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So far large and different data sets revealed the accelerated expansion rateof the Universe, which is usually explained in terms of dark energy. The natureof dark energy is not yet known, and several models have been introduced: a nonzero cosmological constant, a potential energy of some scalar field, effectsrelated to the non homogeneous distribution of matter, or effects due toalternative theories of gravity. Recently, a tension with the flat {\Lambda}CDMmodel has been discovered using a high-redshift Hubble diagram of supernovae,quasars, and gamma-ray bursts. Here we use the Union2 type Ia supernovae (SNIa)and Gamma Ray Bursts (GRB) Hubble diagram, and a set of direct measurements ofthe Hubble parameter to explore different dark energy models. We use theChevallier-Polarski- Linder (CPL) parametrization of the dark energy equationof state (EOS), a minimally coupled quintessence scalar field, and, finally, weconsider models with dark energy at early times (EDE). We perform a statisticalanalysis based on the Markov chain Monte Carlo (MCMC) method, and explore theprobability distributions of the cosmological parameters for each of thecompeting models. We apply the Akaike Information Criterion (AIC) to comparethese models: our analysis indicates that an evolving dark energy, described bya scalar field with exponential potential seems to be favoured by observationaldata.

Prospects of high redshift constraints on dark energy models with the Ep- Eiso correlation in long Gamma Ray Bursts