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Jeffrey Newman, M. Davis (U.C. Berkeley)
The classical dN/dz test allows the determination of fundamental cosmological parameters from the evolution of the cosmic volume element. This test is applied by measuring the redshift distribution of a tracer whose evolution in number density is known. In the past, ordinary galaxies have been used as such a tracer; however, in the absence of a complete theory of galaxy formation, that method is fraught with difficulties. We propose studying instead the evolution of the apparent numbers of dark matter halos as a function of their circular velocity, observable via the linewidths or rotation speeds of visible galaxies. Upcoming redshift surveys will allow the linewidth distribution of galaxies to be determined at both z~1 and the present day. We find that if selection effects are well-controlled and minimal external constraints are applied, the planned DEEP Redshift Survey could allow the measurement of the cosmic equation-of-state parameter w to ±10% (as little as 3% if \Omegam has been well-determined from other observations), providing constraints quite similar to the target for the SNAP satellite, but with completely different potential systematics. This type of test has the potential also to provide a constraint on any evolution of w such as that predicted by ``tracker'' models.
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