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

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Abstract
We review the implications of having a non-trivial matter component in the universe and the potential for detecting such a component through the matter power spectrum and ISW 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 < 0.01 h.Mpc^{-1}). Since these modes have hardly been probed so far by large scale galaxy surveys, we investigate whether joint constraints that can be placed on those two quantities using the recent CMB fluctuations measurements by WMAP 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 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 NVSS radio survey. We believe however that improvements in bias through improved survey scales and depths in the near future will improve hopes of detecting the speed of sound.

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