Early-time cosmological solutions in scalar-Gauss-Bonnet theory

We consider a gravitational theory that contains the Einstein term, a scalar field and the quadratic Gauss-Bonnet term. We focus on the early-universe dynamics, and demonstrate that the Ricci scalar does not affect the cosmological solutions at early times, when the curvature is strong. We then consider a pure scalar-GB theory with a quadratic coupling function: for a negative coupling parameter, we obtain solutions that contain always an inflationary, de Sitter phase, while for a positive coupling function, we find instead expanding singularity-free solutions.Comment: 6 pages, 3 figures, to appear in the proceedings of the Fourteenth Marcel Grossmann Meeting - MG14 (Early Universe session - EU3), Rome, July 12-18, 201

Black Holes in the Framework of the Four-Dimensional Effective Theory of Heterotic Superstrings at Low Energies

In the framework of the four-dimensional effective theory of heterotic superstrings at low energies, which is considered as a generalised theory of gravity, we search for new black hole solutions. The analytical expressions of all the scalar fields of the effective theory is determined in a Kerr-Newman black hole background and the evasion of the existing "no-hair" theorems in the presence of higher curvature gravitational terms is demonstrated. New black hole solutions, such as the Dilatonic and the Coloured black holes, are determined by use of numerical integration and their properties are analysed. Finally, the linear stability of Dilatonic black holes under small time-dependent perturbations is exhibited through a semi-analytic method.Comment: A 4 page version (including summary and contents), PhD thesis, full postscript version (197 pages in Greek) available at http://artemis.sci.uoi.gr/~pkanti/phd.ps.g

5-Dimensional Assisted Inflation and the Remedy of the Fine-Tuning Problem

We extend the idea of assisted inflation to the case of power-law potentials and demonstrate the simultaneous resolution of two major problems that plague chaotic inflation. The implementation of the same idea in the framework of a 5-dimensional, scalar field theory leads to a model of chaotic inflation free of fine-tuning.Comment: 5 pages, Latex file, Talk presented at the "Workshop on Current Issues in String Cosmology", June 21-25, 1999, IHES, Paris and at the "XIth Rencontres de Blois: Frontiers of Matter", June 27-July 3, 1999, Blois, Franc

Effective Temperatures and Radiation Spectra for a Higher-Dimensional Schwarzschild-de-Sitter Black-Hole

The absence of a true thermodynamical equilibrium for an observer located in the causal area of a Schwarzschild-de Sitter spacetime has repeatedly raised the question of the correct definition of its temperature. In this work, we consider five different temperatures for a higher-dimensional Schwarzschild-de Sitter black hole: the bare $T_0$, the normalised $T_{BH}$ and three effective ones given in terms of both the black hole and cosmological horizon temperatures. We find that these five temperatures exhibit similarities but also significant differences in their behaviour as the number of extra dimensions and the value of the cosmological constant are varied. We then investigate their effect on the energy emission spectra of Hawking radiation. We demonstrate that the radiation spectra for the normalised temperature $T_{BH}$ -- proposed by Bousso and Hawking over twenty years ago -- leads to the dominant emission curve while the other temperatures either support a significant emission rate only at a specific $\Lambda$ regime or they have their emission rates globally suppressed. Finally, we compute the bulk-over-brane emissivity ratio and show that the use of different temperatures may lead to different conclusions regarding the brane or bulk dominance.Comment: 31 pages, PDFLatex, 8 figures, 8 Tables, typos corrected, version to appear in Physical Review

Angular profile of Particle Emission from a Higher-dimensional Black Hole: Analytic Results

During the spin-down phase of the life of a higher-dimensional black hole, the emission of particles on the brane exhibits a strong angular variation with respect to the rotation axis of the black hole. It has been suggested that this angular variation is the observable that could disentangle the dependence of the radiation spectra on the number of extra dimensions and angular momentum of the black hole. Working in the low-energy regime, we have employed analytical formulae for the greybody factors, angular eigenvalues and eigenfunctions of fermions and gauge bosons, and studied the characteristics of the corresponding angular profiles of emission spectra in terms of only a few dominant partial modes. We have confirmed that, in the low-energy channel, the emitted gauge bosons become aligned to the rotation axis of the produced black hole while fermions form an angle with the rotation axis whose exact value depends on the angular-momentum of the black hole. In the case of scalar fields, we demonstrated the existence of a "spherically-symmetric zone" that is followed by the concentration of the emission on the equatorial plane, again in total agreement with the exact numerical results.Comment: 27 pages, Latex2e file, 4 figure

