In order for the cosmic microwave background to show hydrogen absorption lines at the redshift corresponding to the universe's age of 380,000 years, what condition would have been necessary?
A)There would have needed to be ionized hydrogen (which would contradict the idea that the CMB originates at the time the universe cooled enough for protons and electrons to combine, making neutral atoms).
B)There would have needed to be cooler hydrogen clouds (which would contradict the observation of a uniform temperature in all directions).
C)There would have needed to be even warmer hydrogen clouds (which would contradict the observation of a uniform temperature in all directions).

Question

Quizplus · Accepted Answer

For hydrogen absorption lines to appear, there would need to be cooler hydrogen clouds to absorb the CMB radiation, which contradicts the observed uniform temperature of the CMB.

In Order for the Cosmic Microwave Background to Show Hydrogen