What do the cosmological supernova data really tell us?

TL;DR

This study reconstructs the universe's expansion history from supernova data using a model-independent approach, finding the transition to acceleration around redshift 0.50 and deriving limits on matter and dark energy densities solely from luminosity distance data.

Contribution

It introduces a cosmographic method to analyze supernova data without assuming a specific cosmological model, and assesses the impact of including GRB data on the results.

Findings

Transition to cosmic acceleration at z ≈ 0.50

Adding GRB data reduces irregularities in reconstruction

Limits on matter and dark energy densities derived from luminosity distances

Abstract

Not much by themselves, aparently. We try to reconstruct the scale factor $a(t)$ of the universe from the SNe Ia data, i.e. the luminosity distance $d_{L}(z)$, using only the cosmological principle and the assumption that gravitation is governed by a metric theory. In our hence "model-independent," or "cosmographic" study, we fit functions to $d_{L}(z)$ rather than $a(t)$, since $d_{L}(z)$ is what is measured. We find that the acceleration history of the universe cannot be reliably determined in this approach due to the irregularity and parametrization-dependence of the results. However, adding the GRB data to the dataset cures most of the irregularities, at the cost of compromising the model-independent nature of the study slightly. Then we can determine the redshift of transition to cosmic acceleration as $z_{\rm t} \sim 0.50 \pm 0.09$ for a flat universe (larger for positive spatial curvature). If Einstein gravity (GR) is assumed, we find a redshift at which the density of the universe predicted from the $d_{L}(z)$ data is independent of curvature. We use this point to derive an upper limit on matter density, hence a lower limit on the density of dark energy. While these limits do not improve the generally accepted ones, they are derived *only using the $d_{L}(z)$ data*.