  Type-Ia supernovae (SN Ia) are powerful stellar explosions that provide
important distance indicators in cosmology. There is significant tension
between values of the Hubble constant (expansion rate of the universe)
determined from SN Ia and from other data. Recently, we proposed a new SN Ia
mechanism that involves a nuclear fission chain reaction in an isolated white
dwarf [PRL 126, 1311010]. We find the average mass of an exploding star
decreases with increasing enrichment f_5 -- the fraction of uranium that is the
isotope U-235. As a result, the average SN Ia luminosity decreases with
increasing f_5. Furthermore, f_5 is likely higher in the host galaxies of SN Ia
observed at large redshift $z$ because of younger galaxy ages. This change of
f_5 leads to the evolution of SN Ia properties with redshift. If f_5 increases
with redshift this results in an increased SN Ia rate, but a lower average SN
Ia luminosity.
