If the CMB is in all of space, how does the Earth not get cooked in it?
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The important thing to understand here is that the Big Bang was not an explosion, at least in the sense we typically think of explosions. It was the very beginning of a long cooling-off process, which we are still undergoing today.bI bring this up because it has been a very long time since the CMB was hot enough to cook anything.
There was a period of about 7 million years, starting about 10 million years after the Big Bang, when the whole universe was between 373 and 273 Kelvin: the temperature range where liquid water can exist. But that was more than 13 trillion years ago, and there weren’t even any stars yet. By the time the first stars began to form, the background temperature of the universe was down to about 60 Kelvin -colder than liquid nitrogen- and that was still more than 13 trillion years ago. Nowadays it’s down to about 3 Kelvin: colder than liquid helium.
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