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J. G. Russo

Publications and source records attributed to J. G. Russo.

At least 37 records · Page 2Linked to original sources
arXiv2005

Classical resolution of singularities in dilaton cosmologies

For models of dilaton-gravity with a possible exponential potential, such as the tensor-scalar sector of IIA supergravity, we show how cosmological solutions correspond to trajectories in a 2D Milne space (parametrized by the dilaton and the scale factor). Cosmological singularities correspond to points at which a trajectory meets the Milne horizon, but the trajectories can be smoothly continued through the horizon to an instanton solution of the Euclidean theory. We find some exact cosmology/instanton solutions that lift to black holes in one higher dimension. For one such solution, the singularities of a big crunch to big bang transition mediated by an instanton phase lift to the black hole and cosmological horizons of de Sitter Schwarzschild spacetimes.

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arXiv2005

New Bubble Decays In Kaluza-Klein Theories

We describe new bubble decays in pure D+1 dimensional Einstein theory with two compact directions. The instanton solution is constructed by analytic continuation of the Kaluza-Klein electrically charged black hole solution. We show that the instanton describes the decay of a Kaluza-Klein vacuum M^{D-1} x T^2 with a non-vanishing torus tilt parameter. The decay is produced by the creation of a bubble of nothing which expands with time. We compute the instanton action, which shows that this Kaluza-Klein vacuum becomes more stable as the torus tilt parameter is increased. As an application, we consider the decay of M-theory torus compactifications leading to type 0A/0B string theories.

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arXiv2005

Cosmological D-instantons and Cyclic Universes

For models of gravity coupled to hyperbolic sigma models, such as the metric-scalar sector of IIB supergravity, we show how smooth trajectories in the `augmented target space' connect FLRW cosmologies to non-extremal D-instantons through a cosmological singularity. In particular, we find closed cyclic universes that undergo an endless sequence of big-bang to big-crunch cycles separated by instanton `phases'. We also find `big-bounce' universes in which a collapsing closed universe bounces off its cosmological singularity to become an open expanding universe.

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arXiv2005

Cosmology as Relativistic Particle Mechanics: From Big Crunch to Big Bang

Cosmology can be viewed as geodesic motion in an appropriate metric on an `augmented' target space; here we obtain these geodesics from an effective relativistic particle action. As an application, we find some exact (flat and curved) cosmologies for models with N scalar fields taking values in a hyperbolic target space for which the augmented target space is a Milne universe. The singularities of these cosmologies correspond to points at which the particle trajectory crosses the Milne horizon, suggesting a novel resolution of them, which we explore via the Wheeler-deWitt equation.

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arXiv2004

Thermal ensemble of string gas in a magnetic field

We study the thermal ensemble of a gas of free strings in presence of a magnetic field. We find that the thermodynamical partition function diverges when the magnetic field exceeds some critical value $B_{\rm cr}$, which depends on the temperature. We argue that there is a first-order phase transition with a large latent heat. At the critical value an infinite number of states -all states in a Regge trajectory- seem to become massless, which may be an indication of recuperation of spontaneously broken symmetries.

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arXiv2004

Semiclassical string spectrum in a string model dual to large N QCD

We explore the string spectrum in the Witten QCD model by considering classical string configurations, thereby obtaining energy formulas for quantum states with large excitation quantum numbers representing glueballs and Kaluza-Klein states. In units of the string tension, the energies of all states increase as the 't Hooft coupling $λ$ is decreased, except the energies of glueballs corresponding to strings lying on the horizon, which remain constant. We argue that some string solutions can be extrapolated to the small $λ$ regime. We also find the classical mechanics description of supergravity glueballs in terms of point-like string configurations oscillating in the radial direction, and reproduce the glueball energy formula previously obtained by solving the equation for the dilaton fluctuation.

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arXiv2003

Cosmological string models from Milne spaces and SL(2,Z) orbifold

The $n+1$-dimensional Milne Universe with extra free directions is used to construct simple FRW cosmological string models in four dimensions, describing expansion in the presence of matter with $p=k ρ$, $k=(4-n)/3n$. We then consider the n=2 case and make SL(2,Z) orbifold identifications. The model is surprisingly related to the null orbifold with an extra reflection generator. The study of the string spectrum involves the theory of harmonic functions in the fundamental domain of SL(2,Z). In particular, from this theory one can deduce a bound for the energy gap and the fact that there are an infinite number of excitations with a finite degeneracy. We discuss the structure of wave functions and give examples of physical winding states becoming light near the singularity.

