12.3 Matter-antimatter asymmetry
2 min read•july 22, 2024
The universe is mostly made of matter, with very little antimatter. This imbalance puzzles scientists because the predicts equal amounts of both. Understanding this asymmetry is crucial for explaining the universe's structure and existence.
Three conditions, proposed by , are needed to create this imbalance: baryon number violation, C and , 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
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- 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 (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
- Baryon number violation
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
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- 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 , then decay producing baryon asymmetry
- Grand Unified Theory (GUT) baryogenesis
- Occurs at very high energies ( 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 ()
- 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 (, 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
Key Terms to Review (18)
Andrei Sakharov: Andrei Sakharov was a prominent Soviet physicist, dissident, and human rights activist, best known for his work on the hydrogen bomb and his advocacy for civil liberties and nuclear disarmament. His contributions to cosmology and fundamental physics also include important insights into matter-antimatter asymmetry, a concept that explores why our universe is predominantly matter rather than antimatter.
B meson decay: B meson decay refers to the process in which a b meson (a type of meson containing a bottom quark) transforms into other particles through weak interactions. This decay process is critical in understanding the behavior of matter and antimatter, as it can lead to asymmetries that are key to exploring why the universe is composed mostly of matter rather than antimatter.
Baryogenesis: Baryogenesis refers to the theoretical processes that explain the imbalance between baryons (matter) and antibaryons (antimatter) in the universe. This phenomenon is crucial for understanding why our universe is predominantly composed of matter rather than an equal mixture of matter and antimatter. Baryogenesis is intricately linked to the early moments following the Big Bang, when conditions allowed for the creation of an excess of baryons over antibaryons, leading to the matter-antimatter asymmetry we observe today.
Baryonic Matter: Baryonic matter refers to the ordinary matter that makes up stars, planets, and living organisms, composed primarily of baryons, which are subatomic particles like protons and neutrons. This form of matter is crucial in understanding the structure and evolution of the universe, as it influences everything from cosmic microwave background radiation to the formation of galaxies and the potential fate of the universe.
Big bang nucleosynthesis: Big bang nucleosynthesis refers to the process that occurred during the first few minutes of the universe's existence, where temperatures and densities were high enough for nuclear reactions to produce the light elements, primarily hydrogen, helium, and trace amounts of lithium and beryllium. This process is crucial in understanding the early universe and its composition, providing insights into alternative theories, predictions of cosmic inflation, and the matter-antimatter asymmetry observed today.
Cosmic evolution: Cosmic evolution refers to the process by which the universe has changed and developed from its earliest moments to its current state. This includes the formation of fundamental particles, the synthesis of elements, the emergence of stars and galaxies, and the evolution of cosmic structures over billions of years. Understanding cosmic evolution is essential for grasping how the universe operates and how matter and energy have transformed through time.
Cp violation: CP violation refers to the phenomenon where the laws of physics are not invariant under the combined operations of charge conjugation (C) and parity transformation (P). This violation is crucial because it provides insight into the differences in behavior between matter and antimatter, which is a key aspect of understanding why our universe is predominantly composed of matter.
Dark Matter: Dark matter is an unseen form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects. It plays a crucial role in the structure and evolution of the universe, influencing galaxy formation, cosmic expansion, and the distribution of galaxies within the cosmic web.
Feynman Diagrams: Feynman diagrams are visual representations used in quantum field theory to depict the interactions between particles. They simplify complex calculations by providing a clear way to visualize particle interactions, including the exchange of force carriers, and are instrumental in understanding processes like scattering and decay. These diagrams use lines and vertices to represent particles and their interactions, making the abstract concepts of particle physics more accessible.
Inflation: Inflation is a rapid expansion of the universe that occurred during the first fraction of a second after the Big Bang, leading to an exponential increase in size and smoothing out irregularities. This phenomenon plays a crucial role in explaining the uniformity of the Cosmic Microwave Background (CMB) radiation, the large-scale structure of the universe, and certain aspects of particle physics, including matter-antimatter asymmetry.
Leptogenesis: Leptogenesis is a theoretical process that explains the generation of an excess of leptons (such as electrons and neutrinos) over anti-leptons in the early universe, which is believed to be a crucial step in explaining the observed matter-antimatter asymmetry. This imbalance is important for understanding why our universe is predominantly made of matter rather than an equal mixture of matter and antimatter. The process involves interactions that can create leptonic asymmetries, which later lead to the baryon asymmetry via processes described by the Sakharov conditions.
Neutrino Oscillation: Neutrino oscillation is a quantum phenomenon where a neutrino changes its flavor as it travels through space. This process implies that neutrinos have mass and are not massless particles, which was a surprising discovery in particle physics. The study of neutrino oscillation is crucial in understanding the behavior of these elusive particles and their role in the universe, particularly concerning the matter-antimatter asymmetry.
Observable universe: The observable universe refers to the part of the universe that we can see and measure from Earth, extending about 93 billion light-years in diameter. This region contains all the celestial objects and cosmic phenomena that can be detected with telescopes and other instruments, which helps us understand the universe's structure and history.
Quantum Field Theory: Quantum Field Theory (QFT) is a fundamental framework in theoretical physics that combines classical field theory, quantum mechanics, and special relativity to describe the behavior of subatomic particles. This approach allows for the understanding of how particles interact and can give rise to phenomena like quantum fluctuations, which are essential in explaining the early universe's structure formation, the cosmological constant problem, and the matter-antimatter asymmetry observed today.
Sakharov Conditions: The Sakharov Conditions are a set of three criteria proposed by physicist Andrei Sakharov in 1967 that explain the matter-antimatter asymmetry observed in the universe. These conditions include baryon number violation, C and CP violation, and thermal equilibrium. They are crucial for understanding why our universe contains more matter than antimatter, which is a key aspect of cosmological models and the evolution of the universe.
Standard Model of Particle Physics: The Standard Model of Particle Physics is a theoretical framework that describes the fundamental particles and forces that constitute the universe, excluding gravity. It classifies all known elementary particles into categories such as quarks, leptons, and gauge bosons, and explains how they interact through the fundamental forces: electromagnetic, weak, and strong interactions. This model provides a comprehensive understanding of how matter and energy behave at the smallest scales, and is essential for discussing phenomena like matter-antimatter asymmetry.
Steven Weinberg: Steven Weinberg was an influential theoretical physicist known for his groundbreaking work in particle physics and cosmology, particularly for developing the electroweak theory. His research helped explain the unification of electromagnetic and weak forces and provided a framework for understanding the matter-antimatter asymmetry observed in the universe.
Supersymmetry: Supersymmetry is a theoretical framework in particle physics that proposes a relationship between two basic classes of particles: bosons and fermions. This concept suggests that every fermion has a corresponding bosonic superpartner and vice versa, which helps in addressing several unresolved issues in physics, including the nature of dark matter, the matter-antimatter asymmetry in the universe, and the cosmological constant problem.