DENSITY-FLUCTUATIONS IN THE INFLATIONARY UNIVERSE

According to general fluctuation-dissipation theorems a nonzero bulk viscosity is necessarily connected with energy density fluctuations. In an expanding universe these fluctuations are proportional to the ratio of the relevant mean free time between the collisions and the Hubble time scale. While the Hubble scale remains constant during an intermediate inflationary stage the mean free collision time may grow. For a fluid with bulk viscosity, moving on the background of a de Sitter spacetime the corresponding fractional energy density fluctuations may be exponentially unstable. Possibly, this provides an alternative approach to the problem of initial conditions for large-scale structure formation in the early universe.