Gravitational particle creation and inflation

Particle creation due to the changing spacetime metric at the end of an inflationary era in the early Universe is discussed. The Universe is assumed to make a transition from de Sitter space to either a radiation-dominated or matter-dominated universe. A perturbation approach is used to calculate the number density and energy density of massless, nonconformally coupled particles created by this transition. It is found that their energy density is typically of the order of ${\rho }_v$ ${\mathrm{}}^2$/${\rho }_{\mathrm{Pl}}$, where ${\rho }_v$ is the value of the cosmological constant in the de Sitter phase and ${\rho }_{\mathrm{Pl}}$ is the Planck energy density. This is approximately the energy density of a thermal bath at the Gibbons-Hawking temperature of de Sitter space. The possible applications of this effect to inflationary models is discussed. It is shown that gravitational particle creation is capable of reheating the Universe after inflation and of being the source of the matter in the Universe. This effect makes it possible to avoid the difficulty with reheating which inflationary models with weakly coupled scalar fields otherwise encounter.