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arXiv2002

Solvable model of strings in a time-dependent plane-wave background

We investigate a string model defined by a special plane-wave metric ds^2 = 2dudv - l(u) x^2 du^2 + dx^2 with l(u) = k/u^2 and k=const > 0. This metric is a Penrose limit of some cosmological, Dp-brane and fundamental string backgrounds. Remarkably, in Rosen coordinates the metric has a ``null cosmology'' interpretation with flat spatial sections and scale factor which is a power of the light-cone time u. We show that: (i) This spacetime is a Lorentzian homogeneous space. In particular, like Minkowski space, it admits a boost isometry in u,v. (ii) It is an exact solution of string theory when supplemented by a u-dependent dilaton such that its exponent (i.e. effective string coupling) goes to zero at u=infinity and at the singularity u=0, reducing back-reaction effects. (iii) The classical string equations in this background become linear in the light-cone gauge and can be solved explicitly in terms of Bessel's functions; thus the string model can be directly quantized. This allows one to address the issue of singularity at the string-theory level. We examine the propagation of first-quantized point-particle and string modes in this time-dependent background. Using certain analytic continuation prescription we argue that string propagation through the singularity can be smooth.

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arXiv2002

A class of exact pp-wave string models with interacting light-cone gauge actions

We find a general class of pp-wave string solutions with NS-NS $H_3$ or R-R $F_3$ field strengths, which are analogous to solutions with non-constant $F_5$ recently considered by Maldacena and Maoz (hep-th/0207284). We show that: (i) all pp-wave solutions supported by non-constant $H_3$ or $F_p$ fields are exact type II superstring solutions to all orders in $\a'$; (ii) the corresponding light-cone gauge Green-Schwarz actions are non-linear in bosons but always quadratic in fermions, and describe UV finite 2-d theories; (iii) the pp-wave backgrounds supported by non-constant $F_3$ field do not have, in contrast to their $F_5$-field counterparts, ``supernumerary'' supersymmetries and thus the associated light-cone GS actions do not possess 2-d supersymmetry. We consider a specific example where the pp-wave $F_3$ background is parametrized by an arbitrary holomorphic function of one complex bosonic coordinate. The corresponding GS action has the same bosonic part, similar Yukawa terms but twice as many interacting world-sheet fermions as the (2,2) supersymmetric model originating from the analogous $F_5$ background. We also discuss the structure of massless scalar vertex operators in the models related to N=2 super sine-Gordon and N=2 super Liouville theories.

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arXiv2002

Quantum entanglement of charges in bound states with finite-size dyons

We show that the presence of finite-size monopoles can lead to a number of interesting physical processes involving quantum entanglement of charges. Taking as a model the classical solution of the N=2 SU(2) Yang-Mills theory, we study interaction between dyons and scalar particles in the adjoint and fundamental representation. We find that there are bound states of scalars and dyons, which, remarkably, are always an entangled configuration of the form |ψ> =|dyon+> |scalar-> +/- |dyon->|scalar+>. We determine the energy levels and the wave functions and also discuss their stability.

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arXiv2002

On solvable models of type IIB superstring in NS-NS and R-R plane wave backgrounds

We consider type IIB string in the two plane-wave backgrounds which may be interpreted as special limits of the AdS_3 x S^3 metric supported by either the NS-NS or R-R 3-form field. The NS-NS plane-wave string model is equivalent to a direct generalization of the Nappi-Witten model, with its spectrum being similar to that of strings in constant magnetic field. The R-R model can be solved in the light-cone gauge, where the Green-Schwarz action describes 4 massive and 4 massless copies of free bosons and fermions. We find the spectra of the two string models and study the asymptotic density of states. We also discuss a more general class of exactly solvable plane-wave models with reduced supersymmetry which is obtained by adding twists in two spatial 2-planes.

