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Baryogenesis

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	Topics Baryogenesis Topic Home In physical cosmologyFacts About Physical cosmology Physical cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned ... , baryogenesis is the generic term for hypothetical physical processes that produced an asymmetrySymmetry Summary Symmetry is a characteristic feature of geometrical shapes, systems, equations, and other real or conceptual objects —t... between baryonBaryon In particle physics, the baryons are the family of subatomic particles which are made of three quarks.... s and anti-baryons in the very early universeBig Bang In physical cosmology, the Big Bang is the scientific theory of how the universe emerged from a tremendously dense and hot s... , resulting in the substantial amounts of residual matterMatter In physics, matter is commonly defined as the substance of which physical objects are composed, not counting the contributio... that make up the universeUniverse The term universe has a variety of meanings, based on the context in which it is used.... today. Baryogenesis theories — the most important being electroweak baryogenesis and GUT baryogenesis — employ sub-disciplines of physicsPhysics Physics , the most fundamental physical science, is concerned with the underlying principles of the natural world.... such as quantum field theoryQuantum field theory Summary Quantum field theory is the quantum theory of fields.... , and statistical physicsStatistical physics Statistical physics is one of the fundamental theories of physics, and uses methods of statistics in solving physical probl... , to describe such possible mechanisms. The fundamental difference between baryogenesis theories is the description of the interactions between fundamental particles. The next step after baryogenesis is the much better understood Big Bang nucleosynthesisBig Bang nucleosynthesis In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than H-1, the normal, light hydrog... , during which light atomic nuclei began to form. Background The Dirac equationDirac equation Summary In physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac... , formulated by Paul DiracFacts About Paul Dirac Paul Adrien Maurice Dirac, OM, FRS was a British theoretical physicist and a founder of the field of quantum physics.... around 1928 as part of the development of relativisticSpecial relativity The special theory of relativity was proposed in 1905 by Albert Einstein in his article "On the Electrodynamics of Moving Bo... quantum mechanicsQuantum mechanics Quantum mechanics is a first quantized quantum theory that supersedes classical mechanics at the atomic and subatomic levels... , predicts the existence of antiparticleAntiparticle Corresponding to each kind of particle, there is an associated antiparticle with the same mass and opposite charges.... s along with the expected solutions for the corresponding particles. Since that time, it has been verified experimentally that every known kind of particle has a corresponding antiparticle. The CPT TheoremCPT symmetry CPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity ... guarantees that a particle and its anti-particle have exactly the same mass and lifetime, and exactly opposite charge. Given this symmetry, it is puzzling that the universe does not have equal amounts of matter and antimatter. Indeed, there is no experimental evidence that there are any significant concentrations of antimatter in the observable universe. There are two main interpretations for this disparity: either when the universe began there was already a small preference for matter, with the total baryonic numberBaryon number In particle physics, the baryon number is an approximate conserved quantum number of a system.... of the universe different from zero ; or, the universe was originally perfectly symmetric , but somehow a set of phenomena contributed to a small imbalance. The second point of view is preferred, although there is no clear experimental evidence indicating either of them to be the correct one. The aforementioned preference is merely based on the following philosophical point-of-view: if the universe encompasses everything (time, space, and matter), nothing exists outside of it and therefore nothing existed before it, leading to the baryonic number . From a more scientific point-of-view, there are reasons to expect that any initial asymmetry would be wiped out to zero during the early history of the universe. One challenge then is to explain how the universe evolves to produce . The Sakharov conditions In 1967, Andrei SakharovAndrei Sakharov Dr. Andrei Dmitrievich Sakharov , was an eminent Soviet nuclear physicist, dissident and human rights activist.... proposed \| doi = 10.1070/PU1991v034n05ABEH002497}} a set of three necessary conditions that a baryonBaryon In particle physics, the baryons are the family of subatomic particles which are made of three quarks.... -generating interaction must satisfy to produce matter and antimatter at different rates. These conditions were inspired by the recent discoveries of the cosmic background radiation and CP-violationCP-violation In physics, and specifically particle physics, CP violation is a violation of the postulated CP symmetry of th... in the neutral kaonKaon In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number ... system. The three necessary "Sakharov conditions" are: Baryon numberBaryon number In particle physics, the baryon number is an approximate conserved quantum number of a system.... violation. C-symmetryC-symmetry In physics, C-symmetry means the symmetry of physical laws under a charge-conjugation transformation.... and CP-symmetry violation. Interactions out of thermal equilibrium. Currently, there is no experimental evidence of particle interactions where the conservation of baryon numberBaryon number In particle physics, the baryon number is an approximate conserved quantum number of a system.... is broken perturbatively: this would appear to suggest that all observed particle reactions have equal baryon number before and after. Mathematically, the commutatorCommutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative... of