Question

Radiation Dominated Universem..

explain..​

Answer 1

In this chapter, we will discuss the Solutions to Friedmann Equations relating to the Radiation Dominated Universe. In the beginning, we compare the energy density of matter to that of radiation. This will enable us to see if our universe is matter dominated or radiation dominated.

Answer 2

There are two factors, first the density of matter falls as (1+z)3 while that of radiation falls as (1+z)4 because geometrical expansion dilutes the number density of matter and photons buy the first term but red-shifting also reduces the energy per photon by another factor of (1+z). that means that even radiation dominated at first, it would eventually transition to matter domination.

However, it would be possible for the radiation density to have started out lower than that for matter so why do we think radiation initially dominated?

We have measured the temperature and spectrum of CMB, which is the majority of the photons, and we also know the photon/baryon ratio from the present day matter content (and indirectly from the ratio of the products of nucleogenesis) and if we project those present day densities back, the numbers say that radiation dominated prior to about 50k years.

Logically, for nucleogenesis to have happened at all, there had to be a time when the average photon had more energy than is needed to create nucleons, so if the photon/baryon ratio was around 1 or even close, the energy density of the radiation would be greater. In fact it's around a billion to one.