Scale hierarchy in Horava-Lifshitz gravity: a strong constraint from   synchrotron radiation in the Crab nebula
  Horava-Lifshitz gravity models contain higher order operators suppressed by a characteristic scale, which is required to be parametrically smaller than the Planck scale. We show that recomputed synchrotron radiation constraints from the Crab nebula suffice to exclude the possibility that this scale is of the same order of magnitude as the Lorentz breaking scale in the matter sector. This highlights the need for a mechanism that suppresses the percolation of Lorentz violation in the matter sector and is effective for higher order operators as well. 
Stefano Liberati, Luca Maccione, Thomas P. Sotiriou
gr-qc astro-ph.HE hep-th