High-redshift investigation on the dark energy equation of state

The understanding of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. Whether or not the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, many models have been proposed to solve this problem, both in the context of General Relativity and alternative theories of gravity. Actually, a further possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state (EOS), w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here, we investigate different models of dark energy, described through several parametrizations of the EOS. Our high-redshift analysis is based on the Union2 Type Ia supernovae data set (Suzuki et al.), the Hubble diagram constructed from some gamma-ray bursts luminosity-distance indicators, and Gaussian priors on the distance from the baryon acoustic oscillations, and the Hubble constant h (these priors have been included in order to help to break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the EOS parameters, we use the Markov Chain Monte Carlo Method. It turns out that, if exact flatness is assumed, the dark energy EOS is evolving for all the parametrizations that we considered. We finally compare our results with the ones obtained by previous cosmographic analyses performed on the same astronomical data sets, showing that the latter ones are sufficient to test and compare the new parametrizations.