I thought the big bang was hot! If the cosmic microwave background radiation is the radiation left over from the big band, why is it only 3k?

a. the big bang was not a hot, but a cold, explosion- its temperature was the same as we observe now from the cosmic background radiation.

b. the cosmic microwave background radiation we observe is not cool – it is very hot, just like the big bang.

c. the universe has expanded and cooled to the temperature we now observe for the background radiation.

d. it is not from the big bang at all – it is from cold, intergalactic hydrogen clouds that cover the sky.