Bulk viscous cosmology

The full causal Müller-Israel-Stewart (MIS) theory of dissipative processes in relativistic fluids is applied to a flat, homogeneous, and isotropic universe with bulk viscosity. It is clarified in which sense the so-called truncated version is a reasonable limiting case of the full theory. The possibility of bulk viscosity-driven inflationary solutions of the full theory is discussed. As long as the particle number is conserved, almost all these solutions exhibit an exponential increase of the temperature. Assuming that the bulk viscous pressure of the MIS theory may also be interpreted as an effective description for particle production processes, the thermodynamical behavior of the Universe changes considerably. In the latter case, the temperature increases at a lower rate or may remain constant during a hypothetical de Sitter stage, accompanied by a substantial growth of the comoving entropy. © 1996 The American Physical Society.