Strong interactions are the ones responsible for nuclear forces between protons and neutrons (the constituents of atomic nuclei), and we will come back to them when we look at the evolution of a star (Sect. 9.1). The remaining type, the weak forces, are experienced by all types of matter, but they are usually overwhelmed by the electromagnetic and strong forces because the weak interactions are, well, weak!
One is used to hear about electrons and protons and, perhaps to a lesser extent, neutrons. All these are constituents of atoms and atomic nuclei. But nature has a much richer population, and among its citizens one of the most intriguing are the neutrinos.
Neutrinos are very light particles and experience only the weak interactions and it is because of this that they are rarely affected by other types of matter. Only in the densest of environments are neutrinos strongly disturbed. These occur in the center of neutron stars (Sec. 9.3.4) or in the early universe. In this last case neutrinos were originally extremely energetic but, just as in the case of radiation, there came a time when the universe expanded to the point that the environment wasn't dense enough for the neutrinos to be affected by it. From that point on the neutrinos have been just cruising along, interacting only very rarely.
Initially these neutrinos lived in a very hot environment, which implies that each of them had a lot of energy and they were in a situation where very large gravitational forces were present. Nowadays they are in an environment where the gravitational forces are very weak. To understand what this implies consider the following analogy.
Imagine that you throw a ball up from the earth: initially the ball has a lot of kinetic energy, that is, energy due to its motion, but as it rises it slows down losing kinetic energy. Of course, this energy does not disappear, it is stored in potential energy (see Fig. 8.21). As the ball falls it will pick up speed so that when you catch it will be moving at the initial velocity (or close to it). In the same way the neutrinos in the present universe will have lost most of their kinetic energy.
So another prediction of the Big Bang theory is that the universe is filled with neutrinos of very small kinetic energy. Unfortunately, out current technology is not sufficiently sophisticated to be able to detect them directly, but this might improve in the future.