I The Universe filled with a "homogeneous perfect fluid"

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The discussion centers on the concept of the universe as a "homogeneous perfect fluid," as described in Schutz's textbook, which replaces the notion of ether. Participants explore the characteristics of this fluid compared to ether, emphasizing that while ether was meant to explain electromagnetism, the perfect fluid model is used to derive the Friedmann-Lemaître-Robertson-Walker (FLRW) solutions in cosmology. The universe is approximated as homogeneous and isotropic on large scales, although local deviations exist due to structure formation. The conversation also touches on the empirical nature of physics, highlighting the importance of observation and experimentation in understanding the universe's structure and dynamics. Ultimately, the discussion reflects on the complexities of cosmological models and the need for refined methods to account for the universe's actual conditions.
  • #31
Well, it applies in a very coarse-grained sense. You have to average over spatial volumes of some 250 Mio. light years:

https://en.wikipedia.org/wiki/Cosmological_principle#Observations

It's not a superfluid, but the "energy content" consists (according to nowadays "concordance model") of 30% of heavy particles ("cold"/"non-relativistic") (25% are "dark matter" made of yet unknown particles and about 5% is made up of the known standard-model particles). The remaining 70% is "dark energy", describable by a cosmological constant, which is the most enigmatic component.

https://en.wikipedia.org/wiki/Lambda-CDM_model
 

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