Friedmann-Robertson-Walker Models

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Discussion Overview

The discussion revolves around the Friedmann-Robertson-Walker (FRW) models, specifically focusing on the behavior of radiation and matter densities in cosmological scenarios such as the Big Bang and Big Crunch. Participants explore the implications of these models on energy densities over time and the conditions under which different types of energy dominate.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether a universe with only radiation density exists solely near the Big Bang and not during a Big Crunch, suggesting that radiation density decreases over time.
  • Another participant asserts that the Big Crunch scenario is incompatible with current cosmology, arguing that radiation would dominate again before a Big Crunch occurs due to how energy densities scale with the scale factor.
  • A different viewpoint suggests that if the universe contained only radiation, it would expand decelerated forever, implying a different behavior than suggested by the initial question.
  • One participant clarifies that while different fluids dominate at different times, the scaling of energy densities means that radiation would again dominate as the universe contracts in a Big Crunch scenario.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between radiation and matter densities in the context of the Big Bang and Big Crunch. There is no consensus on the implications of these models, and the discussion remains unresolved regarding the specifics of energy density behavior in these scenarios.

Contextual Notes

Participants reference hypothetical models and acknowledge that not all proposed models are considered valid. The discussion highlights the complexities of energy density scaling and the conditions under which different types of energy dominate, but does not resolve these complexities.

QuarkDecay
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TL;DR
FRW Models and densities
Can we say that a universe with a radiation density (only) exists only in near a Big Bang time, and not a Big Crunch? Since it gets much smaller as time passes.
For example in here:

http://www.nat.vu.nl/~wimu/Varying-Constants-Pictures/Time-Scale-Universe.jpg
We have different cosmological models for different times. But what about their densities?
For example, in the closed Universe with k=+1 and without a cosmological constant. Can we say that at Big Bang we only have the radiation density, but in the Big Crunch only the matter density?
 
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QuarkDecay said:
Summary: FRW Models and densities

We have different cosmological models for different times.
No we do not. We have a single cosmological model that is considered the current ”gold standard”. The energy density in this model is dominated by different types of fluids at different times.

QuarkDecay said:
Summary: FRW Models and densities

Can we say that at Big Bang we only have the radiation density, but in the Big Crunch only the matter density?
First of all, Big Crunch is incompatible with current cosmology. If it were to occur, radiation would again dominate over matter before it happens as energy densites scales in particular ways with the scale factor, not with time. As the Universe contracts, the CMB would blueshift and eventually become dominant.
 
QuarkDecay said:
Can we say that a universe with a radiation density (only) exists only in near a Big Bang time, and not a Big Crunch? Since it gets much smaller as time passes.
http://www.simondriver.org/Teaching/PHYS3303/obs_cos_lecture6.pdf
No, if the universe contains nothing but radiation it would expand decelerated forever.
 
Orodruin said:
No we do not. We have a single cosmological model that is considered the current ”gold standard”. The energy density in this model is dominated by different types of fluids at different times.

I'm talking about the hypothetical models I listed in that url. I know not all of them are true. But also you said we have different fluids at different times. I know that the matter and radiation density ratio change with time. This is what I was asking. Near the Big Bang time, the radiation density was higher than the matter.
Also in the hypothetical close Universe Model, there is a Big Crunch. What other hypothetical Models use Big Crunch?
 
QuarkDecay said:
But also you said we have different fluids at different times.
This is not what I said. I said that the fluids' energy densities scale differently and therefore different fluids dominate at different times.

Regardless, in the case of a Big Crunch the scale factor would again be small which would mean that radiation again would dominate.
 

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