Probing dark energy perturbations: The dark energy equation of state and speed of sound as measured by WMAP

We review the implications of having a nontrivial matter component in the Universe and the potential for detecting such a component through the matter power spectrum and integrated Sachs-Wolfe effect. We adopt a phenomenological approach and consider the mysterious dark energy to be a cosmic fluid. It is thus fully characterized, up to linear order, by its equation of state and its speed of sound. Whereas the equation of state has been widely studied in the literature, less interest has been devoted to the speed of sound. Its observational consequences come predominantly from very large scale modes of dark matter perturbations $(k<\mathrm{}0.01h{\mathrm{Mpc}}^{-1}).\mathrm{}$ Since these modes have hardly been probed so far by large scale galaxy surveys, we investigate whether joint constraints can be placed on those two quantities using the recent cosmic microwave background (CMB) fluctuations measurements by the Wilkinson Microwave Anisotropy Probe as well as the recently measured CMB large scale structure cross correlation. We find only a tentative 1 sigma detection of the speed of sound, from CMB alone, $c_s^2<0.04\mathrm{}$ at this low significance level. Furthermore, the current uncertainties in bias in the matter power spectrum preclude any constraints being placed using the cross correlation of CMB with the NRAO VLA Sky Survey radio survey.