Testing Gravity Using Type Ia Supernovae Discovered by Next-Generation
  Wide-Field Imaging Surveys
  In the upcoming decade cadenced wide-field imaging surveys will increase the
number of identified $z<0.3$ Type~Ia supernovae (SNe~Ia) from the hundreds to
the hundreds of thousands. The increase in the number density and solid-angle
coverage of SNe~Ia, in parallel with improvements in the standardization of
their absolute magnitudes, now make them competitive probes of the growth of
structure and hence of gravity. The peculiar velocity power spectrum is
sensitive to the growth index $\gamma$, which captures the effect of gravity on
the linear growth of structure through the relation $f=\Omega_M^\gamma$. We
present the first projections for the precision in $\gamma$ for a range of
realistic SN peculiar-velocity survey scenarios. In the next decade the
peculiar velocities of SNe~Ia in the local $z<0.3$ Universe will provide a
measure of $\gamma$ to $\pm 0.01$ precision that can definitively distinguish
between General Relativity and leading models of alternative gravity.
