WHERE DOES MATTER COME FROM? - cont.
We need what are called “out of equilibrium” conditions. In order to “freeze” the net number of baryons produced by the first two conditions, the early universe could not have been always in thermal equilibrium. We are very familiar with systems that are out of thermal equilibrium in our everyday life. An example is condensation of steam. More specifically, imagine a container filled with hot steam which is immersed into a large bucket with cold water. The steam, being too hot compared with the cold water, is out of thermal equilibrium. In order to attain equilibrium it will go through a phase transition; the steam will cool down and condense, going from a gas phase to a liquid phase. As it does so, we will observe the appearance of droplets of the liquid phase that will grow and coalesce. The phase transition ends when the steam is completely converted into water.
How does this reasoning apply to the early universe? Strange as this may sound, the universe also went through phase transitions. Particles—and their properties—are also sensitive to temperature. The standard model of particle physics successfully describes how particles interact at energies over a thousand times larger than nuclear energies. According to this model, at very high temperatures all particles but one, the so-called Higgs particle, have no mass, while at lower temperatures they acquire a mass through their interactions with the Higgs particle. We say that matter has two different “phases,” above and below the temperature at which particles like the quarks and the electron acquire a mass.
Thus, as the temperature of the early universe dropped, it went through a phase transition, and particles gained their mass. Like water droplets in steam, droplets of the low temperature (massive) phase appeared within the high temperature (massless) phase, growing and coalescing, in a typical out-of-equilibrium phase transition. Since only in the high temperature phase are baryons created in excess over anti-baryons (recall that the first two conditions apply only at high temperatures), these excess baryonic particles will penetrate the droplets of the massive phase, like viruses invading cells, becoming the net baryon number in the low temperature phase. As the droplets grow and coalesce, the whole universe is converted into the massive phase, completing the phase transition. According to our current models of “baryogenesis,” the creation of the excess baryons occurred when the universe was about one thousandth of a billionth of a second old. The protons and neutrons we are made of are the fossils of this primordial event.
So is this it? Is our work finished? Far from it. The simplest particle physics models we have do not generate the observed excess of matter over antimatter. Even worse, our true understanding of the complicated dynamics of these phase transitions is at best incomplete, leaving many questions unanswered at the moment. We have the broad outline of an explanation for the generation of matter in the universe, but the details are far from being understood.