arXiv subjects

Panagiota Kanti

Publications and source records attributed to Panagiota Kanti.

At least 37 records · Page 2Linked to original sources

arXiv2017

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 $Λ$ 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.

hep-th↗

arXiv2017

Novel Ansatzes and Scalar Quantities in Gravito-Electromagnetism

In this work, we focus on the theory of Gravito-Electromagnetism (GEM) -- the theory that describes the dynamics of the gravitational field in terms of quantities met in Electromagnetism -- and we propose two novel forms of metric perturbations. The first one is a generalisation of the traditional GEM ansatz, and succeeds in reproducing the whole set of Maxwell's equations even for a dynamical vector potential A. The second form, the so-called alternative ansatz, goes beyond that leading to an expression for the Lorentz force that matches the one of Electromagnetism and is free of additional terms even for a dynamical scalar potential Φ. In the context of the linearised theory, we then search for scalar invariant quantities in analogy to Electromagnetism. We define three novel, 3rd-rank gravitational tensors, and demonstrate that the last two can be employed to construct scalar quantities that succeed in giving results very similar to those found in Electromagnetism. Finally, the gauge invariance of the linearised gravitational theory is studied, and shown to lead to the gauge invariance of the GEM fields E and B for a general configuration of the arbitrary vector involved in the coordinate transformations.

gr-qc↗

arXiv2016

Black holes in Einstein-Gauß-Bonnet-dilaton theory

Generalizations of the Schwarzschild and Kerr black holes are discussed in an astrophysically viable generalized theory of gravity, which includes higher curvature corrections in the form of the Gauss-Bonnet term, coupled to a dilaton. The angular momentum of these black holes can slightly exceed the Kerr bound. The location and the orbital frequency of particles in their innermost stable circular orbits can deviate significantly from the respective Kerr values. Study of the quasinormal modes of the static black holes gives strong evidence that they are mode stable against polar and axial perturbations. Future gravitational wave observations should improve the current bound on the Gauss-Bonnet coupling constant, based on observations of the low-mass x-ray binary A 0620-00.

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arXiv2015

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.

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arXiv2015

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.

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arXiv2015

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

In this work, we consider a generalised 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 a simple choice of the coupling function between the scalar field and the Gauss-Bonnet term and a simplifying assumption regarding the role of the Ricci scalar can lead to new, analytical, elegant solutions with interesting characteristics. We first argue, and demonstrate in the context of two different models, that the presence of the Ricci scalar in the theory at early times, when the curvature is strong, does not affect the actual cosmological solutions. By considering therefore a pure scalar-GB theory with a quadratic coupling function we derive a plethora of interesting, analytic solutions: for a negative coupling parameter, we obtain inflationary, de Sitter-type solutions or expanding solutions with a de Sitter phase in their past and a natural exit mechanism at later times; for a positive coupling function, we find instead singularity-free solutions with no Big-Bang singularity. We show that the aforementioned solutions arise only for this particular choice of coupling function, a result that may hint to some fundamental role that this coupling function may hold in the context of an ultimate theory.

hep-th↗

arXiv2014

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.

hep-th↗

arXiv2014

From GEM to Electromagnetism

In the first part of the present work, we focus on the theory of gravitoelectromagnetism (GEM), and we derive the full set of equations and constraints that the GEM scalar and vector potentials ought to satisfy. We discuss important aspects of the theory, such as the presence of additional constraints resulting from the field equations and gauge condition, the requirement of the time-independence of the vector potential and the emergence of additional terms in the expression of the Lorentz force. We also propose an alternative ansatz for the metric perturbations that is found to be compatible only with a vacuum configuration but evades several of the aforementioned obstacles. In the second part of this work, we pose the question of whether a tensorial theory using the formalism of General Relativity could re-produce the theory of Electromagnetism. We demonstrate that the full set of Maxwell's equations can be exactly re-produced for a large class of models, but the framework has several weak points common with those found in GEM.

gr-qc↗

arXiv2014

Hawking Radiation from Higher-Dimensional Black Holes

We review the quantum field theory description of Hawking radiation from evaporating black holes and summarize what is known about Hawking radiation from black holes in more than four space-time dimensions. In the context of the Large Extra Dimensions scenario, we present the theoretical formalism for all types of emitted fields and a selection of results on the radiation spectra. A detailed analysis of the Hawking fluxes in this case is essential for modelling the evaporation of higher-dimensional black holes at the LHC, whose creation is predicted by low-energy models of quantum gravity. We discuss the status of the quest for black-hole solutions in the context of the Randall-Sundrum brane-world model and, in the absence of an exact metric, we review what is known about Hawking radiation from such black holes.

hep-th↗

arXiv2012

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.

