  In an intriguing recent paper, Crittenden and Turok proposed
cross-correlating the cosmic microwave background (CMB) with tracers of the
matter density to probe the existence of a cosmological constant. Here I
emphasize that a similar cross-correlation arises in an open Universe and,
depending on the redshift distribution of the tracer population and the matter
density, may be comparable to or stronger than that in a flat
cosmological-constant Universe with the same matter density. The two cases can
be distinguished through cross-correlation with tracer populations with
different redshift distributions.
