arXiv subjects

Miguel S. Costa

Publications and source records attributed to Miguel S. Costa.

At least 55 records · Page 3Linked to original sources

arXiv2006

Eikonal Approximation in AdS/CFT: Conformal Partial Waves and Finite N Four-Point Functions

We introduce the impact-parameter representation for conformal field theory correlators of the form A ~ < O_1 O_2 O_1 O_2 >. This representation is appropriate in the eikonal kinematical regime, and approximates the conformal partial-wave decomposition in the limit of large spin and dimension of the exchanged primary. Using recent results on the two-point function < O_1 O_1 >_{shock} in the presence of a shock wave in Anti-de Sitter, and its relation to the discontinuity of the four-point amplitude A across a kinematical branch-cut, we find the high spin and dimension conformal partial- wave decomposition of all tree-level Anti-de Sitter Witten diagrams. We show that, as in flat space, the eikonal kinematical regime is dominated by the T-channel exchange of the massless particle with highest spin (graviton dominance). We also compute the anomalous dimensions of the high-spin O_1 O_2 composites. Finally, we conjecture a formula re-summing crossed-ladder Witten diagrams to all orders in the gravitational coupling.

hep-th↗

arXiv2006

Unitarity in the Presence of Closed Timelike Curves

We conjecture that, in certain cases, quantum dynamics is consistent in the presence of closed timelike curves. We consider time dependent orbifolds of three dimensional Minkowski space describing, in the limit of large AdS radius, BTZ black holes inside the horizon. Although perturbative unitarity fails, we show that, for discrete values of the gravitational coupling, particle propagation is consistent with unitarity. This quantization corresponds to the quantization of the black hole angular momentum, as expected from the dual CFT description. Note, however, that we recover this result by analyzing the physics inside the horizon and near the singularity. The spacetime under consideration has no AdS boundary, and we are therefore not using any assumption regarding a possible dual formulation. We perform the computation at very low energies, where string effects are irrelevant and interactions are dominated by graviton exchange in the eikonal regime. We probe the non-causal structure of space-time to leading order, but work to all orders in the gravitational coupling.

hep-th↗

arXiv2005

D-Brane Dynamics in Constant Ramond-Ramond Potentials, S-Duality and Noncommutative Geometry

We study the physics of D-branes in the presence of constant Ramond-Ramond potentials. In the string field theory context, we first develop a general formalism to analyze open strings in gauge trivial closed string backgrounds, and then apply it both to the RNS string and within Berkovits' covariant formalism, where the results have the most natural interpretation. The most remarkable finding is that, in the presence of a Dp-brane, both a constant parallel NS-NS B-field and R-R C^(p-1)-field do not solve the open/closed equations of motion, and induce the same non-vanishing open string tadpole. After solving the open string equations in the presence of this tadpole, and after gauging away the closed string fields, one is left with a U(1) field strength on the brane given by F=(1/2)(B-*C^(p-1)), where * is Hodge duality along the brane world-volume. One observes that this result differs from the usually assumed result F=B. Technically, this is due to the fact that supersymmetric and bosonic string world-sheet theories are different. Note, however, that the usual F+B combination is still the combination which remains gauge invariant at the sigma-model level. Also, the standard result F=B is, in the D3-brane case, not compatible with S-duality. On the other hand our result, which is derived automatically given the general formalism, offers a non-trivial check of S-duality, to all orders in F, and this leads to a S-dual invariant Moyal deformation. In an appendix, we solve the source equation describing the open superstring in a generic NS-NS and R-R closed string background, within the super Poincare covariant formalism.

hep-th↗

arXiv2005

Hagedorn transition and chronology protection in string theory

We conjecture chronology is protected in string theory due to the condensation of light winding strings near closed null curves. This condensation triggers a Hagedorn phase transition, whose end-point target space geometry should be chronological. Contrary to conventional arguments, chronology is protected by an infrared effect. We support this conjecture by studying strings in the O-plane orbifold, where we show that some winding string states are unstable and condense in the non-causal region of spacetime. The one-loop string partition function has infrared divergences associated to the condensation of these states.

hep-th↗

arXiv2005

Time-dependent Orbifolds and String Cosmology

In these lectures, we review the physics of time-dependent orbifolds of string theory, with particular attention to orbifolds of three-dimensional Minkowski space. We discuss the propagation of free particles in the orbifold geometries, together with their interactions. We address the issue of stability of these string vacua and the difficulties in defining a consistent perturbation theory, pointing to possible solutions. In particular, it is shown that resumming part of the perturbative expansion gives finite amplitudes. Finally we discuss the duality of some orbifold models with the physics of orientifold planes, and we describe cosmological models based on the dynamics of these orientifolds.

hep-th↗

arXiv2005

Coleman meets Schwinger

It is well known that spherical D-branes are nucleated in the presence of an external RR electric field. Using the description of D-branes as solitons of the tachyon field on non-BPS D-branes, we show that the brane nucleation process can be seen as the decay of the tachyon false vacuum. This process can describe the decay of flux-branes in string theory or the decay of quintessence potentials arising in flux compactifications.

