Viscosity in cosmic fluids

Abstract

The effective theory of large-scale structure formation based on $$\Lambda $$ΛCDM paradigm predicts finite dissipative effects in the resulting fluid equations. In this work, we study how viscous effect that could arise if one includes self-interaction among the dark-matter particles combines with the effective theory. It is shown that these two possible sources of dissipation can operate together in a cosmic fluid and the interplay between them can play an important role in determining dynamics of the cosmic fluid. In particular, we demonstrate that the viscosity coefficient due to self-interaction is added inversely with the viscosity calculated using effective theory of $$\Lambda $$ΛCDM model. Thus the larger viscosity has less significant contribution in the effective viscosity. Using the known bounds on $$\sigma /m$$σ/m for self-interacting darkmatter, where $$\sigma $$σ and m are the cross-section and mass of the dark-matter particles respectively, we discuss role of the effective viscosity in various cosmological scenarios.

Global 21-cm brightness temperature in viscous dark energy models

“…The presence of viscosity in the dark energy fluid may exhibit remarkable effects in the dynamics of the Universe [34][35][36][37][38][39]58], and in particular, the global 21-cm scenario. The effective pressure of the viscous dark energy fluid can be expressed as [59] p de = ω de ρ de − 3ζH, (5) where ω de and ρ de are the equation of state parameter and density of dark energy, respectively.…”

“…In some recent treatments, dark energy is considered as a fluid having a certain amount of bulk viscosity [33]. The effect of viscous fluids in evolution of the Universe has evoked considerable interest in cosmology [34][35][36][37][38][39] and viscosity has even been proposed to account for the current accelaration in various models [40][41][42][43]. In the presence of viscosity, not only does the Hubble evolution get modified [33], but additional entropy is produced which heats up both the baryon and dark matter fluid and alters the thermal evolution of the Universe as well [20].…”

Global 21-cm brightness temperature in viscous dark energy models

“…However, the shear viscosity can be ignored at large scales for an isotropic and homogeneous Universe. The effect of bulk viscosity has been widely studied in viscous dark energy [56][57][58][59][60][61][62][63][64][65][66][67], viscous dark matter [68][69][70][71][72], and cosmic inflation [73]. Viscosity has further been proposed to account for the current acceleration in various models [74][75][76][77].…”

Gravitational wave luminosity distance in viscous cosmological models

“…For the sake of completeness, we mention that the interest in the shear viscosity occurs in some particular scenarios such as the Self Interacting Dark Matter (SIDM) model. Indeed, the SIDM model is able to generate the shear viscosity in the Universe [53,55,57] and explain some features in the small scale structure. More generally, other studies consider Viscous Dark Matter where only shear viscosity is considered (see for example [58]).…”