Testing f(R) gravity models with quasar X-ray and UV fluxes

TL;DR

This study uses quasar X-ray and UV fluxes, along with other cosmological data, to test f(R) gravity models as alternatives to dark energy, finding they are consistent with observations and allow small deviations from General Relativity.

Contribution

First application of quasar X-ray and UV fluxes to constrain f(R) gravity models alongside other cosmological data sets.

Findings

Constraints on f(R) model parameters tighten with BAO data.

Results are consistent with LCDM but allow small deviations from GR.

BAO data provides the most restrictive constraints.

Abstract

Recently, Active Galactic Nuclei (AGNs) have been proposed as standardizable candles, thanks to an observed non-linear relation between their X-ray and optical-ultraviolet (UV) luminosities, which provides an independent measurement of their distances. In this paper, we use these observables for the first time to estimate the parameters of f(R) gravity models (specifically the Hu-Sawicki and the exponential models) together with the cosmological parameters. The importance of this type of modified gravity theories lies in the fact that they can explain the late time accelerated expansion of the universe without the inclusion of a dark energy component. We have also included other observable data to the analyses such as estimates of the Hubble parameter H(z) from Cosmic Chronometers, the Pantheon Type Ia supernovae compilation, and Baryon Acoustic Oscillations measurements. Our results show that the allowed space parameter is restricted when both AGN and BAO data are added to CC and SnIa data, being the BAO data set the most restrictive one. We can also conclude that even though our results are consistent with the ones from the LCDM model, small deviations from General Relativity, than can be successfully described by the f(R) models studied in this paper, are also allowed by the considered data sets.