Viscous unified dark matter models under scrutiny: Uncovering inconsistencies from dynamical system analysis
Year: 2024
Type: article
Abstract: Viscous unified dark matter models aim to describe the dark sector of the Universe, as the dark matter fluid itself gives rise to an accelerated expansion due to a negative bulk viscous pressure. However, in most studies, radiation is often disregarded as a minor factor in dynamical system analyses, overlooking whether radiation domination is achievable. In this paper, we rigorously examine this critical aspect for common parametrizations of bulk viscosity, denoted as $\ensuremath{\xi}$, within two general classes of viscous unified models. Our findings reveal significant inconsistencies in models where $\ensuremath{\xi}\ensuremath{\propto}{H}^{1\ensuremath{-}2s}{\ensuremath{\rho}}_{m}^{s}$ with $s\ensuremath{\le}0$, and surprisingly, in models where $\ensuremath{\xi}\ensuremath{\propto}{\ensuremath{\rho}}_{m}^{s}$ with exponents $s<0$, as they both fail to produce a radiation-dominated era---an undesirable outcome for any cosmological model. Moreover, the exponent $s$ must lye within the interval $0\ensuremath{\le}s<1/2$ for the latter model to correctly describes the cosmological evolution. These results underscore the need of including these constraints as a prior in statistical analyses of observational data, with implications for current statistical inferences for the second model, where both prior and best-fit values of $s$ often fall outside the acceptable range.
Authors: Guillermo Palma, Gabriel Gómez, Norman Cruz
Institution: Universidad de Santiago de Chile
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Award: G21973476
OpenAlex ID: W4402270195