Approximately how long after the expansion of the universe began was the radiation that we now see as the 3 K background radiation produced?
A)3 billion years
B)1 microsecond
C)7 minutes
D)300,000 years

Question

Quizplus · Accepted Answer

The radiation that we now see as the 3 K background radiation was produced after approximately 300,000 years of the expansion of the universe. This is known as the time of recombination when the universe cooled enough for electrons and protons to combine into neutral atoms, allowing radiation to escape freely.

Approximately How Long After the Expansion of the Universe Began