Alleviating the Hubble-constant tension and the growth tension via a transition of absolute magnitude favored by the Pantheon+ sample

We establish a cosmological-model-independent method to extract the apparent magnitude and its derivative at different redshifts from the Pantheon+ type Ia supernova sample, and find that the obtained values deviate clearly from the prediction of the ΛCDM model at the lowest redshift. This deviation can be explained as a result of a transition of the absolute magnitude M in the low redshift region. The observations seem to favor this transition since the minimum values of χ² for two ansatzes of a varying M are less than that of a constant M. The Hubble constant tension is alleviated from larger than 5σ to be about $1$ to 2σ for a varying M, and the growth tension can be resolved after attributing the variation of M to a modification of the effective Newton's constant.