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Andreas Karch

Publications and source records attributed to Andreas Karch.

At least 145 records · Page 8Linked to original sources

Hologravity

The dS/dS correspondence provides a holographic description of quantum gravity in d dimensional de Sitter space near the horizon of a causal region in a well defined approximation scheme; it is equivalent to the low energy limit of conformal field theory on de Sitter space in d-1 dimensions coupled to d-1 dimensional gravity. In this work, we extend the duality to higher energy scales by performing calculations of various basic physical quantities sensitive to the UV region of the geometry near the center of the causal patch. In the regime of energies below the d dimensional Planck scale but above the curvature scale of the geometry, these calculations encode the physics of the d-1 dimensional matter plus gravity system above the crossover scale where gravitational effects become strong. They exhibit phenomena familiar from studies of two dimensional gravity coupled to conformal field theory, including the cancellation of the total Weyl anomaly in d-1 dimensions. We also outline how the correspondence can be used to address the issue of observables in de Sitter space, and generalize the correspondence to other space times, such as black holes, inflationary universes, and landscape bubble decays. In the cases with changing cosmological constant, we obtain a dual description in terms of renormalization group flow.

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The dS/dS Correspondence

We present a holographic duality for the de Sitter static patch which consolidates basic features of its geometry and the behavior of gravity and brane probes, valid on timescales short compared to the decay or Poincare recurrence times. Namely de Sitter spacetime $dS_d(R)$ in $d$ dimensions with curvature radius $R$ is holographically dual to two conformal field theories on $dS_{d-1}(R)$, cut off at an energy scale 1/R where they couple to each other and to $d-1$ dimensional gravity. As part of our analysis, we study brane probes in de Sitter and thermal Anti de Sitter spaces, and interpret the terms in the corresponding DBI action via strongly coupled thermal field theory. This provides a dual field theoretic interpretation of the fact that probes take forever to reach a horizon in general relativity.

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Experimental Tests of the Holographic Entropy Bound

Kovtun, Son and Starinets proposed a bound on the viscosity of any fluid in terms of its entropy density. The bound is saturated by maximally supersymmetric theories at strong coupling, but can also easily be challenged experimentally to within a factor of 10 already today. We argue that this bound follows directly from the generalized covariant entropy bound, bringing holography within the reach of experimental investigation.

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Auto-localization in de-Sitter space

We point out that gravity on dS_n gives rise to a localized graviton on dS_{n-1}. This way one can derive a recursion relation for the entropy of dS spaces, which might have interesting implications for dS holography. In the same spirit we study domain walls interpolating between dS spaces with different cosmological constant. Our observation gives an easy way to calculate what fraction of the total entropy can be accessed by an observer stuck on the bubble wall.

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Construction of bosonic string theory on infinitely curved Anti-de Sitter space

Free scalar field theory in the sector with a large number of particles can be interpreted as bosonic string theory on anti-de Sitter space of vanishing radius. Different ways of writing the field theory Hamiltonian translate to different ways of reparametrizing the world-sheet sigma coordinate. Adding a mass term in the field theory corresponds to cutting off the warped AdS direction, with cut-off inversely proportional to the mass. The string theory has neither tachyon, nor critical dimension.

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Defect Conformal Field Theory and Locally Localized Gravity

Gravity may be "locally localized" over a wide range of length scales on a d-dimensional anti-de Sitter (AdS) brane living inside AdS_{d+1}. In this paper we examine this phenomenon from the point of view of the holographic dual "defect conformal field theory". The mode expansion of bulk fields on the gravity side is shown to be precisely dual to the "boundary operator product expansion" of operators as they approach the defect. From the field theory point of view, the condition for localization is that a "reduced operator" appearing in this expansion acquires negative anomalous dimension. In particular, a very light localized graviton exists when a mode arising from the reduction of the ambient stress-energy tensor to the defect has conformal dimension Delta ~ d-1. The part of the stress tensor containing the defect dynamics has dimension Delta = d-1 in the free theory, but we argue that it acquires an anomalous dimension in the interacting theory, and hence does not participate in localization in the regime of small backreaction of the brane. We demonstrate that such an anomalous dimension is consistent with the conservation of the full stress-energy tensor. Finally, we analyze how to compute the anomalous dimensions of reduced operators from gravity at leading order in the interactions with the brane.

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Lightcone Quantization of String Theory Duals of Free Field Theories

We quantize in light cone gauge the bosonic sector of string theory on Anti-de Sitter space in the zero curvature radius limit. We find that the worldsheet falls apart into a theory of free partons and map the Hilbert space of the string theory to the Hilbert space of a free scalar in light-front description. We outline how the string worldsheet reproduces the field theory at weak coupling.

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Deconstructing (2,0) and Little String Theories

We argue that two four-dimensional strongly coupled superconformal field theories, on the Higgs branch in certain large N limits, become respectively (2,0) theory and (1,1) little string theory in six dimensions. We identify the spectrum of states responsible for the generation of the two extra dimensions and string winding modes. We establish the equivalence using orbifold realizations of the field theories and exploiting string dualities. We also speculate on deconstructions of M-theory.

