Baryon asymmetry

Baryon asymmetry is the observed excess of baryons over antibaryons in the universe. This imbalance accounts for the matter that makes up stars, planets, and ordinary material.

Sakharov conditions: Three necessary conditions for generating a baryon asymmetry from an initially symmetric state: baryon-number violation, C and CP violation, and departure from thermal equilibrium. They specify the ingredients any successful dynamical explanation of the asymmetry must provide.
Baryogenesis: The set of physical processes proposed to create a net excess of baryons over antibaryons in the universe. Baryon asymmetry is the outcome that baryogenesis models seek to produce.
Baryon-to-photon ratio: The ratio of the number density of baryons to that of photons in the universe. It quantifies the small surviving baryon excess relative to the cosmic radiation bath.
Standard Model: The quantum field theory describing known elementary particles and three of the four fundamental interactions. Its known CP violation and baryon-number processes do not appear sufficient to explain the observed excess.
Baryon number: A conserved quantum number in many particle interactions, assigned as +1 to baryons and −1 to antibaryons. Creating a net excess requires processes that change the universe’s total baryon number.
Electroweak baryogenesis: A proposed origin of the baryon asymmetry in which CP-violating interactions and a first-order electroweak phase transition generate a baryon excess. This scenario would create the asymmetry during the electroweak transition.
Cosmic microwave background: The relic radiation released when the early universe became transparent, now observed as a nearly uniform microwave glow. Its measured temperature fluctuations constrain the universe’s baryon density.
Neutrino: A neutral elementary particle that interacts through the weak force and gravity and comes in three known flavors. Neutrino properties, including whether neutrinos are their own antiparticles, may illuminate leptogenesis.
CP violation: A failure of physical laws to remain unchanged when particles are replaced by antiparticles and spatial coordinates are inverted. Matter–antimatter differences can help bias processes toward producing more baryons.
Grand unified theory baryogenesis: A proposed origin of baryon asymmetry through baryon-number-violating decays of heavy particles in grand unified theories. It places asymmetry generation at energies far above the electroweak scale.

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