Cosmological gravitational particle production: Scalars and fermions

Particle production in the Early Universe is one of the cornerstones of modern cosmology and plays an important role in dark-matter phenomenology. In this talk, I will discuss the gravitational production of scalars and fermions during inflation. For scalars, I will compare Bogolyubov coefficient and Starobinsky stochastic approaches, showing that they only agree in the limit of infinitely long inflation.

High-scale inflation is very efficient in particle production, leading to constraints on the existence of free, light, and stable scalars: such particles are viable only if their masses are below the eV scale or if the reheating temperature is in the GeV range. For fermions, I will show how the production efficiency depends on the particle mass, which is generated via the Yukawa coupling and sensitive to the corresponding scalar field value.

Scalar fields can experience large quantum fluctuations during inflation, driving the average field to the Hubble scale and above. Thus, fermions can be very heavy during inflation, enhancing particle production. This mechanism can be the leading source of right-handed neutrinos, if they gain a Majorana mass from the Yukawa coupling to a singlet scalar.