Improved derivation of the Smarr formula for Lorentz-breaking gravity

Thermodynamical properties of black holes in gravitational theories without local Lorentz invariance have been subject to intense investigation in past years due to the presence of universal horizons, which are strong causal barriers even for superluminal signals. Here we present a novel general method for deriving the Smarr formula for this class of theories: in particular we show that the Smarr formulas for Einstein-Æther theory and infrared Hořava gravity follow from scale invariance. We not only reproduce straightforwardly previous findings for static black hole solutions, but we are also able to generalize them to the case of stationary rotating black holes. Finally, we apply our results to the rotating black holes with universal horizon as found in three dimensions, from which we shall draw some lessons on the viability of black hole thermodynamics for black hole solutions endowed with universal horizons.