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arXiv2012

Footprints of Higher-Dimensional Decaying Black Holes

We review the current results for the emission of Hawking radiation by a higher-dimensional black hole during the Schwarzschild and the spin-down phases. We discuss particularly the role of the angular variation of the emitted radiation on the brane during the latter phase, the radiation spectra for gravitons in the bulk, and the effect of the mass of the emitted particles in determining the bulk-to-brane energy balance.

hep-th↗

arXiv2012

Stable Lorentzian Wormholes in Dilatonic Einstein-Gauss-Bonnet Theory

We discuss the properties of Lorentzian wormholes in dilatonic Einstein-Gauss-Bonnet theory in four spacetime dimensions. These wormholes do not need any form of exotic matter for their existence. A subset of these wormholes is shown to be linearly stable with respect to radial perturbations. We perform a comprehensive study of their domain of existence, and derive a generalised Smarr relation for these wormholes. We also investigate their geodesics determining all possible particle trajectories, and perform a study of the acceleration and tidal forces that a traveler crossing the wormhole would feel.

hep-th↗

arXiv2012

Wormholes in Dilatonic Einstein-Gauss-Bonnet Theory

We construct traversable wormholes in dilatonic Einstein-Gauss-Bonnet theory in four spacetime dimensions, without needing any form of exotic matter. We determine their domain of existence, and show that these wormholes satisfy a generalised Smarr relation. We demonstrate linear stability with respect to radial perturbations for a subset of these wormholes.

gr-qc↗

arXiv2009

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.

hep-th↗

arXiv2008

Black Holes at the LHC

In these two lectures, we will address the topic of the creation of small black holes during particle collisions in a ground-based accelerator, such as LHC, in the context of a higher-dimensional theory. We will cover the main assumptions, criteria and estimates for their creation, and we will discuss their properties after their formation. The most important observable effect associated with their creation is likely to be the emission of Hawking radiation during their evaporation process. After presenting the mathematical formalism for its study, we will review the current results for the emission of particles both on the brane and in the bulk. We will finish with a discussion of the methodology that will be used to study these spectra, and the observable signatures that will help us identify the black-hole events.

hep-th↗

arXiv2006

Braneworld stars and black holes

We look for spherically symmetric star or black hole solutions on a Randall-Sundrum brane from the perspective of the bulk. We take a known bulk solution, and analyse possible braneworld trajectories within it that correspond, from the braneworld point of view, to solutions of the brane Tolman-Oppenheimer-Volkoff equations. Our solutions are therefore embedded consistently into a full bulk solution. We find the full set of static gravitating matter sources on a brane in a range of bulk spacetimes, analyzing which can correspond to physically sensible sources. Finally, we look at time-dependent trajectories in a Schwarzschild--anti de Sitter spacetime as possible descriptions of time-dependent braneworld black holes, highlighting some of the general features one might expect, as well as some of the difficulties involved in getting a full solution to the question.

hep-th↗

arXiv2004

Black Holes in Theories with Large Extra Dimensions: a Review

We start by reviewing the existing literature on the creation of black holes during high-energy particle collisions, both in the absence and in the presence of extra, compact, spacelike dimensions. Then, we discuss in detail the properties of the produced higher-dimensional black holes, namely the horizon radius, temperature and life-time, as well as the physics that governs the evaporation of these objects, through the emission of Hawking radiation. We first study the emission of visible Hawking radiation on the brane: we derive a master equation for the propagation of fields with arbitrary spin in the induced-on-the-brane black hole background, and we review all existing results in the literature for the emission of scalars, fermions and gauge bosons during the spin-down and Schwarzschild phases of the life of the black hole. Both analytical and numerical results for the greybody factors and radiation spectra are reviewed and exact results for the number and type of fields emitted on the brane as a function of the dimensionality of spacetime are discussed. We finally study the emission of Hawking radiation in the bulk: greybody factors and radiation spectra are presented for the emission of scalar modes, and the ratio of the missing energy over the visible one is calculated for different values of the number of extra dimensions.

hep-ph↗

arXiv2003

Hawking Radiation from a (4+n)-dimensional Black Hole: Exact Results for the Schwarzschild Phase

We start our analysis by deriving a master equation that describes the motion of a field with arbitrary spin $s$ on a 3-brane embedded in a non-rotating, uncharged (4+n)-dimensional black hole background. By numerical analysis, we derive exact results for the greybody factors and emission rates for scalars, fermions and gauge bosons emitted directly on the brane, for all energy regimes and for an arbitrary number $n$ of extra dimensions. The relative emissivities on the brane for different types of particles are computed and their dependence on the dimensionality of spacetime is demonstrated -- we therefore conclude that both the amount and the type of radiation emitted can be used for the determination of $n$ if the Hawking radiation from these black holes is observed. The emission of scalar modes in the bulk from the same black holes is also studied and the relative bulk-to-brane energy emissivity is accurately computed. We demonstrate that this quantity varies considerably with $n$ but always remains smaller than unity -- this provides firm support to earlier arguments made by Emparan, Horowitz and Myers.

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