Cosmology of strongly interacting fermions in the early universe

Understand

In view of growing interest in long range scalar forces in the early universe to generate primordial black holes, we study in detail the general relativistic formulation of a Fermi gas interacting with a scalar field in cosmology.

• Our main finding is that the Yukawa interaction leads to a solution where the scalar field oscillates around zero fermion mass and all energy densities decay as radiation.
• On one hand, we show that if the Fermi gas starts relativistic, it could stay relativistic.
• On the other hand, if the fermions are initially non-relativistic, they remain non-relativistic for all practical purposes.