Looking for a Cosmological Constant with the Rees-Sciama Effect

In models with a cosmological constant a significant component of the cosmic microwave background (CMB) anisotropy is produced at rather low redshifts $z\lesssim 1$. In these models, the gravitational potential perturbations begin to evolve at late times, shifting the frequencies of photons passing though them. Since the potential reflects the matter density, the latter should be correlated with the CMB anisotropy. We examine this correlation and discuss the prospects for using an x-ray/COBE comparison to detect a cosmological constant.