“Think binary. When matter meets antimatter, both vanish, into pure energy. But both existed; I mean, there was a condition we’ll call ‘existence.’ Think of one and minus one. Together they add up to zero, nothing, nada, niente, right? Picture them together, then picture them separating–peeling apart. … Now you have something, you have two somethings, where once you had nothing.” –John Updike
Looking out at our Universe, at the myriad of stars, galaxies, and, well, “stuff” in our Universe, it’s hard not to ask yourself where it all came from.
When we look out at the Universe, each point of light that’s out there, whether a planet, star, galaxy, cluster of galaxies or something even bigger, contains the entire history of the Universe as part of its story.
There’s a great cosmic spider-web of structure that’s traced out by the galaxies in the Universe, with each pixel of light representing the location of a single galaxy.
When we consider our Universe, sure, it’s full of dark matter and dark energy; that’s how you make the structure we observe today. Even if we allowed ourselves to modify the laws of General Relativity, there’s simply no other way to reproduce/recreate the Universe we have today.
Looking at the matchup between simulations and observations, the cosmic web of great clusters, filaments and empty voids fills the entire modern Universe.
How did they get there? It took the billions of years the Universe has been around, the irresistible force of gravity, and the runaway growth of structure in the expanding Universe to bring it all together.
The beautiful simulation below, by Ralf Kähler, scales out the expansion of the Universe so that we can visualize just how matter — both normal and dark — collapses over time into galaxies, filaments and clusters.
Simulation: Oliver Hahn and Tom Abel (KIPAC).
But it’s the normal matter — the protons, neutrons, and electrons — that produce the visible light we pick up with our telescopes. The stars and galaxies that we see are all, as best as we can tell, made out of normal matter. And yet, this, itself, is a puzzle.
Because the laws of physics don’t allow you to create or destroy matter without also creating or destroying an equal amount of antimatter!
At least, this is true experimentally and observationally. But it couldn’t always have been true, otherwise the Universe would have an equal amount of antimatter in it to the matter that’s present.
And it doesn’t. In fact, if there were an equal amount of antimatter created to the amount of matter we presently have in the Universe, the Universe would be so sparsely populated that there’d only be about one subatomic particle per cubic kilometer.
It would be less than one-billionth as dense as the Universe we have today.
So let’s go back to the very early stages of the Universe, when it was filled with a hot, dense plasma, with equal amounts of matter and antimatter, and see if we can’t make the Universe we have today.
Against the background of this hot, dense, fully ionized plasma, an equal quantity of particles and antiparticle flit back-and-forth. They collide with one another, annihilating, while other particles, like photons, interact with one another, producing equal amounts of matter and antimatter when they do.
If the Universe were of a constant size, a constant temperature, and all the particles and antiparticles in it were stable, it would be impossible to create more matter than antimatter, or vice versa. But in our Universe, none of those things are true!
The Universe is expanding and cooling, and what this means is that — when the temperature drops below a certain point — you can no longer create matter/antimatter pairs as quickly as you destroy them! Why’s that? Because E = mc2, and once the energy of your Universe drops below the mass necessary to create the particles/antiparticles you’re looking to make, the ones that already exist simply go away.
How do they go away? They annihilate away, as only matter and antimatter can. But as they do, it gets more and more difficult for the matter and antimatter particles to find one another. Because the Universe is expanding, the density is dropping, and these particles/antiparticles are disappearing, you reach a point where they can no longer find one another. This “leftover” stuff you get, after all the annihilation the Universe can muster, is called freeze-out.
Getting this “frozen out” stuff is a consequence of the Universe being out of thermal equilibrium. For example, for example, at some point, you’re going to be left with a Universe that contains a bunch of muons and anti-muons. Like most particles we know how to make, these are unstable, and will decay. For most particles/antiparticles, like muons/antimuons, this isn’t a big deal. Whatever the particle decays into, the antiparticle will decay into the anti-counterpart, giving you a net gain of nothing.
But some particles are fundamentally different from their antiparticles, and this difference can create more matter than antimatter in the Universe! Here’s how.
Let’s imagine the Universe is filled with a new kind of unstable particle, the positively-charged Q+, and its antiparticle, the negatively-charged Q–. Because of certain conservation laws, they have to have the same mass, the opposite charge, and the same total lifetime.
But they don’t have to be the same in every way. Let’s say the Q+ can decay into either a proton and a neutrino, or into an anti-neutron and a positron. That means the Q– must be allowed to decay into an anti-proton and an anti-neutrino, or into a neutron and an electron.
Although this looks weird, because you sometimes have matter decaying into antimatter and antimatter decaying into matter, there are three important things about this type of decay:
- it allows you to violate the conservation of baryon number. (That is, the number of protons + neutrons combined.)
- This is allowed by the standard model, so long as the number of baryons minus leptons is conserved, and
- it can, if things work out correctly, create more matter than antimatter.
In addition to being out-of-equilibrium, there’s one more thing that we need.
If the percentage of the Q+‘s that become protons and neutrinos is the same as the percentage of Q–‘s that become anti-protons and anti-neutrinos, this won’t help you at all. The protons and anti-protons will be equal in number, and you won’t create any more matter than antimatter.
Same deal with the anti-neutrons/positrons and the neutrons/electrons. But although it’s possible that these individual percentages are equal, it isn’t mandatory. The other possibility is — and this happens in nature — that particles will prefer one type of decay, while antiparticles will prefer a different type!
If this happens, then the Q+‘s would make more protons and neutrinos than the Q–‘s would make anti-protons and anti-neutrinos, while the Q–‘s would make more neutrons and electrons than the Q+‘s would make anti-neutrons and positrons.
Looking solely at the protons/neutrons/anti-protons/anti-neutrons that result from this decay, what would we wind up with?
More matter than anti-matter! In fact, so long as you fulfill these three famous criteria:
- Out-of-equilibrium conditions,
- Baryon-number-violating interactions, and
- C- and CP-violation (the differences in decays, above),
you not only can create more matter than antimatter (or vice versa), but an asymmetry is inevitable. And since something like this is required to create more matter than antimatter in the Universe, and that’s the Universe we have, this is why there’s something instead of nothing!