The explanation of black hole entropy as statistical entropy is one of the big successes of string theory. In this article we review recent progress in this subject, focussing on understanding quantum effects on black hole entropy. Supersymmetry plays a key role in these developments and leads to prototype systems where we can discuss quantum effects to great precision. Our discussion has two strands, both of which involve the gravitational path integral that calculates the supersymmetric index. In the first strand we discuss supersymmetric black holes in the microcanonical ensemble, which are decoupled from the environment and can be treated as independent quantum systems. Using methods of supersymmetric localization one can arrive at the integer quantum degeneracies of such systems purely in terms of the gravitational variables. In the second strand we consider grand-canonical ensembles in gravity in which black holes arise as a finite-action excitation in asymptotically flat or Anti de Sitter space. In this context we discuss the saddle-points of the gravitational index, and how they reproduce the black hole action and entropy that agree with the index of the holographic superconformal field theory even beyond the leading order in the semiclassical approximation. Finally we discuss how the gravitational index informs us about the detailed structure of the non-perturbative sum over saddle points and the resulting phases of the theory. Throughout, we try to highlight the central role played by the methods as well as foundational concepts of supergravity in driving these developments.

We compute the partition function of the WZW model with target a compact Lie group G by adapting a method used by Choi and Takhtajan to compute the heat kernel of the group manifold. The basic idea is to compute the partition function of a supersymmetric version of the WZW model using a form of supersymmetric localization and then use the fact that, since the fermions of the supersymmetric WZW model are actually decoupled from the bosons, this also determines the partition function of the purely bosonic WZW model. The result is a formula for the partition function as a sum over contributions from abelian classical solutions. We verify for G=SU(2) that this formula agrees with the result for the same partition function that comes from the Weyl-Kac character formula. We extend the method of supersymmetric localization to certain related models such as the SL(2,ℝ) WZW model and a Wick-rotated version of this model in which the target space is hyperbolic three-space H_3^+.

Abstract We discuss the Kontsevich–Segal–Witten (KSW) criterion for the allowability of complex metrics, in the context of the gravitational path integral that calculates the supersymmetric index. We focus on the saddle points that capture the contribution of supersymmetric black holes in AdS 5 space. We show that, for such black holes with two independent angular momenta, the conditions imposed on the corresponding saddle point by the KSW criterion are equivalent to the ones arising from the convergence of the microscopic trace form of the supersymmetric index. This result adds to previous results establishing such an equivalence in other, simpler examples of the gravitational index in AdS space and flat space. Along the way, we give a practical algorithm for implementing the KSW criterion in terms of eigenvalues of certain matrices.

We discuss the Kontsevich-Segal-Witten criterion for the allowability of complex metrics, in the context of the gravitational path integral that calculates the supersymmetric index. We focus on the saddle points that capture the contribution of supersymmetric black holes in AdS_5 space. We show that, for such black holes with two independent angular momenta, the conditions imposed on the corresponding saddle point by the KSW criterion are equivalent to the ones arising from the convergence of the microscopic trace form of the supersymmetric index. This result adds to previous results establishing such an equivalence in other, simpler examples of the gravitational index in AdS space and flat space. Along the way, we give a practical algorithm for implementing the KSW criterion in terms of eigenvalues of certain matrices.

A bstract We study the one-loop partition function of superstrings in the AdS 3 × S 3 × S 3 × S 1 background. Specifically, we show that the supergravity spectrum, which contains non-chiral primary states unlike other similar AdS 3 backgrounds, can be recovered by this partition function in the semiclassical limit. We also show how the boundary currents are encoded in the string spectrum. Furthermore, we discuss the effect of these boundary currents in the quantum partition function of near-extremal black holes in theories with large $$ \mathcal{N} $$ N = (4, 4) supersymmetry, recovering results consistent with the analysis of the near-horizon (super-)Schwarzian theory. In particular, we show how the BPS index of the large $$ \mathcal{N} $$ N = (4, 4) theory, which turns out to be temperature dependent, captures the spectrum of excitations around supersymmetric BTZ black holes. Finally, we comment on the limit when the AdS curvature radius is string scale, which is directly accessible within the RNS formalism for this compactification. The spectrum of the limiting theory we obtain differs from the tensionless string spectrum derived in the literature, suggesting that there is not a unique worldsheet CFT for this value of parameters.

