Cosmology with AGN: can we use quasars as standard candles?

TL;DR

This paper explores the potential of using quasars as standard candles for cosmology by leveraging the X-ray and UV luminosity relation, emphasizing the importance of dedicated observations to improve distance measurements and constrain high-redshift universe models.

Contribution

It demonstrates that dedicated X-ray observations can significantly enhance quasar-based cosmological measurements and discusses future prospects with large surveys and high-redshift quasar observations.

Findings

Dedicated X-ray observations improve distance estimate precision.

Large quasar samples can constrain high-redshift cosmology.

Observing ~10 quasars at z~3 is comparable to discovering a supernova at that redshift.

Abstract

The non-linear relation between X-ray and UV luminosity in quasars can be used to estimate their distance. Recently, we have shown that despite the large dispersion of the relation, a Hubble Diagram made of large samples of quasars can provide unique constraints on cosmology at high redshift. Furthermore, the dispersion of the relation is heavily affected by measurement errors: until now we have used serendipitous X-ray observations, but dedicated observations would significantly increase the precision of the distance estimates. We discuss the future role of XMM in this new field, showing (1) the fundamental contribution of the Serendipitous Source Catalogue and of large surveys, and (2) the breakthrough advancements we may achieve with the observation of a large number of SDSS quasars at high redshift: every ~10 quasars observed at z$\sim$3 would be equivalent to discovering a supernova at that redshift.