  The light curve of a type Ia supernova decays at a rate set by the beta-decay
lifetimes of the Ni-56 and Co-56 produced in the explosion. This makes such a
light curve sensitive to the value of the Fermi constant G_F at the time of the
supernova. Using data from the CfA Supernova Archive, we measure the dependence
of the light curve decay rate on redshift and place a bound on the time
variation of G_F of |(dG_F/dt)/G_F| < 10^(-9) / y.