hep-th↗

arXiv2002

A New Cosmological Scenario in String Theory

We consider new cosmological solutions with a collapsing, an intermediate and an expanding phase. The boundary between the expanding (collapsing) phase and the intermediate phase is seen by comoving observers as a cosmological past (future) horizon. The solutions are naturally embedded in string and M-theory. In the particular case of a two-dimensional cosmology, space-time is flat with an identification under boost and translation transformations. We consider the corresponding string theory orbifold and calculate the modular invariant one-loop partition function. In this case there is a strong parallel with the BTZ black hole. The higher dimensional cosmologies have a time-like curvature singularity in the intermediate region. In some cases the string coupling can be made small throughout all of space-time but string corrections become important at the singularity. This happens where string winding modes become light which could resolve the singularity. The new proposed space-time casual structure could have implications for cosmology, independently of string theory.

hep-th↗

arXiv2001

Flux-branes and the Dielectric Effect in String Theory

We consider the generalization to String and M-theory of the Melvin solution. These are flux p-branes which have (p+1)-dimensional Poincare invariance and are associated to an electric (p+1)-form field strength along their worldvolume. When a stack of Dp-branes is placed along the worldvolume of a flux (p+3)-brane it will expand to a spherical D(p+2)-brane due to the dielectric effect. This provides a new setup to consider the gauge theory/gravity duality. Compactifying M-theory on a circle we find the exact gravity solution of the type IIA theory describing the dielectric expansion of N D4-branes into a spherical bound state of D4-D6-branes, due to the presence of a flux 7-brane. In the decoupling limit, the deformation of the dual field theory associated with the presence of the flux brane is irrelevant in the UV. We calculate the gravitational radius and energy of the dielectric brane which give, respectively, a prediction for the VEV of scalars and vacuum energy of the dual field theory. Consideration of a spherical D6-brane probe with n units of D4-brane charge in the dielectric brane geometry suggests that the dual theory arises as the Scherk-Schwarz reduction of the M5-branes (2,0) conformal field theory. The probe potential has one minimum placed at the locus of the bulk dielectric brane and another associated to an inner dielectric brane shell.

hep-th↗

arXiv2000

The Kaluza-Klein Melvin Solution in M-theory

We study some aspects of the Kaluza-Klein Melvin solution in M-theory. The associated magnetic field has a maximal critical value $B=\pm 1/R$ where $R$ is the radius of the compactification circle. It is argued that the Melvin background of type IIA with magnetic field $B$ and of type 0A with magnetic field $B'=B-1/R$ are equivalent. Evidence for this conjecture is provided using a further circle compactification and a `9-11' flip. We show that partition functions of nine-dimensional type IIA strings and of a $(-1)^Fσ_{1/2}$ type IIA orbifold both with NS-NS Melvin fluxtubes are related by such shift of the magnetic field. Then the instabilities of both IIA and 0A Melvin solutions are analyzed. For each theory there is an instanton associated to the decay of spacetime. In the IIA case the decay mode is associated to the nucleation of $D6/D\bar{6}$-brane pairs, while in the 0A case spacetime decays through Witten's bubble production.

hep-th↗

arXiv2000

Interacting Black Holes

We revisit the geometry representing l collinear Schwarzschild black holes. It is seen that the black holes' horizons are deformed by their mutual gravitational attraction. The geometry has a string like conical singularity that connects the holes but has nevertheless a well defined action. Using standard gravitational thermodynamics techniques we determine the Free energy for two black holes at fixed temperature and distance, their entropy and mutual force. When the black holes are far apart the results agree with Newtonian gravity expectations. This analyses is generalized to the case of charged black holes. Then we consider black holes embedded in String/M-theory as bound states of branes. Using the effective string description of these bound states and for large separation we reproduce exactly the semi-classical result for the entropy, including the correction associated with the interaction between the holes.

hep-th↗

arXiv2000

Absorption by Double-Centered D3-Branes and the Coulomb Branch of N=4 SYM Theory

We calculate the classical cross-section for absorption of a minimally coupled scalar in the double-centered D3-brane geometry. The dual field theory has gauge symmetry broken to S(U(N_1)*U(N_2)) and is on the Coulomb branch of N=4 Super Yang-Mills theory. Our analysis is valid at energy scales much smaller than the W particles mass, giving logarithmic corrections to the cross section calculated at the IR conformal fixed points. These corrections are associated with deformations of the N=4 Super Yang-Mills theory by irrelevant operators that break conformal invariance and correspond to processes where a virtual pair of gauge particles or a virtual pair of W bosons interact with the incident wave to create a pair of gauge particles.

hep-th↗

arXiv2000

A Test of the AdS/CFT Duality on the Coulomb Branch

We consider the N=4 SU(N) Super Yang Mills theory on the Coulomb branch with gauge symmetry broken to S(U(N_1) x U(N_2)). By integrating the W particles, the effective action near the IR SU(N_i) conformal fixed points is seen to be a deformation of the Super Yang Mills theory by a non-renormalized, irrelevant, dimension 8 operator. The correction to the two-point function of the dilaton field dual operator near the IR is related to a three-point function of chiral primary operators at the conformal fixed points and agrees with the classical gravity prediction, including the numerical factor.

