Generalized Free Energy Landscapes from Iyer-Wald Formalism

Abstract: The generalized free energy landscape plays a pivotal role in understanding black hole thermodynamics and phase transitions. In general relativity, one can directly derive the generalized free energy from the contributions of black holes exhibiting conical singularities. In this work, we extend this idea to general covariant theories. By employing Noether's second theorem, we present an alternative formulation of the Lagrangian, which can elucidate the role of conical singularities. We demonstrate that, in general, the contribution from conical singularities depends on the specific implementation of the regularization scheme and is not uniquely determined; this feature is explicitly exhibited and confirmed in three-dimensional new massive gravity. Nevertheless, these ambiguities can be absorbed into the second-order (and higher) corrections induced by conical singularities when the gravitational theory is described by the Lagrangian $L(g_{ab},R_{abcd})$. Moreover, for certain theories such as general relativity and Bumblebee gravity, this contribution simplifies to a well-defined result. However, the interpretation of the generalized free energy in Bumblebee gravity is somewhat different, with its extrema corresponding to the geometry of conical singularities. Our results uncover the particular properties of the generalized free energy beyond general relativity.