For refcode 2003ApJ...597....9D: Retrieve 176 NED objects in this reference. Please click here for ADS abstract
NED Abstract
Copyright by American Astronomical Society.
Reproduced by permission
2003ApJ...597....9D
A ModelIndependent Determination of the Expansion and Acceleration Rates
of the Universe as a Function of Redshift and Constraints on Dark Energy
Ruth A. Daly and S. G. Djorgovski
Abstract. Determination of the expansion and acceleration history of the
universe is one of the fundamental goals of cosmology. Detailed
measurements of these rates as a function of redshift can provide new
physical insights into the nature and evolution of the dark energy, which
apparently dominates the global dynamics of the universe at the present
epoch. We present here dimensionless coordinate distances y(z) to 20 radio
galaxies reaching out to z~1.8, the redshift range currently not covered by
supernova standardcandle observations. There is very good agreement
between coordinate distances to radio galaxies and supernovae for the
redshift range in which these measurements overlap, suggesting that neither
is plagued at this level by unknown systematic errors. We develop a simple
numerical method for a direct determination of the expansion and
acceleration rates, E(z) and q(z), from the data, which makes no
assumptions about the underlying cosmological model or the equation of
state parameter w. This differential method is in contrast to the
traditional cosmological tests, in which particular model equations are
integrated and then compared with the observations. The new approach is
model independent, but at a cost of being noisier and highly sensitive to
the amount and quality of the available data. We illustrate the method by
applying it to the currently available supernova data and the data on radio
galaxies presented here. We derive the expansion rate of the universe as a
function of redshift, E(z), and for the first time we obtain a direct
estimate of the acceleration rate of the universe as a function of
redshift, q(z), in a way that is independent of assumptions regarding the
dark energy and its redshift evolution. The current observations indicate
that the universe makes a transition from acceleration to deceleration at a
redshift greater than 0.3, with a bestfit estimate of about 0.45; this
transition redshift and our determinations of E(z) are broadly in agreement
with the currently popular FriedmannLemaitre cosmology with {OMEGA}_m_ =
0.3 and {OMEGA}_{LAMBDA}_=0.7, even though no model assumptions are made in
deriving the fits for E(z) and q(z). With the advent of much better and
richer data sets in the future, our direct method can provide a useful
complementarity and an independent check on the traditional cosmological
tests.
Keywords: Cosmology: Cosmological Parameters, Cosmology: Observations,
Cosmology: Theory, Dark Matter, Equation of State
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