  We investigate the matter creation processes during the reheating period at
the end of inflation in the early Universe, by using the irreversible
thermodynamic of open systems. The matter content of the Universe is assumed to
consist of the inflationary scalar field, which, through its decay, generates
relativistic matter, and pressureless dark matter, respectively. At the early
stages of reheating the inflationary scalar field transfers its energy to the
newly created matter particles, with the field energy decreasing to near zero.
The general equations governing the irreversible matter creation during
reheating are obtained by combining the thermodynamics description of the
matter creation and the gravitational field equations. A dimensionless form of
the general system of the reheating equations is also introduced. The role of
the different inflationary scalar field potentials is analyzed by using
analytical and numerical methods, and the evolution of the matter and scalar
field densities, as well as of the cosmological parameters during reheating,
are obtained. Typically, the values of the energy densities of relativistic
matter and dark matter reach their maximum when the Universe is reheated up to
the reheating temperature, which is determined for each case, as a function of
the scalar field decay width, the scalar field particle mass, and of the
cosmological parameters. An interesting result is that particle production
leads to the acceleration of the Universe during the reheating phase, with the
deceleration parameter showing a complex dynamics. Once the energy density of
the scalar field becomes negligible with respect to the matter densities, the
expansion of the Universe decelerates, and inflation has a graceful exit after
reheating.
