Four Dimensional Gravitational Backgrounds Based on N=4 c=4 Superconformal Systems

We propose two new realizations of the N=4, $\hat{c}=4$ superconformal system based on the compact and non-compact versions of parafermionic algebras. The target space interpretation of these systems is given in terms of four-dimensional target spaces with non-trivial metric and topology different from the previously known four-dimensional semi-wormhole realization. The proposed $\hat{c}=4$ systems can be used as a building block to construct perturbatively stable superstring solutions with covariantized target space supersymmetry around non-trivial gravitational and dilaton backgrounds.Comment: LateX, 21p

Construction of Superstrings in Wormhole-Like Backgrounds

We construct a class of superstring solutions in non trivial space-time. The existence of an $N=4$ world-sheet superconformal symmetry stabilizes our solutions under perturbative string loop corrections and implies in target space some unbroken space-time supersymmetries.Comment: 8 pages, no figures, preprint (Talk given at the International Europhysics Conference on High Energy Physics, Marseille, 22-28 July 1993) CERN-TH.7169/94 (Missprints corrected and title changed

Superstring Extension of the Standard Model and Gravitation

Some of the four-dimensional Superstring solutions provide a consistent framework for a Supersymmetric Unification of all interactions including gravity. A class of them extends successfully the validity of the standard model up to the string scale ${\cal O}(10^{17})~GeV$. We stress the importance of string corrections which are relevant for low energy ${\cal O}(1)~TeV$ predictions of gauge and Yukawa couplings as well as the spectrum of the supersymmetric particles after supersymmetry breaking.Comment: plain LATEX, no figures, 22 page

Introduction to Superstring Cosmology

This is a summary of lectures in superstring cosmology given by the author at the CORFU 2009 School and Workshops "Theory - Cosmology - Phenomenology", Corfu Institute, Greece, Sept 6-13, 2009. These lectures are based on some recent developments and ideas, in the framework of superstring theory, concerning the evolution and structure of the universe in (i) the very early "non-geometric"' cosmological era, (ii) the intermediate "radiation-like" era and (iii) the late time cosmological era characterized by the electroweak phase transition.Comment: Summary of lectures in superstring cosmology given by the author at the CORFU 2009 School and Workshops "Theory - Cosmology - Phenomenology". 6 page

Universal Thermal Instabilities and the High-Temperature Phase of the N=4 Superstrings

Using the properties of gauged N=4 supergravity, we show that it is possible to derive a universal thermal effective potential that describes all possible high-temperature instabilities of the known N=4 superstrings. These instabilities are due to non-perturbative dyonic modes, which become tachyonic in a region of the thermal moduli space M={s,t,u}; M is common to all non-perturbative dual-equivalent N=4 superstrings in five dimensions. We analyse the non-perturbative thermal potential and show the existence of a phase transition at high temperatures corresponding to a condensation of 5-branes. This phase is described in detail, using an effective non-critical string theory.Comment: 15 pages, LATEX file, no figure

Construction of String Solutions around Non-trivial Backgrounds

We present a way of constructing string solutions around non-trivial gravitational backgrounds. The proposed solutions are constructed using $N = 4$ superconformal building blocks with $\hat c = 4$. We give two different and inequivalent realizations of non-trivial four-dimensional subspaces, and we show the emergence of the $N = 4$ globally defined superconformal symmetry. The existence of $N = 4$ world-sheet symmetry stabilizes our solutions and implies in target space a number of covariantized supersymmetries around space-time dependent gravitational and dilaton backgrounds.Comment: Latex file, 15pp, CERN-TH.6790/9

Non-singular superstring cosmology in two dimensions

We review a recently proposed approach to construct superstring cosmological evolutions, which are free of Hagedorn instabilities and initial singularities. We illustrate these ideas in hybrid models in two dimensions.Comment: 8 pages. Based on a talk given at the "10th Hellenic School and Workshops on Elementary Particle Physics and Gravity", Corfu, Greece, September 4 - 18, 201

Gravitational Phenomena from Superstrings in Curved Spacetime

The four-dimensional superstring solutions define at low energy effective supergravity theories. A class of them extends successfully the validity of the standard model up to the string scale (${\cal O}(10^{17})~TeV$). We stress the importance of string corrections which are relevant for low energy (${\cal O}(1)~TeV$) predictions of gauge and Yukawa couplings as well as the spectrum of the supersymmetric particles. A class of exact string solutions are also presented, providing non trivial space-time backgrounds, from which we can draw some lessons concerning the regions of space-time where the notion of the effective field theory prescription make sense. We show that the string gravitational phenomena may induce during the cosmological evolution, transitions from one effective field theory prescription to a different one where the geometrical and topological data, as well as the relevant observable states are drastically different. Talk presented by C. Kounnas in the XXIX Moriond Meeting, M\'eribel, France.Comment: LateX, 11pp., CERN-TH.7332/9

Instabilities in Strong Magnetic Fields in String Theory

We construct groundstates of the string with non-zero mass gap and non-trivial chromo-magnetic fields as well as curvature. The exact spectrum as function of the chromo-magnetic fields and curvature is derived. We examine the behavior of the spectrum, and find that there is a maximal value for the magnetic field $H_{\rm max}\sim M_{\rm Plank}^2$. At this value all states that couple to the magnetic field become infinitely massive and decouple. We also find tachyonic instabilities for strong background fields of the order ${\cal O}(\mu M_{\rm Plank})$ where $\mu$ is the mass gap of the theory. Unlike the field theory case, we find that such ground states become stable again for magnetic fields of the order ${\cal O}(M^2_{\rm Plank})$. The implications of these results are discussed.Comment: To appear in the proceedings of the Conference on Gauge Theories, Applied Supersymmetry and Quantum Gravity, Leuven, July 199