Everything about Baryogenesis totally explained
In
physical cosmology,
baryogenesis is the generic term for hypothetical physical processes that produced an
asymmetry between
baryons and anti-baryons in the
very early universe, resulting in the substantial amounts of residual
matter that make up the
universe today.
Baryogenesis theories — the most important being electroweak baryogenesis and GUT baryogenesis — employ sub-disciplines of
physics such as
quantum field theory, and
statistical physics, 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 nucleosynthesis, during which light
atomic nuclei began to form.
Background
The
Dirac equation, formulated by
Paul Dirac around 1928 as part of the development of
relativistic quantum mechanics, predicts the existence of
antiparticles 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 Theorem guarantees that a particle and its anti-particle have exactly the same mass and lifetime, and exactly opposite charge. Given this symmetry, it's puzzling that the universe doesn't have equal amounts of matter and antimatter. Indeed, there's 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 number 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's 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 Sakharov proposed 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 background radiation
and
CP-violation in the neutral
kaon system.
The three necessary "Sakharov conditions" are:
Currently, there's no experimental evidence of particle interactions where the conservation of
baryon number is broken perturbatively: this would appear to suggest that all observed particle reactions have equal baryon number before and after. 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 non-perturbatively: a global U(1) anomaly.
Baryon number violation can also result from physics beyond the Standard Model (see
supersymmetry and
Grand Unification Theories).
The second condition — violation of
CP-symmetry — was discovered in 1964 (direct CP-violation, that's 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-symmetry.
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 don't 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,
» ,
for bosons and fermions with
and
degrees of freedom at temperatures
and
respectively. At the present era,
.
A naive estimation of the baryon asymmetry of the universe
Observational results yield that
η is approximately equal to 10
−10 — more precisely, 2.6 <
η × 10
10 < 6.2. This means that for every 10 billion pairs of particle and antiparticle, there was one extra particle that was left without an antiparticle with which to annihilate into background radiation. This is a very small number, and explaining
how to obtain it's very difficult: one is trying to make predictions to the very large (
large-scale structure of the cosmos) based on the laws of the very small (
particle physics)!
A reasonable idea of how this number is found experimentally follows. The
Hubble Space Telescope surveys report that the
observable universe contains approximately 125 billion (1.25×10
11) galaxies. Assuming that they are, in average, similar to our own
galaxy, each contains around 100 billion (10
11) stars. The mass of the
Sun, which is a typical star, is around 2×10
30 kg. Making the approximation that our Sun is composed only of
hydrogen atoms, each of which weighs approximately 1.67×10
−27 kg, the Sun contains 1.2×10
57 atoms. The total number of atoms in the
observable universe is then approximately 1.5×10
79. The radius of the observable universe is about 16 billion light years, or 4.4×10
26 m. This means that the observable universe is a sphere of 3.6×10
80 m
3. The atom density would then be around 4.2×10
−2 m
−3. On the other hand,
statistical physics tells us that a gas of photons in thermal equilibrium at the temperature of the cosmic background radiation, 2.73 K, has a number density of 4.1×10
8 m
−3. The resulting estimate of
η is 1.0×10
−10. This isn't a bad approximation; it's only slightly outside the range quoted in the literature. The exact experimental value involves measuring the concentration of chemical elements in the universe not originating from stellar synthesis.
Philosophical considerations
It should be noted that, were there not a disparity between baryons and anti-baryons of the kind observed, it's questionable whether in fact there would be matter which would allow
life capable of observing it.
This is a common argument presented in answer to issues of "why is the universe as it is", known as the
anthropic principle. In essence, it answers the question by saying that in those universes or visible sections of the cosmos which didn't have conditions favorable for life, no life would have emerged to notice it. If an asymmetry between baryons and anti-baryons was an essential prerequisite for the material existence of stars, planets and life, then (the argument goes) there may have been countless universes or sections of the cosmos in which life couldn't emerge, until a section with the appropriate asymmetries came about eventually by chance, in which observers could exist. These observers would then notice the conditions, however atypical, which allowed their existence.
Similar arguments are also considered by some scientists when answering the question of why our planet within the cosmos is as it is, or why life exists on
Earth.
Further Information
Get more info on 'Baryogenesis'.
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