The X-ray Background as a Cosmological Tool

Abstract

We review the use of the X-ray background (XRB) as a cosmological tool with emphasis on three techniques: multipole analysis, autocorrelation functions and excess fluctuations. Although these methods of analysis of XRB fluctuations, and in particular the excess fluctuations method, are quite sensitive to cosmological parameters, the poor knowledge of a possibly redshift dependent bias parameter for the sources that dominate the X-ray background prevents the derivation of firm conclusions at present. Conversely, knowledge of the cosmological parameters could be used together with XRB fluctuations to assess the clustering properties of AGN as the dominant XRB sources.