Caption: A log-log plot of cosmic temperature (in mega-electron-volts (MeV): 1 MeV ≅ 10**10 K) versus cosmic time 10**(-10) s -- 10**16 s ≅ 0.3 Gyr.

Features:

1. Note the Boltzmann contant k = 1.380649*10**(-23) J/K = (0.8617333262 ...)*10**(-4) eV/K (exact) ≅ 10**(-4) eV/K = 10**(-10) Mev/K (see NIST: Fundamental Physical Constants). Thus, 1 mega-electron-volt (MeV) corresponds to about 10**(10) K.

2. The cosmic temperature at all cosmic times later than the quark era (10**(-12) -- 10**(-6) s) at least is the temperature of cosmic background radiation (CBR). The cosmic temperature is, in fact, the only temperature in the observable universe (other than that of the dark matter) until recombination era t = 377,770(3200) y during which occurs decoupling era when CBR baryonic matter decoupled and ceased to have the same temperature.

3. The present cosmic time CBR is mainly in the microwave band (fiducial range 0.1--100 cm, 0.01--10 cm**(-1)) and is the cosmic microwave background (CMB) with CMB T = 2.72548(57) K (Fixsen 2009) (see also Wikipedia: Cosmic microwave background radiation: Features).

4. The cosmic temperature obeys a very simple relation:

     T = T_0*(a_0/a) = T_fiducial*(a_fiducial/a)  or  T ∝ 1/a  , 

   where subscript 0 indicates present cosmic time, a_0 = 1 conventionally, and subscript fiducial indicates any fiducial cosmic time. Since there is the universal expansion, cosmic temperature falls with cosmic time.

5. For cosmic time less than the radiation-matter equality 49200 y = 1.55*10**12 s a ∝ t**(1/2) and after a ∼ ∝ t**(2/3). Thus,

     T ∝ t**(-1/2)    before radiation-matter equality;
     T ∝ ∼ t**(-2/3)    after radiation-matter equality.  

   Recall on log-log plots, the exponent (AKA power) becomes the slope. Hence, the slope of the cosmic temperature is -1/2 before the radiation-matter equality and about -2/3 after.

6. Cosmic history eras (all times are approximate):
   1. very early universe (t < 10**(-12) s)
      1. Planck era t < Planck time t_plank = sqrt(ħG/c**5) = 5.39125*10**(-44) s
   2. early universe (10**(-12) s -- 377.770(3200) kyr)
      1. quark era (10**(-12) -- 10**(-6) s)
      2. neutrino decoupling t ≅ 1 s
      3. Big Bang nucleosynthesis era (cosmic time ∼ 10--1200 s ≅ 0.17--20 m)
      4. radiation-matter equality 49.200 kyr = 1.55*10**12 s
      5. recombination era t = 377.770(3200) kyr = 1.192*10**13 s
   3. dark ages and large-scale structure emergence era (377.770(3200) kyr -- 1 Gyr)
      1. dark ages (377.770(3200) kyr -- 150 Myr)
      2. cosmic dawn (AKA reionization era, z∼6--20, cosmic time ∼ 150 Myr--1 Gyr in Λ-CDM model)
   4. cosmic noon (cosmic time t ∼ 3.5 Gyr, z ∼ 1.9): This is the peak of the cosmic star formation rate (SFR).
   5. cosmic present t_0 = to the age of the observable universe = 13.797(23) Gyr (Planck 2018)