About what was the temperature of the universe when a typical CBR photon was no longer able to spontaneously create an electron and positron pair?
A) 1000 K
B) 100,000 K
C) 1 million K
D) 1 billion K

Question

Quizplus · Accepted Answer

At temperatures above 1 billion K, the energy density of photons was too high for them to travel freely without being absorbed and recreating an electron and positron pair. As the universe cooled below this temperature, the density of photons decreased enough for them to no longer have sufficient energy to create electron-positron pairs spontaneously. This occurred when the temperature of the universe was around 1 billion K. Therefore, the correct option is D.

About What Was the Temperature of the Universe When a Typical