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, the observation that only matter (baryons) and not antimatter (antibaryons) is detected in universe other than in cosmic ray collisions.
[3][4]: 22.3.6 Since it is assumed in cosmology that the particles we see were created using the same physics we measure today, and in particle physics experiments today matter and antimatter are always symmetric, the dominance of matter over antimatter is unexplained.
[5] 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).
[8] 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.
[9] These estimates predict that a large volume of material will occasionally exhibit a spontaneous proton decay, which has not been observed.
Baryogenesis theories are based on different descriptions of the interaction between fundamental particles.
Quantum field theory and statistical physics are used to describe such possible mechanisms.
Baryogenesis is followed by primordial nucleosynthesis, when atomic nuclei began to form.
The majority of ordinary matter in the universe is found in atomic nuclei, which are made of neutrons and protons.
In the universe about 1 in 10,000 protons are antiprotons, consistent with ongoing production due to cosmic rays.
Possible domains of antimatter in other parts of the universe is inconsistent with the lack of measurable of gamma radiation background.
The match between the predictions and observations of the nucleosynthesis model constrains the value of this baryon asymmetry factor.
[5]: 37 There are two main interpretations for this disparity: either the universe began with a small preference for matter (total baryonic number of the universe different from zero), or the universe was originally perfectly symmetric, but somehow a set of particle physics phenomena contributed to a small imbalance in favour of matter over time.
In 1967, Andrei Sakharov proposed[11] 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[12] 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.
This ensures the particles and their corresponding antiparticles do not achieve thermal equilibrium due to rapid expansion decreasing the occurrence of pair-annihilation.
The interactions must be out of thermal equilibrium at the time of the baryon-number and C/CP symmetry violating decay occurs to generate the asymmetry.
Baryogenesis within the Standard Model requires the electroweak symmetry breaking to be a first-order cosmological phase transition, since otherwise sphalerons wipe out any baryon asymmetry that happened up to the phase transition.
[15] Due to CP-symmetry violating electroweak interactions, some amplitudes involving quarks are not equal to the corresponding amplitudes involving anti-quarks, but rather have opposite phase (see CKM matrix and Kaon); since time reversal takes an amplitude to its complex conjugate, CPT-symmetry is conserved in this entire process.
[16] Thus certain sums of amplitudes for quarks have different absolute values compared to those of anti-quarks.
[17] However, sphalerons are rare enough in the broken phase as not to wipe out the excess of baryons there.
[18] 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.
[19] 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.
After the decoupling, the total number of CBR photons remains constant.
because the entropy density of the universe remained reasonably constant throughout most of its evolution.
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.