Gravitational waves in the presence of viscosity
Abstract
We analyze gravitational waves propagating in an isotropic cosmic fluid endowed with a bulk viscosity ζ and a shear viscosity η, assuming these coefficients to vary with fluid density ρ as ρλ, with λ = 1/2 favored by experimental evidence. We give the general governing equation for the gravitational waves and focus thereafter on two examples. The first concerns waves in the very late universe, close to the Big Rip, where the fate of the cosmic fluid is dependent highly on the values of the parameters. Our second example considers the very early universe, the lepton era, the motivation for this choice being that the microscopical bulk viscosity as calculated from statistical mechanics is then at maximum. We find that the gravitational waves on such an underlying medium are damped, having a decay constant equal to the inverse of the conformal Hubble parameter. Our results turn out to be in good agreement with other viscositybased approaches.
 Publication:

International Journal of Modern Physics D
 Pub Date:
 2019
 DOI:
 10.1142/S0218271819501335
 arXiv:
 arXiv:1901.00767
 Bibcode:
 2019IJMPD..2850133B
 Keywords:

 Dark energy;
 gravitational wave;
 viscous cosmology;
 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Theory
 EPrint:
 Version to appear in IJMPD. LaTeX 14 pages