Constraints on generalized dark energy from recent observations

Effects of a generalized dark energy fluid are investigated on cosmic density fluctuations such as a cosmic microwave background. As a general dark energy fluid, we take into consideration the possibility of the anisotropic stress for dark energy, which has not been discussed much in the literature. We comprehensively study its effects on the evolution of density fluctuations along with that of the nonadiabatic pressure fluctuation of dark energy, then give constraints on such a generalized dark energy from current observations. We show that, though we cannot find any stringent limits on the anisotropic stress or the nonadiabatic pressure fluctuation themselves, the constraints on the equation of state of dark energy can be affected in some cases by the nature of dark energy fluctuation characterized by these properties. This may have important implications in the strategy to study the nature of dark energy.