Welcome back to The Greatest Story Ever Told, where we’re bringing you the story of the Universe. We’re going to go from the very beginning — before the big bang — up through the present day, and tell you how we got here. This is part seven, and you can always go back for parts 1, 2, 3, 4, 5, and 6. Last time, we got rid of all the antimatter in the Universe by letting it cool enough that every particle-antiparticle pair in the Universe annihilated with one another, producing a huge excess of radiation.
But there was also a little bit of normal matter — protons, neutrons, and electrons — that got left over. When we ask “how much”, we found that for every ten billion photons (particles of light), we have only six protons and neutrons left over, with exactly as many electrons as there are protons to keep the Universe electrically neutral.
The Universe is also still very hot and dense, although it has been expanding and cooling. When we ended the last part, the Universe was around one second old, which means the temperature was only about a thousand times as hot as the center of the Sun. (And that’s right; it took the first six parts of this story just to cover the first second!) So what do you think these protons and neutrons are going to do at these high temperatures and high densities?
Did you guess “fuse to form heavier elements” like they do in the center of the Sun? It’s a good guess. After all, you certainly have enough protons and neutrons, enough energy, and a high enough density to make heavier elements. But there’s a problem. For every six nucleons (protons and neutrons) that you have, you have ten billion photons. So even though you can fuse a proton and a neutron into deuterium — the first step in our staircase towards the heavier elements — a whole slew of photons will run into your newly formed deuterium before anything else can happen, effectively blasting it apart immediately.
If we want to make anything that lasts, we need the Universe to cool down so that these photons won’t have enough energy to blast our nuclei apart. The protons will be fine; they’re patient, and they live forever, so they can wait for the Universe to cool. But the neutrons? They’re not so patient.
You’ve all heard of radioactive decay before. Sometimes, you get a collection of protons and neutrons together in a way that’s unstable, and they radiate some energy away, becoming a different atomic nucleus in the process. Believe it or not, free neutrons are radioactive, too! If you leave a neutron on its own, after about ten minutes (its half-life), it will decay into a proton, an electron, and an anti-neutrino. So for every second that we wait for the Universe to cool down, so that these photons will stop blasting our nuclei apart, we lose neutrons, and replace them with protons and electrons.
If the neutrons have to wait too long for the Universe to cool, then they’ll all decay, and our Universe will be 100% protons and electrons, with no neutrons at all. But our Universe is an impatient place, and when it expands, it cools fast. Every time the Universe doubles in size, the temperature halves.
But what’s really been changing quickly? The density of the Universe. Every time the Universe doubles in size, the matter density drops by a factor of eight, and the energy density (because things are also cooling) drops by a factor of sixteen. So by time the Universe is a little more than three minutes old, you can form deuterium without it being blasted apart! And once you make deuterium, everything else goes super quickly, bringing you to Helium-3, and then all the way up to Helium-4.
So, if you work out the math, theoretically, you find that this story should get you a Universe that was about 76% Hydrogen and 24% Helium-4, with trace amounts of Deuterium, Helium-3, and Lithium-7 from the Big Bang.
So what do you do, if you’re an astronomer? You go and look for regions of space that haven’t been touched by star formation since the big bang, and you try to measure how much of each element you have!
This is the earliest test we’re able to perform on the Big Bang Theory: you know what elements you ought to get from the Big Bang, but what do you actually see when you go out and measure these elements? What do you find for Hydrogen, Helium-4, Deuterium, Helium-3, and Lithium-7? Let’s have a look.
Amazingly, we measure almost exactly what we predict! This was a huge confirmation of the Big Bang Theory, and is still one of the best tools we have today!
So the Universe manages to save the neutrons by locking them up inside of — mostly — Helium nuclei, which are stable! We’ve been telling you a story that’s been almost entirely theoretical, because we haven’t been able to observe anything this early, until now. So our first chance to test whether this “Greatest Story Ever Told” matches reality or not is a smashing success! Not only that, but our Universe is only three minutes old at this point! What comes next? Come back for part eight…