Assisted Chaotic Inflation in Higher Dimensional Theories

We address the problem of the large initial field values in chaotic inflation and propose a remedy in the framework of the so-called assisted inflation. We demonstrate that a 4-dimensional theory of multiple, scalar fields with initial field values considerably below the Planck scale, can give rise to inflation even though none of the individual scalar fields are capable of driving inflation. The problems arising from the presence of possible non-renormalizable interactions is therefore removed. As a concrete example of a theory with multiple scalar fields, we consider a (4+d)-dimensional field theory of a single, non-interacting massive scalar field whose KK modes play the role of the assisted sector. For the KK modes to assist inflation, the extra dimensions must have a size larger than the inverse (4D) Planck scale.Comment: 11 pages, LaTeX fil

Brane-World Black Holes

In this talk, I present and discuss a number of attempts to construct black hole solutions in models with Warped Extra Dimensions. Then, a contact is made with models with Large Extra Dimensions, where black-hole solutions are easily constructed -- here the focus will be on the properties of microscopic black holes and the possibility of using phenomena associated with them, such as the emission of Hawking radiation, to discover fundamental properties of our spacetime.Comment: 16 pages, 6 figures, to appear in the Proceedings of the "Recent Developments in Gravity" NEB-XIII Conference, Thessaloniki, Greece, 4-6 June 200

Linear Stability of Dilatonic Black Holes

In this talk, we recall the most important features of the Dilatonic Black Holes which arise in the framework of the Einstein-Dilaton-Gauss-Bonnet theory and which are dressed with a classical long range dilaton field in contradiction with the existing "no-hair" theorems of the Theory of General Relativity. We demonstrate linear stability of these black hole solutions under small spacetime-dependent perturbations by making use of a semi-analytic method based on the Fubini-Sturm's theorem. As a result, the Dilatonic Black Holes constitute one of the very few examples of stable black hole solutions with non-trivial "hair" that arise in the framework of a more generalised theory of gravity

Greybody Factors for Scalar Fields emitted by a Higher-Dimensional Schwarzschild-de-Sitter Black-Hole

In this work, we consider the propagation of scalar particles in a higher-dimensional Schwarzschild-de-Sitter black-hole spacetime, both on the brane and in the bulk. Our analysis applies for arbitrary partial modes and for both minimal and non-minimal coupling of the scalar field. A general expression for the greybody factor is analytically derived in each case, and its low-energy behaviour is studied in detail. Its profile in terms of scalar properties (angular-momentum number and non-minimal coupling parameter) and spacetime properties (number of extra dimensions and cosmological constant) is thoroughly investigated. In contrast to previous studies, the effect of the cosmological constant is taken into account both close to and far away from the black-hole horizon. The dual role of the cosmological constant, that may act either as a helping agent to the emission of scalar particles or as a deterring effect depending on the value of the non-minimal coupling parameter, is also demonstrated.Comment: Latex file, 28 pages, 10 figures, references added, typos corrected, version to appear in Physical Review

Gauss-Bonnet Inflation

We consider an Einstein-Scalar-Gauss-Bonnet gravitational theory, and argue that at early times the Ricci scalar can be safely ignored. We then demonstrate that the pure scalar-Gauss-Bonnet theory, with a quadratic coupling function, naturally supports inflationary -- de Sitter -- solutions. During inflation, the scalar field decays exponentially and its effective potential remains always bounded. The theory contains also solutions where these de Sitter phases possess a natural exit mechanism and are replaced by linearly expanding -- Milne -- phases.Comment: 6 pages, Revtex, 3 figures, introduction updated, a section added, typos corrected, accepted as a Rapid Communications article in Physical Review