the baryon number quantum operatorOperator In mathematics, an operator is a function, usually of a special kind depending on the topic.... with the (perturbative) Standard ModelStandard Model The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces,... hamiltonianHamiltonian (quantum mechanics) The quantum Hamiltonian is the physical state of a system, which may be characterized as a ray in an abstract Hilbert space ... is zero: . However, the Standard Model is known to violate the conservation of baryon number non-perturbatively: a global U(1) anomaly. Baryon number violation can also result from physics beyond the Standard Model (see supersymmetrySupersymmetry In particle physics, supersymmetry is a physical theory which proposes a physical symmetry between bosons and fermions.... and Grand Unification TheoriesGrand unification theory Grand unification, grand unified theory, or GUT is one of several very similar theories or models in physics tha... ). The second condition — violation of CP-symmetry — was discovered in 1964 (direct CP-violation, that is violation of CP-symmetry in a decay process, was discovered later, in 1999). If CPT-symmetry is assumed, violation of CP-symmetry demands violation of time inversion symmetry, or T-symmetryT-symmetry T-symmetry is the symmetry of physical laws under a time reversal transformation—... . The last condition states that the rate of a reaction which generates baryon-asymmetry must be less than the rate of expansion of the universe. In this situation the particles and their corresponding antiparticles do not achieve thermal equilibrium due to rapid expansion decreasing the occurrence of pair-annihilation. Matter content in the universe The baryon asymmetry parameter The challenges to the physics theories are then to explain how to produce this preference of matter over antimatter, and also the magnitude of this asymmetry. An important quantifier is the asymmetry parameter, . This quantity relates the overall number density difference between baryons and anti-baryons ( and , respectively) and the number density of cosmic background radiation photon . According to the Big Bang model, matter decoupled from the cosmic background radiation (CBR) at a temperature of roughly 3000 kelvins, corresponding to an average kinetic energy of . After the decoupling, the total number of CBR photons remains constant. Therefore due to space-time expansion, the photon density decreases. The photon density at equilibrium temperature , per cubic kelvinKelvin The Kelvin scale is a temperature scale where absolute zero—the coldest possible temperature where there is no heat en... and per cubic centimeter, is given by , with as the Boltzmann constantBoltzmann constant The Boltzmann constant is the physical constant relating temperature to energy.... , as the Planck constant divided by and as the speed of light in vacuum. In the numeric approximation at the left hand side of the equation, the convention was used, and for in kelvins the result is given in . At the current CBR photon temperature of , this corresponds to a photon density of around CBR photons per cubic centimeter. Therefore, the asymmetry parameter , as defined above, is not the "good" parameter. Instead, the preferred asymmetry parameter uses the entropyEntropy In thermodynamics, entropy, symbolized by S, is a state function of a thermodynamic system defined by the differential q... density , because the entropy density of the universe remained reasonably constant throughout most of its evolution. The entropy density is with and as the pressure and density from the energy density tensor , and as the effective number of degrees of freedom for "massless" particle (inasmuch as holds) at temperature , , for bosons and fermions with and degrees of freedom at temperatures and respectively. At the present era, . Philosophical considerations It should be noted that, were there not a disparity between baryons and anti-baryons, there may never have been lifeLife Life is the characteristic state of organisms.... capable of observing it. Our own existence is the product of many specific conditions. In investigating nature, physicists attempt to divide such factors into natural constants of mathematical relationships, and variables of history and chance. The sole aim of scienceScience Science in the broadest sense refers to any system of knowledge attained by verifiable means.... is to produce analytical structures from observations. PhilosophyPhilosophy Philosophy is a field of study that includes diverse subfields such as aesthetics, epistemology, ethics, logic, and metaphys... , on the other hand, directly aims to answer specific questions, such as "why are we here?" As there may be no possible direct observations that lead to a natural theory of baryogenesis, it may also have no scientific explanation. The most advanced theory, therefore, would be that baryogenesis certainly happened because we are here to observe it—the "anthropic principleAnthropic principle In physics and cosmology, the Anthropic Principle begins with the observation that the universe appears surprisingly hospita... ." Although this theory is verified by the simple act of posing it, unfortunately it cannot suggest a hypothesis of the causes of the physical process. No matter what physical laws might be extrapolated to investigate the Big Bang, or what fundamental forces we find underlying our existence, the possibility that all our laws and theory, and the universe we know, could have resulted from an unobservable event, forever obscure to science, is sure to forever delight philosophers and frustrate scientists. See also LeptonLepton In physics, a lepton is a particle with spin, 1/2 that does not experience the strong nuclear force.... LeptogenesisFacts About Leptogenesis (physics) In the strict sense, leptogenesis is a process which creates leptons.... CP-symmetry, CP violation Articles Textbooks External links A. D. Dolgov (July 1997). "Baryogenesis, 30 years after". arXivArXiv arXiv is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science a... , . A. Riotto (July 1998). "Theories of baryogenesis". arXiv, . Also, CERN preprint CERN-TH/98-204. M. Trodden (March 1998). "Electroweak baryogenesis". arXiv, .