G.A. Diamandis

Cosmological evolution in a type 0 string theory

We study the cosmological evolution of a type-0 string theory by employing non-criticality, which may be induced by fluctuations of the D3 brane worlds. We check the consistency of the approach to in the corresponding σ-model. The ten-dimensional theory is reduced to an effective four-dimensional model, with only time dependent fields. We show that the four-dimensional universe has an inflationary phase and graceful exit from it, while the other extra dimensions are stabilized to a constant value, with the fifth dimension much larger than the others. We pay particular attention to demonstrating the role of tachyonic matter in inducing these features. The universe asymptotes, for large times, to a nonaccelerating linearly-expanding universe with a time-dependent dilaton and a relaxing to zero vacuum energy a la quintessence. Our perturbative string solution appears to have initial singularities (Big Bang type), which however, we believe, may be lifted in a nonperturbative way, and they do not represent true singularities of the string theory.

On the stability of the classical vacua in a minimal SU(5) five-dimensional supergravity model

We consider a five-dimensional supergravity model with SU(5) gauge symmetry and the minimal field content. Studying the arising scalar potential we find that the gauging of the U(1)R symmetry of the five-dimensional supergravity causes instabilities. Lifting the instabilities, the vacua are of anti-de-Sitter type and SU(5) is broken along with supersymmetry. Keeping the U(1)R ungauged the potential has flat directions along which supersymmetry is unbroken.

Dissipative Liouville cosmology : A case study

We consider solutions of the cosmological equations pertaining to a dissipative, dilaton-driven off-equilibrium Liouville cosmological model, which may describe the effective field theoretic limit of a non-critical string model of the Universe. The non-criticality may be the result of an early-era catastrophic cosmic event, such as a big-bang, brane-world collision, etc. The evolution of the various cosmological parameters of the model are obtained, and the effects of the dilaton and off-shell Liouville terms, including briefly those on relic densities, which distinguish the model from conventional cosmologies, are emphasised.

Vanishing scalar masses in no-scale supergravity

Abstract Radiative corrections to the effective scalar potential are studied in no-scale supergravity models with local supersymmetry spontaneously broken by a gravitino mass. A simple proof is given that gauge non-singlet scalar fields acquire no supersymmetry breaking masses at the one-loop level, and a general argument is given extending this result to all orders of perturbation theory in the effective low-energy theory, proving also that no trilinear soft supersymmetry breaking terms in the effective potential are generated. These results are applicable to the four-dimensional no-scale supergravity theory obtained from the superstring after compactification, and support suggestions that the dominant source of global supersymmetry breaking in this theory may be the gaugino mass.

Inflation in R 2 supergravity with non-minimal superpotentials

a b s t r a c t We investigate the cosmological inflation in a class of supergravity models that are generalizations of non-supersymmetric R2 models. Although such models have been extensively studied recently, especially after the launch of the PLANCK and BICEP2 data, the class of models that can be constructed has not been exhausted. In this note, working in a supergravity model that is a generalization of Cecottis model, we show that the appearance of new superpotential terms, which are quadratic in the superfield � that couples to the Ricci supermultiplet, alters substantially the form of the scalar potential. The arising potential has the form of the Starobinsky potential times a factor that is exponential in the inflaton field and dominates for large inflaton values. We show that the well-known Starobinsky inflation scenario is maintained only for unnaturally small fine-tuned values of the coupling describing the � 2 superpotential terms. A welcome feature is the possible increase of the tensor to scalar ratio r, within the limits set by the new Planck and BICEP2 data.

TACHYON EFFECTS ON THE TWO-DIMENSIONAL BLACK HOLE GEOMETRY

We study solutions of the tree level string effective action in the presence of the tachyon mode. In the case of static fields we find numerically that the full system has a black hole solution with the tachyon regular at the horizon. We also find a nonstatic exact solution of the equations of motion having a black hole structure with a past singularity.

On the brane coupling of unified orbifolds with gauge interactions in the bulk

Abstract In the on-shell formulation of D = 5 , N = 2 supergravity, compactified on S 1 / Z 2 , we extend the results of Mirabelli and Peskin describing the interaction of the bulk fields with matter which is assumed to be confined on the brane. The novel characteristics of this approach are: (i) Propagation of both gravity and gauge fields in the bulk, which offers an alternative for a unified description of models in extra dimensions and (ii) use of the on-shell formulation avoiding the complexity of off-shell schemes which involve numerous auxiliary fields. We also allow for non-trivial superpotential interactions of the chiral matter fields. The method we employ uses the Nother procedure and our findings are useful for building models advocating propagation of the gauge degrees of freedom in the bulk, in addition to gravity.

Tachyon field quantization and Hawking radiation.

We quantize the tachyon field in a static two dimensional dilaton gravity black hole background,and we calculate the Hawking radiation rate. We find that the thermal radiation flux, due to the tachyon field, is larger than the conformal matter one. We also find that massive scalar fields which do not couple to the dilaton, do not give any contribution to the thermal radiation, up to terms quadratic in the scalar curvature.

Time-dependent perturbations in two-dimensional String Black Holes

Abstract We discuss time-dependent perturbations (induced by matter fields) of a black-hole background in tree-level two-dimensional string theory. We analyse the linearized case and show the possibility of having black-hole solutions with time-dependent horizons. The latter exist only in the presence of time-dependent “tachyon” matter fields, which constitute the only propagating degrees of freedom in two-dimensional string theory. For real tachyon field configurations it is not possible to obtaine solutions with horizons shrinking to a point. On the other hand, such a possibility seems to be realized in the case of string black-hole models formulated on higher world-sheet genera. We connect this latter result with black-hole evaporation/decay at a quantum level.

Bogoliubov Coefficients of 2D Charged Black Holes

We exactly calculate the thermal distribution and temperature of Hawking radiation for a two-dimensional charged dilatonic black hole after it has settled down to an “equilibrium” state. The calculation is carried out using the Bogoliubov coefficients. The background of the process is furnished by a preexisting black hole and not by collapsing matter as considered by Giddings and Nelson for the case of a Schwarzschild black hole. Furthermore, the vanishing of the temperature and/or the Hawking radiation in the extremal case is obtained as a regular limit of the general case.