When this transition occurs — when the Universe cools enough to form neutral atoms — the radiation that was around at that time should finally be free to stream unimpeded through the Universe. Because of how expansion works in General Relativity, that leftover radiation from the primeval fireball should still be around today, although it should be redshifted to just a few degrees above absolute zero.

It should be almost exactly the same in all directions, and it should have the spectrum of a perfect blackbody. If we look for it, guess what we find?

Image credit: COBE / FIRAS, retrieved from Fermilab.

It fits perfectly. And finally, there’s one more prediction, going back even farther. If the Universe could have been hot-and-dense enough to prevent the formation of neutral atoms, then it must have been hot enough even earlier to blast apart any heavy atomic nuclei that formed!

Image credit: me, modified from Lawrence Berkeley Labs.

So if this were the case, we would expect that there were only free protons and neutrons for the first few minutes, unable to stably form anything heavier. When the Universe finally cooled below a critical temperature, they could fuse to form deuterium, helium and a little bit of Lithium, and we should be able to measure the pristine gas left over from this event today.

Image credit: NASA, WMAP Science Team and Gary Steigman.

And just as you’d hope, our predictions and our observations match. In fact we have detected this pristine gas, billions of light years away, and it contains (and doesn’t contain) exactly the elements we expect. (Although, to be fair, it would probably take many years to find it if we just started looking today.)

There are plenty of other predictions of this scenario that have been supported by observation, but these pieces of evidence are more than sufficient. You see, this is what the Big Bang is! It’s this set of initial conditions — a hot, dense, uniform, matter-and-radiation-filled Universe — that expands and cools with General Relativity as our theory of gravity, that gives rise to all of these phenomena and more, including the sophisticated structure we see today.