# Temperature after the big bang

I was just wondering about possible extreme temperatures. It came to my mind that shortly after the supposed big bang the universe was probably at its hottest in its entire Lifetime. But does it even make sense to speak about a temperature Right at the instant of the Big Bang? I mean the Big Bang Theory still Needs a lot of Investigation and Explanation. Or is it possible to create hotter temperatures than at the instant of the BB. Correct me if I am wrong, but the answer to that last Question is probably no anyway.

• You are asking about the temperature at the moment of the Big Bang, and that is probably undefined in all current physical theories since it is a space-time singularity and most physical measures go to infinity as it is approached. – Anders Sandberg Jun 12 '18 at 21:11

Here is the current history of the universe with the modified Big Bang model :

Note the fuzziness at the Big Bang. This is because the current model needs to use effective quantization of gravity up to the end of the inflaton period , in order to fit observations. Temperature has no meaning in a quantum mechanical system, it is a thermodynamic intensive variable. The total energy of the universe was there, but cannot be modeled with classical thermodynamics without further assumptions.

In this plot , one sees a temperature for the "beginning of the BB":

The mathematical model used to get a temperature gives a correlation with radiation energy density used for the image above.

For early stages of the expansion of the universe, it's energy density was dominated by radiation, with matter present only as a negligible contaminant.

So at most one can extrapolate to the fuzzy region, not beyond, keeping in mind that all these are very tentative extrapolations, imo of course.

• So, how long time after BB does it make sense to talk about a temperature? You often hear something like "At $t=$ a Planck time, it was a Planck temperature hot", i.e. ~1e32 K. Or, "During inflation, the Universe supercooled from 1e27 to 1e22 K". Do these temperatures make sense? – pela Jun 13 '18 at 12:22
• you know the saying "beauty is in the eye of the beholder"? I would say "sense is in the brain of the thinker" :). In this link for example, a temperature is given way into the inflationary period hyperphysics.phy-astr.gsu.edu/hbase/Astro/timlin.html . imo once one can model "paticles" in the soup, one can start talking of temperature. They do have a rational mathematical model hyperphysics.phy-astr.gsu.edu/hbase/Astro/expand.html#c2 . OK, I will modify my answer – anna v Jun 13 '18 at 12:55