The above illustrates
pair production and pair annihilation
The figure shows
annihilation and pair
production in a bubble chamber.
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
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
In the early Universe, this allows the annihilation
reactions to persist long after pair production
- 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) ~ 1,100 photons per cubic centimeter
- Number of matter particles in the Universe (are there anti-matter
N(m) ~ 6 x 10-7 particles per cubic
The ratio of photons to matter particles is then
N(photons)/N(m) ~ 1,900,000,000 !
This is a huge number; note that
There must have been 1,000,000,000 and 1 electrons for every 1,000,000,000
positrons in the early Universe. If an asymmetry at this level did not
exist, then we would not be here.
1,000,000,001 electrons + 1,000,000,000 positrons ---> 2,000,000,000
photons + 1 electron
The small matter/anti-matter asymmetry is another mystery of the
Universe which we can either simply accept or try to explain
Return to Lecture 6