Ask Ethan: How did matter come to exist in our Universe? | by Ethan Siegel | Starts With A Bang! | Mar, 2024

In the very early Universe, there were tremendous numbers of quarks, leptons, anti-quarks, and anti-leptons of all species. After only a tiny fraction-of-a-second has elapsed since the hot Big Bang, most of these matter-antimatter pairs annihilate away, leaving a very tiny excess of matter over antimatter. How that excess came about is a puzzle known as baryogenesis, and is one of the greatest unsolved problems in modern physics. (Credit: E. Siegel/Beyond the Galaxy)

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.

The early Universe was full of matter and radiation, and was so hot and dense that it prevented all composite particles, like protons and neutrons from stably forming for the first fraction-of-a-second. There was only a quark-gluon plasma, as well as other particles (such as charged leptons, neutrinos, and other bosons) zipping around at nearly the speed of light. This primordial soup consisted of particles, antiparticles, and radiation: a highly symmetric state. (Credit: Models and Data Analysis Initiative/Duke University)

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…

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