On the Hubble Constant Tension in the SNe Ia Pantheon Sample

The Hubble constant (H-0) tension between Type Ia supernovae (SNe Ia) and Planck measurements ranges from 4 to 6 sigma. To investigate this tension, we estimate H-0 in the Lambda CDM and w(0)w(a)CDM (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 H-0 values with a function mimicking the redshift evolution: g(z) = H-0 (z) = (H) over tilde (0)/(1 + z)(alpha), where alpha indicates an evolutionary parameter and (H) over tilde (0) = H-0 at z = 0. We set the absolute magnitude of SNe Ia so that H-0 = 73.5 km s(-1) Mpc(-1), and we fix fiducial values for Omega(Lambda CDM)(0m) = 0.298 and Omega(w0,waCDM)(0m) = 0.308. We find that H-0 evolves with redshift, showing a slowly decreasing trend, with alpha coefficients consistent with zero only from 1.2 to 2.0 sigma. Although the alpha coefficients are compatible with zero in 3 sigma, this however may affect cosmological results. We measure locally a variation of H-0 (z = 0) - H-0 (z = 1) = 0.4 km s(-1) Mpc(-1) in three and four bins. Extrapolating H-0 (z) to z = 1100, the redshift of the last scattering surface, we obtain values of H-0 compatible in 1 sigma with Planck measurements independent of the cosmological models and number of bins we investigated. Thus, we have reduced the H-0 tension in the range from 54% to 72% for both cosmological models. If the decreasing trend of H-0 (z) is real, it could be due to astrophysical selection effects or to modified gravity.