Estimate of the Bulk Viscosity in the Cosmic Fluid
Year: 2016
Type: preprint
Abstract: We first give a review of recent works on bulk viscous cosmology. Then, we derive general solutions of the Friedmann equations when bulk viscosity is included in the energy-momentum tensor, both for a single-component and a multicomponent fluid, showing that these general solutions reduce to those found in the literature in special cases. We solve the energy conservation equation for the three cases $\zeta$=const., $\zeta\propto \sqrt{\rho}$, and $\zeta\propto \rho$, often studied in previous investigations, and find the best-fit values for the present day viscosity $\zeta_0$ in each of the three cases. Taking into account constraints from thermodynamics we find, in agreement with previous works, that the present day viscosity is less than about $ 10^7~$Pa s. In fact the best fit values suggest $\zeta_0\sim 10^6$Pa s. We point out that this magnitude is acceptable from a hydrodynamic point of view. Altogether, we give preference to the model $\zeta\propto \sqrt{\rho}$, which by now seems to be widely accepted, although the differences between the predictions of the various models are not very large. Having established reasonable numerical values for the bulk viscosity in the present universe, we use this information to analyze the fate of the late universe, especially the occurrence of a big rip in the far future. Of particular interest is the case $\zeta \propto \sqrt{\rho}$ 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.
Source: arXiv (Cornell University)
Authors: Ben David Normann, Iver Brevik
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2337336488