Constraints on generalized dark energy from recent observations

Effects of a generalized dark energy fluid is investigated on cosmic densityfluctuations such as cosmic microwave background. As a general dark energyfluid, we take into consideration the possibility of the anisotropic stress fordark energy, which has not been discussed much in the literature. Wecomprehensively study its effects on the evolution of density fluctuationsalong with that of non-adiabatic pressure fluctuation of dark energy, then giveconstraints on such a generalized dark energy from current observations. Weshow that, though we cannot find any stringent limits on the anisotropic stressor the non-adiabatic pressure fluctuation themselves, the constrains on theequation of state of dark energy can be affected in some cases by the nature ofdark energy fluctuation characterized by these properties. This may haveimportant implications to the strategy to study the nature of dark energy.Comment: 17 pages, 12 figure