The strongly-coupled 3-dimensional theory, holographically dual to black branes at fixed chemical potential μ and temperature T ≪ μ is considered in AdS4 Einstein-Maxwell theory. The retarded Green’s functions at frequency ω is calculated using holography in the regime ω, T ≪ μ but otherwise arbitrary. When the transverse space has finite volume, there is a non-zero energy scale Egap, scaling as 1/μ for large μ, below which quantum-gravitational corrections due to the fluctuations of the nearly-gapless Schwarzian modes become important. Such corrections to the retarded Green’s function are calculated at different relative values of ω, T, and Egap. The ω → 0 limit is used to define the shear viscosity η. As the temperature is lowered below μ, quantum corrections are found to increase the value of η with respect to its semiclassical value. The quantum-corrected result for η diverges as √(E_gap/T) at T ≪ Egap, in accord with corresponding results for the absorption cross section. The quantum result for the ratio η/s, where s is the entropy density, dips below the semiclassical limit of 1/4π when Egap ≪ T ≪ μ, then turns back to increase towards lower temperatures, and finally diverges at temperatures much below Egap.

We study invariants of bosonic and fermionic (Grassmann-valued) matrices under the adjoint action of U(N), weighted by the fermion number. Such models naturally appear as the supersymmetric indices of supersymmetric gauge theories and are captured by U(N) matrix models. We discuss two features of the fermionic models that are qualitatively different from bosonic models. Firstly, the 2N^th power of a Grassmann matrix vanishes, which gives rise to many new trace relations. Secondly, trace relations in models involving fermions could cause an increase in the supersymmetric index as N decreases, in contrast with purely bosonic models. We focus on a simple model involving one fermion and one derivative that corresponds to a 1/4-BPS supersymmetric index in 𝒩=4 SYM theory, in which we find that the index is independent of N. We prove this rank-independence analytically, and experimentally study the cancellations between bosonic and fermionic trace relations that lead to it. Based on these observations, we make some conjectures on resulting algebraic structures, including the analogue of the polarized Cayley-Hamilton identities and the Second Fundamental Theorem of invariants in the presence of Grassmann matrices. Finally, we present various (smooth and singular) limits of the most general supersymmetric index in 𝒩=4 SYM theory, and study some patterns in their behavior as a function of N.

We present saddle-points of the Euclidean Gravitational Path Integral (GPI) corresponding to the supersymmetric index of the D1-D5-P black string. These saddles are complex, supersymmetric, non-extremal solutions of 10-dimensional IIB supergravity theory with arbitrary inverse temperature β. The solutions carry fixed monopole charges Q_1, Q_5, Q_n, and angular momentum J_L equal to the extremal supersymmetric black string. Crucially, the solutions have fixed angular velocity Ω_R = - 2 πi /β, which is the chemical potential dual to J_R. This implements the insertion of (-1)^F in the GPI. The Gibbons-Hawking on-shell action is temperature-independent and agrees with the supersymmetric index of the extremal black string carrying the same monopole charges. Upon taking a finite-temperature near-horizon decoupling limit, we obtain solutions of the form S^3 fibered over BTZ. Although these near-horizon solutions have finite holomorphic and anti-holomorphic modular parameters, their on-shell action reproduces the Cardy formula of the holomorphic elliptic genus of the dual D1-D5 SCFT_2.

The count of microstates for supersymmetric black holes is typically obtained from a supersymmetric index in weakly-coupled string theory. We find the saddles in the gravitational path integral corresponding to this index in a general theory of N=2 supergravity in asymptotically flat space. This saddle exhibits a new attractor mechanism which explains the agreement between the string theory index and the macroscopic entropy. These saddles are smooth, complex Euclidean spinning black holes that are supersymmetric but not extremal, i.e., they are formally finite-temperature solutions. With this new mechanism, the scalars and the electromagnetic fields get attracted to temperature- and moduli-independent values at the north and south poles of the rotating black hole, although they vary along the Euclidean horizon in a non-universal way. Further, although the area and the spin of the black hole depend non-trivially on the temperature and on the moduli, the free energy is essentially a function only of the black hole charges (apart from a trivial dependence on the temperature and the moduli through the BPS mass), and agrees with the string theory index.

We revisit the computation of the string partition function in AdS_3 focussing on the appearance of spacetime (super) symmetries. We show how the asymptotic symmetries of the AdS_3 spacetime, which generate the boundary (super) Virasoro currents, are captured by the one-loop partition sum. We use this to argue that the recent understanding of near-extremal black hole thermodynamics based on the gravitational path integral continues to hold for finite string length. Along the way we clarify some aspects of the AdS_3/CFT_2 duality and, in particular, deduce which bulk gauge fields lead to boundary currents. We also explain how one can interpolate between supersymmetric and thermal (Atick-Witten) fermion boundary conditions in the target space by suitably tuning rotational chemical potentials in the string partition function.

