Friedmann-Robertson-Walker Models

In summary: So yes, it is possible for a universe with only radiation density to exist near a Big Bang time, but not necessarily in a Big Crunch scenario.
  • #1
QuarkDecay
47
2
TL;DR Summary
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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  • #2
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.
 
  • #3
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.
 
  • #4
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?
 
  • #5
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.
 

Related to Friedmann-Robertson-Walker Models

What is the Friedmann-Robertson-Walker (FRW) model?

The FRW model is a mathematical framework used in cosmology to describe the evolution of the universe. It is based on Einstein's theory of general relativity and assumes that the universe is homogeneous and isotropic on large scales.

What are the key assumptions of the FRW model?

The FRW model assumes that the universe is homogeneous and isotropic, meaning that it looks the same at every point and in every direction. It also assumes that the universe is expanding and that matter is distributed uniformly throughout.

What is the significance of the FRW model in cosmology?

The FRW model is significant because it provides a way to study the large-scale structure and evolution of the universe. It has been used to make predictions about the expansion rate, age, and composition of the universe, and has been supported by observational evidence such as the cosmic microwave background radiation.

How does the FRW model explain the expansion of the universe?

The FRW model explains the expansion of the universe through the use of the Hubble parameter, which describes the rate at which the universe is expanding. According to the model, the expansion is driven by the presence of dark energy, a mysterious force that counteracts the gravitational pull of matter and causes the universe to accelerate.

What is the role of dark energy in the FRW model?

In the FRW model, dark energy is responsible for the accelerating expansion of the universe. It is thought to make up about 70% of the total energy in the universe and its presence is necessary to explain the observed behavior of galaxies and other large-scale structures.

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