Evolution of spatially averaged energy density and pressure in FRW cosmology

초록

Our universe, though homogeneous and isotropic globally, is filled with irregularities. The shape of these irregularities varies with time as the universe expands. It is a very complicated job to follow the evolution of small fluctuations within the general relativistic framework beyond first-order approximations. Instead we present a way to extract the change in time of spatially averaged quantities such as energy density and pressure up to second order. Radiative modes and compressional modes are dealt with separately. We discuss the implication of the results in the context of FRW cosmology.