When did the universe cool to a temperature of 3 K?
A) 1 second after the start of the Big Bang, when pair production ceased
B) 500,000 years after the Big Bang, when the universe became transparent to radiation
C) 3 minutes after the start of the Big Bang, when primordial nuclear reactions ceased
D) very recently

Question

Quizplus · Accepted Answer

The universe cooled to a temperature of about 3 K very recently, as it is the current temperature of the cosmic microwave background radiation, which has been cooling since the Big Bang.

When Did the Universe Cool to a Temperature of 3