Matter–antimatter asymmetry (baryogenesis)
Why does the observable universe consist almost entirely of matter with essentially no primordial antimatter, when the Big Bang should have produced matter and antimatter in equal amounts that would have mutually annihilated?
History and references
Still open in the literature as of 2026.
Andrei Sakharov identified in 1967 the three conditions necessary for a universe to end up with more matter than antimatter: baryon number violation, C and CP violation, and departure from thermal equilibrium. The Standard Model technically satisfies all three conditions, but the measured amount of CP violation (seen in kaon and B-meson decays) is far too small to account for the observed matter excess, meaning some additional, as-yet-undiscovered source of CP violation or baryon-number-violating physics (possibly involving neutrinos, via leptogenesis) must be at work. This remains one of the central unsolved problems connecting particle physics and cosmology.
You, this page, and every star in the sky exist only because of a staggeringly tiny imbalance — about one extra particle of matter for every billion matter-antimatter pairs — that physics still cannot fully explain.
Explanations that circulated
- That the known amount of CP violation in the Standard Model (from the CKM quark-mixing matrix) is sufficient to explain the observed matter-antimatter asymmetry — it is known to be many orders of magnitude too small, meaning additional, undiscovered sources of CP violation are required
References
- Violation of CP Invariance, C Asymmetry, and Baryon Asymmetry of the Universe, A. D. Sakharov, JETP Letters, 1967
Solutions 0
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