Before the decoupling:
A)the Universe was transparent to radiation.
B)the Universe was opaque to radiation.
C)protons and electrons combined to form atoms.
D)there was more helium than hydrogen.
E)deuterium produced electrons and positrons.

Question

Quizplus · Accepted Answer

Before the decoupling, the Universe was opaque to radiation because photons could not travel freely due to the high density of ionized particles. Before the decoupling, the Universe was filled with a dense plasma of protons, electrons, and photons. At this time, the Universe was opaque to radiation, as the photons could not travel freely without being scattered by the dense plasma. It was only after decoupling, when the Universe cooled and expanded enough for protons and electrons to combine to form neutral atoms, that the Universe became transparent to radiation.

Before the Decoupling