Equation of state for cosmological fluids: Direct dynamical derivation

Abstract

The derivation of the “baroenergetic” equation of state is reconsidered in rather a general form following Bernoulli’s kinetic approach but with Lorentz energy-momentum dependence. It is shown that uniform versions of the equation of state generally used in cosmology are valid only in extreme limits: an ultrarelativistic photon gas (electromagnetic radiation) and a nonrelativistic gas of massive particles. Possible ways of generalization of these results are pointed out and briefly discussed.