Gravity on Noncommutative D-Branes

The effective action for the low energy scattering of two gravitons with a D-brane in the presence of a constant antisytmetric $B$ field in bosonic string theory is calculated and the modification to the standard D-brane action to first order in $\alpha'$ is obtained.Comment: 18 pages, Latex file, accepted in Int. J. Mod. Phys.

New Examples of Flux Vacua

Type IIB toroidal orientifolds are among the earliest examples of flux vacua. By applying T-duality, we construct the first examples of massive IIA flux vacua with Minkowski space-times, along with new examples of type IIA flux vacua. The backgrounds are surprisingly simple with no four-form flux at all. They serve as illustrations of the ingredients needed to build type IIA and massive IIA solutions with scale separation. To check that these backgrounds are actually solutions, we formulate the complete set of type II supergravity equations of motion in a very useful form that treats the R-R fields democratically.Comment: 38 pages, LaTeX; references updated; additional minor comments added; published versio

Nonabelian D-branes and Noncommutative Geometry

We discuss the nonabelian world-volume action which governs the dynamics of N coincident Dp-branes. In this theory, the branes' transverse displacements are described by matrix-valued scalar fields, and so this is a natural physical framework for the appearance of noncommutative geometry. One example is the dielectric effect by which Dp-branes may be polarized into a noncommutative geometry by external fields. Another example is the appearance of noncommutative geometries in the description of intersecting D-branes of differing dimensions, such as D-strings ending on a D3- or D5-brane. We also describe the related physics of giant gravitons.Comment: 21 pages, Latex, ref. adde

Observational constraints on braneworld inflation: the effect of a Gauss-Bonnet term

High-energy modifications to general relativity introduce changes to the perturbations generated during inflation, and the latest high-precision cosmological data can be used to place constraints on such modified inflation models. Recently it was shown that Randall-Sundrum type braneworld inflation leads to tighter constraints on quadratic and quartic potentials than in general relativity. We investigate how this changes with a Gauss-Bonnet correction term, which can be motivated by string theory. Randall-Sundrum models preserve the standard consistency relation between the tensor spectral index and the tensor-to-scalar ratio. The Gauss-Bonnet term breaks this relation, and also modifies the dynamics and perturbation amplitudes at high energies. We find that the Gauss-Bonnet term tends to soften the Randall-Sundrum constraints. The observational compatibility of the quadratic potential is strongly improved. For a broad range of energy scales, the quartic potential is rescued from marginal rejection. Steep inflation driven by an exponential potential is excluded in the Randall-Sundrum case, but the Gauss-Bonnet term leads to marginal compatibility for sufficient e-folds.Comment: 10 pages, 10 figures, version to appear in Physical Review

Intersecting Attractors

We apply the entropy formalism to the study of the near-horizon geometry of extremal black p-brane intersections in D>5 dimensional supergravities. The scalar flow towards the horizon is described in terms an effective potential given by the superposition of the kinetic energies of all the forms under which the brane is charged. At the horizon active scalars get fixed to the minima of the effective potential and the entropy function is given in terms of U-duality invariants built entirely out of the black p-brane charges. The resulting entropy function reproduces the central charges of the dual boundary CFT and gives rise to a Bekenstein-Hawking like area law. The results are illustrated in the case of black holes and black string intersections in D=6, 7, 8 supergravities where the effective potentials, attractor equations, moduli spaces and entropy/central charges are worked out in full detail.Comment: 1+41 pages, 2 Table

Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua

We investigate a string-inspired scenario associated with a rolling massive scalar field on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the massive scalar field on the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of the warp factor of the anti-D3 brane, it is possible to obtain the required level of amplitude of density perturbations. We study the spectra of scalar and tensor perturbations generated during the rolling scalar inflation and show that our scenario satisfies the observational constraint coming from the Cosmic Microwave Background anisotropies and other observational data. We also implement the negative cosmological constant arising from the stabilization of the modulus fields in the KKLT vacua and find that this leads to a successful reheating in which the energy density of the scalar field effectively scales as a pressureless dust. The present dark energy can be also explained in our scenario provided that the potential energy of the massive rolling scalar does not exactly cancel with the amplitude of the negative cosmological constant at the potential minimum.Comment: RevTex4, 15 pages, 5 eps figures, minor clarifications and few references added, final version to appear in PR

What is needed of a tachyon if it is to be the dark energy?

