Antimatter-matter asymmetry, Law of charge

[Sciencelad2798]

TL;DR Summary

When matter was created at a slightly larger volume than anti matter, why did the universe stay net neutral?

I'm new to Astrophysics and I'm just now learning about Antimatter-Matter asymmetry. I understand some of the theories involving how this imbalance couldve occurred, but I'm confused on why the net charge of the universe stayed at zero. It seems to me like if the antimatter particle changed to a matter particle, it would create a slight imbalance in terms of charge. Or at least that statistically speaking, one particle, for example a proton, would be made at a greater volume than the electron might. Of course those are interchangeably but it seems impossibly unlikely that it would be exactly zero, and I don't think law of conversion of change applies to this. I could very well be wrong but I was hoping there were theoretical explanations for this.

 

[Orodruin]

It is not that a number of positrons randomly change to being electrons or anything similar. The processes still follow certain rules (exactly which depend on the theory).

 

[mathman]

The discrepancy seems to based on slightly different decay mechanisms. Charge is not affected.

 

[Sciencelad2798]

The discrepancy seems to based on slightly different decay mechanisms. Charge is not affected.

Ok that makes sense. Is there any explanation for why the ratio of protons and electrons seems to be near identical though? I may be misinterpreting but that seems unrelated to your point about decay

 

[mathman]

Universe started out with neutral charge,

 

[Sciencelad2798]

Universe started out with neutral charge,

Yes but I'm still confused on the whole process of it. Like how do electrons know when there's an excess of protons, and vice versa?

 

[Orodruin]

They don’t need to know. Charge is conserved.

 

[Drakkith]

Yes but I'm still confused on the whole process of it. Like how do electrons know when there's an excess of protons, and vice versa?

Assuming the universe started with a neutral charge, then charge conservation ensures that it can never develop a charge imbalance. For every negative charge created, a positive charge will also be created, and vice-versa. The same is true when charges are destroyed. No particle has to know anything at all since charge conservation is 'built in' to the creation and destruction rules.

 

[ohwilleke]

The discrepancy seems to based on slightly different decay mechanisms. Charge is not affected.

I think it would be more accurate to say that we do not know the cause of the matter-antimatter asymmetry of the universe.

Electromagnetic charge is always conserved. But there are charge parity (CP) violating processes that can, together with an extremely high energy theoretical process known as a sphaleron process, change the matter-antimatter asymmetry of the universe.

There are only three processes in the Standard Model relevant to matter-antimatter imbalance.

One involves quarks being transformed into different kinds of quarks in weak force mediated interactions (via W+ and W- bosons quantified by something known as the CKM matrix) which is slightly CP violating.

One involves neutrino oscillations (quantified by something known as the PMNS matrix). This is probably massively CP violating but the exact extent that this is so has only been measured extremely crudely.

The third involves a theoretically possible extremely high energy process called a sphaleron process, which is understood mathematically but has never been observed experimentally because none of our experiments (even the Large Hadron Collider) are powerful enough to make this occur.

General relativity does not have a CP violating process on its face, but it is possible that for some unknown reason antimatter has disproportionately ended up inside black holes where it cannot be observed, while ordinary matter has disproportionately ended up outside black holes.

These known Standard Model and General Relativity processes alone are widely acknowledged to be insufficient to lead to the currently observed matter-antimatter imbalance (among quarks and charged leptons at least), if the initial state of the universe was one in which there were equal amounts of matter and antimatter, or if the initial state of the universe was pure energy.

The two CP violating Standard Model processes are too small in magnitude and rare (even extrapolating them to high energies according to the rule for doing that that applies at energy scales we can measure). The length of time when the universe would have had a temperature high enough to cause sphaleron processes to take place (conventionally believed to be a mere fraction of a single second), meanwhile, would have been too short to produce the matter-antimatter asymmetry that we observe, even when combined with the two CP violating Standard Model processes.

Black holes, meanwhile, make up only a tiny fraction of all of the matter in the known universe, so they couldn't explain the matter-antimatter asymmetry even if they where made 100% out of antimatter, and furthermore, we have observed many black holes form out of predominantly ordinary matter, so we know that the source of black holes can't be anything close to 100% antimatter.

The cause for the matter-antimatter imbalance in the universe must be, therefore, either New Physics at high energies, or an initial state in which there was an excess of matter over antimatter, or some other explanation that is not yet widely recognized.

A Somewhat More Technical Analysis In Terms Of Baryon Number And Lepton Number

The matter-antimatter balance of the universe can be described with two quantities, the "baryon number of the universe" and the "lepton number of the universe". If B+L is positive then there are more matter than antimatter particles in the universe. If B+L is negative then there are more antimatter than matter particles in the universe.

Each quark has a baryon number of 1/3. Each anti-quark has a baryon number of -1/3. Each electron, muon, tau lepton and neutrino in the universe (muons and tau leptons are heavy versions of electrons) has a lepton number of 1. Each positron (i.e. anti-electron), anti-muon, anti-tau lepton, and anti-neutrino has a lepton number of -1.

The baryon number (B) of the universe is approximately the total number of protons and neutrons minus the total number of antiprotons and antineutrons in the universe. The lepton number of the universe is the total number of electrons, muons, tau leptons and neutrinos in the universe minus the number of positrons, anti-muons, anti-tau leptons, and anti-neutrinos in the universe.

If the number of protons and neutrons were equal to the number of antiprotons and antineutrons in the universe, we would have B=0, but in reality, B is a large positive number.

Likewise, if the number of leptons and antileptons in the universe were equal, we would have L=0. The charged leptons (electrons, positrons, muons, anti-muons, tau leptons, and anti-tau leptons) combined have a positive aggregate L number equal almost exactly to the number of protons in the universe.

In processes that we can observe, B and L are conserved separately. Every process has the same B number and the same L number in the starting start that it does in the finishing state. But, in the Standard Model, in extremely high temperature "sphaleron" processes, which we have not yet observed, in theory, only B-L is conserved.

We don't know what the relative proportions of neutrinos and antineutrinos in the universe are with sufficient precision to evaluate this matter-antimatter imbalance meaningfully in neutrinos. It appears to be roughly 50-50, but the answers don't get any more clear than that, although there are slight hints that there may be more anti-neutrinos than neutrinos in the universe.

The total value of B+L and B-L depends almost completely on the relative number of neutrinos and antineutrinos, because there are vastly more of them in the universe than there are quarks or charged leptons.

One possibility that would be particularly interesting is that the number of antineutrinos in the universe slightly exceeds the number of neutrinos in the universe, by exactly the sum of (1) the "baryon number of the universe" (B) and (2) the number of electrons (and heavy electrons called muons and tau leptons) minus the number of positrons (and heavy positrons called anti-muons and anti-tau leptons) in the universe (which is almost exactly equal to the number of protons in the universe).

If this were true, B-L for the universe as a whole would equal zero, which would be a nice fit to the fact that the Standard Model conserves B-L in all circumstances. But we aren't very close to knowing if that is true or not.