Poets say science takes away from the beauty of the stars — mere globs of gas atoms. I, too, can see the stars on a desert night, and feel them. But do I see less or more? -Richard Feynman
Ahh, the stars that make up our galaxy, and the galaxies that make up our Universe. It’s the simple power of the humble atom — the protons, neutrons, and electrons that are the building blocks of everything on our world — that powers all the stars and galaxies in the Universe.
But it didn’t need to be this way! Why is it that all the stars in our galaxy, and in every other galaxy we observe are powered by matter, and not by anti-matter?
Because we know what antimatter looks like from its telltale signature from when it runs into matter: they annihilate!
And the annihilation signature would be extremely different if there were entire galaxies of antimatter out there; there aren’t. And yet, on the other hand, whenever we run our powerful supercolliders, the only way we’ve found to make matter involves making an equal amount of antimatter, too.
So how do we do it? How do we wind up with a Universe full of matter and virtually devoid of antimatter?
- You need an interaction that violates Baryon number conservation, which means you need to be able to make more protons than antiprotons or something akin to that.
- You need to violate CP-symmetry, which means (in English) that you need particles and antiparticles to decay into their various products at different rates.
- You need to be out of thermal equilibrium.
Anything where — for example — the rate of expansion is faster than the rate of heat transfer. A supernova explosion (remnants shown above and below) does this, and immediately following the Big Bang, our entire Universe does this.
Small parts of our Universe are in thermal equilibrium, of course, but back when the Universe was young? Forget about it! For our Universe, being out of thermal equilibrium is easy.
It turns out that the first condition — not conserving baryon number — is easy too, so long as we’re at high enough energies.
These particles and antiparticles, above, are the ones that make up matter and antimatter in the Universe. The quarks (and antiquarks) make up protons, neutrons, and the other baryons (and antibaryons), and these comprise the totality of the understood matter and antimatter in the Universe.
While in our low-energy world, we don’t see this happening very often, even the boring old standard model lets us do this through well-known interactions. This happens very efficiently at high energies, which — coincidentally — is what the early Universe, right after the Big Bang, is known for!
So, the last thing that we need is for the unstable matter particles we make to decay differently from the unstable antimatter particles that we make. We initially thought this was impossible, and indeed it is, if you look at the earliest unstable particles we made. The neutron, the muon, and the particles made up of up and down (and anti-up and anti-down) quarks only do not do this.
But once we found the third quark — the strange quark — we found a very strange set of particles: the Kaons.
And they do! There is a fundamental difference between the decays of one type of kaon and the decays of its antiparticle. Let this happen in the Early Universe, and we should wind up with more matter than antimatter!
But, although this was discovered back in the 1960s, a huge problem remains: it isn’t enough! The physics from this predicts that we’d only wind up with one ten-billionth the amount of matter we have in our Universe today.
So we need for there to be a bigger asymmetry, one we hadn’t detected. But we always suspected that maybe one of the heavier quarks — the charm, bottom, or top — would have a big enough asymmetry to explain our Universe. And — thanks to the D0 team at Fermilab (see paper) — we may have just hit the jackpot.
While the asymmetries due to the strange quarks were a tiny fraction of a percent, these asymmetries they’re finally discovering from bottom quarks are huge!
Many orders of magnitude greater than the Kaons — and far greater than the standard model predicts — this discovery could be the missing piece of the puzzle to where the matter in the Universe comes from.
This is currently a 4-σ effect, almost good enough to be deemed an unequivocal discovery. While you should always be skeptical of initial reports of new discoveries — especially of discoveries that indicate new physics — this is a very well-motivated one.
The origin of the matter in our Universe — what physicists call the Baryon Asymmetry — is wikipedia’s #2 unsolved problem in cosmology, behind “Why does the Universe exist?” If we can answer this question correctly, this is the biggest news for cosmology since the discovery of Dark Energy in 1998. We know it’s going to take new physics to solve this problem, and this could be our first hint of it.
And now you’re among the first to know!