The supersymmetric index in string theory can sometimes have a discontinuous integer-valued jump at co-dimension one surfaces in moduli space called walls of marginal stability. When the index counts black hole microstates, crossing such walls of marginal stability amounts to the appearance or disappearance of a large number of such states. While wall-crossing has been understood in string theory and through the disappearance of extremal Lorentzian supergravity solutions as the moduli are varied, there has been no understanding about how the discontinuous changes in the index occur at the level of the gravitational path integral. In this paper, we find the finite-temperature saddles in 4d flatspace supergravity in which fermionic fields are periodic when going around the thermal circle that correspond to the multi-center black hole contributions to the index. By analyzing these saddles, we can explain how wall-crossing occurs: as the scalar moduli in supergravity are varied at the asymptotic boundary, for a given split of the charges, the saddle point equations can no longer be solved and, consequently, the corresponding multi-center saddle no longer contributes to the index. While the values of the scalars and the jump in the index when a wall is crossed all agree with the prediction from previously found Lorentzian supergravity solutions, the saddles in the index exhibit a much richer moduli space, which we analyze in detail.

We construct an infinite set of conserved tensor currents of rank $2n$, $n=1,2,\dots$, in the two-dimensional theory of free massive fermions, which are bilinear in the fermionic fields. The one-point functions of these currents on the torus depend on the modular parameter $\tau$ and spin structure $(\alpha,\beta)$. We show that, upon scaling the mass $m$ so as to keep the combination $m^2$Im($\tau$) invariant, the one-point functions are non-holomorphic Jacobi forms of weights $(2n,0)$ or $(0,2n)$ and index 0, with respect to the modular parameter $\tau$ and elliptic parameter $z=\alpha\tau+\beta$. In particular, we express the one-point functions as Kronecker-Eisenstein-type sums over the lattice $\mathbb{Z}\tau+\mathbb{Z}$, which makes the modular symmetry manifest. We show that there is an action of three differential operators on these Jacobi forms which form an $\mathfrak{sl}_2(\mathbb{R})$ Lie algebra. Further we show that these Jacobi forms obey three differential equations arising from the representation theory of the Jacobi group.

The 1/2 -BPS indices of 𝒩 = 4 Super Yang-Mills theory with unitary, orthogonal, and symplectic groups all admit q-expansions suggesting an interpretation in terms of D- branes in the dual bulk AdS5 string theories. We present a derivation of these expansions in the corresponding bulk duals by quantizing the moduli space of 1/2 -BPS giant gravitons using supersymmetric localization, extending and clarifying our study in arxiv:2312.14921. We perform a detailed analysis of the one-loop fluctuations around the maximal giants (the fixed points), and show how the Hamiltonian analysis is recovered from the functional integral for the equivariant index. We show that the analytic continuation for these giant graviton expansions observed in the literature maps precisely to a wall-crossing phenomenon for the index. In the case of orthogonal and symplectic gauge groups, the ℤ2 quotient in the bulk leads to a corresponding projection in the q-expansion. Additional terms in the expansion related to the Pfaffian operator arise from topologically stable branes in the bulk dual on AdS5 × RP^5 .

We construct a series of novel Euclidean multi-black-hole, black ring, black Saturn, and black lens solutions to 5d supergravity that contribute as saddle-points to the 5d gravitational supersymmetric index, either in asymptotically flat space or in asymptotically AdS_3× S^2. All these solutions are supersymmetric, have finite temperature, and an appropriate angular velocity turned on that makes fermionic fields periodic around the thermal circle. They contribute either to the helicity supertrace of supergravity in 5d flat space or to the elliptic genus of a supergravity theory in AdS_3 × S^2. Their on-shell actions are independent of temperature, as consistent with the computation of a protected index, and equal to the entropy of the corresponding extremal black object. Our construction relies on uplifting saddles that can be singular in 4d, but which are desingularized in 5d. The resulting saddles exhibit a novel “index enigma”, not encountered in previous Lorentzian solutions. One example of this enigma is that, in the computation of the index in asymptotically flat space, less symmetric black ring saddles dominate over the contributions from 5d black holes.

Logarithmic corrections to the entropy of extremal black holes have been successfully used to accurately match degeneracies from microscopic constructions to calculations of the gravitational path integral. In this paper, we revisit the problem of deriving such corrections for the case of extremal black holes, either non-supersymmetric or supersymmetric, and for near-extremal black holes. The zero-modes that are present at extremality are crucial, since their path integral cannot be treated quadratically and needs to be regulated. We show how the regulated result can be obtained by taking the zero-temperature limit of either the 4d Einstein-Maxwell or 4d supergravity path integral to find the Schwarzian or super-Schwarzian theories. This leads to drastically different estimates for the degeneracy of extremal black hole: it vanishes when the extremal limit does not preserve supersymmetry, while it reproduces Bekenstein-Hawking in the BPS case. In a companion paper, we discuss how such zero-modes affect the calculation of BPS black holes degeneracies, using supersymmetric localization for an exact computation of the gravitational path integral.