We study a dark energy scenario in the presence of a tachyon field $\phi$ with potential $V(\phi)$ and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity $\lambda=-M_pV_\phi/V^{3/2}$. If $\lambda$ is a constant, which corresponds to an inverse square potential $V(\phi) \propto \phi^{-2}$, there exists one stable critical point that gives an acceleration of the universe at late times. When $\lambda \to 0$ asymptotically, we can have a viable dark energy scenario in which the system approaches an ``instantaneous'' critical point that dynamically changes with $\lambda$. If $|\lambda|$ approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where $|\lambda|$ is smaller than of order unity.Comment: 11 pages and 3 figures, minor clarifications added; final version to appear in PR

Holographic RG flow dual to attractor flow in extremal black holes

We extend the discussion of the "Kerr/CFT correspondence" and its recent developments to the more general gauge/gravity correspondence in the full extremal black hole space-time of the bulk by using a technique of the holographic renormalization group (RG) flow. It is conjectured that the extremal black hole space-time is holographically dual to the chiral two dimensional field theory. Our example is a typical four dimensional Reissner-Nordstrom black hole, a system in which the M5-brane is wrapped on four cycles of Calabi-Yau threefold. In five dimensional supergravity view point this near horizon geometry is $AdS_3\times S^2$, and three dimensional gravity coupled to moduli fields is effectively obtained after a dimensional reduction on $S^2$. Constructing the Hamilton-Jacobi equation, we define the holographic RG flow from the three dimensional gravity. The central charge of the Virasoro algebra is calculable from the conformal anomaly at the point where the beta function defined from gravity side becomes zero. In general, we can also identify the c-function of the dual two dimensional field theory. We show that these flow equations are completely equivalent to not only BPS but also non-BPS attractor flow equations of the muduli fields. The attractor mechanism by which the values of the moduli fields are fixed at the event horizon of the extremal black hole can be understood equivalently to the fact that the RG flows are fixed at the critical points in the dual field theory.Comment: 19 pages, references adde

Off-shell extension of S-matrix elements and tachyonic effective actions

We show that the on-shell S-matrix elements of four open string massless scalars, two scalars and two tachyons, and four open string tachyons in the super string theory can be written in a unique form. We then propose an off-shell extension for the S-matrix element of four scalars which is consistent, in the low energy limit, with the Dirac-Born-Infeld effective action. Using a similar off-shell extension for the S-matrix element of two scalars and two tachyons and for the S-matrix element of four tachyons, we show that they are fully consistent with the tachyonic DBI action.Comment: Latex, 17 pages, v3:a paragraph comparing off-shell and on- shell amplitudes added, reference adde

Non Abelian Geometrical Tachyon

We investigate the dynamics of a pair of coincident D5 branes in the background of $k$ NS5 branes. It has been proposed by Kutasov that the system with a single probing D-brane moving radially in this background is dual to the tachyonic DBI action for a non-BPS Dp brane. We extend this proposal to the non-abelian case and find that the duality still holds provided one promotes the radial direction to a matrix valued field associated with a non-abelian geometric tachyon and a particular parametrization for the transverse scalar fields is chosen. The equations of motion of a pair of coincident D5 branes moving in the NS5 background are determined. Analytic and numerical solutions for the pair are found in certain simplified cases in which the U(2) symmetry is broken to $U(1) \times U(1)$ corresponding to a small transverse separation of the pair. For certain range of parameters these solutions describe periodic motion of the centre of mass of the pair 'bouncing off' a finite sized throat whose minimum size is limited by the D5 branes separation.Comment: 18 pages, 2 figures, PdfLatex: references added.accepted for publication in JHE