Cosmological two-fluid bulk viscosity

Abstract

A simple two-fluid model of cosmological bulk viscosity, in which small deviations from thermal equilibrium account for the viscous bulk pressure, is substantiated by kinetic theory. Some peculiar issues regarding its relation to the radiative fluid model are discussed. The microphysical picture underlying the viscous dissipation is made precise. We also consider a reactive 'cross' viscosity associated with deviations from detailed balance, which includes the so-called creation pressure of the cosmological fluid. For collisional interactions between the fluid components, the reactive viscous pressure is not an independent mechanism for entropy production. Entropy from cross effects may be generated through an effective isentropic particle source. In both instances new results are obtained for the reactive viscosity, and applied to a representative case of non-equilibrium decay.

Non-minimal derivative coupling scalar field and bulk viscous dark energy

“…Some examples to describe the source of the viscosity are as follows: moving cosmic strings through the cosmic magnetic fields and magnetic monopoles in monopole interactions effectively experience various viscous phenomena. Various mechanisms for primordial quantum particle productions and their interactions were also major arguments for viscous fluids [16][17][18][19][20][21][22][23][24].…”

“…In other words, to recognize the mechanism of heat generation and characteristic of smallest fluctuations one possibility is revealing the presence of dissipative processes [24]. In classical fluid dynamics, viscosity is trivial consequence of internal degree of freedom of the associated particles [25]. The existence of any dissipative process for isotropic and homogeneous Universe, should undoubtedly be scalar, consequently, at the background level, we deal with bulk viscous model [18,19,21,[26][27][28][29].…”

Remarks on Cosmological Bulk Viscosity in Different Epochs

“…There is also another reason why anisotropy (p i = 0) is problematic in the Kasner model. From Equation (38), it follows that if w < 1, which is the case for the usual fluids, the combination (ζ + 4η/3) becomes negative. Although negative viscosities are occasionally considered in cosmology (cf.…”

“…We consider the development of the fluid from t = t in onwards; the initial energy and pressure being ρ in and p in . It is notable that the governing Equation (38) actually fix the later time dependence to be:…”

“…By now, the literature is rich with contributions on the topic of viscous cosmology. The contributions include investigations of the early universe [13][14][15][16], the late universe [17][18][19], the phantom divide [20][21][22][23][24][25], models for the dark sector [26][27][28][29], and others [30][31][32][33][34][35][36][37][38][39].…”