Hubble speed from first principles

We introduce a way of measuring H₀ from a combination of independent geometrical data sets, without the need for calibration or the choice of a cosmological model. We build on the distance duality relation, which sets the ratio of the luminosity and angular diameter distance to a fixed scaling in redshift for any metric theory of gravity with standard photon propagation and constitutes a founding block of any theory describing our Universe. Our method allows us to determine H₀ from first principles, unleashing the measurement of this fundamental constant from calibration and the assumption of a cosmological model. We find H₀=69.5±1.7 km/s/Mpc at 68% C.L., showing that the Hubble constant can be constrained at the percent level with minimal assumptions.