# How did the Universe thermalize to the same temperature before inflation?

Inflation is speculated to take place at about $$10^{-35}$$ seconds after BigBang. The idea is that at that moment the universe was small enough so that photons could thermalize to reach the same temperature. For the source see Our Baby Universe: Ed Copeland (6.30 minutes) Then that region underwent an exponential expansion. How does this argument fit with the fact that there were no photons at that time?

• "The idea is that at that moment the universe was small enough so that photons could thermalize to reach the same temperature" - Source? – probably_someone Nov 25 '19 at 15:48
• "there were no photons at that time" - Source? – probably_someone Nov 25 '19 at 15:48
• @probably_someone added. For the second, we know that there was nothing except the field responsible for inflation. – mithusengupta123 Nov 25 '19 at 15:59
• "we know that there was nothing except the field responsible for inflation" - Source? – probably_someone Nov 25 '19 at 16:11
• Please see my comment above. The idea of inflation (though I know very little) is that there were only a scalar field filling the Universe at the time of inflation which then decayed to produce every other particles of the Universe. You can see this en.wikipedia.org/wiki/Inflation_(cosmology)#Reheating – mithusengupta123 Nov 25 '19 at 16:19

This is the timeline in the standard cosmological model:

There are no photons before the spontaneous weak symmetry breaking time, so there is no way they define an ensemble to have a temperature, which is a thermodynamic variable, i.e. the statistics of very many particles.

During the inflationary epoch, the inflaton field dominates , a theoretical model formed to reconcile the observations with the theory, and explain the cosmic microwave background radiation (CMB)small inhomogeneities, but that background comes at the transparency point of light, 200.000 years or so after the Big Bang.

The idea is that at that moment the universe was small enough so that photons could thermalize to reach the same temperature.

It is not photons that homogenize the universe at inflation time, but inflatons, and the small inhomogeneities result in the small deviations of the CMB from the perfect uniformity, which explains galactic clusters and galaxies.

If you watch the timeline after photons appear , a bit before $$10^{-10}sec$$, they can start interacting with quarks and then with hydrogen and deuterium until the universe expands enough that the photons do not find scattering centers and mainly decouple, at the transparency time. This give a snapshot of that time,of how matter was distributed. The $$10^{-5}$$ inhomogeneity seen in the CMB spectrum correlates with clusters of galaxies.

One has to spend some time studying the models and the mathematics of the models.

• The summary of your answer is that the claim made in that video is wrong. – mithusengupta123 Nov 25 '19 at 17:29
• I looked at the time you gave for the video and did not see any plot about photons. Maybe you have mixed up something stated with the CMB , which I am trying to explain here. The inflation time inhomogeneity is seen as a snapshot in the photon spectrum of the CMB . – anna v Nov 25 '19 at 17:32
• Let me put it in order. 1) people observed inhomegeneity in the distribytion of CMB photons.This could not be explained in the simple big bang starting from a point singularity 2) The universe now is not homogenious in matter, there are galaxies and clusters of galaxies. These cannot also be explained by a simple Big Bang. So the inflation time was proposed, which would be homogeneous but due to the inflaton particles some quantum fluctuation could explain how there were centers for generation of clusters of galaxies and galaxies + also explain the CMB inhomogeneity at the $10^{-5}$ level. – anna v Nov 25 '19 at 19:27