Cosmological inflation

This chapter explains the shortcomings of the historical Hot Big Bang model and how they can be solved by a phase of inflation. It shows that a shrinking comoving Hubble radius is equivalent, in an expanding universe, with the expansion of the latter being accelerated, hence the name commonly used to refer to such a phase: cosmological inflation. The chapter details the mechanism by which primordial density fluctuations and gravitational waves are generated during inflation, computes their statistical properties, and compares them to observations. It explains that inflation is an ultraviolet sensitive phenomenon, in the sense that physics at energies even much higher than the one of inflation strongly affects it. The chapter shows how the deviation from Gaussian statistics of the latter, so-called primordial non-Gaussianities, provide cosmologists with an invaluable source of information about the physics active in the early universe, allowing them to discriminate between different scenarios that would be otherwise indistinguishable.