[albertzeyer]

For the matter-antimatter ratio, you would not expect 50:50, or would you? Because 50:50 would be a highly unstable system? In any case, you would expect that unstable states would be highly unlikely, and it would converge into a stable state.

I'm not sure about the other examples. But maybe it's a similar reason that it is not a 50:50 ratio?

[hhjinks]

A 50/50 matter/anti-matter system could still house stable local pockets of mostly matter or anti-matter. The problem is, from what I understand, that the universe seems to have sprung into existence with way more matter than anti-matter, and we don't know why.

[nabakin]

Could it be that the observable universe is one of these stable local pockets and the antimatter to balance it out is simply not observable to us?

[ben_w]

Mathematically possible.

If you flip 2n fair coins, you expect n+δ heads and n-δ tails, where δ is (IIRC) sqrt(n/2). Going much away from that becomes infintessimally unlikely.

Probability is a subject famously easy to get wrong, so be careful with what I'm about to suggest: I *think* you could argue that in the moment prior to the inflation epoch spreading everything out just enough that pair production stops*, any given particle in our horizon is a coin toss of matter or antimatter.

Number of observed atoms in the universe is about 6e79 (http://www.wolframalpha.com/input/?i=how%20many%20atoms%20in...), so 6e79 = sqrt(n/2) -> n = 7.2e159 due to protons, and the same again for electrons; as we don't see significant signs of antimatter, any around must have annihilated a long time ago, so in this scenario we should expect to see ~7e159 (red-shifted) photons from the supermajority of particles which have annihilated.

It's outside my field to know how that compares to cosmologist's observations.

* won't that be at different times for protons/neutrons and electrons?

I can't get good answers on the expectations for either "why are protons and electrons counts the same" or "what is the observable consequence if they're not?"

[SkyBelow]

Potentially, but it seems like an untestable hypothesis by itself.

[BlarfMcFlarf]

Not a physicist, but here is my understanding of the cosmology physics:

High energy can spontaneously form matter antimatter pairs. In the early universe, the heat of the universe was very high, so this was common, constantly happening.

The problem as always if fine tuning. If the early universe was 60-40, that would be understandable. If the early universe was precisely 50-50, that’s fine too. But the universe was 50.0001-49.9999 or something like that, and then all annihilated. It’s too big a difference to easily be random chance, and too small a difference to be easily explained by a starting condition what wasn’t precisely tuned by some mechanism.

[raxxorraxor]

Not a physicist either but pair production also occurs in "non extreme" conditions and is still quite common.

If find this question fascinating. Matter can only ever exist with respective anti-matter. Question is where has all the antimatter gone? Are there processes were it does indeed behave different from matter? So where is it? Since a photon and antiphoton are the same and do not absorb each other, we should be able to see it, shouldn't we?

I still want to believe in the antimatter universe where there is some evil twin of mine.

[btilly]

In all known physical processes, the baryon number is conserved. Particles with a positive baryon number are the heavy particles in matter. Think protons, neutrons, and so on. Particles with a negative number are antimatter. Think antiprotons, and antineutrons. And particles with a 0 baryon number are not made of quarks. Think leptons like electrons and neutrinos, or bosons like photons and the Higgs boson.

This means that all known ways to create or destroy matter, also creates or destroys an equal amount of antimatter.

It turns out that most attempts to extend the Standard Model allow violations of baryon conservation. This could explain the dominance of matter in our universe. However none of those attempts have been able to make any predictions that matched experiment. And so it remains true that all known physical processes perfectly conserve the baryon number.

(It is also possible that baryon number really is conserved, and dark matter is actually dark antimatter. But we lack a theory of what dark matter could be that predicts this.)

[adrian_b]

Even if in the strong, weak and electromagnetic interactions the baryon number is conserved, there is the interesting fact that for the 8 particle set composed of the 3 kinds of u quarks, 3 kinds of d quarks, 1 electron and 1 neutrino, the sum of all kinds of quantities that are expected to be conserved, like electric charge, color charges and spin sum to zero. This set of 8 particles is also equivalent with the set of the components of one proton, one neutron, one electron and one neutrino.

This property of this set of 8 particles is analogous to the similar property of the set of 2 particles composed of a particle and its anti-particle, and to the similar property of the sets of 4 particles that can be involved in a weak interaction (the intermediate weak bosons convert one 4-particle interaction into a couple of 3-particle interactions, but when looking at the overall inputs and outputs, all the weak interactions are 4-particle interactions), which ensure the conservation of various quantities over such interactions.

This means that it is possible to conceive an additional kind of interaction, which unlike electromagnetic interactions between 2 particles and weak interactions between 4 particles, involves 8 particles, so it has a much smaller probability of occurring, i.e. it is a much weaker interaction than the weak interaction, and through which, when provided with enough energy, quarks + electrons + neutrinos could be generated simultaneously without generating anti-matter.

While there is no evidence yet for such an interaction, it is conceivable that at least during the circumstances of the Big Bang, such an interaction could have existed, so all the quarks and leptons could have been generated from some unknown bosons, just with enough initial energy and with conservation of all quantities for which there are solid reasons to believe that they must always be conserved, like energy, linear momentum, angular momentum, electric charge and color charges. (Unlike for the baryon number, for which there is no other reason to believe that it must be conserved, except that the strong, weak and electromagnetic interactions happen to have this behavior.)

[mystified5016]

You would expect a 50/50 ratio because when energy is converted into matter, it's typically in the form of matter/antimatter pairs.

There's nothing special about matter or antimatter. Same energy, just opposite charge. All else being equal, they should be created in equal amounts. As far as we're aware, there is no special property that would make the universe preferentially create more matter than antimatter.

There's also no requirement that the configuration of matter and antimatter be "stable" for whatever definition you want to apply. The only rule is that conserved quantities stay conserved.

[permo-w]

but I was under the impression that equal parts of matter and antimatter annihilate, which would make a 50:50 system remain as such, which is why its such a mystery?