Direct Constraints on the Properties and Evolution of Dark Energy
  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 about 0.4, and the present value of deceleration parameter is -0.35 +/- 0.15. The standard ``concordance model'' 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 a redshift of about one. They are generally consistent with the concordance model, at least out to a redshift of about 0.5, but the existing data preclude any stronger conclusions at this point. 
Ruth A. Daly and S. G. Djorgovski