arXiv subjects

Emil J. Martinec

Publications and source records attributed to Emil J. Martinec.

At least 19 recordsLinked to original sources

arXiv2026

The black hole S-matrix in gauge/gravity duality

A variety of examples of gauge/gravity duality have a Coulomb branch for the gauge theory dynamics. We exploit this feature to construct an S-matrix, in which the initial state is a shell of branes converging on the origin of the Coulomb branch. In the gravitational dual, the shell of branes sources a geometry with a capped throat; the cap descends from the asymptotic region to larger and larger redshift. A trapped surface forms when the redshift of the cap reaches the point where the excitation of strings stretching between the branes is unsuppressed, and the dual gauge theory deconfines. The intermediate state is a long-lived black hole, which then decays via the slow emission of branes back onto the Coulomb branch. We compare and contrast the descriptions of this process in the bulk effective field theory and the dual gauge theory; and discuss the consequences of this construction for the black hole information paradox.

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arXiv2025

BPS Fivebrane Stars and BTZ Black Holes

We construct a large new class of BPS supergravity solutions parametrized by arbitrary chiral wave profiles on fundamental string and Neveu-Schwarz fivebrane sources. The effective action approach we employ describes the regime where the brane sources are slightly separated. String probes see these backgrounds as smooth and effectively capped off at the scale of that separation. Within the space of solutions is a large ensemble of ultracompact objects exhibiting many of the features of extremal BTZ black holes, in particular having a deep $AdS_2$ throat. We present the details of solutions with circular source profiles, and compare them to known superstratum geometries. The separation of the sources makes them structurally more like BPS fivebrane stars than like extremal black hole microstates. When evaluated on the supergravity background, the brane effective action governs (chaotic) near-BPS dynamics, and exhibits a collective mode analogous to the Schwarzian mode of $AdS_2$ black holes. Finally, we discuss the approach to the black hole phase as the sources coalesce, in which the semiclassical approximation breaks down due to the appearance of light D-brane excitations; this bulk realization of the fivebrane deconfinement transition connects directly to the Hawking-Page transition.

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arXiv2025

On the string theory of a single NS5-brane

We formulate a worldsheet description of string theory in the background of a single decoupled NS5-brane, which is a particularly simple example of ``little string'' holography. The worldsheet theory involves a gauged Wess-Zumino-Witten model for the group coset $\mathcal{G}/\mathcal{H}$, where $\mathcal{G} = {PSU(1,1|2)} \times \mathbb R^{5,1}$ and $\mathcal{H}=U(1) \times U(1)$. A variant of the embedding of $\mathcal{H}$ into $\mathcal{G}$ describes a background that interpolates between the fivebrane throat in the UV and $AdS_3$ in the IR, which is dual to the (single trace) $T\bar T$-deformed symmetric product orbifold $(\mathbb T^4)^N/S_N$. Well-known difficulties with a worldsheet theory for $n_5=1$ fivebranes are bypassed by the incorporation of an extra constraint on the dynamics. This constraint arises naturally from consideration of the effects of deforming the theory by one of the four Ramond-Ramond moduli of the spacetime background, which for $n_5=1$ imposes the constraint that our analysis has identified. Along the way, we construct a map between the supergroup formulation of the $n_5=1$ worldsheet theory and the more conventional RNS worldsheet formalism.

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arXiv2025

Effective Microstructure

In AdS$_3$/CFT$_2$ duality, there are large families of smooth, horizonless microstate geometries that correspond to heavy pure states of the dual CFT. The metric and fluxes are complicated functions of up to five coordinates. There are also many duals of heavy pure states that cannot be described in supergravity, but only admit a worldsheet description. Extracting the physical properties of these solutions is technically challenging. In this paper, we show that there are much simpler effective descriptions of these solutions that capture many of their stringy and geometrical features, at the price of sacrificing supergravity smoothness. In particular, the effective description of some families of superstrata, and of certain worldsheet solutions, is given by easy-to-construct three-center solutions. For example, the effective description of a superstratum with a long AdS$_2$ throat is a scaling, three-center solution in which the momentum wave is collapsed to a singular source at one of the three centers. This also highlights how momentum migrates away from the supertube locus in the back-reacted geometry. Our results suggest that effective descriptions can be extended to more general microstates, and that many singular multi-center solutions can in fact correspond to effective descriptions of smooth horizonless microstructure.

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arXiv2025

Cogito, ergo - strings: Supersymmetric ergoregions and their stringy excitations

Supergravity solutions describing supersymmetric rotating bound states of NS fivebranes, fundamental strings and momentum can sometimes have an ergoregion but no horizon. In such supersymmetric ergoregions, there is an unusual feature that there exist BPS "excitations" that cost negative (or zero) energy as measured from asymptotic infinity. We study the spectrum of supergravity and string excitations of these backgrounds using their worldsheet description as gauged Wess-Zumino-Witten (WZW) models, and construct their holographic map to transitions in the dual spacetime CFT. The backgrounds generically contain orbifold singularities, and we exhibit vertex operators corresponding to twisted sector ground states localized at the orbifold fixed points. The gauged WZW model thus provides a valuable tool to explore the stringy structure of such heavy BPS states in $AdS_3/CFT_2$.

