# Spontaneous symmetry breaking and the photon/baryon ratio

1. ### nSlavingBlair

31
Hi,

I am looking into symmetry breaking and how it (may have) affected the photon/baryon ratio in the primordial universe. I found this wonderful encyclopaedia of cosmology which relates the grand unified theory to an orthorhombic crystal, making analogies for symmetry, spontaneous symmetry breaking, low energy physics and high energy physics.

While I feel I am getting somewhere with understanding symmetry breaking, I cannot work out how it is supposed to relate to the ratio of baryons to photons.

Is anyone able to give me a clue on this, or know of a good reference for it? Or even if there have been any studies into exactly this?

I keep getting a lot of information about the still recent Higgs boson experiments, and while they are related, I do not believe they are quite what I am looking for.

PS. I'm not sure if this belongs in the particle physics section, or the beyond the standard model section?

2. ### ChrisVer

2,403
The Higgs Boson which is supposedly detected is thought of as the Standard Model's Higgs Boson, so it does not contain any information about a GUT's Spontaneous Symmetry Breaking (SSB).
The SSB is a procedure which occurs when a bigger group G is somehow broken to some other subgroup. The best theory which describes that SSB is the Higgs (and a long line of names) mechanism, which introduces a scalar complex field and lets it acquire a non vanishing vacuum expectation value. That way symmetries are broken "naturally", so from a bigger gauge group you end up in a smaller one (if they are to explain everything, that most certainly is still true) $G_{big} \rightarrow G_{small} \times U(1)$

So I'd say that the SSB and its mechanism have nothing to do with the baryons/photons ratio. What most certainly plays a role is which bigger group there was before the breaking. Because for any bigger group, you could have "new" interactions and ratios (for example the proton decay of the supersymmetric $SU(5)$ -the minimalistic SU(5) remains just a game and has no physical significance since it gives us wrong proton decay lifetimes). So if for some period some interactions were allowed, things would have differently evolved from we know today...

3. ### Bill_K

4,157
I believe the symmetry involved in this case is Charge symmetry. The observed baryon/photon ratio is about 1:1 billion. Since in the very early universe we expect all particle abundances to be roughly equal, this indicates that all but one in a billion baryons that once existed have annihilated with antiparticles. Apparently an unknown C-breaking process created 1,000,000,001 baryons for every 1,000,000,000 antibaryons.

4. ### nSlavingBlair

31
Okay, so here is what I understand of this currently. Corrections/explanations to it would be very much appreciated. I'm going to start from the BB. I'm also going to say "stuff" a lot, as I am not aware of a word that encompasses matter and antimatter and whatever it was that was present at the very birth of the Universe.

1. For some unknown reason, the Universe appeared. According to our models it was infinitely dense, hot, and small; a singularity. This could be a breakdown of our models, or not. No one knows, but plenty of people have ideas. There was also no distinguishing between one type of stuff and another, or even distinction between interactions. It was completely invariant.
2. The Universe began to expand (inflation). This caused a cooling as stuff couldn't interact with other stuff as easily.
3. Somewhere between 10^-35s and 10^-10s matter started to appear. The previous invariance broke and one thing was distinguishable from another thing. Electrons, neutrinos and quarks got "frozen out", along with their antiparticle versions and equivalent interactions.
4. These anti/particles oscillated between matter and antimatter a lot before decaying. For some unknown reason, they preferred decaying as matter (well, 1 billionth of a preference).
5. Most of the matter and antimatter annihilated producing photons, but that 1 in 1billion particle that had a preference for matter, instead of going for a prefect 50/50, could not annihilate with anything. Hence there is now a 1 baryon:1 billion photons ratio.

I know I am missing a lot, and this is probably in a weird order that's not quite right.. I have not mentioned CP invariance being violated, as I'm not quite sure how to fit that in yet.

Also, there are some cool experiments being done to try to work this out.. One did my head in, it's called GERDA. It looks at Germanium-76 which has a lower limit on its half life at 34 trillion trillion years, and is supposed to be able to decay into Selenium-76 through neutrinoless double beta decay, which basically is 2 decay processes that occur simultaneously where one produces a neutrino and the other uses an antineutrino, and this can only work if the neutrino is its own antiparticle. But what did my head in was their proposed timeline for finding this; 3 years!!! They need 40kg of this stuff and only 3 years to see a decay, if its half life is less than 100 trillion trillion years.. that's just nuts, and my brain does not want to comprehend this basically infinite quantity being measurable by 2 quantities I can comprehend.
Anyway, these particles that can do this (majorana particles), be their own antiparticle, come in 2 types; heavy and lightweight. The heavy ones can only exist in the early universe, and form slightly less leptons than antileptons. Then the excess of antileptons causes the excess in baryons we see today.. but I don't understand how an antilepton is supposed to produce baryons? shouldn't leptons produce baryons, as they're both matter, not one matter and the other antimatter?

Thank you so much for your help already. I am very grateful :)

### Staff: Mentor

Okay, let's ignore this part as our understanding there is really limited.
Why do you think something cooled? During inflation, things stayed relatively similar - so similar that we don't know how long inflation happened (we just have a lower limit).
Not sure if you mean the right things here.
It doesn't have to be an oscillation, different decay probabilities work fine.
Yes.

They have a lot of atoms, so even small decay probabilities (like 3 atoms out of 100 trillion trillions in 3 years) lead to some decays.

Actually, seeing decays is not an issue, the double beta decay with two neutrinos is well-measured. The search for a neutrinoless decay (if it is possible - we don't know) is the interesting part.

The definitions of "matter" and "antimatter" is purely historical (where everything we see today was called "matter" and the other parts "antimatter"). It would be better to think of leptons as antimatter and antileptons as matter, if you want to study symmetries and so on.

6. ### nSlavingBlair

31
Thank you very much, that's fantastic! I'll re-write number 3 and see if it makes more sense.