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 A (z) is expected to reach its maximum value. This turning point in d A (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.

A lower limit of 50 microgauss for the magnetic field near the Galactic Centre.

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of approximately 100 parsecs (pc), fields of approximately 1,000 microgauss (microG; refs 1-3) have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas, the contested 'very hot' plasma, and the magnetic field) suggests fields of approximately 100 microG over approximately 400 pc size scales. Finally, assuming equipartition, fields of only approximately 6 microG have been inferred from radio observations for 400 pc scales. Here we report a compilation of previous data that reveals a downward break in the region's non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least approximately 50 microG on 400 pc scales, lest the synchrotron-emitting electrons produce too much gamma-ray emission, given other existing constraints. Other considerations support a field of 100 microG, implying that over 10% of the Galaxy's magnetic energy is contained in only less than or approximately 0.05% of its volume.