Baryongenesis

Baryongenesis :
As the Universe cools a weak asymmetry in the direction towards matter becomes evident. Matter that is
massive is unstable, particularly at the high temperature in the early Universe. Low mass matter is stable,
but susceptible to destruction by high energy radiation (photons).

As the volume of the Universe increases, the lifetime of stable matter (its time between collisions with
photons) increases. This also means that the time available for matter to interact with matter also increases.

The Universe evolves from a pure, energy dominated domain to a more disordered, matter dominated
domain, i.e. entropy marches on.

The last two stages of matter construction is the combining of three quark groups into baryons (protons and
neutrons), then the collection of electrons by proton/neutron atomic nuclei to form atoms. The construction
of baryons is called baryongenesis.
Baryongenesis begins around 1 second after the Big Bang. The equilibrium process at work is the balance
between the strong force binding quarks into protons and neutrons versus the splitting of quark pairs into
new quark pairs. When the temperature of the Universe drops to the point that there is not enough energy
to form new quarks, the current quarks are able to link into stable triplets.

As all the anti-particles annihilate by colliding with their matter counterparts (leaving the small percentage
of matter particles, see next lecture) leaving the remaining particles in the Universe to be photons,
electrons, protons and neutrons. All quark pairs have reformed into baryons (protons and neutrons). Only
around exotic objects, like black holes, do we find any anti-matter or mesons (quark pairs) or any of the
other strange matter that was once found throughout the early Universe.
Soon after the second symmetry breaking (the GUT era), there is still lots of energy available to produce
matter by pair production, rather than quark confinement. However, the densities are so high that every
matter and anti-matter particle produced is soon destroyed by collisions with other particles, in a cycle of
equilibrium.

Note that this process (and quark confinement) produces an equal number of matter and anti-matter
particles, and that any particular time, if the process of pair production or quark confinement were to stop,
then all matter and anti-matter would eventual collide and the Universe will be composed only of photons.
In other words, since there are equal numbers of matter and anti-matter particles created by pair
production, then why is the Universe made mostly of matter? Anti-matter is extremely rare at the present
time, yet matter is very abundant.
This asymmetry is called the matter/anti-matter puzzle. Why if particles are created symmetrically as
matter and anti-matter does matter dominate the Universe today. In theory, all the matter and anti-matter
should have canceled out and the Universe should be a ocean of photons.

It is not the case that the Universe is only filled with photons (look around the room). And it is not the case
that 1/2 the Universe is matter and the other half is anti-matter (there would be alot of explosions).
Therefore, some mechanism produced more matter particle than anti-matter particles. How strong was this
asymmetry? We can't go back in time and count the number of matter/anti-matter pairs, but we can count
the number of cosmic background photons that remain after the annihilations. That counting yields a value
of 1 matter particle for every 1010 photons, which means the asymmetry between matter and anti-matter
was only 1 part in 10,000,000,000.
This means that for every 10,000,000,000 anti-matter particles there are 10,000,000,001 matter particles,
an asymmetry of 1 particle out of 10 billion. And the endresult is that every 10 billion matter/anti-matter
pairs annihilated each other leaving behind 1 matter particle and 10 billion photons that make up the
cosmic background radiation, the echo of the Big Bang we measure today. This ratio of matter to photons
is called the baryon number.

Even though the baryon number is extremely small (10-10) why isn't it zero? In Nature, there are only three
natural numbers, 0, 1 and infinity. All other numbers require explanation. What caused the asymmetry of
even one extra matter particle for every 10 billion matter/anti-matter pairs?
One answer is that the asymmetry occurs because the Universe is out of equilibrium. This is clearly true
because the Universe is expanding, and a dynamic thing is out of equilibrium (only static things are stable).
And there are particular points in the history of the Universe when the system is out of equilibrium, the
symmetry breaking moments. Notice also that during the inflation era, any asymmetries in the microscopic
world would be magnified into the macroscopic world. One such quantum asymmetry is CP violation.
CP Violation:
As the Universe expands and cools and the process of creation and annihilation of matter/anti-matter pairs
slows down. Soon matter and anti-matter has time to undergo other nuclear processes, such as nuclear
decay. Many exotic particles, massive bosons or mesons, can undergo decay into smaller particles. If the
Universe is out of equilibrium, then the decay process, fixed by the emergent laws of Nature, can become
out of balance if there exists some asymmetry in the rules of particle interactions. This would result in the
production of extra matter particles, rather than equal numbers of matter and anti-matter.
In the quantum world, there are large numbers of symmetric relationships. For example, there is the
symmetry between matter and anti-matter. For every matter particle, there is a corresponding anti-matter
particle of opposite charge. In the 1960's, it was found that some types of particles did not conserve left or
right-handedness during their decay into other particles. This property, called parity, was found to be
broken in a small number of interactions at the same time the charge symmetry was also broken and
became known as CP violation.

The symmetry is restored when particle interactions are considered under the global CPT rule (charge -
parity - time reversal), which states that that a particle and its anti-particle may be different, but will behave
the same in a mirror-reflected, time-reversed study. During the inflation era, the rapid expansion of
spacetime would have thrown the T in CPT symmetry out of balance, and the CP violation would have
produced a small asymmetry in the baryon number.
This is another example of how quantum effects can be magnified to produce large consequences in the
macroscopic world.