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 remaining particles in the Universe to be photons, electrons, protons and neutrons. All quark pairs have reformed into baryons (protons and neutrons). Today, only around exotic objects like black holes, do we find any anti-matter, anti-mesons (quark pairs) or any of the other strange matter that was once found throughout the early Universe.

Matter versus Anti-Matter :

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.