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Measuring the Cosmic Equation of State with Counts of
Galaxies: Prospects for the DEEP Redshift Survey
J.A. Newman et al.
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 distribu-
tion of a tracer whose evolution in number density is known. In
the past, ordinary galaxies have been used as such a tracer; how-
ever, in the absence of a complete theory of galaxy formation, that
method is fraught with diфculties. 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 nd that if selection eects 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
m has been well-determined from other observations), provid-
ing constraints quite similar to the target for the SNAP satellite,
but with completely dierent 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.
Co-author: Marc Davis
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