The universe is mostly made of matter, with very little antimatter. This imbalance puzzles scientists because the standard model of particle physics predicts equal amounts of both. Understanding this asymmetry is crucial for explaining the universe's structure and existence.
Three conditions, proposed by Andrei Sakharov, are needed to create this imbalance: baryon number violation, C and CP violation, and departure from thermal equilibrium. Scientists explore various mechanisms to explain how these conditions led to the matter-dominated universe we see today.
Matter-Antimatter Asymmetry in Cosmology
Matter-antimatter asymmetry in cosmology
- Observed imbalance between matter and antimatter in the universe
- Universe composed almost entirely of matter, with very little antimatter
- Cannot be explained by the standard model of particle physics
- Standard model predicts equal amounts of matter and antimatter created during Big Bang
- Understanding origin of asymmetry crucial for explaining existence of matter and structure of universe
Sakharov conditions for asymmetry
- Andrei Sakharov proposed three conditions for baryogenesis (generation of matter-antimatter asymmetry):
- Baryon number violation
- Processes that do not conserve baryon number must exist
- Allows creation of excess baryons over antibaryons
- C and CP violation
- Charge conjugation (C) and charge-parity (CP) symmetry must be violated
- Produces different rates for matter and antimatter processes
- Departure from thermal equilibrium
- Universe must have been out of thermal equilibrium at some point
- Prevents asymmetry from being erased by equilibrium processes
Mechanisms of early universe baryogenesis
- Electroweak baryogenesis
- Occurs during electroweak phase transition in early universe
- Requires first-order phase transition and sufficient CP violation in electroweak sector
- Leptogenesis
- Generates asymmetry in leptons, later converted into baryon asymmetry through sphaleron processes
- Relies on decay of heavy Majorana neutrinos that violate lepton number and CP symmetry
- Affleck-Dine mechanism
- Utilizes scalar fields (squarks, sleptons) in supersymmetric theories
- Scalar fields acquire large vacuum expectation values during inflation, then decay producing baryon asymmetry
- Grand Unified Theory (GUT) baryogenesis
- Occurs at very high energies ($10^{16}$ GeV) in context of grand unified theories
- Involves decay of heavy gauge bosons or Higgs bosons that violate baryon number and CP symmetry
CP violation in matter-antimatter imbalance
- CP violation necessary condition for baryogenesis (Sakharov conditions)
- Allows matter and antimatter processes to occur at different rates, leading to asymmetry
- Standard model CP violation observed in quark sector through complex phase in Cabibbo-Kobayashi-Maskawa (CKM) matrix
- Amount of CP violation in standard model insufficient to explain observed matter-antimatter asymmetry
- Theories beyond standard model (supersymmetry, grand unified theories) introduce new sources of CP violation
- New sources can potentially provide necessary amount of CP violation to generate observed asymmetry
- Experimental searches for CP violation in various systems (B mesons, neutrinos) help constrain and guide theories of baryogenesis