Testing viable f(R) models with the angular-diameter distance to compact quasar cores

We consider here some popular f(R) models generally viewed as possible alternatives to the existence of dark energy in General Relativity. For each of these, we compute the redshift z _max at which the angular diameter distance d (z) is expected to reach its maximum value. This turning point in d (z) was recently measured in a model-independent way using compact quasar cores and was found to occur at z _max = 1.70 ± 0.20. We compare the predictions of z _max for the f(R) models with this observed value to test their viability at a deeper level than has been attempted thus far, thereby quantifying an important observational difference between such modified gravity scenarios and standard Lambda Cold Dark Matter (ΛCDM) cosmology. Our results show that, while the most popular f(R) models today are consistent with this measurement to within 1σ, the turning point z _max will allow us to prioritize these alternative gravity theories as the measurement precision continues to improve, particularly with regard to how well they mitigate the tension between the predictions of ΛCDM and the observations. For example, while the Hu-Sawicki version of f(R) increases this tension, the Starobinky model reduces it.