

Paper: 
Direct Constraints on the Properties and Evolution of Dark Energy 
Volume: 
339, Observing Dark Energy 
Page: 
117 
Authors: 
Daly, R.A.; Djorgovski, S.G. 
Abstract: 
We describe a method to derive the expansion and acceleration rates directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy. If one also specifies a theory of gravity, we can also determine the pressure, energy density, and equation of state of the dark energy as functions of redshift. We then apply this methodology on a modern data set of distances to supernovae and to radio galaxies. We find that the universe transitions from deceleration to acceleration at a redshift of z _{T} ≈ 0.4, and the present value of the deceleration parameter is q^{0} = −0.35 ± 0.15. The standard "concordance model" with Ω_{0} = 0.3 and Λ = 0.7 provides a reasonably good fit to the dimensionless expansion rate as a function of redshift, though it fits the dimensionless acceleration rate as a function of redshift less well. Adopting General Relativity as the theory of gravity, we obtain the redshift trends for the pressure, energy density, and equation of state of the dark energy out to z ~ 1. They are generally consistent with the concordance model, at least out to z ~ 0.5, but the existing data preclude any stronger conclusions at this point. For the present values of these quantities, we obtain p_{0} = −0.6 ± 0.15, f_{0} = 0.62 ± 0.05, and w_{0} = −0.9 ± 0.1. Application of this methodology to richer data sets in the future may provide valuable new insights into the physical nature and evolution of the dark energy. 



