Non-Abelian Duality for Open Strings

We examine non-abelian duality transformations in the open string case. After gauging the isometries of the target space and developing the general formalism, we study in details the duals oftarget spaces with SO(N) isometries which, for the SO(2) case, reduces to the known abelian T-duals. We apply the formalism to electrically and magnetically charged 4D black hole solutions and, as in the abelian case, dual coordinates satisfy Dirichlet conditions.Comment: 18 pages, Latex. Some formulas are added. Final version to appear in Nucl. Phys.

Semiclassical quantization of rotating superstring in AdS_5 x S^5

Motivated by recent proposals in hep-th/0202021 and hep-th/0204051 we develop semiclassical quantization of superstring in $AdS_5 x S^5$. We start with a classical solution describing string rotating in $AdS_5$ and boosted along large circle of $S^5$. The energy of the classical solution $E$ is a function of the spin $S$ and the momentum $J$ (R-charge) which interpolates between the limiting cases S=0 and J=0 considered previously. We derive the corresponding quadratic fluctuation action for bosonic and fermionic fields from the GS string action and compute the string 1-loop (large \lambda= {R^4\over \a'^2}) correction to the classical energy spectrum in the $(S,J)$ sector. We find that the 1-loop correction to the ground-state energy does not cancel for non-zero $S$. For large $S$ it scales as $\ln S$, i.e. as the classical term, with no higher powers of $\ln S$ appearing. This supports the conjecture made in hep-th/0204051 that the classical $E-S = a \ln S$ scaling can be interpolated to weak coupling to reproduce the corresponding operator anomalous dimension behaviour in gauge theory.Comment: harvmac, 35p. v2,3: minor corrections; v4: added remarks about higher-loop corrections in section 4 and an argument suggesting the absence of higher than log S corrections to the energy to all orders in string tension in section 6.1; v5: factor 1/2 misprints corrected in eqs. (6.6) and (6.8) and thus in (6.5) and (6.9

Dynamical Instability of Self-Tuning Solution with Antisymmetric Tensor Field

We consider the dynamical stability of a static brane model that incorporates a three-index antisymmetric tensor field and has recently been proposed as a possible solution to the cosmological constant problem. Ultimately, we are able to establish the existence of time-dependent, purely gravitational perturbations. As a consequence, the static solution of interest is ``dangerously'' located at an unstable saddle point. This outcome is suggestive of a hidden fine tuning in what is an otherwise self-tuning model.Comment: 16 Pages, Latex; Discussion added but conclusions unchange

Brane Universes and the Cosmological Constant

The cosmological constant problem and brane universes are reviewed briefly. We discuss how the cosmological constant problem manifests itself in various scenarios for brane universes. We review attempts - and their difficulties - that aim at a solution of the cosmological constant problem.Comment: corrected typos, added references, 13 pages, accepted by MPLA as brief revie

Model for self-tuning the cosmological constant

The vanishing cosmological constant in the four dimensional space-time is obtained in a 5D Randall-Sundrum model with a brane (B1) located at $y=0$. The matter fields can be located at the brane. For settling any vacuum energy generated at the brane to zero, we need a three index antisymmetric tensor field $A_{MNP}$ with a specific form for the Lagrangian. For the self-tuning mechanism, the bulk cosmological constant should be negative.Comment: LaTeX file of 4 pages, to appear in Phys. Rev. Let

A criterion for admissible singularities in brane world

When gravity couples to scalar fields in Anti-de Sitter space, the geometry becomes non-AdS and develops singularities generally. We propose a criterion that the singularity is physically admissible if the integral of the on-shell Lagrangian density over the finite range is finite everywhere. For all classes of the singularities studied here, the criterion suggested in this paper coincides with an independent proposal made by Gubser that the potential should be bounded from above in the solution. This gives a reason why Gubser's conjecture works.Comment: 14 pages, revtex, 1 table, references added, two other criteria include

Wave function of the radion in the brane background with a massless scalar field and a self-tuning problem

We consider flat solutions in the brane background with a massless scalar field appearing in 5D $H^2_{MNPQ}$. Since there exist bulk singularities or arises the divergent 4D Planck mass, we should introduce a compact extra dimension, the size of which is then fixed by brane tension(s) and a bulk cosmological constant. Inspecting scalar perturbations around the flat solutions, we find that the flat solutions are stable vacua from the positive mass spectrum of radion. We show that the massless radion mode is projected out by the boundary condition arising in cutting off the extra dimension. Thus, the fixed extra dimension is not alterable, which is not useful toward a self-tuning of the cosmological constant.Comment: Latex file of 18 pages including 1 eps figur

Wilson loops from supergravity and string theory

We present a theorem that determines the value of the Wilson loop associated with a Nambu-Goto action which generalizes the action of the $AdS_5\times S_5$ model. In particular we derive sufficient conditions for confining behavior. We then apply this theorem to various string models. We go beyond the classical string picture by incorporating quadratic quantum fluctuations. We show that the bosonic determinant of $D_p$ branes with 16 supersymmetries yields a Luscher term. We confirm that the free energy associated with a BPS configuration of a single quark is free from divergences. We show that unlike for a string in flat space time in the case of $AdS_5\times S_5$ the fermionic determinant does not cancel the bosonic one. For a setup that corresponds to a confining gauge theory the correction to the potential is attractive. We determine the form of the Wilson loop for actions that include non trivial $B_{\mu\nu}$ field. The issue of an exact determination of the value of the stringy Wilson loop is discussed. Talk presented in string 99 Potsdam.Comment: 12 pages Late

Cosmological Constant of the (p+1)(p+1)-Dimensional World, Embedded in the dd-Dimensional Bulk Space

In this manuscript we study the cosmological constant of a $(p+1)$-dimensional world, which lives in the higher dimensional bulk space. We assume the extra dimensions are compact on tori. We consider two cases: positive and negative bulk cosmological constant. It is pointed out that the tiny cosmological constant of our world can be obtained by the dynamics of a scalar field and adjusting the parameters of the model. The cosmological constant of the dual world also will be discussed. We obtain the Dirac quantization of these cosmological constants.Comment: 11 pages, Latex, No figure. In the revised version, major changes have been introduced and also references have been adde