On the Hubble Constant Tension in the SNe Ia Pantheon Sample

The Hubble constant (H0) tension between Type Ia supernovae (SNe Ia) and Planck measurements ranges from 4 to 6σ. To investigate this tension, we estimate H0 in the ΛCDM and w0waCDM (cold dark matter) models by dividing the Pantheon sample, the largest compilation of SNe Ia, into 3, 4, 20, and 40 bins. We fit the extracted H0 values with a function mimicking the redshift evolution: g(z) = H0 (z) = H∼0 (1 + z)a, where α indicates an evolutionary parameter and H∼0 = H0 at z=0. We set the absolute magnitude of SNe Ia so that H0 = 73.5 km s-1Mpc-1, and we fix fiducial values for Ω0mΛCDM= 0.298 and Ω0mw0waCDM = 0.308. We find that H0 evolves with redshift, showing a slowly decreasing trend, with α coefficients consistent with zero only from 1.2 to 2.0σ. Although the α coefficients are compatible with zero in 3σ, this however may affect cosmological results. We measure locally a variation of H0(z = 0)-H0(z = 1) = 0.4 km s-1Mpc-1in three and four bins. Extrapolating H0 (z) to z=1100, the redshift of the last scattering surface, we obtain values of H0 compatible in 1σ with Planck measurements independent of the cosmological models and number of bins we investigated. Thus, we have reduced the H0 tension in the range from 54% to 72% for both cosmological models. If the decreasing trend of H0 (z) is real, it could be due to astrophysical selection effects or to modified gravity.