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arXiv2025

BPS Fivebrane Stars III: Effective Actions

An effective action for NS5-branes coupled to supergravity is used to derive the full 10d form of horizon-free BPS solutions of fivebranes carrying momentum waves, including both transverse scalar and internal gauge excitations of the branes. When internal modes are highly excited, we find solutions that plausibly mediate the transition between the Coulomb phase of NS5-branes and the black hole phase. We also compute the two-point functions of fivebrane density fluctuations and of gravitons absorbed by the branes. Finally, we begin an exploration of near-BPS perturbations of the fivebrane ensemble, and propose the use of the brane+bulk effective action as a tool to explore the black hole phase, even in the AdS decoupling limit.

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arXiv2024

NS5-brane backgrounds and coset CFT partition functions

Worldsheet string theory is solvable for a variety of backgrounds involving Neveu-Schwarz fivebranes, in terms of gauged nonlinear sigma models on group manifolds. We compute the worldsheet torus partition function of these models, and propose gauging of null isometries as a unifying principle and conceptual framework for this large family of string backgrounds. In the process, we explain how partition functions of asymmetrically gauged Wess-Zumino-Witten models can be computed from the path integral, and organize and systematize various results scattered throughout the literature.

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arXiv2023

BPS Fivebrane Stars I: Expectation Values of Observables

We study ensembles of 1/2-BPS bound states of fundamental strings and NS-fivebranes (NS5-F1 states) in the AdS decoupling limit. We revisit a solution corresponding to an ensemble average of these bound states, and find that the appropriate duality frame for describing the near-source structure is the T-dual NS5-P frame, where the bound state is a collection of momentum waves on the fivebranes. We find that the fivebranes are generically well-separated; this property results in the applicability of perturbative string theory. The geometry sourced by the typical microstate is not close to that of an extremal non-rotating black hole; instead the fivebranes occupy a ball whose radius is parametrically much larger than the "stretched horizon" scale of the corresponding black hole. These microstates are thus better characterized as BPS fivebrane stars than as small black holes. When members of the ensemble spin with two fixed angular potentials about two orthogonal planes, we find that the spherical ball of the non-rotating ensemble average geometry deforms into an ellipsoid. This contrasts with ring structures obtained when fixing the angular momenta instead of the angular potentials; we trace this difference of ensembles to large fluctuations of the angular momentum in the ensemble of fixed angular potential.

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arXiv2023

BPS Fivebrane Stars II: Fluctuations

We investigate quantum fluctuations of metric components in coherent 1/2-BPS bound states of $n_1$ fundamental strings and $n_5$ NS5-branes. The leading order contribution in an expansion in $1/(n_1n_5)$ is calculated via a combination of analytical and numerical methods. We find that the fluctuations are small away from a tiny distance from the source, comparable to the 6d Planck scale. Comparing this result with an analysis in the literature of fluctuations in the maximally mixed state, we conclude that the large fluctuations previously found for the latter are statistical rather than quantum in nature, and that perturbative string theory provides an accurate description of these backgrounds.

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arXiv2023

AdS3 Orbifolds, BTZ Black Holes, and Holography

Conical defects of the form $(AdS_3\times S^3)/Z_k$ have an exact orbifold description in worldsheet string theory, which we derive from their known presentation as gauged Wess-Zumino-Witten models. The configuration of strings and fivebranes sourcing this geometry is well-understood, as is the correspondence to states/operators in the dual $CFT_2$. One can analytically continue the construction to Euclidean $AdS_3$ (i.e. $H_3^+$) and consider the orbifold by any infinite discrete (Kleinian) group generated by a set of elliptic (rotational) elements. The resulting geometry consists of multiple conical defects traveling along geodesics in $H_3^+$, and provides a semiclassical bulk description of correlation functions in the dual CFT involving the corresponding defect operators, which is nonperturbatively exact in $α'$. The Lorentzian continuation of these geometries describes a collection of defects colliding to make a BTZ black hole. We comment on a recent proposal to use such correlators to prepare a basis of black hole microstates, and elaborate on a picture of black hole formation and evaporation in terms of the underlying brane dynamics in the bulk.

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arXiv2023

Charge currents, rare decays, and black holes

Radiation of conserved charges carried by a black hole is a rare process, whose probability is suppressed by the change in the entropy between the initial and final states. This universal result provides insight into the black hole's internal structure and dynamics.

