Redshift-space density versus real-space velocity comparison

I propose to compare the redshift-space density field directly to the real-space velocity field. Such a comparison posesses all of the advantages of the conventional, redshift-space analyses, while at the same time it is free of their disadvantages. In particular, the model-dependent reconstruction of the density field in real space is unnecessary, and so is the reconstruction of the velocity field in redshift space. The redshift-space velocity field can be reconstructed only at linear order, because only at this order is it irrotational. Unlike the conventional redshift-space density-velocity comparisons, the comparison proposed here does not have to be restricted to the linear regime. Nonlinear effects can then be used to break the Omega-bias degeneracy plaguing the analyses based on linear theory. I present a degeneracy-breaking method for the case of nonlinear but local bias.