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Baryogenesis
--------------
In the early stages of the universe there were equal numbers of
particles and antiparticles of every kind. When the universe
decoupled, the photons that existed became free and formed the
cosmic background radiation we see today. The protons and
antiprotons annihilated each other and should have left a universe
containing equal amounts of matter and antimatter. However, this
is not what happened. Instead, what we see today is a universe
that has more matter than antimatter. In fact, there is no
experimental evidence to date that there are any significant
concentrations of antimatter anywhere in the observable universe.
The challenge is to explain how this preference for matter over
antimatter, and also how the magnitude of this asymmetry, came
about. An important quantifier is the asymmetry parameter, Y_{p},
defined as:
Y_{p} = (N_{p+} - N_{p-})/N_{γ}
Where N_{γ} is the density of the cosmic background radiation photons.
For a blackbody spectrum the number of photons is equivalent to the
entropy. Thus, we can also write, N_{γ} = S.
The measured value of Y_{p} is about 10^{-10}. Although this is a tiny
number, it is should be 0. We would expect the universe to be full
of photons and nothing else!
In order for the observed asymmetry to be explained, Andrei Sakharov
proposed that the following symmetry conditions would need to have
been violated. These became known as the SAKHAROV CONDITIONS.
1. Baryon number violation.
The Baryon number is defined as:
B = (1/3)(n_{q+} - n_{q-})
- Baryon number violation has never been observed in nature.
However, it could be possible that this symmetry breaking is
extremely weak. It is conceivable that a gauge boson with
extremeky high mass could mediate a decay of an antiproton
into an electron/photon pair.
e^{+}
/
p^{-} -----
\
γ
The Feynman propagator predicts the probability amplitude
of such a process is ∝ 1/(m - E)^{2}. At low temperatures the
probability is extremely remote but, at the high temperatures
(E) of the early universe, this probability could become very
much larger. However, a massive gauge boson such as this is
not part of the current Standard Model.
2. C, CP violation.
- This could be possible. While the electromagnetic and strong
interactions are invariant under the parity transformation, the
weak interaction is not. CP symmetry breaking has been observed
experimentally with beta decay of Cobalt-60 nuclei where the P
symmetry is violated. The electrons ejected along with
antineutrinos are predominantly left-handed. Asymmetry has
also been observed when neutral K-mesons transform into their
antiparticles (where each quark is replaced with the other's
antiquark) and vice versa, but such transformations do not occur
with exactly the same probability in both directions.
The implication is that the following processes may not happen
with the same probability, and an imbalance is created.
e^{-}
/
p^{+} -----
\
γ
e^{+}
/
p^{-} -----
\
γ
3. CPT violation/interactions out of thermal equilibrium .
- True. The universe is expanding extremely rapidly and
temperatures were dropping. Processes were certainly not
in equilibrium. Forward time and backward time were different.
The hypothetical process that produced the baryon/antibaryon
asymmetry (imbalance) in the early stages of the universe are
referred to as BARYOGENESIS. While the conditions have been
established, there is currently insufficient observational evidence
to explain why the imbalance came about, however. In addition,
we have very little undertanding of physics at the extremely high
temperatures under which baryogenesis took place.