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Open AccessArticle

Remarks on Cosmological Bulk Viscosity in Different Epochs

Department of Energy and Process Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
Author to whom correspondence should be addressed.
Symmetry 2020, 12(7), 1085;
Received: 16 April 2020 / Revised: 22 May 2020 / Accepted: 16 June 2020 / Published: 1 July 2020
(This article belongs to the Special Issue Viscous Cosmology)
The intention of this paper is mainly two-fold. First, we point out a striking numerical agreement between the bulk viscosity in the lepton era calculated by Husdal (2016) and our own calculations of the present-day bulk viscosity when the functional form is ζ ρ . From a phenomenological point of view, we thus seem to have an ansatz for the viscosity, which bridges the infancy of the Universe (∼1 s) with the present. This can also be looked upon as a kind of symmetry between the early-time cosmology and the present-day cosmology: it is quite remarkable that the kinetic theory-based bulk viscosity in the early universe and the experimentally-based bulk viscosity in the present universe can be covered by the same simple analytical formula. Second, we consider the Kasner universe as a typical anisotropic model of Bianchi-Type I, investigating whether this geometrical model is compatible with constant viscosity coefficients in the fluid. Perhaps surprisingly, the existence of a shear viscosity turns out to be incompatible with the Kasner model. By contrast, a bulk viscosity is non-problematic in the isotropic version of the model. In the special case of a Zel’dovich (stiff) fluid, the three equal exponents in the Kasner metric are even determined by the bulk viscosity alone, independent of the value of the fluid energy density. We also give a brief comparison with some other recent approaches to viscous cosmology. View Full-Text
Keywords: viscous cosmology; kasner universe; anisotropic cosmology viscous cosmology; kasner universe; anisotropic cosmology
MDPI and ACS Style

Brevik, I.; Normann, B.D. Remarks on Cosmological Bulk Viscosity in Different Epochs. Symmetry 2020, 12, 1085.

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