Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

Today, the universe has expanded by a factor of 1,000 from the time photons were set free, and so the cosmic background has, in turn, cooled by a factor of 1,000. All the visible light photons from that epoch have become 1/1,000th as energetic.