CMB-Galaxy Correlation in Unified Dark Matter Scalar Field Cosmologies

We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar field cosmologies. We work out the predicted cross-correlation function in UDM models, which depends on the speed of sound of the unified component, and compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of the speed of sound and perform a likelihood analysis, finding that the UDM model is as likely as the ΛCDM, and is compatible with observations for a range of values of c_∞ (the value of the sound speed at late times) on which structure formation depends. In particular, we obtain an upper bound of (c_∞)^2 ≤ 0.009 at 95% confidence level, meaning that the ΛCDM model, for which (c_∞)^2 = 0, is a good fit to the data, while the posterior probability distribution peaks at the value (c_∞)^2 = 10^(-4) . Finally, we study the time dependence of the deviation from ΛCDM via a tomographic analysis using a mock redshift distribution and we find that the largest deviation is for low-redshift sources, suggesting that future low-z surveys will be best suited to constrain UDM models.