Quasilocal approach to general universal horizons

Understand

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.