The CMB can be regarded as a "wall" beyond which we cannot see light because
A) photons couldn't leave the hot plasma before the recombination period.
B) we haven't discovered the technology to detect light from earlier epochs.
C) galaxies absorb all the photons from earliest times, closer to the Big Bang.
D) there existed only massive particles within the universe in the first 380,000 years of existence; there were no photons.
E) behind the CMB "wall" there is a huge black hole that doesn't allow light to escape.

Question

Quizplus · Accepted Answer

Before the recombination period, the universe was a hot, dense plasma where photons were constantly scattered by free electrons, preventing them from traveling freely. After recombination, neutral atoms formed, allowing photons to travel freely, creating the Cosmic Microwave Background (CMB).

The CMB Can Be Regarded as a "Wall" Beyond Which