  The luminosity of fading type Ia supernovae is governed by radioactive decays
of 56Ni and 56Co. The decay rates are proportional to the Fermi coupling
constant G_F and, therefore, are determined by the vacuum expectation value v
of the Brout-Englert-Higgs field. We use publicly available sets of light
curves of type Ia supernova at various redshifts to constrain possible
spacetime variations of the 56Ni decay rate. The resulting constraint is not
very tight; however, it is the only direct bound on the variation of the decay
rate for redshifts up to z~1. We discuss potential applications of the result
to searches for non-constancy of G_F and v.
