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Mukund Rangamani

Publications and source records attributed to Mukund Rangamani.

125 records · Page 7Linked to original sources
arXiv2003

Causal inheritance in plane wave quotients

We investigate the appearance of closed timelike curves in quotients of plane waves along spacelike isometries. First we formulate a necessary and sufficient condition for a quotient of a general spacetime to preserve stable causality. We explicitly show that the plane waves are stably causal; in passing, we observe that some pp-waves are not even distinguishing. We then consider the classification of all quotients of the maximally supersymmetric ten-dimensional plane wave under a spacelike isometry, and show that the quotient will lead to closed timelike curves iff the isometry involves a translation along the u direction. The appearance of these closed timelike curves is thus connected to the special properties of the light cones in plane wave spacetimes. We show that all other quotients preserve stable causality.

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arXiv2003

Black strings in asymptotically plane wave geometries

We present a class of black string spacetimes which asymptote to maximally symmetric plane wave geometries. Our construction will rely on a solution generating technique, the null Melvin twist, which deforms an asymptotically flat black string spacetime to an asymptotically plane wave black string spacetime while preserving the event horizon.

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arXiv2003

Generating asymptotically plane wave spacetimes

In an attempt to study asymptotically plane wave spacetimes which admit an event horizon, we find solutions to vacuum Einstein's equations in arbitrary dimension which have a globally null Killing field and rotational symmetry. We show that while such solutions can be deformed to include ones which are asymptotically plane wave, they do not posses a regular event horizon. If we allow for additional matter, such as in supergravity theories, we show that it is possible to have extremal solutions with globally null Killing field, a regular horizon, and which, in addition, are asymptotically plane wave. In particular, we deform the extremal M2-brane solution in 11-dimensional supergravity so that it behaves asymptotically as a 10-dimensional vacuum plane wave times a real line.

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arXiv2002

Causal structures of pp-waves

We discuss the causal structure of pp-wave spacetimes using the ideal point construction outlined by Geroch, Kronheimer, and Penrose. This generalizes the recent work of Marolf and Ross, who considered similar issues for plane wave spacetimes. We address the question regarding the dimension of the causal boundary for certain specific pp-wave backgrounds. In particular, we demonstrate that the pp-wave spacetime which gives rise to the N = 2 sine-Gordon string world-sheet theory is geodesically complete and has a one-dimensional causal boundary.

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arXiv2002

No horizons in pp-waves

We argue that pp-wave backgrounds can not admit event horizons. We also comment on pp-wave generalizations which would admit horizons and show that there exists a black string solution which asymptotes to a five dimensional plane wave background.

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arXiv2002

Penrose Limits and Non-local theories

We investigate Penrose limits of two classes of non-local theories, little string theories and non-commutative gauge theories. Penrose limits of the near-horizon geometry of NS5-branes help to shed some light on the high energy spectrum of little string theories. We attempt to understand renormalization group flow in these theories by considering Penrose limits wherein the null geodesic also has a radial component. In particular, we demonstrate that it is possible to construct a pp-wave spacetime which interpolates between the linear dilaton and the AdS regions for the Type IIA NS5-brane. Similar analysis is considered for the holographic dual geometry to non-commutative field theories.

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arXiv2002

Strings from Quivers, Membranes from Moose

We consider N=2 moose/quiver gauge theories corresponding to N_1 D3-branes at a C^2/Z_{N_2} singularity in the ``large moose'' limit where N_1 and N_2 are scaled to infinity together. In the dual holographic description, this scaling gives rise to a maximally supersymmetric pp-wave background with a compact light-cone direction. We identify the gauge theory operators that describe the Discrete Light-Cone Quantization (DLCQ) of the string in this background. For each discrete light-cone momentum and winding sector there is a separate ground state and Fock space. The large moose/quiver diagram provides a useful graphical representation of the string and its excitations. This representation has a natural explanation in a T-dual language. The dual theory is a non-relativistic type IIA string wound around the T-dual direction, and bound by a quadratic Newtonian potential. We end with some comments on D-string/D-particle states, a possible lift to M-theory and the relation to deconstruction.

