Acoustic oscillations and viscosity - art. no. 123507

Using a simple thermo-hydrodynamic model that respects relativistic causali ty, we reexamine the analysis of the qualitative features of acoustic oscil lations in the photon-baryon fluid. The growing photon mean free path intro duces transient effects that can be modelled by the causal generalization o f relativistic Navier-Stokes-Fourier theory. Causal thermodynamics provides a more satisfactory hydrodynamic approximation to kinetic theory than the quasi-stationary (and non-causal) approximations arising from standard ther modynamics or from expanding the photon distribution to first order in Thom son scattering time. The causal approach introduces small corrections to th e dispersion relation obtained in quasi-stationary treatments. A dissipativ e contri bution to the speed of sound slightly increases the frequency of t he oscillations. The diffusion damping scale is slightly increased by the c ausal corrections; Thus quasi-stationary approximations tend to over-estima te the spacing and under-estimate the damping of acoustic peaks. In our sim ple model, the fractional corrections at decoupling are greater than or sim ilar to 10(-2). [S0556-2821(98)05224-2].