Observational Cosmology. Pair Production: the Creation of a Matter Particle and its Antimatter Counterpart

Cosmology is, by its very nature, an observational science. It is passive in that we must wait for events to happen in the Universe which we can then watch and learn from. Not for the practising cosmologist is the relative ease of making a universe in the laboratory, although simulations of the early Universe are partially possible in particle accelerators.

Luckily, as pointed out in chapter 1, astronomers have physics, an experimental science, on their side. Every piece of cosmological insight gained is achieved by painstakingly modelling observed celestial characteristics, using the physics gleaned from laboratory experiments.

The first fundamental observation which can be made about the Universe, as a whole, is that it contains matter. The radiation content is not so surprising, as we shall see, but the fact that solid lumps of matter abound is rather a shock! The reason for the surprise is because of the results from experiments currently being conducted in particle accelerators, mentioned in chapter 2.

In these experiments, the interchangeability of mass and energy is explored. Energy, carried by photons, can be changed into mass under certain conditions: for example, if the photon gets close to a heavy atomic nucleus. When this happens, two particles are produced - one of matter, the other of its anti-matter counterpart.

This is necessary to conserve charge among other quantities. For the purposes of this example, imagine that the two particles created are an electron and its counterpart, a positron. Eventually and usually sooner rather than later, the positron comes into contact with the electron (or another which is just like it) and they annihilate each other, returning their energy back into photons (see Fig. 3. 1).

Fig. 3. 1. Pair production: the creation of a matter particle and its antimatter counterpart, resulting from photon interaction

If this process holds true universally, then for every particle of created matter, there should be an antimatter equivalent. Eventually, mutual annihilation events will return all the mass energy back into photons, leaving nothing from which to make stars, planets, you or me. Could it be that the antimatter has somehow been segregated from the matter?

This is a rather unsatisfactory explanation, since there is no observational evidence to support it. It is also unsatisfactory from the point of view of the cosmological principle, as will be explained shortly. Some way has to be devised, within the known laws of physics, which enables the ratio of matter particles to antimatter particles to be greater than 1.

Current approximations, based upon the ratio of photons to matter particles, suggest that the asymmetry of matter to antimatter is probably only one part in a billion! Very small indeed but, as insignificant as one in a billion may sound, it has led to a profoundly different Universe than one filled with radiation alone.