In physical cosmology, baryogenesis (also known as baryosynthesis[1][2]) is the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe.
[3] One of the outstanding problems in modern physics is the predominance of matter over antimatter in the universe.
A number of theoretical mechanisms are proposed to account for this discrepancy, namely identifying conditions that favour symmetry breaking and the creation of normal matter (as opposed to antimatter).
Experiments reported in 2010 at Fermilab, however, seem to show that this imbalance is much greater than previously assumed.
Most grand unified theories explicitly break the baryon number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive X bosons (X) or massive Higgs bosons (H0).
[6] The rate at which these events occur is governed largely by the mass of the intermediate X or H0 particles, so by assuming these reactions are responsible for the majority of the baryon number seen today, a maximum mass can be calculated above which the rate would be too slow to explain the presence of matter today.
[7] These estimates predict that a large volume of material will occasionally exhibit a spontaneous proton decay, which has not been observed.
The majority of ordinary matter in the universe is found in atomic nuclei, which are made of neutrons and protons.
These nucleons are made up of smaller particles called quarks, and antimatter equivalents for each are predicted to exist by the Dirac equation in 1928.
Hypotheses investigating the first few instants of the universe predict a composition with an almost equal number of quarks and antiquarks.
[9] Once the universe expanded and cooled to a critical temperature of approximately 2×1012 K,[3] quarks combined into normal matter and antimatter and proceeded to annihilate up to the small initial asymmetry of about one part in five billion, leaving the matter around us.
Likewise, there is no experimental evidence that there are any significant concentrations of antimatter in the observable universe.
The second point of view is preferred, although there is no clear experimental evidence indicating either of them to be the correct one.
In 1967, Andrei Sakharov proposed[10] a set of three necessary conditions that a baryon-generating interaction must satisfy to produce matter and antimatter at different rates.
These conditions were inspired by the recent discoveries of the cosmic microwave background[11] and CP-violation in the neutral kaon system.
CP-symmetry violation is similarly required because otherwise equal numbers of left-handed baryons and right-handed anti-baryons would be produced, as well as equal numbers of left-handed anti-baryons and right-handed baryons.
Finally, the interactions must be out of thermal equilibrium, since otherwise CPT symmetry would assure compensation between processes increasing and decreasing the baryon number.
Mathematically, the commutator of the baryon number quantum operator with the (perturbative) Standard Model hamiltonian is zero:
However, the Standard Model is known to violate the conservation of baryon number only non-perturbatively: a global U(1) anomaly.
[14] To account for baryon violation in baryogenesis, such events (including proton decay) can occur in Grand Unification Theories (GUTs) and supersymmetric (SUSY) models via hypothetical massive bosons such as the X boson.
In this situation the particles and their corresponding antiparticles do not achieve thermal equilibrium due to rapid expansion decreasing the occurrence of pair-annihilation.
[20] The central question to baryogenesis is what causes the preference for matter over antimatter in the universe, as well as the magnitude of this asymmetry.
[21] According to the Big Bang model, matter decoupled from the cosmic background radiation (CBR) at a temperature of roughly 3000 kelvin, corresponding to an average kinetic energy of 3000 K / (10.08×103 K/eV) = 0.3 eV.
for bosons and fermions with gi and gj degrees of freedom at temperatures Ti and Tj respectively.
This phenomenon suggests that in the early universe, particles such as the B-meson decay into a visible Standard Model baryon as well as a dark antibaryon that is invisible to current observation techniques.
[24] These oscillating mesons then decay down into the baryon-dark antibaryon pair previously mentioned,
is any extra light meson daughters required to satisfy other conservation laws in this particle decay.
[22] If this process occurs fast enough, the CP-violation effect gets carried over to the dark matter sector.
However, this contradicts (or at least challenges) the last Sakharov condition, since the expected matter preference in the visible universe is balanced by a new antimatter preference in the dark matter of the universe and total baryon number is conserved.
[23] B-mesogenesis results in missing energy between the initial and final states of the decay process, which, if recorded, could provide experimental evidence for dark matter.