I've noticed what seems to be a fine-tuning issue. To be more specific, electric neutrality implies that the amount of lepton asymmetry produced through some unknown processes precisely cancels out the baryon asymmetry produced by some unknown processes. This precise cancellation appears to me as a fine-tuning issue. Perhaps this is a basic question, but I'm convinced there must be an explanation grounded in particle physics that doesn't rely on the fine-tuning explanation. That is, the mechanism producing matter-antimatter asymmetry must keep our universe electrically neutral, which doesn't appear trivial to me. I'm not sure whether mechanisms proposed so far can all explain this. I think this condition should be a supplement to the Sakharov conditions for successful theory of baryogenesis.
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$\begingroup$ a description of the problem here home.cern/science/physics/matter-antimatter-asymmetry-problem . seems it is a basic problem studied in Cern experiments, still open. $\endgroup$– anna vCommented Jan 6 at 17:20
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1$\begingroup$ @annav Thanks. But the link only contains generic description without mentioning the electrical neutrality problem. I'm looking for a general argument for why the mechanism producing matter-antimatter asymmetry keeps our universe electrically neutral. $\endgroup$– BababelumaCommented Jan 6 at 17:31
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$\begingroup$ I don't have a complete answer but here's an outline. We expect that the Sakrov conditions are roughly correct for explaining why there are Baryons and Leptons despite baryon/lepton number conservation (en.wikipedia.org/wiki/Baryogenesis). Because of CP violation, at the nonperturbative level these things are no longer conserved, but we haven't found sufficient CP violation yet in the standard model. In the same theory, because of anomaly cancellation the same effect should not occur for charge, which is more perfectly conserved (en.wikipedia.org/wiki/Anomaly_(physics)). $\endgroup$– AXensenCommented Jan 7 at 1:40
2 Answers
There's no fine-tuning issue here because there is an exact symmetry associated with electric charge conservation. At a basic level you can think about, for example, Maxwell's equations resulting in the 'continuity equation'. But because the electromagnetic gauge symmetry is unbroken, such a fact must extend to our full formulation of the Standard Model.
Baryon plus lepton number symmetry is already violated in the Standard Model by sphalerons, as mentioned in another answer. Baryon minus lepton number symmetry may or may not be violated by short-distance physics. But whichever of these dynamics you make use of in some early baryogenesis mechanism, electric charge will be conserved because the electromagnetic symmetry remains unbroken.
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$\begingroup$ Thanks, charge conservation as you mentioned is related to the U(1) gauge symmetry. I haven't yet understood how such a symmetry manifest itself before EW phase transition, which is the period where matter-antimatter is supposedly produced. Maybe I'll understand it after my advanced QFT class. $\endgroup$ Commented Jan 7 at 9:21
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$\begingroup$ Before the electroweak phase transition, the electric charge is to be understood as a linear combination of $SU(2)_L$ and $U(1)_Y$ charges, roughly $Q = (T^3_L + Y)/2$. This is still conserved. $\endgroup$– SethKCommented Jan 7 at 10:56
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I believe you can find some good leads by reading the references in article below. The most substantive is that the standard model makes explicit assumptions about charge conservation and the conservation of the difference between baryon and lepton numbers. This serves to strongly tie baryogenesis to leptogenesis. This may be something you are already aware of. Since sphalerons allow conversion of leptons to baryons, or rather between respective particles and antiparticles, while preserving qauntities, then you do have what is argubly a contrivance to ensure charge conservation is ensured, thus overall neutral charge. While this may be justified for a variety of reasons, symmetry being the most obvious, I suspect if you start going down these lines there is some body of work that relies heavily on non-pertabative and non-linear analysis.
