You can only create or destroy matter by creating or destroying equal amounts of antimatter. So how did we become a matter-rich Universe?
Everywhere we look in the Universe, we find structure of some sort: planets, stars, gas, dust, plasma, and galaxies galore. Every indication we have is that each one of those structures we see is overwhelmingly composed of normal matter, like quarks and leptons, with only trace amounts of antimatter, made of things like antiquarks and antileptons. And yet, when we perform our particle experiments here on Earth, at low energies and at high energies, using particles from colliders as well as particles from cosmic rays, we always find the same thing: that our reactions, while they can create and destroy matter, can only do so at the expense of creating and destroying an equivalent amount of antimatter.
So how, then, did all the matter, and not an equal amount of antimatter, come to exist in our Universe? That’s the topic of this week’s Ask Ethan question thanks to Mateen Khan, who writes in wanting to know:
“Can you help me understand, how matter came to existence from almost nothing (at the moment of big bang) to it’s vast abundance everywhere in the universe and still growing. I am puzzled with the fact that matter is not a living organism so how come it has multiplied in a sense to keep growing?”
Although it isn’t quite the case that the matter abundance is growing or multiplying with time, it is true that our Universe must have generated a matter-antimatter asymmetry, somehow, in the distant past. Here’s what science knows about that puzzle as of today.
To start thinking about how matter came into existence, you have to understand that “almost nothing” is not a good description of what the Universe was like at the start of the hot Big Bang. In fact, it was likely just the opposite, as “almost everything,” at least in terms of what is allowed by the incredibly high energies and temperatures present at the start of the hot Big Bang, is…