hep-th↗

arXiv2000

Radiation from Accelerated Branes

The radiation emitted by accelerated fundamental strings and D-branes is studied within the linear approximation to the supergravity limit of string theory. We show that scalar, gauge field and gravitational radiation is generically emitted by such branes. In the case where an external scalar field accelerates the branes, we derive a Larmor-type formula for the emitted scalar radiation and study the angular distribution of the outgoing energy flux. The classical radii of the branes are calculated by means of the corresponding Thompson scattering cross sections. Within the linear approximation, the interaction of the external scalar field with the velocity fields of the branes gives a contribution to the observed gauge field and gravitational radiation.

hep-th↗

arXiv1999

Intersecting D-branes and black holes in type 0 string theory

We study intersecting D-branes in type 0 string theories and show that the D$p_{\pm}$-brane bound states obey similar intersecting rules as the D$p$-branes of the type II theories. The D$5_{\pm}$-D$1_{\pm}$ brane system is studied in detail. We show that the corresponding near-horizon geometry is the $AdS_3\times S^3\times T^4$ space and that there is no tachyon instability in this background. The Bekenstein-Hawking entropy is calculated. The worldvolume theory on the D$5_{\pm}$-D$1_{\pm}$ system is also studied. This theory contains both bosons and fermions and it is seen to arise as a projection of the supersymmetric gauge theory related to the D5-D1 system of the type IIB theory. The Bekenstein-Hawking entropy formula is reproduced exactly using the dual conformal field theory.

hep-th↗

arXiv1998

Black hole dynamics from instanton strings

A D-5-brane bound state with a self-dual field strength on a 4-torus is considered. In a particular case this model reproduces the D5-D1 brane bound state usually used in the string theory description of 5-dimensional black holes. In the limit where the brane dynamics decouples from the bulk the Higgs and Coulomb branches of the theory on the brane decouple. Contrasting with the usual instanton moduli space approximation to the problem the Higgs branch describes fundamental excitations of the gauge field on the brane. Upon reduction to 2-dimensions it is associated with the so-called instanton strings. Using the Born-Infeld action for the D-5-brane we determine the coupling of these strings to a minimally coupled scalar in the black hole background. The supergravity calculation of the cross section is found to agree with the D-brane absorption probability rate calculation. We consider the near horizon geometry of our black hole and elaborate on the corresponding duality with the Higgs branch of the gauge theory in the large N limit. A heuristic argument for the scaling of the effective string tension is given.

hep-th↗

arXiv1998

Torons and black hole entropy

We consider a supersymmetric system of D-5-branes compactified on a 5-torus with a self-dual background field strength on a 4-torus and carrying left-moving momentum along a circle. The corresponding supergravity solution describes a 5-dimensional black hole with a regular horizon. The entropy of this black hole may be explained in terms of the Landau degeneracy for open strings stretching between different branes. In the gauge theory approximation this D-5-brane system is described by a super Yang-Mills theory with a t'Hooft twist. By choosing a supersymmetric branch of the theory we obtain perfect agreement with the entropy formula. The result relies on the number of massless torons associated with the gauge field components that obey twisted boundary conditions.

hep-th↗

arXiv1998

Landau degeneracy and black hole entropy

We consider the supergravity solution describing a configuration of intersecting D-4-branes with non-vanishing worldvolume gauge fields. The entropy of such a black hole is calculated in terms of the D-branes quantised charges. The non-extreme solution is also considered and the corresponding thermodynamical quantities are calculated in terms of a D-brane/anti-D-brane system. To perform the quantum mechanical D-brane analysis we study open-strings with their ends on branes with a magnetic condensate. Applying the results to our D-brane system we managed to have a perfect agreement between the D-brane entropy counting and the corresponding semi-classical result. The Landau degeneracy of the open string states describing the excitations of the D-brane system enters in a crucial way. We also derive the near-extreme results which agree with the semi-classical calculations.

hep-th↗

arXiv1997

Non-threshold D-brane bound states and black holes with non-zero entropy

We start with BPS-saturated configurations of two (orthogonally) intersecting M-branes and use the electro-magnetic duality or dimensional reduction along a boost, in order to obtain new p-brane bound states. In the first case the resulting configurations are interpreted as BPS-saturated non-threshold bound states of intersecting p-branes, and in the second case as p-branes intersecting at angles and their duals. As a by-product we deduce the enhancement of supersymmetry as the angle approaches zero. We also comment on the D-brane theory describing these new bound states, and a connection between the angle and the world-volume gauge fields of the D-brane system. We use these configurations to find new embeddings of the four and five dimensional black holes with non-zero entropy, whose entropy now also depends on the angle and world-volume gauge fields. The corresponding D-brane configuration sheds light on the microscopic entropy of such black holes.

hep-th↗