On the cosmological impact of a quantized scalar field

We explicitly compute the leading order contribution to the stress-energy- momentum tensor of a scalar field, which propagates in a Friedmann-Robertson-Walker (FRW) universe and placed in the infinite adiabatic vacuum. Since the adiabatic regularization requires removing the adiabatic zeroth, second and fourth order terms, one ends up with the sixth order expressions as the leading order contribution to the stress-energy-momentum tensor. This is a good approximation for all modes of a massive field when the mass m of the field is larger than the Hubble parameter H and only for the high energy (subhorizon) modes of a massless field satisfying k>k*, where k* is a fixed comoving momentum scale obeying k*>> aH. To determine the magnitude of the vacuum energy density, we consider the spacetimes undergoing power-law expansion and discuss the implications for different cosmologically relevant backgrounds. We next consider the backreaction effects of a quantized massless real scalar field propogating in a FRW universe. This can be viewed as a semiclassical approach, where gravity is treated classically. We manage to obtain a new first order differential equation, which approximately determines the evolution of the vacuum energy density of the massless scalar field. In this procedure, we utilize an approximation that uses the fact that the subhorizon modes evolve nearly adiabatically and the superhorizon modes freeze out. To check the validity of our method, we take fixed backgrounds of cosmological interest such as de Sitter space and our findings are shown to agree with the known results in the literature. We also analyse the possible implications of the backreaction effects for the slow-roll inflationary models. We find out that although these are negligible during slow-roll regime, the vacuum energy density created might have a cosmological significance at subsequent stages, since it decreases slower than radiation or dust.