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arXiv2001

Supersymmetric fluxbrane intersections and closed string tachyons

We consider NS-NS superstring model with several ``magnetic'' parameters $b_s$ (s=1, ...,N) associated with twists mixing a compact $S^1$ direction with angles in $N$ spatial 2-planes of flat 10-dimensional space. It generalizes the Kaluza-Klein Melvin model which has single parameter $b$. The corresponding U-dual background is a R-R type IIA solution describing an orthogonal intersection of $N$ flux 7-branes. Like the Melvin model, the NS-NS string model with $N$ continuous parameters is explicitly solvable; we present its perturbative spectrum and torus partition function explicitly for the N=2 case. For generic $b_s$ (above some critical values) there are tachyons in the $S^1$ winding sector. A remarkable feature of this model is that while in the Melvin N=1 case all supersymmetry is broken, a fraction of it may be preserved for $N >1$ by making a special choice of the parameters $b_s$. Such solvable NS-NS models may be viewed as continuous-parameter analogs of non-compact orbifold models. They and their U-dual R-R fluxbrane counterparts may have some ``phenomenological'' applications. In particular, in N=3 case one finds a special 1/4 supersymmetric R-R 3-brane background. Putting Dp-branes in flat twisted NS-NS backgrounds leads to world-volume gauge theories with reduced amount of supersymmetry. We also discuss possible ways of evolution of unstable backgrounds towards stable ones.

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arXiv2001

Magnetic backgrounds and tachyonic instabilities in closed superstring theory and M-theory

Models of closed superstrings in certain curved NS-NS magnetic flux backgrounds are exactly solvable in terms of free fields. They interpolate between free superstring theories with periodic and antiperiodic boundary conditions for fermions around some compact direction, and, in particular, between type 0 and type II string theories. Using ``9-11 flip'', this interpolation can be extended to M-theory and provides an interesting setting for a study of tachyon problem in closed string theory. Starting with a general 2-parameter family of such Melvin-type models, we present several new magnetic flux backgrounds in 10-d string theory and 11-d M-theory and discuss their tachyonic instabilities. In particular, we suggest a description of type 0B theory in terms of M-theory in curved magnetic flux background, which supports its conjectured SL(2,Z) symmetry, and in which the type 0 tachyon appears to correspond to a state in 11-d supergravity multiplet. In another ``T-dual'' description, the tachyon is related to a winding membrane state.

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arXiv2001

Free energy and critical temperature in eleven dimensions

We compute the one-loop contribution to the free energy in eleven-dimensional supergravity, with the eleventh dimension compactified on a circle of radius $R_{11}$. We find a finite result, which, in a small radius expansion, has the form of the type IIA supergravity free energy plus non-perturbative corrections in the string coupling $g_A$, whose coefficients we determine. We then study type IIA superstring theory at finite temperature in the strong coupling regime by considering M-theory on $R^9\times T^2$, one of the sides of the torus being the euclidean time direction, where fermions obey antiperiodic boundary conditions. We find that a certain winding membrane state becomes tachyonic above some critical temperature, which depends on $g_A$. At weak coupling, it coincides with the Hagedorn temperature, at large coupling it becomes $T_{\rm cr} \cong 0.31 l_P^ {-1} $ (so it is very small in string units).

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arXiv2000

Strong Coupling Effects in non-commutative spaces from OM Theory and supergravity

We show that a four-parameter class of 3+1 dimensional NCOS theories can be obtained by dimensional reduction on a general 2-torus from OM theory. Compactifying two spatial directions of NCOS theory on a 2-torus, we study the transformation properties under the $SO(2,2;Z)$ T-duality group. We then discuss non-perturbative configurations of non-commutative super Yang-Mills theory. In particular, we calculate the tension for magnetic monopoles and (p,q) dyons and exhibit their six-dimensional origin, and construct a supergravity solution representing an instanton in the gauge theory. We also compute the potential for a monopole-antimonopole in the supergravity approximation.

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arXiv2000

Large N field theories from superstrings

These are introductory lectures on the correspondence between SU(N) gauge theories and Superstring Theory in anti-de Sitter geometries (AdS). The subject combines a number of different topics, including supersymmetric field theory, classical and quantum physics of black holes, string theory, string dualities, conformal field theories (CFT), and quantum field theory in anti-de Sitter spaces. We also discuss applications of this AdS/CFT correspondence to the large $N$ dynamics of pure QCD.

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arXiv2000

On Noncommutative Open String Theories

We investigate new compactifications of OM theory giving rise to a 3+1 dimensional open string theory with noncommutative $x^0$-$x^1$ and $x^2$-$x^3$ coordinates. The theory can be directly obtained by starting with a D3 brane with parallel (near critical) electric and magnetic field components, in the presence of a RR scalar field. The magnetic parameter permits to interpolate continuously between the $x^0$-$x^1$ noncommutative open string theory and the $x^2$-$x^3$ spatial noncommutative U(N) super Yang-Mills theory. We discuss $SL(2,Z)$ transformations of this theory. Using the supergravity description of the large $N$ limit, we also compute corrections to the quark-antiquark Coulomb potential arising in the NCOS theory.

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