Matter-microwave correlations in an open universe

In an intriguing recent paper, Crittenden and Turok proposed cross-correlating the cosmic microwave background 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.