Constraints on Generalized Dark Energy from Recent Observations

Effects of a generalized dark energy fluid is investigated on cosmic density fluctuations such as 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 non-adiabatic 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 non-adiabatic pressure fluctuation themselves, the constrains 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 to the strategy to study the nature of dark energy.Comment: 17 pages, 12 figure

Dark matter kinetic decoupling with a light particle

We argue that the acoustic damping of the matter power spectrum is not a generic feature of the kinetic decoupling of dark matter, but even the enhancement can be realized depending on the nature of the kinetic decoupling when compared to that in the standard cold dark matter model. We consider a model that exhibits a ${\it sudden}$ kinetic decoupling and investigate cosmological perturbations in the ${\it standard}$ cosmological background numerically in the model. We also give an analytic discussion in a simplified setup. Our results indicate that the nature of the kinetic decoupling could have a great impact on small scale density perturbations.Comment: 19 pages, 7 figure

Effects of Cosmological Moduli Fields on Cosmic Microwave Background

We discuss effects of cosmological moduli fields on the cosmic microwave background (CMB). If a modulus field \phi once dominates the universe, the CMB we observe today is from the decay of \phi and its anisotropy is affected by the primordial fluctuation in the amplitude of the modulus field. Consequently, constraints on the inflaton potential from the CMB anisotropy can be relaxed. In particular, the scale of the inflation may be significantly lowered. In addition, with the cosmological moduli fields, correlated mixture of adiabatic and isocurvature fluctuations may be generated, which results in enhanced CMB angular power spectrum at higher multipoles relative to that of lower ones. Such an enhancement can be an evidence of the cosmological moduli fields, and may be observed in future satellite experiments.Comment: 11pages, 4 figures, numerical error correcte