What is the cosmic microwave background?

What is the cosmic microwave background?
The cosmic microwave background (CMB) is really the first light that emerged in the Universe, and that is still traveling across this Universe. It is called cosmic because it is of cosmic origin. It is called background because it is everywhere. And it is called microwave because it is an electromagnetic radiation (what we call losely “light”) which is in the microwave radiation frequency range. In this sense, it is similar to the microwave radiation present in your microwave oven. This microwave radiation is close in frequency to the electromagnetic radio waves. This is why the CMB can be detected nowadays by a simple dish antenna (an experiment that we do routinely on the roof of our lab in Paris).

Finally, let us stress that the CMB was produced at the time of Hydrogen recombination in a different frequency range but was redshifted to the microwave range by the expansion of the Universe.

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