Cosmic chronometers with photometry: a new path to H(z)

We present a proof-of-principle determination of the Hubble parameter H(z) from photometric data, obtaining a determination at an effective redshift of z = 0.75 (0.65 < z < 0.85) of H(0.75) = 105.0+/-7.9(stat)+/-7.3(sys) km s-1 Mpc-1, with 7.5% statistical and 7% systematic (10% with statistical and systematics combined in quadrature) accuracy. This is obtained in a cosmology model-independent fashion, but assuming a linear age-redshift relation in the relevant redshift range, as such, it can be used to constrain arbitrary cosmologies as long as H(z) can be considered slowly varying over redshift. In particular, we have applied a neural network, trained on a well-studied spectroscopic sample of 140 objects, to the COSMOS2015survey to construct a set of 19 thousand near-passively evolving galaxies and build an age-redshift relation. The Hubble parameter is given by the derivative of the red envelope of the age-redshift relation. This is the first time the Hubble parameter is determined from photometry at <~ 10% accuracy. Accurate H(z) determinations could help shed light on the Hubble tension; this study shows that photometry, with a reduction of only a factor of two in the uncertainty, could provide a new perspective on the tension.