Bulk viscous matter and recent acceleration of the Universe
  We consider a cosmological model dominated by bulk viscous matter with total bulk viscosity coefficient proportional to the velocity and acceleration of the expansion of the universe in such a way that $\zeta=\zeta_{0}+\zeta_{1}\frac{\dot{a}}{a}+\zeta_{2}\frac{\ddot{a}}{\dot{a}}.$ We show that there exist two limiting conditions in the bulk viscous coefficients, ($\zeta_{0}$, $\zeta_{1}$, $\zeta_{2}$) which corresponds to a universe having a Big-Bang at the origin, followed by an early decelerated epoch and then making a smooth transition into an accelerating epoch. We have constrained the model using the type Ia Supernovae data, evaluated the best estimated values of all the bulk viscous parameters and the Hubble parameter corresponding to the two limiting conditions. We found that even though the evolution of the cosmological parameters are in general different for the two limiting cases, they show identical behavior for the best estimated values of the parameters from both the limiting conditions. A recent acceleration would occur if $\tilde{\zeta}_{0}+\tilde{\zeta}_{1}>1$ for the first limiting conditions and if $\tilde{\zeta}_{0}+\tilde{\zeta}_{1}<1$ for the second limiting conditions. The age of the universe predicted by this model is found to be less than that predicted from the oldest galactic globular clusters. The total bulk viscosity seems to be negative in the past and becomes positive when $z\leq0.8$. So the model violates the local second law of thermodynamics. However, the model satisfies the generalized second law of thermodynamics at the apparent horizon throughout the evolution of the universe. We also made a statefinder analysis of the model and found that it is distinguishably different from the standard $\Lambda$CDM model at present, but shows a de Sitter type behavior in the far future of the evolution. 
Athira Sasidharan and Titus K. Mathew
gr-qc