Cosmology with gamma-ray bursts

II. Cosmography challenges and cosmological scenarios for the accelerated Universe

¹ Institute for Theoretical Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland
² Department of Astronomy, Williams College, Williamstown, MA 01267, USA
³ Dipartimento di Fisica, Università degli Studi di Napoli Federico II, Compl. Univ. Monte S. Angelo, 80126 Naples, Italy
⁴ INFN, Sez. di Napoli, Compl. Univ. Monte S. Angelo, Edificio 6, via Cinthia, 80126 Napoli, Italy
e-mail: ester@na.infn.it
⁵ Department of Physics and Earth Sciences, University of Ferrara, Block C, via Saragat 1, 41122 Ferrara, Italy
⁶ Department of Physics, University of Nice Sophia Antipolis, Parc Valrose 06034, Nice Cedex 2, France
⁷ INAF-IASF, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy

Received: 12 May 2016
Accepted: 17 September 2016

Abstract

Context. Explaining the accelerated expansion of the Universe is one of the fundamental challenges in physics today. Cosmography provides information about the evolution of the universe derived from measured distances, assuming only that the space time geometry is described by the Friedman-Lemaitre-Robertson-Walker metric, and adopting an approach that effectively uses only Taylor expansions of basic observables.

Aims. We perform a high-redshift analysis to constrain the cosmographic expansion up to the fifth order. It is based on the Union2 type Ia supernovae data set, the gamma-ray burst Hubble diagram, a data set of 28 independent measurements of the Hubble parameter, baryon acoustic oscillations measurements from galaxy clustering and the Lyman-α forest in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), and some Gaussian priors on h and Ω_M.

Methods. We performed a statistical analysis and explored the probability distributions of the cosmographic parameters. By building up their regions of confidence, we maximized our likelihood function using the Markov chain Monte Carlo method.

Results. Our high-redshift analysis confirms that the expansion of the Universe currently accelerates; the estimation of the jerk parameter indicates a possible deviation from the standard ΛCDM cosmological model. Moreover, we investigate implications of our results for the reconstruction of the dark energy equation of state (EOS) by comparing the standard technique of cosmography with an alternative approach based on generalized Padé approximations of the same observables. Because these expansions converge better, is possible to improve the constraints on the cosmographic parameters and also on the dark matter EOS.

Conclusions. The estimation of the jerk and the DE parameters indicates at 1σ a possible deviation from the ΛCDM cosmological model.