Hi, what are the most popular theories for the antimatter asymmetry that explains why matter dominates the universe and antimatter does not?
The most popular theory is almost certainly God made it that way.
As far as I recall there are three physics notions (I won’t call them theories) which I guess might be subscribed to in the following order of likelihood:
Weak nuclear force interactions can violate combined charge-parity symmetry. This somehow leads to more production of matter than antimatter.
If fundamental particles have an electric dipole this wold violate parity and might allow for antimatter to decay preferentially to matter.
There really isn’t a difference. There are antimatter dominated regions, but they are far separated from matter-dominated regions. (I believe current observations indicate the antimatter regions would have to be outside the observable universe.)
I don’t think anyone has really proposed a generally accepted specific mechanism for 1 or 2.
(IMfez: Since I’m not sure your background, I’m shooting somewhat blindly at the technical level here. I’m happy to clarify anything as needed.)
Theories come in two main types: (1) There is matching antimatter somewhere, but it’s very far away, possibly due to unknown physics; (2) The particles that were around in the early universe violated appropriate symmetries that led to the imbalance. In terms of popularity, theories of the second type win out and are far more numerous.
To generate an asymmetry that survives to today, the theory needs to have a handful of properties. Each of those properties can be introduced in as many ways as there are theorists. Multiply that all up and you get a lot of workable models. Some highlights:
(1) A candidate theory needs to violate baryon number. That is, some decay or interaction or transition in the theory has to change the number of baryons. The Standard Model doesn’t allow this at low energies (i.e., today), but it does at the high energies/temperatures present in the early universe if you also allow lepton number to be violated (though there are issues). Separately, so-called “grand unified theories” (GUTs) that aim to unify the strong, weak, and electromagnetic forces into a single force tend to have baryon number violation. (If fact, experimental searches for proton decay [a manifestly baryon-number-violating process] are usually cast as limits on possible GUTs.) Another popular approach takes advantage of a technical feature of supersymmetric models that can imprint a net baryon number on the universe during cosmological inflation.
(2) A candidate theory also needs to violate certain discrete symmetries related to particles and antiparticles (technical terms: C and CP symmetries). Roughly, some decay or interaction or transition has to have different rates for particles and antiparticles. The Standard Model has this, but not at a high enough level. Supersymmetry augments the Standard Model in a way that makes it easy to add additional sources of CP symmetry violation. Certain GUTs do the same. In fact, it’s actually a mystery why the Standard Model has so little CP violation given it’s mathematical structure. A different but increasingly popular approach to the C/CP violation question is “leptogenesis”. Leptogenesis takes advantages of very heavy neutrinos (partners, of a sort, to the everyday neutrinos) to (usually) generate the everyday neutrinos’ masses and also inject CP violating processes into the early universe, when these heavy neutrino would be around in large numbers. Leptogenesis also needs a process that can convert lepton number violation into baryon number violation, which the Standard Model can do if certain timescales are right.
(3) A candidate theory also needs to have the relavant processes happening out of equilibrium. That is, the universe needs to be changing rapidly enough so that asymmetries don’t just continually wash themselves out. If the relevant processes involve very heavy particles, this can come “for free” simply because these particles will be doing their thing very early in the universe’s evolution. If the important actions happen at lower temperatures, you need an event to change circumstances in a fairly quick way. Some theories muck with cosmological evolution itself. A fairly popular approach extends the Standard Model to ensure that electroweak symmetry breaking (the process whereby the Higgs gets its vacuum expectation value) is a first-order phase transition (which here just means “fast”).
So, the question of how to generate the matter/antimatter imbalance of the universe isn’t really one question but rather three questions. And, there are lots of reasonable approaches to each question, and when you mix it all together you get a lot of possible theories. If I had to pick out some popular pieces, I’d say leptogenesis; SUSY extensions to the Standard Model; GUTs; and a fast electroweak transition. But keep in mind that each of these isn’t a full theory but rather a piece, embedded in a (hopefully) quantitatively viable theory that satisfies all three conditions above (the “Sakharov conditions”). Also, theories that aim to explain the matter/antimatter asymmetry using these or other pieces are often trying to solve other problems at the same time.