Looking for $\Lambda$ with the Rees-Sciama Effect

In models with a cosmological constant, a significant component of the large scale cosmic microwave background (CMB) anisotropy is produced at rather low redshifts, z < 1. In these models, the gravitational potential perturbations begin to evolve at late times. Photons passing through these time varying potentials aquire frequency shifts, an effect first noted by Rees and Sciama. Since the gravitational potential is associated with matter at observable redshifts, the local matter density (or some tracer thereof) is correlated with the CMB anisotropy on the sky. We outline the optimal method of measuring this correlation, and discuss the prospects for using an X-ray/COBE comparison to detect a cosmological constant.