Gravitational wave luminosity distance in viscous cosmological models

Abstract

We study the so-called Gravitational Wave luminosity distance-redshift relation d L GW(z) during cosmological eras driven by non-perfect fluids. In particular, we show that the presence of a shear viscosity in the energy momentum tensor turns out to be the most relevant effect. Within this scenario, a constant shear viscosity imprints the gravitational wave propagation through a friction term δ(z) with a uniquely given redshift dependence. This peculiar evolution predicts a specific shape for the ratio d L GW/d L EM which tends to a constant value when the sources are at z ≳ 1, whereas scales linearly with the shear viscosity at lower redshifts, regardless of the value of Ω m0. According to our final discussion, the predicted redshift dependence δ(z) provided by a shear viscosity could be tested by upcoming surveys of multi-messenger sources against analogous scenarios provided by some widely studied theories of modified gravity.

On the stability of string-hole gas

“…Moreover, it is important to note that a perfect fluid description is not stable under general O(d, d) transformations [7,19], indeed a general transformation could induce bulk/shear viscosity terms. Here, we will focus only on the contribution of the bulk viscosity term, since we are considering spatial homogeneous and isotropic backgrounds (but see, for example, [20,21] for possible effects of shear viscosity in anisotropic scenarios). We leave the investigation of more general description with shearviscosity for future analysis.…”