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Since their proposal, Lorentz violating theories of gravity have posed apotential threat to black hole thermodynamics, as superluminal signals appearedto be incompatible with the very black hole notion. Remarkably, it was soonrealized that in such theories causally disconnected regions of space-time canstill exist thanks to the presence of universal horizons: causal barriers forsignals of arbitrary high speed. Several investigations, sometimes withcontrasting results, have been performed so to determine if these horizons canbe associated with healthy thermodynamic properties similar to those associatedwith Killing horizons in General Relativity. In this work we offer what we deemto be the final picture emerging from this and previous studies. In summary weshow that: 1) there is a thermal, and most of all species-independent, emissionassociated to universal horizons, determined by their surface gravity; 2) dueto the modified dispersion relation of the matter fields, the low energy partof the emitted spectrum is affected by the presence of the Killing horizon, ina way similar to an effective refractive index, leading at low energies (w.r.t.the Lorentz breaking scale) to an emission that mimics a standard Hawkingspectrum (i.e. one determined by the Killing horizon surface gravity); 3) thewhole picture is compatible with a globally well defined vacuum state i.e. anUnruh state associated with preferred observers, which however at very lowenergies it is basically indistinguishable from the standard Unruh vacuumassociated to metric free-falling observers. One can then conclude that Hawkingradiation is remarkably resilient even within the context of gravitationaltheories entailing the breakdown of local Lorentz invariance.

Hawking Radiation in Lorentz Violating Gravity: A Tale of Two Horizons