A positive energy theorem for Einstein-aether and Ho\v{r}ava gravity
  Energy positivity is established for a class of solutions to Einstein-aether theory and the IR limit of Ho\v{r}ava gravity within a certain range of coupling parameters. The class consists of solutions where the aether 4-vector is divergence free on a spacelike surface to which it is orthogonal (which implies that the surface is maximal). In particular, this result holds for spherically symmetric solutions at a moment of time symmetry. 
David Garfinkle and Ted Jacobson
gr-qc hep-th