In the early Universe, matter is created via pair production and destroyed by annihilation. Since an equal amount of matter and anti-matter is produced in pair production (at least based on simple theories), there could be a problem.

• Pair Production -- To make pairs, the temperature must be high so that photons (or whatever) can make virtual particles real ===> since the Universe cools as it expands, at some point a threshold is reached beyond which pairs can no longer be created.

• Annihilations -- Annihilations don't require high temperatures. All that is needed is a density that is sufficiently high so that the probability that a matter particle and its anti-matter twin will re-unite is high.
  In the early Universe, this allows the annihilation reactions to persist long after pair production halts.

• Because of the symmetry of pair production and annihilation, calculations suggest that annihilations would remove all of the matter/anti-matter from the Universe leading to a Universe which is filled entirely with photons. Is this true? Well, obviously not since we are here.

The problem is more than a little vexing because the asymmetry is small.

• Number of photons in the Universe. The number of photons in the Universe is essentially the number of photons in the CMBR, N(photons) ~ 1.1 billion photons per cubic meter

• Number of matter particles in the Universe (are there anti-matter galaxies?) is roughly N(m) ~ 0.6 particles per cubic meter

The ratio of photons to matter particles is then

N(photons)/N(m) ~ 1,900,000,000 !

This is a huge number; note that

1,000,000,001 protons + 1,000,000,000 anti-protons ---> 2,000,000,000 photons + 1 proton

There must have been 1,000,000,000 and 1 proton for every 1,000,000,000 anti-protons in the early Universe. If an asymmetry at this level did not exist, then we would not be here.

The small matter/anti-matter asymmetry is another mystery of the Universe which we can either simply accept or try to explain

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