Baryogenesis

Baryogenesis refers to the set of physical processes that produced the matter–antimatter asymmetry of the universe. Observations show that the universe today contains far more matter than antimatter, a fact quantified by the baryon-to-photon ratio eta ≈ 6×10−10, inferred from the cosmic microwave background and primordial element abundances.

The problem is framed by the Sakharov conditions: for a net baryon number to develop from an almost symmetric early universe, three requirements must be met: baryon number violation, C and CP violation, and departure from thermal equilibrium.

Several theoretical frameworks have been proposed. In electroweak baryogenesis, the asymmetry is generated during the electroweak phase transition; this mechanism typically requires new physics to render the transition strongly first order and to supply sufficient CP violation. Leptogenesis generates a lepton asymmetry through CP-violating decays of heavy neutrinos; sphaleron processes then partially convert this lepton asymmetry into a baryon asymmetry. Grand unified theories offered an early view, in which heavy gauge or scalar bosons decay in a B-violating way. The Affleck-Dine mechanism uses scalar fields carrying baryon number to generate large asymmetries after inflation. Spontaneous baryogenesis invokes a time-dependent field that biases baryon-number-violating processes.

In the Standard Model, baryon-number violation occurs nonperturbatively through sphalerons, and CP violation from the CKM matrix is too small to explain the observed asymmetry. Moreover, the electroweak transition in the Standard Model is a crossover, not a strong first-order transition, motivating new physics beyond the Standard Model.

Ongoing tests include searches for CP violation in the lepton sector, proton decay experiments, and probes of early-universe phase transitions through gravitational waves or precise cosmological measurements.