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