Cosmology: 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

The universe first became transparent to most light when it was about 380,000 years old, which coincides with the origin of the cosmic microwave background radiation. At this time, the temperature of the universe had cooled enough to allow the formation of neutral atoms from protons and electrons. This allowed light to travel freely through space instead of being scattered by free electrons, making the universe transparent. The formation of atomic nuclei occurred earlier, within the first few minutes after the Big Bang. The formation of protons and neutrons was also an early process in the universe's history. The formation of stars and galaxies occurred much later, after the universe had expanded and cooled further.

Cosmology: the Universe First Became Transparent to Most Light, Coinciding