The LSST project anticipates detecting 300,000 Type-1a supernovae over 10 years. Since these are standardizable candles, their estimated distance and redshift can be used to estimate peculiar velocities. I will describe how such a velocity catalog can be used to constrain dark energy and modified gravity. The mean pairwise velocity statistic provides notable control over systematic errors. If supernova host galaxy redshifts are measured spectroscopically with BigBOSS, the resulting constraints on the dark energy evolution parameter (w_a) and the growth index parameter (gamma) describing modifications of gravity are among the strongest of any proposed methods.