Friedmann-Robertson-Walker Models

[QuarkDecay]

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?

 

[Orodruin]

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.

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.

 

[timmdeeg]

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.

 

[QuarkDecay]

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?

 

[Orodruin]

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.