[en] We investigate the thermodynamic behaviour of AdS quasitopological black hole solutions in the context of extended thermodynamic phase space, in which the cosmological constant induces a pressure with a conjugate volume. We find that the third order exact quasitopological solution exhibits features consistent with the third order Lovelock solutions for positive quasitopological coupling, including multiple reentrant phase transitions and isolated critical points. For negative coupling we find the first instances of both reentrant phase transitions and thermodynamic singularities in five dimensions, along with other modified thermodynamic behaviour compared to Einstein-AdS-Gauss Bonnet gravity.

[en] Thermalons can mediate phase transitions between different vacua in higher curvature gravity, potentially changing the asymptotic structure of the spacetime. Treating the cosmological constant as a dynamical parameter, we study these phase transitions in the context of extended thermodynamic phase space. We find that in addition to the AdS to dS phase transitions previously studied, thermal AdS space can undergo a phase transition to an asymptotically flat black hole geometry. In the context of AdS to AdS transitions, we comment on the similarities and differences between thermalon transitions and the Hawking-Page transition.

[en] Minimal D = 5 supergravity admits asymptotically globally AdS₅ gravitational solitons (stationary, geodesically complete, horizonless spacetimes with positive mass). We show that, like asymptotically flat gravitational solitons, these solutions satisfy mass and mass variation formulas analogous to those satisfied by AdS black holes. A thermodynamic volume associated to the non-trivial topology of the spacetime plays an important role in this construction. We then consider these solitons within the holographic ‘complexity equals action’ and ‘complexity equals volume’ conjectures as simple examples of spacetimes with nontrivial rotation and topology. We find distinct behaviours for the volume and action, with the counterterm for null boundaries playing a significant role in the latter case. For large solitons we find that both proposals yield a complexity of formation proportional to a power of the thermodynamic volume, V ^3/4. In fact, up to numerical prefactors, the result coincides with the analogous one for large black holes. (paper)

[en] We obtain charged and rotating black hole solutions to the novel 3D Gauss–Bonnet theory of gravity recently proposed, both of which generalize the Banados–Teitelboim–Zanelli (BTZ) metric. The charged solutions are obtained in the Maxwell and Born–Infeld theories and feature ‘universal thermodynamics’—identical to the thermodynamics of their Einstein cousins. The rotating Gauss–Bonnet BTZ black holes are quite exceptional and are not of constant curvature. They possess an ergoregion and outer horizon, but do not have an inner horizon. We present their basic properties and show that they break the universality of thermodynamics present for their static charged counterparts. Extending our considerations to higher dimensions, we also obtain novel 4D Gauss–Bonnet rotating black strings. (letter)

[en] As shown recently Hennigar et al (2019 (arXiv:1903.0866)), consistent thermodynamics of the Lorentzian Taub–NUT solutions with Misner strings present can be formulated provided a new pair of conjugate quantities (related to the NUT parameter) is introduced. In Hennigar et al (2019 (arXiv:1903.0866)) this pair was calculated from the Euclidean action but no geometrical interpretation for the new quantities was provided. In this paper we propose that the potential should be identified with the surface gravity of the Misner string and the conjugate Misner charge N can be obtained by a Komar-type integration over the tubes surrounding the string singularities. We show that similar tube contributions also modify the Komar formula for the thermodynamic volume. To render the integrals finite we employ the method of Killing co-potentials. By construction the new charges then satisfy the Smarr relation. Equipped with these geometrical notions, we generalize the first law for the (possibly charged) Taub–NUT spacetimes to account for asymmetric distributions of Misner strings and their potential variable strengths. (paper)

[en] We perform the first study of holographic heat engines where the working material is a rotating black hole, obtaining exact results for the efficiency of a rectangular engine cycle. We also make general considerations in the context of benchmarking these engines on circular cycles. We find an exact expression that is valid for black holes with vanishing specific heat at constant volume and derive an upper bound, below the Carnot efficiency and independent of spacetime dimension, which holds for any black hole of this kind. We illustrate our results with applications to a variety of black holes, noting the effects of spacetime dimension, rotation, and higher curvature corrections on the efficiency of the cycle. (paper)

[en] We construct a class of five dimensional black hole solutions to cubic quasi-topological gravity with conformal scalar hair and study their thermodynamics. We find these black holes provide the second example of black hole λ-lines: a line of second order (continuous) phase transitions, akin to the fluid/superfluid transition of ⁴He. Examples of isolated critical points are found for spherical black holes, marking the first in the literature to date. We also find various novel and interesting phase structures, including an isolated critical point occurring in conjunction with a double reentrant phase transition. The AdS vacua of the theory are studied, finding ghost-free configurations where the scalar field takes on a non-zero constant value, in notable contrast to the five dimensional Lovelock case.

[en] A new class of higher-curvature modifications of )-dimensional Einstein gravity has been recently identified. Densities belonging to this ‘Generalized quasi-topological’ class (GQTGs) are characterized by possessing non-hairy generalizations of the Schwarzschild black hole satisfying and by having second-order equations of motion when linearized around maximally symmetric backgrounds. GQTGs for which the equation of the metric function is algebraic are called ‘Quasi-topological’ and only exist for . In this paper we prove that GQTG and Quasi-topological densities exist in general dimensions and at arbitrarily high curvature orders. We present recursive formulas which allow for the systematic construction of nth order densities of both types from lower order ones, as well as explicit expressions valid at any order. We also obtain the equation satisfied by for general D and n. Our results here tie up the remaining loose end in the proof presented in Bueno et al (2019 (arXiv:1906.00987)) that every gravitational effective action constructed from arbitrary contractions of the metric and the Riemann tensor is equivalent, through a metric redefinition, to some GQTG. (paper)

[en] We investigate the thermodynamics of AdS black holes in Generalized Quasi-topological Gravity with and without electric charge, concentrating on the version of the theory that is cubic in curvature. We study new aspects of Hawking-Page transitions that occur for these black holes. Working within the framework of black hole chemistry, we find a variety of familiar and new critical behaviour and phase transitions in four and higher dimensions for the charged black holes. We also consider some holographic aspects of our work, demonstrating how the ratio of viscosity to entropy is modified by inclusion of these cubic curvature terms.

[en] By employing the new ultraspinning limit we construct novel classes of black holes with non-compact event horizons and finite horizon area and study their thermodynamics. Our ultraspinning limit can be understood as a simple generating technique that consists of three steps: i) transforming the known rotating AdS black hole solution to a special coordinate system that rotates (in a given 2-plane) at infinity ii) boosting this rotation to the speed of light iii) compactifying the corresponding azimuthal direction. In so doing we qualitatively change the structure of the spacetime since it is no longer possible to return to a frame that does not rotate at infinity. The obtained black holes have non-compact horizons with topology of a sphere with two punctures. The entropy of some of these exceeds the maximal bound implied by the reverse isoperimetric inequality, such black holes are super-entropic.