Why did the formation of particle-antiparticle pairs (such as protons and antiprotons)from gamma rays come to an end in the first second after start of the Big Bang?
A) Expansion of the universe carried the gamma rays too far apart for them to collide and form particle-antiparticle pairs.
B) The gamma-ray energy decreased as the universe expanded.
C) The universe became transparent to radiation, so gamma rays and particles no longer interacted.
D) The formation of particle-antiparticle pairs exhausted the supply of gamma rays, preventing further formation of particle-antiparticle pairs.

Question

Quizplus · Accepted Answer

As the universe expanded, the temperature decreased, causing the energy of gamma rays to decrease as well. This meant that the gamma rays no longer had enough energy to create particle-antiparticle pairs. Therefore, the formation of particle-antiparticle pairs from gamma rays came to an end within the first second of the Big Bang.

Why Did the Formation of Particle-Antiparticle Pairs (Such as Protons