Open closed flat confused about what universe counts

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SUMMARY

The discussion centers on the concept of omega (Ω), which represents the ratio of the current observed density of the universe to its critical density. Participants clarify that while our observable universe is approximately 13 billion years old, the calculations of omega rely on the assumption of homogeneity and isotropy across the universe. This means that the density measured in our observable region is sufficient to infer the overall density of the universe, regardless of regions beyond our particle and event horizons. The importance of the observable universe in determining omega's value is thus established as a fundamental aspect of cosmological models.

PREREQUISITES
  • Understanding of cosmological principles, specifically density ratios.
  • Familiarity with the concepts of homogeneity and isotropy in cosmology.
  • Knowledge of particle and event horizons in the context of the universe.
  • Basic grasp of critical density and its significance in cosmology.
NEXT STEPS
  • Research the implications of homogeneity and isotropy in cosmological models.
  • Study the concept of critical density and its role in determining the fate of the universe.
  • Explore the effects of particle and event horizons on observable universe measurements.
  • Learn about advanced cosmological metrics, such as the Friedmann-Lemaître-Robertson-Walker (FLRW) metric.
USEFUL FOR

Astronomers, cosmologists, and physics students interested in understanding the structure and dynamics of the universe, particularly those focused on density measurements and cosmological models.

blumfeld0
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hello. I know when omega is equal to, less than or greater than one our universe is either flat open or closed respectively.
omega is the ratio of the current observed density to critical denisty in the universe.
my question is that we can only see our part of the universe (that is 13 billion years old approximately) but the universe is actually much larger than that. we can't see it because light in that part hasnt reached us and some of it never will. there are particle and event horizons.
so when we calculate omega we are only looking at this part of the universe. but isn't the part greater than 13 billion years need to be taken into account to calculate observed omega. why is our part of the universe, up to our horizon, the important part to determine omegas value.
does this question make sense. i hope so
thank you

blumfeld
 
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As you correctly stated, omega is a density ratio. If the universe at large is homogeneous and isotropic, then all we need to know is the density in one part and we know the density of the whole, is it not?
 
yes you are right. thank you
 

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