On `Graceful Exit' from inflationary phase in two-dimensional Liouville String Cosmology

Within the context of a super-critical (Liouville) string, we discuss (target-space) two-dimensional string cosmology. A numerical analysis indicates that the identification of time with the Liouville mode results in an expanding universe with matter which exhibits an inflationary phase, and `graceful exit' from it, tending asymptotically to a flat-metric fixed point.This fixed point is characterized by a dilaton configuration which, depending on the initial conditions, either decreases linearly with the cosmic time, or is a finite constant. This implies that, in contrast to the critical string case, the string coupling remains bounded during the exit from the inflationary phase, and, thus, the pertinent dynamics can be reliably described in terms of a tree-level string effective action. The r\^ole of matter in inducing such phenomena is emphasized. It is also interesting to note that the asymptotic value of the vacuum energy, which in the $\sigma$-model framework is identified with the `running' central charge deficit, depends crucially on the set of initial conditions. Thus, although preliminary, this toy model seems to share all the features expected to characterize a phenomenologically acceptable cosmological string model.Comment: 23 pages LATEX, six eps figures incorporate

Cosmological String Solutions in 4 Dimensions from 5d Black Holes

We obtain cosmological four dimensional solutions of the low energy effective string theory by reducing a five dimensional black hole, and black hole--de Sitter solution of Einstein gravity down to four dimensions. The appearance of a cosmological constant in the five dimensional Einstein--Hilbert action produces a special dilaton potential in the four dimensional effective string action. Cosmological scenarios implemented by our solutions are discussed.Comment: Latex, 3 figures, epsfig.sty, DESY 93-11

Tachyon Effects on the 2-Dim Black Hole Geometry

We study solutions of the tree level string effective action in the presence of the tachyon mode.We find that the 2-dim. static black hole is stable against tachyonic perturbations.For a particular ansatz for the tachyon field we find an exact solution of the equations of motion which exhibits a naked singularity.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.Comment: NTUA 45/94, Pages 11, Latex, 3 Figures on reques

Value-at-Risk for long and short trading positions: The case of the Athens Stock Exchange

This paper provides Value-at-Risk estimates for daily stock returns with the application of various parametric univariate models that belong to the class of ARCH models which are based on the skewed Student distribution. We use daily data for three stock indexes of the Athens Stock Exchange (ASE) and three stocks of Greek companies listed in the ASE. We conduct our analysis with the adoption of the methodology suggested by Giot and Laurent (2003). Therefore, we estimate an APARCH model based on the skewed Student distribution to fully take into account the fat left and right tails of the returns distribution. We show that the estimated VaR for traders having both long and short positions in the Athens Stock Exchange is more accurately modeled by a skewed Student APARCH model that by a normal or Student distributions.Value-at-Risk, risk management, APARCH models, skewed Student distribution