The observed dominance of matter over antimatter requires physical processes that produced a net baryon number in the early Universe. Andrei Sakharov at the Lebedev Physical Institute identified three necessary conditions now known as the Sakharov conditions: baryon number violation, C and CP violation, and departure from thermal equilibrium. These principles frame modern proposals for how the asymmetry arose.
Mechanisms
One broad class is baryogenesis at the electroweak scale, where non-perturbative processes called sphalerons in the Standard Model can violate baryon number. Gerard 't Hooft at Utrecht University showed that electroweak interactions can change baryon and lepton numbers through anomaly-related processes. Electroweak baryogenesis requires sufficiently strong first-order phase transitions and extra sources of CP violation beyond the Kobayashi and Maskawa mechanism proposed by Makoto Kobayashi and Toshihide Maskawa at Kyoto University. Experimental measurements of CP violation in quark systems confirm the Kobayashi-Maskawa picture but quantitatively fall short of producing the observed cosmic asymmetry.
A second major proposal is leptogenesis, introduced by Masataka Fukugita and Tsutomu Yanagida at the University of Tokyo. Heavy right-handed neutrinos decay out of equilibrium in the early Universe, generating a net lepton number through CP-violating decays. Electroweak sphalerons then partially convert that lepton asymmetry into a baryon asymmetry. Leptogenesis ties the asymmetry to neutrino mass generation and the seesaw mechanism, connecting cosmology to laboratory neutrino experiments.
Other frameworks include baryogenesis via grand unified theories, Affleck-Dine mechanisms in supersymmetric theories, and scenarios invoking new particles or interactions during inflation or reheating. Each relies on some combination of the Sakharov conditions and typically requires physics beyond the minimal Standard Model.
Relevance and consequences
The matter-antimatter asymmetry is essential: without it, protons and electrons would annihilate and the Universe could not form galaxies, planets, or life, making baryogenesis a central question for cosmology and fundamental physics. Observational constraints from the cosmic microwave background and primordial element abundances restrict viable models, while terrestrial experiments at accelerators and neutrino facilities probe required CP violation and new particles. Cultural and territorial aspects matter too: major experimental efforts are international—experiments in Europe, Japan, and North America collaborate to measure CP violation and neutrino properties—so progress depends on global scientific infrastructure. Resolving the asymmetry will likely require combining cosmological observations with particle physics discoveries to reveal which of the proposed mechanisms, if any, produced the matter-dominated Universe we observe today.