Sabtu, 14 Desember 2013

More on Matter vs. Antimatter

More on Matter vs. Antimatter

Yesterday, I wrote to you about part 5 of The Greatest Story Ever Told, about how the Universe came to have more matter than antimatter in it. And many of you correctly responded that I had given too much detail and not enough explanation.
So, I want to try again for all of you. Here’s the explanation, starting at the beginning.

The Universe inflated first, stretching it flat and making it uniform, both everywhere in space and in all directions equally. Then inflation ended, and all the energy that was making it inflate got dumped into particles and radiation. This part, when inflation ended, could be called the “classical” big bang.

The radiation and these particles were incredibly energetic, even compared to anything we’ve ever created in a laboratory. It was so energetic that there were no atoms, nuclei, or even protons or neutrons. What you need to do, if you want to understand what the Universe looked like at this time, is imagine the tiniest point particles and all of their antiparticles, flying around in great abundance, as close to the speed of light as the conservation of energy allows them. This means electrons, muons, taus, all six quarks, neutrinos, and all of their antiparticles in equal abundance, in addition to whatever else may be out there.

They’re also way more abundant at this time — by about a factor of a billion — than matter particles are today. But if this was all we had in the Universe — matter and antimatter in equal amounts — they would annihilate away (like matter and antimatter are wont to do), until there were so few particles left that they couldn’t find each other in this huge and expanding Universe.
But this didn’t happen; if it did, only one in every hundred billion particles that exist now would still be here, and half of them would be antiparticles. (This is down by a factor of about 1020 from the particles & antiparticles that existed at the end of inflation.) So something had to have happened that made the Universe choose matter over antimatter, and at the level of about one extra matter particle for every billion matter/antimatter particles out there, otherwise nearly everything would have annihilated away into photons (i.e., light) by now.

So what happened? Well, we know that there are a few constraints between matter and antimatter. First off, if you have an unstable matter particle, its corresponding antimatter particle is also unstable. Because of symmetries between matter and antimatter, they need to have the same total decay rate, which means they need the same lifetime and they need to be able to decay into the opposite, corresponding particles. And finally, whatever it is that they decay into, there is a conservation law telling us that the net number of baryons (protons & neutrons) must equal the net number of leptons (electrons and neutrinos) that you create.
This works out to be amazingly convenient, because we do have a Universe with equal numbers of protons and electrons! But, I wanted to tell you how this happened. There are many different ways, including (here come some scientific names) GUT baryogenesis, leptogenesis, electroweak baryogenesis, and the Affleck-Dine mechanism.
But let’s make up a way to do this that’s even simpler than any of these, just to show you how this is possible. Imagine that we have a new particle called Y, which is neutral and unstable, and its antiparticle, Y*. This is both necessary and reasonable; in fact, it seems unreasonable to imagine that we’ve discovered every single particle in the Universe, considering that there’s such a significant energy range left to explore.
We need to produce a lepton for every baryon we produce, and an anti-lepton for each anti-baryon we produce. We also have to conserve charge, and we need for the Y and the Y* to allow the same types of decays. The one way they’re allowed to be different is known as CP violation, which means that particles can decay through one decay route more frequently than their antiparticles do, and antiparticles can decay through an alternate route more frequently.
So let’s say the Universe is full of — among other things — Y’s and Y*’s. The Y’s can decay into either a proton (one baryon) and an electron (one lepton), or an anti-neutron (one anti-baryon) and one anti-neutrino (one anti-lepton). In both cases charge is conserved and we meet our conservation law about baryon number equaling lepton number. What do the Y*’s do? Well, our conservation laws tell us they must decay into either an anti-proton (one anti-baryon) and a positron (an anti-lepton) or into a neutron (one baryon) and a neutrino (one lepton).
But remember what CP-violation lets us do: it allows the Y’s to decay into protons and electrons more frequently than they decay into anti-neutrons and anti-neutrinos, and it allows the Y*’s to decay into neutrons and neutrinos more frequently than they decay into anti-protons and anti-electrons. It means that if we have a thousand Y’s and a thousand Y*’s, 501 Y’s can become protons and electrons while 499 become anti-neutrons and anti-neutrinos, while 501 Y*’s can become neutrons and neutrinos, while 499 become anti-protons and positrons. The protons and antiprotons, neutrons and anti-neutrons, and electrons and positrons will all find each other, annihilating away. But what will be left over? Two extra protons, two extra electrons, two extra neutrons, and a bunch of neutrinos and anti-neutrinos.
In other words, more matter than antimatter. I realize this is a difficult topic and I’m sorry I don’t have a simpler way to explain it, but that may be because I don’t have a simpler way that I understand it. So ask your questions here, and if there are enough good ones that are answerable in a reasonable length, I’ll take them on at the end of the week. In the meantime, I hope this clarification helps!


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