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Hadron Masses and Screening from AdS Wilson Loops

We show that in strongly coupled N=4 SYM the binding energy of a heavy and a light quark is independent of the strength of the coupling constant. As a consequence we are able to show that in the presence of light quarks the analog of the QCD string can snap and color charges are screened. The resulting neutral mesons interact with each other only via pion exchange and we estimate the masses

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Adding flavor to AdS/CFT

Coupling fundamental quarks to QCD in the dual string representation corresponds to adding the open string sector. Flavors therefore should be represented by space-time filling D-branes in the dual 5d closed string background. This requires several interesting properties of D-branes in AdS. D-branes have to be able to end in thin air in order to account for massive quarks, which only live in the UV region. They must come in distinct sets, representing the chiral global symmetry, with a bifundamental field playing the role of the chiral condensate. We show that these expectations are born out in several supersymmetric examples. To analyze most of these properties it is not necessary to go beyond the probe limit in which one neglects the backreaction of the flavor D-branes.

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Absence of a VVDZ Discontinuity in AdS_AdS

We clarify the role of gauge invariance for the theory of an AdS4 brane embedded in AdS5. The presence of a nonvanishing mass parameter even for the lightest KK mode of the graviton indicates that all of the spin-2 modes propagate five polarization states. Despite this fact, it was shown earlier that the classical theory has a smooth limit as the mass parameter is taken to zero. We argue that locality in the fifth dimension ensures that this property survives at the quantum level.

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Supersymmetric Brane-Antibrane Configurations

We find a class of flat supersymmetric brane-antibrane configurations. They follow from ordinary brane-antibrane systems by turning on a specific worldvolume background electric field, which corresponds to dissolved fundamental strings. We have clarified in detail how they arise and identified their constituent charges as well as the corresponding supergravity solutions. Adopting the matrix theory description, we construct the worldvolume gauge theories and prove the absence of any tachyonic degrees. We also study supersymmetric solitons of the worldvolume theories.

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Open and Closed String Interpretation of SUSY CFT's on Branes with Boundaries

We consider certain supersymmetric configurations of intersecting branes and branes ending on branes and analyze the duality between their open and closed string interpretation. The examples we study are chosen such that we have the lower dimensional brane realizing an n+1 dimensional conformal field theory on its worldvolume and the higher dimensional one introducing a conformal boundary. We also consider two CFTs, possibly with different central charges, interacting along a common conformal boundary. We show with a probe calculation that the dual closed string description is in terms of gravity in an AdS_{n+2} bulk with an AdS_{n+1} defect or two different AdS_{n+2} spaces joined along a defect. We also comment briefly on the expected back-reaction.

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Localized Gravity in String Theory

We propose a string realization of the AdS4 brane in AdS5 that is known to localize gravity. Our theory is M D5 branes in the near horizon geometry of N D3 branes, where M and N are appropriately tuned.

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Locally Localized Gravity

We study the fluctuation spectrum of linearized gravity around non-fine-tuned branes. We focus on the case of an AdS4 brane in AdS5. In this case, for small cosmological constant, the warp factor near the brane is essentially that of a Minkowski brane. However, far from the brane, the metric differs substantially. The space includes the AdS5 boundary, so it has infinite volume. Nonetheless, for sufficiently small AdS4 cosmological constant, there is a bound state graviton in the theory, and four-dimensional gravity is reproduced. However, it is a massive bound state that plays the role of the four-dimensional graviton.

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Mirror Symmetry in 2+1 and 1+1 Dimensions

We study the Coulomb-Higgs duality of N=2 supersymmetric Abelian Chern-Simons theories in 2+1 dimensions, by compactifying dual pairs on a circle of radius R and comparing the resulting N=(2,2) theories in 1+1 dimensions. Below the compactification scale, the theory on the Higgs branch reduces to the non-linear sigma model on a toric manifold. In the dual theory on the Coulomb branch, the Kaluza-Klein modes generate an infinite tower of contributions to the superpotential. After resummation, in the limit R->0 the superpotential becomes that of the Landau-Ginzburg model which is the two-dimensional mirror of the toric sigma model. We further examine the conjecture of all-scale three-dimensional mirror symmetry and observe that it is consistent with mirror symmetry in 1+1 dimensions.

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Orientifold dual for stuck NS5 branes

We establish T-duality between NS5 branes stuck on an orientifold 8-plane in type I' and an orientifold construction in type IIB with D7 branes intersecting at angles. Two applications are discussed. For one we obtain new brane constructions, realizing field theories with gauge group a product of symplectic factors, giving rise to a large new class of conformal N=1 theories embedded in string theory. Second, by studying a D2 brane probe in the type I' background, we get some information on the still elusive (0,4) linear sigma model describing a perturbative heterotic string on an ADE singularity.

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Calabi-Yau Mirror Symmetry as a Gauge Theory Duality

We show that there are two different dualities of two dimensional gauge theories with N=(2,2) supersymmetry. One is basically a consequence of 3d mirror symmetry. The non-linear sigma model with Calabi-Yau target space on the Higgs branch of the gauge theory is mapped into an equivalent non-linear sigma model on the Coulomb branch of the dual theory, realizing a T-dual target space with torsion. The second duality is genuine to two dimensions. In addition to swapping Higgs and Coulomb branch it trades twisted for untwisted multiplets, implying a sign flip of the left moving U(1)_R charge. Succesive application of both dualities leads to geometric mirror symmetry for the target space Calabi-Yau.

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