system
Given the following post, respond as if you're commenting on Reddit:
human
What was the temperature at Planck time and GUT transition?

During the early Universe the relation between time and temperature is given by
t = 2.4 * g^-1/2 * (T/1 MeV)^-2 s,
where g is relativistic degrees of freedom (which should be 106.75 at above the electroweak transition).

Is this the right formula to use to get the temperature at Planck time (5.39 * 10^-44 s) and similar to get time at GUT transition at e.g T=10^15 GeV? 

The degrees of freedom, g,  should grow as we go back in time. Are there any estimates for this at the Grand Unified Epoch and Planck Epoch?

The reason I started thinking about this was that I kind of always though the temperature at the Planck time was just the Planck temperature...

PS: Short answers are welcome :)
gpt
That formula applies exclusively for the radiation-dominated era. You can see that by reviewing the derivation. So it clearly only applies from the end of inflation up to matter-radiation equivalence.

Many people assume the GUT scale is around the end of inflation; in that case the formula should work for GUT and should not apply for Planck.