  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.
