The universe first became transparent to most light,coinciding with the origin of the cosmic background radiation,when the universe cooled enough to allow the formation of
A) protons and neutrons.
B) atomic nuclei.
C) neutral atoms.
D) stars and galaxies.

Question

Quizplus · Accepted Answer

When the universe cooled down sufficiently, electrons combined with protons and formed neutral hydrogen atoms, causing the universe to become transparent to most light. Prior to this, the universe was filled with a highly ionized plasma that scattered light. The origin of the cosmic background radiation also coincides with the timeframe when neutral atoms formed.

Therefore, the correct choice is C - neutral atoms.

The Universe First Became Transparent to Most Light,coinciding with the Origin