Origin of bulk viscosity in cosmology and its thermodynamic implications

The purpose of the present work is twofold: (1) we propose a new mechanism for the origin of bulk viscosity in cosmological context, and then, (2) we address the thermodynamic implications of viscous cosmology based on the thermodynamics of the apparent horizon. In particular, we show that the velocity gradient of the comoving expansion of the Universe (along the distance measured from a comoving observer) in turn leads to a viscouslike pressure of the fluid inside the apparent horizon. This points to the importance of the bulk viscosity of fluid during the cosmological evolution of the Universe, as the comoving expansion itself generates the viscosity. Therefore, the thermodynamic interpretation of viscous cosmology becomes important in its own right. In this regard, it turns out that the cosmological evolution of the Universe with a bulk viscosity naturally satisfies the first and the second laws of thermodynamics of the apparent horizon, without imposing any exotic condition and for a general form of the coefficient of viscosity. This in turn affirms the thermodynamic correspondence of viscous cosmology.