How warm was the universe 880 million years after the Big Bang?
13.8 billion years ago, the universe was hotter than hot. Then it expanded and cooled down - to 2.725 Kelvin today, the temperature of the cosmic background radiation. From the moment the cosmic background radiation was released until today, the universe has expanded by a factor of about 1100. The cosmic background radiation, which originally had a temperature of about 3000 Kelvin and whose thermal radiation thus looked at that time as similar as the light of a halogen lamp, cooled down by the same factor. Of course, the entire universe was never equally warm everywhere. This makes it…