This concluding chapter recaps what has been learnt in the previous chapters about the Standard Model. This model is highly successful in describing particle physics phenomena. Some of its successes are briefly underlined, such as the number of light neutrino families. However, as with any model, it also has its weaknesses, which are also provided. The most important open questions of particle physics are addressed in the second part of the chapter, in particular, the matter–antimatter asymmetry, the hypothetical presence of the dark matter. Possible extensions of the Standard Model are presented to incorporate massive neutrinos.

This chapter reviews the Boltzmann equation, which is a starting point for some of the key results in cosmology. We introduce a general version of the Boltzmann equation, then study its implications in the simple scenario of a few interacting particles. We introduce the concept of a freezeout of particle species, and illustrate it using a simple example. We end the chapter by discussing baryogenesis (the process that generated the excess of baryons over antibaryons), and Sakharov conditions for successful baryogenesis to take place.