For every one billion particles of antimatter there were one billion and one particles of matter. And when the mutual annihilation was complete, one billionth remained – and that’s our present universe. -Albert Einstein
Welcome back to our series, The Greatest Story Ever Told, where we’re recounting the physical history of the Universe, from before the big bang up through the present day. We’re currently in a hot, dense, expanding Universe, filled with equal parts matter and antimatter, bathed in radiation, and it’s been only a tiny fraction of a microsecond for all of this to happen.
But the Universe we live in today isn’t equal parts matter and antimatter. In fact, every galaxy we observe in the Universe is made out of matter and not antimatter. The laws of nature that we’ve discovered are pretty symmetric between matter and antimatter, and we believe that the Universe started out with equal amounts of matter and antimatter. So how are we here? If there were equal amounts and the Universe was very dense, eventually nearly all of the matter and antimatter would find their antiparticles, and would annihilate, leaving a Universe that was practically empty except for radiation (photons).
Although there are many different ways to make slightly more matter than antimatter, they all have the following properties, known as Sakharov conditions:
- You need to be able to create or destroy baryons (protons, neutrons, etc.),
- You need particles and antiparticles to have slightly different properties from one another, and
- You need to be out of thermal equilibrium.
This is not hard. First off, if you’re in an expanding, cooling Universe, you’re always going to go out of thermal equilibrium, so that one’s a given. But what about the other two? How could this possibly happen, and still obey all the laws of physics we currently observe?
Let me lay out the simplest scenario for you of how to make more matter than antimatter, and if you want to know the word physicists use when we talk about this process, it’s called baryogenesis.
And it doesn’t take anything divine, either. I’m going to assume that we have electrons (charge -1), positrons (charge +1), and that protons and neutrons are made up of quarks.
A proton has two up quarks (charge +2/3 each) and one down quark (charge -1/3), while a neutron is made up of one up quark and two down quarks, while antiprotons and antineutrons are made up of two anti-up quarks (charge -2/3) and one anti-down (charge +1/3), and antineutrons are two anti-down and one anti-up. So if we want more matter than antimatter, we need to make more quarks than antiquarks, and more electrons than positrons.
How can we get this? Imagine a particle — I’ll call it X — that has a charge of +4/3, and can decay either into two up quarks or one positron and one anti-down quark. It also has an anti-particle — X* — that has a charge of -4/3, and can decay into two anti-up quarks or an electron and a down quark.
So our possibilities are:
- X –> up + up
- X –> positron + anti-down
- X* –> anti-up + anti-up
- X* –> electron + down
The early Universe is full of all the particles that can exist, including X’s (or things very much like them.) If the X goes into two ups 50% of the time and into a positron and anti-down 50% of the time, then we’ll get the Universe we want if the X* goes into two anti-ups 49.99997% of the time and into an electron and a down quark 50.00003% of the time.
Is this possible? Yes; it’s called CP-violation, and we’ve observed it in many different cases.
So even if everything starts out perfectly symmetric between matter and anti-matter, all you need is a slight difference between particles and anti-particles, consistent with what we observe, and you’ll be guaranteed to have a Universe with either more matter than anti-matter or more anti-matter than matter!
(And if it were the other way around, you’d never know, except you’d be made of anti-matter, and you’d likely be calling it matter!)
Once you’ve got this problem solved — making the matter in the Universe — you can get on to turning it from a hot, dense, expanding soup into the Universe we see today.
Come back for part 6, where we’ll take the next step on our journey!