The first 380,000 years of our universe, before matter and radiation decoupled, were dominated by the primordial fireball filled with an immense quantity of radiant energy. What was the source of these photons?
A)They were created during the cosmic singularity and bounced around until the universe became transparent to radiation.
B)They were given off when the free electrons were captured by protons to form the first hydrogen atoms.
C)As the universe expanded and the radiation cooled, photons no longer had enough energy to create particles by pair production. But particles continued to annihilate and produce additional photons.
D)The "fireball" that we see in the distant past is really a reflection of all the radiation produced before that time bouncing back from the universe in its early opaque state.

Question

Quizplus · Accepted Answer

As the universe expanded and cooled, photons lost energy and could no longer create particle-antiparticle pairs, but existing particles continued to annihilate, producing photons.

The First 380,000 Years of Our Universe, Before Matter and Radiation