New Constraint on Open Cold-Dark-Matter Models

We calculate the large-angle cross correlation between the cosmic-microwave-background temperature and the x-ray-background intensity expected in an open universe with cold dark matter (CDM) and a nearly scale-invariant spectrum of adiabatic density perturbations. Results are presented as a function of the nonrelativistic-matter density ${\Omega }_0$ and the x-ray bias $b_x$ for both an open universe and a flat cosmological-constant universe. Recent experimental upper limits to the amplitude of this cross correlation provide a new constraint to the ${\Omega }_0-b_x$ parameter space that open-CDM models (and the open-inflation models that produce them) must satisfy.