A Hubble diagram for quasars

TL;DR

This paper introduces a novel method using quasars as standard candles via their UV and X-ray emission relation, extending the Hubble diagram to higher redshifts and constraining cosmological parameters.

Contribution

It demonstrates that quasars can serve as standard candles for cosmology, filling the observational gap between supernovae and the CMB.

Findings

Constructed a quasar Hubble diagram up to z ~ 5.

Provided constraints on matter and dark energy content.

Showed potential for future dark energy studies with larger quasar samples.

Abstract

The cosmological model is at present not tested between the redshift of the farthest observed supernovae (z ~ 1.4) and that of the Cosmic Microwave Background (z ~ 1,100). Here we introduce a new method to measure the cosmological parameters: we show that quasars can be used as "standard candles" by employing the non-linear relation between their intrinsic UV and X-ray emission as an absolute distance indicator. We built a sample of ~ 1,900 quasars with available UV and X-ray observations, and produced a Hubble Diagram up to z ~ 5. The analysis of the quasar Hubble Diagram, when used in combination with supernovae, provides robust constraints on the matter and energy content in the cosmos. The application of this method to forthcoming, larger quasar samples, will also provide tight constraints on the dark energy equation of state and its possible evolution with time.