Cosmologists presently consider the universe to be matter dominated. In this stage the number of photons
A) vastly outnumbers the number of particles.
B) is vastly outnumbered by the number of particles.
C) is greatly outnumbered by the number of particles, but is less than the number of particles plus antiparticles.
D) is close to being equal to the number of particles.

Question

Quizplus · Accepted Answer

In the matter-dominated era of the universe, the number of photons vastly outnumbers the number of particles, as the universe has cooled and expanded since the Big Bang, leaving a large number of relic photons from the cosmic microwave background.

Cosmologists Presently Consider the Universe to Be Matter Dominated