arXiv subjects

Panagiota Kanti

Publications and source records attributed to Panagiota Kanti.

53 records · Page 3Linked to original sources

arXiv2003

Reading the Number of Extra Dimensions in the Spectrum of Hawking Radiation

After a brief review of the production and decay of Schwarzschild-like (4+n)-dimensional black holes in the framework of theories with Large Extra Dimensions, we proceed to derive the greybody factors and emission rates for scalars, fermions and gauge bosons on the brane. We present and discuss analytic and numerical methods for obtaining the above results, and demonstrate that both the amount and type of Hawking radiation emitted by the black hole can help us to determine the number of spacelike dimensions that exist in nature.

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arXiv2003

Challenges and Obstacles for a Bouncing Universe in Brane Models

A Brane evolving in the background of a charged AdS black-hole displays in general a bouncing behaviour with a smooth transition from a contracting to an expanding phase. We examine in detail the conditions and consequences of this behaviour in various cases. For a cosmological-constant-dominated Brane, we obtain a singularity-free, inflationary era which is shown to be compatible only with an intermediate-scale fundamental Planck mass. For a radiation-dominated Brane, the bouncing behaviour can occur only for background-charge values exceeding those allowed for non-extremal black holes. For a matter-dominated Brane, the black-hole mass affects the proper volume or the expansion rate of the Brane. We also consider the Brane evolving in an asymmetric background of two distinct charged AdS black hole spacetimes being bounded by the Brane and find that, in the case of an empty critical Brane, bouncing behaviour occurs only if the black-hole mass difference is smaller than a certain value. The effects of a Brane curvature term on the bounce at early and late times are also investigated.

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arXiv2003

Calculable Corrections to Brane Black Hole Decay II: Greybody Factors for Spin 1/2 and 1

The production of black holes in extra-dimensional brane-world theories can lead to detectable signals via the Hawking evaporation of the black hole to brane-localised Standard Model modes. We calculate, as a function of partial wave number and number of toroidally compactified extra dimensions, the leading correction to the energy spectrum of such Hawking radiation (the greybody factors) for decay into spin 1/2 fermions and spin 1 gauge fields localised on the Standard Model brane. We derive the associated improved differential emission rates for both types of fields. We provide both simple expressions for the leading behavior of the greybody factors in the low-energy limit $\om r_H \ll 1$, and numerical evaluation of our full analytic expressions for the emission rates, valid for $\om r_H \sim 1$. The full analytic expressions demonstrate that both the greybody factors and emission rates are enhanced as the number of extra dimensions increases.

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arXiv2002

Stable, Time-Dependent, Exact Solutions for Brane Models with a Bulk Scalar Field

We derive two classes of brane-world solutions arising in the presence of a bulk scalar field. For static field configurations, we adopt a time-dependent, factorizable metric ansatz that allows for radion stabilization. The solutions are characterized by a non-trivial warping along the extra dimension, even in the case of a vanishing bulk cosmological constant, and lead to a variety of inflationary, time-dependent solutions of the 3D scale factor on the brane. We also derive the constraints necessary for the stability of these solutions under time-dependent perturbations of the radion field, and we demonstrate the existence of phenomenologically interesting, stable solutions with a positive cosmological constant on the brane.

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arXiv2002

On the Stabilization of the Size of Extra Dimensions

We derive a general formalism for constructing a four-dimensional effective action for the radion field in 5 dimensions, taking into account the possible dependence of the scale factor on the internal coordinate. First and second variations of the action with respect to the size of the extra dimension reveals two types of algebraic constraints on the combination of brane and bulk Lagrangians that extremize the action, and provide an effective mass for the radion field. We derive a compact formula for the radion mass squared in terms of brane and bulk potentials for the case of a bulk scalar field. We further specialize these constraints for several, five-dimensional realizations of brane world models.

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arXiv2002

Calculable Corrections to Brane Black Hole Decay I: The Scalar Case

In the context of brane-world theories, the production cross-section for black holes may be greatly enhanced. Such black holes can in principle lead to detectable signals via their Hawking evaporation to brane-localized modes. We calculate, in the semiclassical approximation, the leading corrections to the energy spectrum (the greybody factors) for decay into scalar fields, as a function of the number of toroidally compactified extra dimensions, and partial wave number.

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arXiv2002

Quest for Localized 4-D Black Holes in Brane Worlds

We investigate the possibility of obtaining localized black hole solutions in brane worlds by introducing a dependence of the four-dimensional line--element on the extra dimension. An analysis, performed for the cases of an empty bulk and of a bulk containing either a scalar or a gauge field, reveals that no conventional type of matter can support such a dependence. Considering a particular ansatz for the five-dimensional line--element that corresponds to a black hole solution with a ``decaying'' horizon, we determine the bulk energy--momentum tensor capable of sustaining such a behaviour. It turns out that an exotic, shell-like distribution of matter is required. For such solutions, the black hole singularity is indeed localized near the brane and the spacetime is well defined near the AdS horizon, in contrast to the behaviour found in black string type solutions.

