What is the cosmic microwave background?

Cambridge University physicist Fran Day tackled this cosmic question from Martin, who got in touch via Facebook...

Fran - I think what's being referred to here is the cosmic microwave background. This is radiation that was produced 400,000 years after the Big Bang, but in cosmic timescales that's nothing. And it was produced at a time in the universe when electrons and protons were combining to make neutral hydrogen, and that produces all of these photons. And this happened everywhere in the universe at once. So it didn't just happen at a point. It happened across the universe.

Now since that time, four hundred thousand years after the Big Bang, the universe has been expanding. So this radiation, these photons, have just been travelling through the universe as it expanded, and the effect that that has is to stretch them. So the effect of the expansion of the universe is that we see the radiation, the light, from the cosmic microwave background as having a longer wavelength. But because the universe is expanding, everything just expands in that together. So the radiation is everywhere in the universe. And the stuff that we see is the stuff that just happens to be where you are. But it is just travelling through this ever expanding universe.

Chris - One argument I heard for this that put it I thought quite well, is that for something to be sending us radiation like this from all directions, argues that the stuff which is across the universe must have at some time all been together in one place and shared the energy of the Big Bang equally amongst all those particles, so that when the universe then inflated very very fast - which it did far faster than light in everything else in the very early phases of the universe - it took all those bits with it that had shared all that energy. And now they are busy radiating that energy in all directions at us. And as you say because the universe is also inflated it stretched the light out. So we see this stuff at very long wavelengths. But it is coming at us from all directions because the material that it was all part of in the very early universe is now everywhere, and so it's now sending us radiation whichever way we look.

Fran - Yes that's exactly right and that's really why the cosmic microwave background is very uniform in all directions. Which is quite surprising because it's spread across the whole universe, and this is one of the reasons we think we need this period of inflation where the universe expanded very very quickly at the start of the universe, so that it can all have been together at one stage.