The following quote is from the Wikipedia article https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis#Characteristics . The temperature/time relation in this era can be given by the equation: where t is time in seconds, T is temperature in MeV, and g* is the effective number of particle species. (g* includes contributions of 2 from photons, 7/2 from electron-positron pairs and 7/4 from each neutrino flavor. In the standard model g* is 10.75).I would much appreciate seeing a reference citation or an explanation from someone about how the value 0.74 is calculated for the nucleosynthesis era. By the way, when I searched the internet for this information, all I could find were articles that quoted the same material from Wikipedia.