Unification of the Fundamental Forces

Unification of the Fundamental Forces (2) ...

We mentioned earlier that we don't have a very clear understanding of why the Universe that we see appears to be composed of matter. Most of the basic physics that we think governed the early Universe would have favored the creation of equal amounts of matter and antimatter, in clear contradiction to the observed dominance of matter over antimatter. There is a potential solution to this problem, involving GUTs. To explain it, we must first take a small detour and discuss the role of symmetries and conservation laws in the evolution of the Universe.

Symmetries and Conservation Laws

A cornerstone of modern science is the idea of symmetries and that the preservation of those symmetries leads to certain quantities being absolutely conserved (that is, they do not change with time). An example is angular momentum, which we have discussed in many contexts and which, we have asserted, is always conserved. It may be shown that the conservation of angular momentum is associated with a deep symmetry of nature, the isotropy of space, which means that the laws governing the physical Universe are the same no matter what the orientation of the experimental apparatus. Another way of stating this symmetry is that there are no preferred directions.

Now of course there can be preferred directions in local regions of space. For example, if we drop a ball it falls downward, not sideways. But that is because there is a gravitational field in our local region of space. If we take the entire system into account (the ball, the gravitational field, and anything else involved), we would find that space appears to be isotropic. This symmetry under rotations (that is, with respect to directions in space) can be shown mathematically to be equivalent to the requirement that the angular momentum of the entire system cannot change with time; that is, it is conserved.

Symmetries Important for the Early Universe

The preceding example is termed a spacetime symmetry, because the symmetry operation involves spacetime. But there are other more abstract symmetries that play a central role in modern physics and in particular in the early Universe. Some examples critical to our present discussion are:

Charge conjugation symmetry, denoted by C, which asserts that physical laws are unchanged by the replacement of a particle by its antiparticle, or vice versa.

Parity symmetry, denoted by P, which asserts that physical laws are unchanged if the experiment is replaced by its mirror image (for example, a right hand is the mirror image of a left hand).

The product of parity and charge conjugation, denoted by CP, which asserts that physical laws are unchanged if the experiment is replaced by its mirror image and all particles are replaced by antiparticles and vice versa.

Baryon conservation, which asserts that a quantity called the baryon number (usually denoted by B) is conserved in all possible reactions. Every baryon can be assigned a unique baryon number. The corresponding antiparticle has a baryon number that is the negative of the particle baryon number. For example, a proton has baryon number B = 1 and an antiproton has baryon number B = -1. If a particle is not a baryon (for example, electrons, neutrinos, photons, and their antiparticles), they have baryon number zero. The baryon number then is the number obtained by summing the baryon number for each individual particle. For example, a system consisting of two protons, one antiproton, and one electron would have baryon number B = 1 + 1 - 1 + 0 = 1.

What is the experimental status of these conservation laws? Parity is conserved by all interactions except the weak interactions, which violate parity strongly. The same is true of charge conjugation. The product of parity and charge conjugation, CP, is conserved in most reactions but is violated a small amount in one particular type of weak interaction. The baryon number is observed to be conserved in all experiments performed to date, but there are theoretical reasons to believe that it is not an absolute conservation law but is violated at some level that is not yet detectable. In particular, grand unified theories (GUTs) generally predict that baryon number is not conserved.

The Matter-Antimatter Asymmetry Problem

We may now address the issue of why the Universe is composed of matter with little antimatter (or why equal amounts of matter and antimatter have not annilated to leave us a Universe with only photons and neither baryons nor antibaryons). It can be shown that for the big bang to produce such a universe three conditions must be fulfilled (These conditions were first clearly stated by the Russian physicist Andrei Sakharov, who was the "father" of the Russian hydrogen bomb but became a famous political dissident late in his life and was awarded the 1975 Nobel Peace Prize.):

There must exist in nature reactions that do not conserve baryon number.

There must exist reactions that violate both C and CP symmetries.

In the early part of the big bang, the Universe must pass through periods in which it is not in thermal equilibrium.

As we have noted, there do exist reactions in nature that violate both C and CP symmetry. Furthermore, the standard big bang passes through periods when it is not in complete thermal equilibrium (for example, when various types of particles decouple from the equilibrium). The one additional ingredient that is required to generate a baryon asymmetry (that is, a Universe with more baryons than antibaryons) is baryon nonconserving reactions. But grand unified theories generally do not conserve baryon number, so if GUTs played a role in the big bang, it is possible to arrange a scenario where slightly more baryons than antibaryons are created in the early Universe. Then, because particles and antiparticles annihilate to produce photons, essentially all the antibaryons annihilate with baryons to produce photons, leaving the Universe as observed today containing many photons, many fewer baryons, and almost no antibaryons.