Quasilocal approach to general universal horizons
  Theories of gravity with a preferred foliation usually display arbitrarily
fast signal propagation, changing the black hole definition. A new inescapable
barrier, the universal horizon, has been defined and many static and
spherically symmetric examples have been studied in the literature. Here, we
translate the usual definition of the universal horizon in terms of an optical
scalar built with the preferred flow defined by the preferred spacetime
foliation. The new expression has the advantages of being of quasilocal nature
and independent of specific spacetime symmetries in order to be well defined.
Therefore, we propose it as a definition for general quasilocal universal
horizons. Using the new formalism we show that there are no universal analog of
cosmological horizons for FLRW models for any scale factor function, and we
also state that quasilocal universal horizons are restricted to trapped regions
of the spacetime. Using the evolution equation, we analyze the formation of
universal horizons under a truncated Horava-Lifshitz theory, in spherical
symmetry, showing the existence of regions in parameter space where the
universal horizon formation cannot be smooth from the center, under some
physically reasonable assumptions. We conclude with our view on the next steps
for the understanding of black holes in nonrelativistic gravity theories.
