High-Redshift Cosmography

TL;DR

This paper uses a minimal assumption cosmographic approach with recent supernovae and gamma-ray burst data to constrain the universe's kinematic parameters up to high redshift, revealing a negative deceleration parameter.

Contribution

It extends cosmographic analysis to redshifts up to 6.6 using the y-parameter expansion and demonstrates the constraining power of gamma-ray bursts at high redshift.

Findings

Gamma Ray Bursts improve high-redshift constraints.

Deceleration parameter q_0 is definitively negative at 3σ.

Cosmographic parameters can be reliably estimated up to z=6.6.

Abstract

We constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires a very minimal set of assumptions (namely to specify a metric) and does not rely on the dynamical equations for gravity. On the data side, we consider the most recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows to further extend the cosmographic fit up to $z = 6.6$, i.e. up to redshift for which one could start to resolve the low z degeneracy among competing cosmological models. In order to reliably control the cosmographic approach at high redshifts, we adopt the expansion in the improved parameter $y = z/(1+z)$. This series has the great advantage to hold also for $z > 1$ and hence it is the appropriate tool for handling data including non-nearby distance indicators. We find that Gamma Ray Bursts, probing higher redshifts than Supernovae, have constraining power and do require (and statistically allow) a cosmographic expansion at higher order than Supernovae alone. Exploiting the set of data from Union and GRBs catalogues, we show (for the first time in a purely cosmographic approach parametrized by deceleration $q_0$, jerk $j_0$, snap $s_0$) a definitively negative deceleration parameter $q_0$ up to the 3$\sigma$ confidence level. We present also forecasts for realistic data sets that are likely to be obtained in the next few years.