In the beginning t equaled zero seconds (t=0). A Big Bang-like thing happened and the universe began expanding. It was very
hot and very dense cold and empty. The universe expanded exponentially in a period known as inflation. Inflation ended around t=0.000000000000000000000000000000001 seconds.The universe was so hot that everything was relativistic and “radiation like”, so the universe was considered radiation-dominated.
At around t=1s neutrinos decoupled, meaning they stopped interacting with matter or radiation and just continued on their way forever. This is the elusive CNB — the cosmic neutrino background. Around the same time the ratio of protons to neutrons “froze out”.
At around t=200 seconds the temperature had dropped enough that deuterium could start to form. These were the first nuclei in the universe. This is the beginning of the era known as Big Bang Nucleosynthesis (or BBN). After less than 15 minutes, BBN ceased and the initial ratio of helium to hydrogen was established. The universe was now about 20 minutes old and for the next 50,000 years it was a hot bath of radiation and light elements that continued to expand and cool.
At t=50,000 years or so, the temperature got low enough that matter started to dominate the universe and by t=350,000 years the universe was entirely transparent to radiation, giving rise to the “last scattering surface” which we see as the CMB (cosmic microwave background).
By t=250,000,000 years or so, stars and galaxies had formed. The universe continued to expand and cool even as the the universe evolved to what we see today.
At about t=10,000,000,000 years, about 75% of the present age of the universe, matter ceased to dominate the universe and Λ (lambda) know as the ominous “Dark Energy” began to take over. Thus, the universe is not just expanding, it’s expansion is accelerating.
Currently t=13.7 billion years and the energy density of the universe is comprised of roughly 0.00008% radiation, 30% matter and 70% Λ, the temperature is T=2.7 degrees Kelvin and the Hubble rate, which is a measure of the expansion of the universe, is H=70 km/s per megaparsec.
 My cosmology professor was kind enough to read my post and to this point he said “The universe wasn’t hot during inflation. It was actually cold and empty; the inflaton field was a condensate, totally different from a thermal bath; the best analogy is a Bose-Einstein condensate at T = 0. The condensate at the time t =0.000000….1 you say decayed. The decay products then thermalized to very high temperature.”
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