Confused after reading the following article.
https://www.cbc.ca/radio/ideas/particle-astrophysics-studying-origin-of-universe-1.6733153
In: 17
There’s a fundamental problem physicists have been trying to solve for a long time: why is the universe made out of matter?
We know that antimatter exists, we know that matter and antimatter annihilate each other. What we don’t know is why the universe is made almost entirely of matter.
If matter and antimatter ‘came into existence’ in the same way at the same time, hypothetically there would have been an equal amount of each, they would have annihilated each other, and the universe would not exist as we know it.
This didn’t happen, therefore there must be something about matter and antimatter that we don’t understand. The article doesn’t go into detail, but the behavior of the neutrino is giving us clues as to why that imbalance exists.
As far as scientists can figure out, there is fundamentally no difference between matter and antimatter, other than the reversed charges. They have the same mass, have the same interactions with gravity and the other forces, they have the same charge strength (just reversed). There’s nothing really important about protons having *positive* charge and electrons having negative, other than they are opposites so they attract. There’s no particular reason that if you flipped the charges for every single particle in the universe that you would notice a difference. It’s all *exactly* the same.
And, anything that creates matter also creates an equal amount of antimatter – again, because they’re not special either way. If you smash particles together in the Large Hadron Collider and protons come out, an equal number of antiprotons will also come out with an equal amount of energy between them.
In the earliest fractions of fractions of seconds in the universe, everything was so hot and so dense that it was like the entire universe was like a collider colliding particles at all points everywhere all the time. It was all just energy smashing around. Whatever particles were created *should* have been an equal amount of matter and antimatter.
Why, then, is the universe made almost entirely of what we call normal matter? Where is all the antimatter? If the universe *had* created an equal amount of antimatter, there would be no matter of any sort because it would have all annihilated before the universe could expand and spread stuff apart. You might (very reasonably) wonder if maybe there were just big clumps of one or the other that, sure, annihilated at the boundaries when they touched, but then spread out and now the universe just has big patches of antimatter. In fact, if some distant galaxies were made entirely out of antimatter there would be no difference to see in our telescope…*except*…
Space isn’t *really* empty. There is dust and gas everywhere, even though it’s very very sparse. If huge regions of the universe were made out of antimatter, that dust and gas would inevitably mix and annihilate and give off high-energy light rays, and we would see that. But we *don’t*.
What that all means is that for *some reason*, during the earliest moments of the universe, there was *slightly* more matter than antimatter created and after all the antimatter combined and annihilated with all the normal matter that it could, there was just a teeny tiny bit of regular matter left over. That is all the matter that we see in the universe. That must mean that there is *something* different about normal matter, some property that we haven’t figured out yet, where one or more forces interacts with matter just slightly differently.
Neutrinos are…weird. It’s probably the case that they’re massless particles, but they come in different “flavors” and it’s possible that they spontaneously change “flavor”. One of the flavors may have mass. If so, and if they *do* change, then where does their mass come from? And if neutrinos are their own antiparticle – their own version of antimatter – then as all the neutrinos were annihilating themselves it could explain why the universe ended up with more matter than antimatter.
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I’m not sure there is an ELI5 answer, but [this article](https://www.quantamagazine.org/neutrino-experiment-intensifies-effort-to-explain-matter-antimatter-asymmetry-20131015/) gives the following explanation:
> A fraction of a second after the Big Bang, those primordial heavy neutrinos would have undergone a process known as leptogenesis: Calculations show they would have decayed asymmetrically, generating slightly fewer leptons (electrons, muons and tau particles) than antileptons. By a conventional Standard Model process, the antilepton excess would then have cascaded into a one-part-per-billion excess of baryons (protons and neutrons) over antibaryons. “The baryons and antibaryons annihilated each other, and then the tiny imbalance left over is the matter we have today,” Caldwell said.
>
> “If the neutrino is its own antiparticle, then the so-called leptogensis mechanism to explain the matter-antimatter asymmetry will be very plausible,” Schwingenheuer said.
Basically, there is a complicated decay chain that could result in an excess of matter, but only if you’re allowed to treat neutrinos and antineutrinos the same. If one stage needs a neutrino as an input, and another stage produces an antineutrino as an output, you can chain them together only if an antineutrino is the same as a neutrino, i.e., if a neutrino is its own antiparticle.
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