General bulk-viscous solutions and estimates of bulk viscosity in the cosmic fluid

Abstract

We derive a general formalism for bulk viscous solutions of the energy-conservation equation for ρ(a,ζ) ρ(a,ζ) , both for a single-component and a multicomponent fluid in the Friedmann universe. For our purposes, these general solutions become valuable in estimating the order of magnitude of the phenomenological viscosity in the cosmic fluid at present. H(z) H(z) observations are found to put an upper limit on the magnitude of the modulus of the present-day bulk viscosity. It is found to be ζ 0 ∼10 6 ζ0∼106 Pa·s, in agreement with previous works. We point out that this magnitude is acceptable from a hydrodynamic point of view. Finally, we bring new insight by using our estimates of ζ to analyze the fate of the future universe. Of special interest is the case ζ∝ρ √ ζ∝ρ for which the fluid, originally situated in the quintessence region, may slide through the phantom barrier and inevitably be driven into a big rip. Typical rip times are found to be a few hundred Gy.