Spatial curvature sensitivity to local $H_0$ from the Cepheid distance ladder

Over the last few years, low- and high-redshift observations set off a tension in the measurement of the present-day expansion rate, $H_0$. Adding to the riddle, observational data from the Planck mission point to a $3.4\sigma$ evidence for a closed universe, further challenging the $\Lambda$CDM concordance model of cosmology. Recently, a direct-observational test has been proposed to discriminate effects of the spatial curvature in the cosmological model. The test is based on the fundamental distance--flux--redshift relation of the luminosity distance modulus, $\Delta \mu$. We reexamine the outcomes of this test and show that achieving the required $\Delta \mu$ sensitivity to discriminate among cosmological models is materially far more challenging than previously thought. Armed with supernova type Ia (SN Ia) data, calibrated using Cepheid measured distances, we apply the test to archetypal spatially non-flat models that ameliorate the $H_0$ tension and show that the $3\sigma$ contour of $\Delta \mu$ predicted by these models overlaps the 68\% CL SN Ia residuals with respect to $\Lambda$CDM. This implies that the spatial curvature remains insensitive to local $H_0$ measurements from the Cepheid distance ladder.