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arXiv2002

Unstable Horizons

We investigate some of the issues relating to the dynamical instability of general static spacetimes with horizons. Our paper will be partially pedagogical and partially exploratory in nature. After discussing the current understanding, we generalize the proposal of Gubser and Mitra, which identifies dynamical instability of black branes with local thermodynamic instability, to include all product spacetimes with the horizon uniformly smeared over an internal space. We support our conjecture by explicitly exhibiting a threshold unstable mode for Schwarzschild-AdS_5 x S^5 black hole. Using the AdS/CFT correspondence, this simultaneously yields a prediction for a phase transition in the dual gauge theory. We also discuss implications for spacetimes with cosmological horizons.

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arXiv2001

Hot Little String Correlators: A View From Supergravity

We study the propagation of a massless minimally coupled scalar in the near horizon geometry of non-extremal NS5-branes. Using the holographic principle for dilatonic backgrounds we compute the two-point function of an operator in Little String Theory at the Hagedorn temperature. We then comment on relations with correlation functions in two dimensional string theory.

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arXiv2001

Reverse Engineering ADHM Construction from Non-Commutative Instantons

We study the non-commutative instanton solution proposed in hep-th/0009142 and obtain the spectrum of small oscillations. The spectrum thus obtained is in exact agreement with the spectrum of stringy excitations in a configuration of point like D0 branes sitting on top of D4-branes with a uniform magnetic field turned on in the world-volume of the D4-branes in the Seiberg-Witten decoupling limit. This provides further evidence for the solution of hep-th/0009142 and also enables us recover the ADHM data from the 0-4 string spectrum. Generalizations to higher co-dimension solitons are also discussed.

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arXiv2001

Little String Thermodynamics

We argue that the holographic dual to little string theories at finite temperature suffers from a Gregory-Laflamme like instability, providing an alternative explanation to the results of hep-th/0012258.

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arXiv2000

D-brane Dynamics and the Quantum Hall Effect

We study the recently proposed D-brane configuration [hep-th/0010105] modeling the quantum Hall effect, focusing on the nature of the interactions between the charged particles. Our analysis indicates that the interaction is repulsive, which it should be for the ground state of the system to behave as a quantum Hall liquid. The strength of interactions varies inversely with the filling fraction, leading us to conclude that a Wigner crystal is the ground state at small nu. For larger rational nu (still less than unity), it is reasonable to expect a fractional quantum Hall ground state.

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arXiv2000

The Hagedorn transition in non-commutative open string theory

The Hagedorn transition in non-commutative open string theory (NCOS) is relatively simple because gravity decouples. For NCOS theories in no more than five spacetime dimensions, the Hagedorn transition is second order, and the high temperature phase involves long, nearly straight fundamental strings separating from the D-brane on which the NCOS theory is defined. Above five spacetime dimensions interaction effects become important below the Hagedorn temperature. Although this complicates studies of the transition, we believe that the high temperature phase again involves long strings liberated from the bound state.

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arXiv2000

Noncommutative Gauge Dynamics From The String Worldsheet

We show how string theory can be used to reproduce the one-loop two-point photon amplitude in noncommutative U(1) gauge theory. Using a simple realization of the gauge theory in bosonic string theory, we extract from a string cylinder computation in the decoupling limit the exact one loop field theory result. The result is obtained entirely from the region of moduli space where massless open strings dominate. Our computation indicates that the unusual IR/UV singularities of noncommutative field theory do not come from closed string modes in any simple way.

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arXiv1998

Dibaryons, Branes, And Strings In AdS Orbifold Models

A generalization of the Maldacena conjecture asserts that Type IIB string theory on $§^5/\Z_3$ is equivalent to a certain supersymmetric $SU(N)^3$ gauge theory with bifundamental matter. To test this assertion, we analyze the wrapped branes on $§^5/\Z_3$ and their interpretation in terms of gauge theory. The wrapped branes are interpreted in some cases as baryons or dibaryons of the gauge theory and in other cases as strings around which there is a global monodromy. In order to successfully match the brane analysis with field theory, we must uncover some aspects of $S$-duality which are novel even in the case of four-dimensional free field theory.

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arXiv1998

Three-Point Functions of Chiral Operators in D=4, $\mathcal{N}=4$ SYM at Large N

We study all three-point functions of normalized chiral operators in D=4, $\mathcal{N}=4$, U(N) supersymmetric Yang-Mills theory in the large $N$ limit. We compute them for small 't Hooft coupling $λ=g_{YM}^2N<<1$ using free field theory and at strong coupling $λ=g_{YM}^2>>1$ using the $AdS$/CFT correspondence. Surprisingly, we find the same answers in the two limits. We conjecture that at least for large $N$ the exact answers are independent of $λ$ .

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