The supersymmetric index of 5d black strings and spinning black holes in M-theory is related to that of 4d black holes in type IIA supergravity when both theories are compactified on the same Calabi-Yau threefold. We find the finite-temperature saddles for the 5d gravitational supersymmetric index by uplifting the recently found attractor saddles of the corresponding 4d index. We study uplifts for two types of geometries: 5d black holes and 5d black strings. For 5d black holes, the uplift guarantees that the index of 4d and 5d black holes match. For 5d black strings, the saddle reproduces the microscopic index at leading order in G_N , even without the conventional decoupling limit taken in AdS/CFT. In particular, when the temperature is set to be finite in the 5d flat space region, the black string index is computed from an asymptotically flat solution where the AdS throat is absent. Further, as the temperature is lowered and eventually becomes infinitesimally small in the flat space region, the solution admits a novel decoupling limit in which the AdS_3 throat takes the form of a finite-temperature BTZ black hole that is known to compute the index in AdS_3/CFT_2. This represents the first step towards understanding holography for supersymmetric observables in flat space, away from the decoupling limit.

The study of black holes in string theory has led to the discovery of deep and surprising connections between black holes and modular forms -- which are two classical, a priori unrelated, subjects. This article explains the main physical and mathematical ideas behind these connections. It is known from the pioneering work of J.Bekenstein and S.Hawking in the 1970s that black holes have thermodynamic entropy, and should therefore be made up of a collection of microscopic quantum states. Superstring theory provides a framework wherein we can associate a number of microscopic states that make up the quantum-statistical system underlying a black hole, thus explaining their thermodynamic behavior from a more fundamental point of view. %The above-mentioned connections arise from the observation that, i The basic connection to modular forms arises from the observation that, in the simplest superstring-theoretic construction, the generating function of the number of microscopic states is a modular form. In one direction, modular symmetry acts as a powerful guide to the calculation of quantum-gravitational effects on the black hole entropy. In the other direction, the connection has led to the discovery of surprising relations between Ramanujan's mock modular forms and a class of string-theoretic black holes, thus providing an infinite number of new examples of mock modular forms.

Reproducing the integer count of black hole micro-states from the gravitational path integral is an important problem in quantum gravity. In this paper, we show that, by using supersymmetric localization, the gravitational path integral for $\frac{1}8$-BPS black holes in $\mathcal{N}=8$ supergravity reproduces the index obtained in the string theory construction of such black holes, including all non-perturbatively suppressed geometries. A more refined argument then shows that, not only the black hole index, but also the total number of black hole microstates within an energy window above extremality that is polynomially suppressed in the charges, also matches this string theory index. To achieve such a match we compute the one-loop determinant arising in the localization calculation for all $\mathcal{N}=2$ supergravity supermultiplets in the $\mathcal{N}=8$ gravity supermultiplet. Furthermore, we carefully account for the contribution of boundary zero-modes, that can be seen as arising from the zero-temperature limit of the $\mathcal{N}=4$ super-Schwarzian, and show that performing the exact path integral over such modes provides a critical contribution needed for the match to be achieved. A discussion about the importance of such zero-modes in the wider context of all extremal black holes is presented in a companion paper.

We study the Konstevich–Segal–Witten criterion for allowable complex metrics, in the context of the gravitational path integral corresponding to the supersymmetric index. In various theories of supergravity in asymptotically flat and asymptotically AdS space, the exponential growth of states of the corresponding microscopic index in string theory is known to be captured by complex saddle points of this path integral. We compare the KSW criterion for these complex saddles against constraints from geometric consistency and the convergence of microscopic indices for the same saddles. In all four-dimensional situations we find that the three criteria precisely agree with each other. However, in the AdS_5 dual to the superconformal index with unequal chemical potentials for the two angular momenta, we find that this agreement does not hold. The region of convergence of microscopic index in parameter space is a strict subset of the region allowed by the KSW criterion, which in turn is a strict subset of the geometric consistency conditions. We conclude that the KSW criterion is necessary but not sufficient for the allowability of complex metrics contributing to the superconformal index.

The fundamental heterotic string has a tower of BPS states whose supersymmetric index has an exponential growth in the charges. We construct the saddle-point of the gravitational path integral corresponding to this index. The saddle-point configuration is a supersymmetric rotating non-extremal Euclidean black hole. This configuration is singular in the two-derivative theory. We show that the addition of higher-derivative terms in four-dimensional $N=2$ supergravity resolves the singularity. In doing so, we extend the recently-developed "new attractor mechanism" to include the effect of higher-derivative terms. Remarkably, the one-loop, four-derivative F-term contribution to the prepotential leads to a precise match of the gravitational and microscopic index. We also comment, using the effective theory near the horizon, on the possibility of a string-size near-extremal black hole. Our results clarify the meaning of different descriptions of this system in the literature. The thermal state transitions to a winding condensate and a gas of strings without ever reaching a small black hole, while the index is captured by the rotating Euclidean black hole solution and is constant and thus smoothly connected to the microscopic ensemble.