Okay, so In 1964, Arno Penzias and Robert Wilson at Bell Labs were working on a radio telescope. It looked like this.
So. they got a kind of noisy signal. (incidentally, the same noisy signal you get when you plug a tv antenna in… that static screen) They thought, at first, that the noise was caused by pigeon and pigeon poop. So they cleaned it out. but it was still there.
Long story short, they’d discovered cosmic microwave background radiation. the residue of the big bang!
so what is cosmic microwave background radiation?
CMB radiation is a kind of electromagnetic radiation (light) that has a super-long wavelength (about 2 mm). The kicker is that it’s super uniform. anywhere you look in the sky, at any time, the light coming in has the same colour and intensity. Also, it originates from a time (400,000 years after the big bang) when the universe was still so hot that it glowed like a fluorescent lightbulb.
It’s so uniform, in fact, that physicists have been taking super-precise measurements of it. We use the slight (very slight) differences in the colour and intensity to learn about the early universe!
Just looking at a map of the actual colour/intensity maps is super boring. it just looks like a big orange sphere. If you look at differences in differences in differences in the intensity, you get a map like this:
Okay. so what is the cosmic microwave background? Where does it come from? how does it work?
Well. to be technically correct, it’s photons remaining from the surface of last scattering from the time when the universe had cooled enough to go from being full of plasma to being full of neutrally charged atoms.
but what does all that mean? there’s an easy way to talk about these things and it involves a BOAT PARTY which is floating down a widening river.
So. the big bang. er… Imagine that instead of looking at the universe as a whole, I’m just going to look at a small chunk of it. In this case, the expansion that the bits and bots in the universe feel is not unlike how bits and bobs floating down a widening river will spread apart… they aren’t being pushed apart by the river, exactly. they just drift apart because more water moves between them.
So. as the different boats drift apart, the music on the water gets quieter, and the kids calm down and get back into the boats.
So in the early universe, the density is really high, and so it’s really hot. so hot that the nuclei can’t hold onto the electrons (this is called a plasma). the free electrons scatter light super-aggressively, and the photons get batted around by all the electrons and the average photon will have the same temperature as the electrons and nuclei. But then, as the universe expands, and the density of the gas decreases and it cools. At some point it cools so much that all of the free electrons get captured by nucleii, making neutrally charged hydrogen, helium (and a few heavier elements). The photons hardly interact with these gasses, and so they’ve just hung out from then until now. the expansion of the universe has been cooling them ever since.