The cosmic microwave background or CMB is my second feature for this series. The CMB is formed by the original photons produced in the Big Bang and was created at the last scattering surface. For about 380000 years after the Big Bang the universe was really hot. A dense expanding plasma. It was so hot that every time a proton and an electron got together an energetic photon broke them apart again. But as the universe expanded it cooled down, and eventually it reached a time where the photons weren’t energetic enough to ionise hydrogen anymore and in their last scatter on the atoms they formed the CMB.

The expansion continued and still is continuing, and the CMB is only detectable with microwave instruments.

This the most detailed map of the CMB yet. The differences between a dark area and a light area amount to 1/10000th of a degree kelvin and they were formed from the fluctuations of matter density of the primordial universe.

After the CMB was emitted the most mysterious epoch of the universe begun: The dark ages when no stars were shining.