Matter/Microwave Correlations in an Open Universe
  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. 
Marc Kamionkowski (Columbia University)