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
(SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts
(GRBs) luminosity distance indicators, and Gaussian priors on the distance from
the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors
have been included in order to help 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
(MCMC). It turns out that the dark energy equation of state is evolving for all
the parametrizations that we considered. We finally compare our results with
the ones obtained by previous cosmographic analysis performed on the same
astronomical datasets, showing that the latter ones are sufficient to test and
compare the new parametrizations.
