There's a diagram on Ned Wright's site. It's a gradual transition - the larger the scale, the more homogeneous.
 
skydivephil said:
When we say the universe is homogenous and isotropic . How many Mega PArsecs are we talking about? Preusmably we all agree its not homogenous or isotropic on small scales. So where is the border and how when was it established? Is the current estimate, whatever it is, as recent as Sloan Digital Sky survey or much older than that?
Roughly 80Mpc or so.
 
Agh thanks for that, wanted it for my cosmology exams which was yesterday ,think I aced it and you guys have been enormous help in general, Physics forums rock
 

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