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arXiv2001

Relic Dark energy from Trans-Planckian Regime

As yet, there is no underlying fundamental theory for the transplanckian regime. There is a need to address the issue of how the observables in our present universe are affected by processes that may have occured during the transplanckian regime. A particular feature of the family of dispersion functions chosen is the production of ultralow frequencies at very high momenta $k> M_P$. We name the range of the ultralow energy modes (of very short distances) that have frequencies equal or less than the current Hubble rate $H_0$ as the $\it{tail}$ modes. These modes are still frozen today due to the expansion of the universe. We calculate their energy today and show that the $tail$ provides a strong candidate for the {\it dark energy} of the universe. During inflation, their energy is about 122-123 orders of magnitude smaller than the total energy. We present the exact solutions and show that: the CMBR spectrum is that of a (nearly) black body, and that the adiabatic vacuum is the only choice for the initial conditions. Finally, some of these results can also be applied to black hole physics.

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arXiv2001

(De)Stabilization of an extra dimension due to a Casimir force

We study the stabilization of one spatial dimension in (p+1+1)-dimensional spacetime in the presence of $p$-dimensional brane(s), a bulk cosmological constant and the Casimir force generated by a conformally coupled scalar field. We find general static solutions to the metric which require the fine-tuning of the inter-brane distance and the bulk cosmological constant (leaving the two brane tensions as free parameters) corresponding to a vanishing effective cosmological constant and a constant radion field. Taking these solutions as a background configuration, we perform a dimensional reduction and study the effective theory in the case of one- and two-brane configurations. We show that the radion field can have a positive mass squared, which corresponds to a stabilization of the extra dimension, only for a repulsive nature of the Casimir force. This type of solution requires the presence of a negative tension brane. The solutions with one or two positive tension branes arising in this theory turn out to have negative radion mass squared, and therefore are not stable.

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arXiv2001

A 6-D Brane World Model

We consider a 6D space-time which is periodic in one of the extra dimensions and compact in the other. The periodic direction is defined by two 4-brane boundaries. Both static and non-static exact solutions, in which the internal spacetime has constant radius of curvature, are derived. In the case of static solutions, the brane tensions must be tuned as in the 5D Randall-Sundrum model, however, no additional fine-tuning is necessary between the brane tensions and the bulk cosmological constant. By further relaxing the sole fine-tuning of the model, we derive non-static solutions, describing de Sitter or Anti de Sitter 4D spacetimes, that allow for the fixing of the inter-brane distance and the accommodation of pairs of positive-negative and positive-positive tension branes. Finally, we consider the stability of the radion field in these configurations by employing small, time-dependent perturbations around the background solutions. In analogy with results drawn in 5 dimensions, the solutions describing a de Sitter 4D spacetime turn out to be unstable while those describing an Anti de Sitter geometry are shown to be stable.

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arXiv2000

Solving the Hierarchy Problem in Two-Brane Cosmological Models

We analyze cosmological solutions in the class of two-brane models with arbitrary tensions which contain matter with general equations of state. We show that the mass hierarchy between the two branes is determined by the ratio of the lapse functions evaluated on the branes. This ratio can be sufficiently small without fine-tuning the brane separation, once the transverse dimension is stabilized. For suitably large interbrane separations, both brane tensions are positive. We also find that the cosmological evolution obeys the standard four-dimensional Friedman equation up to small corrections.

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arXiv2000

Static Solutions for Brane Models with a Bulk Scalar Field

We present static solutions of the 5-dimensional Einstein equations in the brane-world scenario by using two different approaches for the stabilization of the extra dimension. Assuming a ``phenomenological'' stabilization mechanism, that creates a non-vanishing $\hat T^5_5$ in the bulk, we construct a two-brane model, which allows both branes to have positive self-energies. We then consider a candidate theory for the dynamical stabilization, through the introduction of a massless scalar field in the bulk, which interacts with the branes. We find exact static solutions for the metric and scalar field in the bulk and demonstrate that the inter-brane distance is determined by the parameters of scalar field-brane interactions. However, these solutions are always accompanied by a correlation between the bulk cosmological constant, the brane self-energies and the interaction terms of the scalar field with the branes and thus cannot be considered as candidates for the phenomenologically viable stabilized geometry. We find that the aforementioned correlation cannot be avoided even in the case of a single-brane solution with positive self-energy where the fifth dimension ends on a singularity.

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arXiv1999

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.

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arXiv1999

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.

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arXiv1999

On the Realization of Assisted Inflation

We consider conditions necessary for a successful implementation of so-called assisted inflation. We generalize the applicability of assisted inflation beyond exponential potentials as originally proposed to include standard chaotic (λϕ^4 or m^2 ϕ^2) models as well. We also demonstrate that in a purely 4-dimensional theory, unless the assisted sector is in fact decoupled, the additional fields of the assisted sector actually impede inflation. As a specific example of an assisted sector, we consider a 5-dimensional KK model for which the extra dimension may be somewhat or much larger than the inverse Planck scale. In this case, the assisted sector (coming from a KK compactification) eliminates the need for a fine-tuned quartic coupling to drive chaotic inflation. This is a general result of models with one or more "large" extra dimensions.

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arXiv1998

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.

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arXiv1998

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.

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