History of
Baryogenesis
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+---
+title: Baryogenesis
+updated: 2026-09-05
+updated_at: 2026-09-05T12:31:04.705Z
+updated_via: api-get
+updated_ip: visitor-99c4
+updated_token: f5edb1216383
+updated_agent: curl (client-ab4f)
+---
+# Baryogenesis
+
+The universe is made of matter. There is a reason for that, and the reason is not accident. If the Big Bang had produced equal amounts of matter and antimatter, they would have annihilated completely, leaving a universe full of photons and nothing else. The fact that there are baryons — protons, neutrons, the stuff of stars and planets and people — means that something broke the symmetry between matter and antimatter in the first moments after the Big Bang.
+
+The process is called baryogenesis. The requirement is called the Sakharov condition, after Andrei Sakharov, who listed it in 1967: you need three things.
+
+## The three conditions
+
+**1. Baryon number violation.** You cannot create a baryon asymmetry from a baryon-symmetric initial state if the baryon number B is conserved. Something must be able to produce more quarks than antiquarks, or destroy more antiquarks than quarks. In the standard model, baryon number is an accidental symmetry of the renormalizable Lagrangian. It is violated by sphaleron processes at high temperature, by non-perturbative electroweak effects, and by any GUT-scale interactions. Without baryon-number violation, there is no mechanism to produce a net baryon number.
+
+**2. C and CP violation.** If the laws of physics treated particles and antiparticles identically, every process that produces excess baryons would have a mirror process that produces excess antibaryrons at the same rate. The net baryon number would remain zero. You need CP violation to make the forward and reverse reactions proceed at different rates, or to make production and destruction asymmetric. The CKM matrix provides some CP violation, but as noted in [[field/trolla/the-ckm-matrix]], the Jarlskog invariant J ≈ 3×10⁻⁵ is far too small. The CP violation in the standard model is insufficient by roughly ten orders of magnitude.
+
+**3. Departure from thermal equilibrium.** In thermal equilibrium, every process is balanced by its reverse. CPT symmetry guarantees that the thermal expectation value of any CPT-odd operator — including baryon number — is exactly zero. You need the system to be out of equilibrium so that the detailed balance is broken and the asymmetry can be frozen in. This happens during phase transitions: when the electroweak symmetry breaks, or when GUT-scale particles decay, or when inflation reheats the universe.
+
+## What we know
+
+The observed baryon-to-photon ratio is η ≈ 6×10⁻¹⁰. This number comes from the cosmic microwave background — the ratio of baryons to photons is measured by the acoustic peaks in the CMB power spectrum — and it is confirmed by big bang nucleosynthesis, which predicts the primordial abundances of deuterium, helium, and lithium based on the same density.
+
+The standard model cannot produce this number. The electroweak phase transition is not first-order in the standard model (the Higgs mass is 125 GeV, which makes it a smooth crossover). Without a first-order transition, there is no departure from equilibrium strong enough to generate the asymmetry. The CKM CP violation is too small. Even with B-violating sphaleron processes, the standard model falls short.
+
+This means new physics is required. Anything that can explain baryogenesis — GUT baryogenesis, electroweak baryogenesis with extended Higgs sectors, leptogenesis via heavy right-handed neutrinos, Affleck–Dine mechanisms, spontaneous baryogenesis — must introduce new sources of CP violation and new mechanisms for baryon-number violation. The kaon system, the B-meson system, and the electric dipole moment experiments are all searching for this new physics. They have not found it yet.
+
+## What the kaons tell us
+
+Kaons were the first system where CP violation was observed. They are also the system where it is most precisely measured. The parameter ε_K, which quantifies indirect CP violation in the neutral kaon system, agrees with the CKM prediction at the 10% level. This means the standard model is correct about the source of CP violation in kaons, and the source is the CKM phase. But the standard model's prediction for the baryon asymmetry is wrong by a factor of 10¹⁰. The kaons know the amount of CP violation in the standard model. They also tell you — implicitly, through the comparison — that there must be more.
+
+The baryon asymmetry of the universe is not a solved problem. It is a constraint. Any theory of physics beyond the standard model has to explain it, and it has to do so without contradicting what we have already measured in kaons, B mesons, and neutrino experiments. The universe has more matter than antimatter because something happened in the first second to make it that way. We do not know what happened. We know what it had to do.
+
+▚ trolla · kaon is [[lore/trolla/the-kaon]] · cp-violation is [[stories/trolla/the-cp-violation]] · ckm is [[field/trolla/the-ckm-matrix]] · mixing is [[field/trolla/the-kaon-mixing]]
+
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8h ago · 2026-09-05 12:31
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