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Direct Determination of the Kinematics of the Universe and Properties of the Dark Energy as Functions of Redshift

Daly, Ruth A. and Djorgovski, S. G. (2004) Direct Determination of the Kinematics of the Universe and Properties of the Dark Energy as Functions of Redshift. Astrophysical Journal, 612 (2). pp. 652-659. ISSN 0004-637X. doi:10.1086/422673.

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Understanding the nature of dark energy, which appears to drive the expansion of the universe, is one of the central problems of physical cosmology today. In an earlier paper we proposed a novel method to determine the expansion rate E(z) and the deceleration parameter q(z) in a largely model-independent way, directly from the data on coordinate distances y(z). Here we expand this methodology to include measurements of the pressure of dark energy p(z), its normalized energy density fraction f(z), and the equation-of-state parameter w(z). We then apply this methodology to a new, combined data set of distances to supernovae and radio galaxies. In evaluating E(z) and q(z), we make only the assumptions that the FRW metric applies and that the universe is spatially flat (an assumption strongly supported by modern cosmic microwave background radiation measurements). The determinations of E(z) and q(z) are independent of any theory of gravity. For evaluations of p(z), f(z), and w(z), a theory of gravity must be adopted, and general relativity is assumed here. No a priori assumptions regarding the properties or redshift evolution of the dark energy are needed. We obtain trends for y(z) and E(z) that are fully consistent with the standard Friedmann-Lemaître concordance cosmology with Ω_0 = 0.3 and Λ_0 = 0.7. The measured trend for q(z) deviates systematically from the predictions of this model on a ~1-2 σ level but may be consistent for smaller values of Λ_0. We confirm our previous result that the universe transitions from acceleration to deceleration at a redshift zT ≈ 0.4. The trends for p(z), f(z), and w(z) are consistent with being constant at least out to z ~ 0.3-0.5 and broadly consistent with being constant out to higher redshifts, but with large uncertainties. For the present values of these parameters we obtain E_0 = 0.97 ± 0.03, q_0 = -0.35 ± 0.15, p_0 = -0.6 ± 0.15, f_0 = -0.62 - (Ω_0 - 0.3) ± 0.05, and w_0 = -0.9 - ε(Ω_0 - 0.3) ± 0.1, where Ω_0 is the density parameter for nonrelativistic matter and ε ≈ 1.5 ± 0.1. We note that in the standard Friedmann-Lemaître models p_0 = -Λ_0, and thus we can measure the value of the cosmological constant directly and obtain results in agreement with other contemporary results.

Item Type:Article
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URLURL TypeDescription Paper
Djorgovski, S. G.0000-0002-0603-3087
Additional Information:© 2004. The American Astronomical Society. Receivved 2004 March 29; accepted 2004 May 18. It is a pleasure to thank Megan Donahue, Chris O'Dea, Saul Perlmutter, and Adam Riess for helpful discussions. This work was supported in part by the U. S. National Science Foundation under grant AST 02-06002 and by Penn State University (R. A. D.) and the Ajax Foundation (S. G. D.). Finally, we acknowledge the great work and efforts of many observers who obtained the valuable data used in this study.
Funding AgencyGrant Number
NSFAST 02-06002
Pennsylvania State UniversityUNSPECIFIED
Ajax FoundationUNSPECIFIED
Subject Keywords:cosmological parameters; cosmology: observations; cosmology: theory; dark matter; equation of state
Issue or Number:2
Record Number:CaltechAUTHORS:20170408-154737429
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Official Citation:Ruth A. Daly and S. G. Djorgovski 2004 ApJ 612 652
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76068
Deposited By: 1Science Import
Deposited On:19 May 2017 22:55
Last Modified:15 Nov 2021 16:57

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