Thermal Equilibrium of the Universe
That the cosmic background radiation has nearly a perfect blackbody spectrum tells us that at some point in its history, the universe was in nearly perfect thermal equilibrium, with everything at one uniform temperature. That is certainly not the case today. The cosmic radiation is at a temperature of 2.735 kelvin, Hawaii has an average temperature of 295 kelvin, and the temperature inside of stars ranges up to billions of degrees. There must have been a dramatic change in the nature of the universe sometime in the past. That change occurred when the universe suddenly became transparent at an age of about 700,000 years.
To see why the universe suddenly became transparent, and why this was such an important event, it is instructive to reconstruct what the universe must have been like at still earlier times.