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arXiv2023

The Holar Wind

String theory in AdS3 with purely NS-NS fluxes and vanishing RR moduli has a continuum of winding string excitations in radial plane wave states. BTZ black holes can emit such strings, which then flow out toward the AdS3 boundary as a stream of massive quanta, and form a black hole analogue of the solar wind. The winding string sector thus provides a decay channel for the black hole to evaporate without having either to couple the system to an external reservoir or to match the AdS3 throat onto an asymptotically flat region. We compute the emission amplitude of this "holar wind" in the semi-classical approximation, and consider the associated version of the black hole information paradox.

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arXiv2022

On the BPS sector in AdS_3/CFT_2 Holography

The BPS sector in AdS_3/CFT_2 duality has been fertile ground for the exploration of gauge/gravity duality, from the match between black hole entropy and the CFT elliptic genus to the construction of large families of geometrical microstates and the identification of the corresponding states in the CFT. Worldsheet methods provide a tool to further explore the relation between string theory in the bulk and corresponding CFT quantities. We show how to match individual BPS strings to their counterparts in the symmetric product orbifold CFT. In the process, we find an exact match between known constructions of microstate geometries and condensates of BPS supergraviton strings, and discuss their role in the broader collection of BPS states. In particular, we explore how microstate geometries develop singularities; and how string theory resolves these singularities through the appearance of ``tensionless'' string dynamics, which is the continuation of structures found in the weak-coupling CFT into the strongly coupled regime described by string theory in the bulk. We argue that such ``tensionless'' strings are responsible for black hole microstructure in the bulk description.

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arXiv2022

Fuzzballs and Microstate Geometries: Black-Hole Structure in String Theory

The black-hole information paradox provides one of the sharpest foci for the conflict between quantum mechanics and general relativity and has become the proving-ground of would-be theories of quantum gravity. String theory has made significant progress in resolving this paradox, and has led to the fuzzball and microstate geometry programs. The core principle of these programs is that horizons and singularities only arise if one tries to describe gravity using a theory that has too few degrees of freedom to resolve the physics. String theory has sufficiently many degrees of freedom and this naturally leads to fuzzballs and microstate geometries: The reformation of black holes into objects with neither horizons nor singularities. This not only resolves the paradox but provides new insights into the microstructure of black holes. We summarize the current status of this approach and describe future prospects and additional insights that are now within reach. This paper is an expanded version of our Snowmass White Paper arXiv:2203.04981.

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arXiv2022

Trouble in Paradox

Recent developments in holography have suggested a potential resolution to the black hole information paradox within the context of gravitational effective field theory. We emphasize the non-local nature of this proposed resolution, and highlight the ways in which observations outside the black hole can detect it and conclude that the black hole is not radiating like an ordinary body would.

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arXiv2022

Snowmass White Paper: Micro- and Macro-Structure of Black Holes

The black-hole information paradox provides a stringent test of would-be theories of quantum gravity. String theory has made significant progress toward a resolution of this paradox, and has led to the fuzzball and microstate geometry programs. The central thesis of these programs is that only string theory has sufficiently many degrees of freedom to resolve black-hole microstructure, and that horizons and singularities are artifacts of attempting to describe gravity using a theory that has too few degrees of freedom to resolve the physics. Fuzzballs and microstate geometries recast black holes within string theory as horizonless and singularity-free objects that not only resolve the paradox but provide new insight into the underlying microstructure. We give an overview of this approach, summarize its current status and describe future prospects and insights that are now within reach.

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arXiv2022

A Defect in AdS3/CFT2 Duality

$AdS_3$ string theory in the stringy regime $k=(R_{AdS}/\ell_{str})^2 < 1$ provides a laboratory for the study of holography in which both sides of AdS/CFT duality are under fairly good control. Worldsheet string theory is solvable, and for closed strings the dual spacetime CFT is a deformation of a symmetric product orbifold. Here we extend this construction to include open strings by adding a probe D-string, described semi-classically by an $AdS_2$ D-brane in $AdS_3$. The dual defect or boundary conformal field theory (BCFT) is again a deformed symmetric product, which now describes the Fock space of long open and closed strings near the AdS boundary, with a boundary deformation implementing the open/closed transition in addition to the symmetric product ${\mathbb Z}_2$ twist deformation that implements closed string joining/splitting. The construction thus provides an explicit example of an $AdS_3/BCFT_2$ duality.

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arXiv2022

String dynamics in NS5-F1-P geometries

String theory dynamics on certain fivebrane supertube backgrounds is described by an exactly solvable null-gauged WZW model. We use this description to compute the spectrum of closed string excitations on the three-charge non-supersymmetric solution found by Jejjala, Madden, Ross and Titchener, as well as its supersymmetric limit. The low-lying spectrum matches that of supergravity modes in the effective geometry and exhibits an underlying group-theoretic structure. Winding sectors describe strings carrying the same charges as the background; processes whereby strings turn into flux or vice-versa are mediated by large gauge transformations on the worldsheet. The S-matrix of such wound strings probes microstructure of the fivebrane source that is hidden in the supergravity approximation.

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