At what time did the universe cool to a temperature of about 3 K?
A) the end of the inflationary era
B) the end of the Planck time
C) very recently
D) the era of recombination

Question

Quizplus · Accepted Answer

The universe cooled to about 3 K very recently, as it has been expanding and cooling over billions of years since the Big Bang. The current temperature of the cosmic microwave background radiation is approximately 2.7 K.

At What Time Did the Universe Cool to a Temperature