Papers by Panagiota Kanti
The present article discusses magnetic confinement of the Dirac excitations in graphene in presen... more The present article discusses magnetic confinement of the Dirac excitations in graphene in presence of inhomogeneous magnetic fields. In the first case a magnetic field directed along the z axis whose magnitude is proportional to 1/r is chosen. In the next case we choose a more realistic magnetic field which does not blow up at the origin and gradually fades away from the origin. The magnetic fields chosen do not have any finite/infinite discontinuity for finite values of the radial coordinate. The novelty of the two magnetic fields is related to the equations which are used to find the excited spectra of the excitations. It turns out that the bound state solutions of the two-dimensional hydrogen atom problem are related to the spectra of graphene excitations in presence of the 1/r (inverse-radial) magnetic field. For the other magnetic field profile one can use the knowledge of the bound state spectrum of a two-dimensional cut-off Coulomb potential to dictate the excitation spectra of graphene. The spectrum of the graphene excitations in presence of the inverse-radial magnetic field can be exactly solved while the other case cannot be. In the later case we give the localized solutions of the zero-energy states in graphene.
We discuss the properties of Lorentzian wormholes in dilatonic Einstein-Gauss-Bonnet theory in fo... more 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.
We review the current results for the emission of Hawking radiation by a higherdimensional black ... more We review the current results for the emission of Hawking radiation by a higherdimensional 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.
During the spin-down phase of the life of a higher-dimensional black hole, the emission of partic... more 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 "sphericallysymmetric zone" that is followed by the concentration of the emission on the equatorial plane, again in total agreement with the exact numerical results.
In this talk, we recall the most important features of the Dilatonic Black Holes which arise in t... more 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.
In the framework of the four-dimensional effective theory of heterotic superstrings at low energi... more 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.
We extend the idea of assisted inflation to the case of power-law potentials and demonstrate the ... more 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.
We consider the embedding of the Standard Model fields in a $(4+d)$-dimensional theory while grav... more We consider the embedding of the Standard Model fields in a $(4+d)$-dimensional theory while gravitons may propagate in $d'$ extra, compact dimensions. We study the modification of strengths of the gravitational and gauge interactions and, for various values of $d$ and $d'$, we determine the energy scale at which these strengths are unified. Special cases where the unification of strengths is characterized by the absence of any hierarchy problem are also presented.
We compute the external moduli and dilaton hair to $O(\alpha')$ in the framework of the one-loop ... more We compute the external moduli and dilaton hair to $O(\alpha')$ in the framework of the one-loop corrected superstring effective action for a rotating black hole background.
We consider the combined Yang Mills-Dilaton-Gravity system in the presence of a Gauss-Bonnet term... more We consider the combined Yang Mills-Dilaton-Gravity system in the presence of a Gauss-Bonnet term as it appears in the 4D Effective Superstring Action. We give analytical arguments and demonstrate numerically the existence of black hole solutions with non-trivial dilaton and Yang Mills hair for the particular case of SU(2) gauge fields. The thermodynamical properties of the solutions are also discussed.
We construct traversable wormholes in dilatonic Einstein-Gauss-Bonnet theory in four spacetime di... more 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.
Physical Review D, 1995
We extend existing treatments of black hole solutions in String Gravity to include moduli fields.... more We extend existing treatments of black hole solutions in String Gravity to include moduli fields. We compute the external moduli and dilaton hair, as well as of their associated axions,to $O(\alpha')$ in the framework of the loop corrected superstring effective action for a Kerr-Newman black hole background.
Physical Review D, 1995
We extend existing treatments of black hole solutions in String Gravity to include moduli fields.... more We extend existing treatments of black hole solutions in String Gravity to include moduli fields.We compute the external moduli and dilaton hair, as well as of their associated axions,to O(α ′ ) in the framework of the loop corrected superstring effective action for a Kerr-Newman black hole background.
Physical Review D, 2001
We consider a 6D space-time which is periodic in one of the extra dimensions and compact in the o... more 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 finetuning 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.
Journal of Physics: Conference Series, 2011
The 14th conference in the series "Recent Developments in Gravity" was organized by the Theoretic... more The 14th conference in the series "Recent Developments in Gravity" was organized by the Theoretical Physics group of the Department of Physics, University of Ioannina, Greece. NEB-XIV (NEB-14) took place in Ioannina, Greece, from 8-11 June, 2010 at the Grand Serai Hotel. The abstracts of all the talks, along with other details of the meeting, are to be found on the website of the conference http://neb14.physics.uoi.gr/index.htm. The NEB series (initials from the Greek: Νɛώτɛρɛς Εξɛλίξɛις στην Βαρύτητα) is a series of meetings initiated in 1984 in Crete by the late Professor Vassilis Xanthopoulos and a group of Greek relativists. Since then this meeting has been held every other year at various cities in Greece, organized by different groups of Greek relativists and cosmologists. This was the third time it was organized at Ioannina (the other two times were in 1988 and 2000). It was our pleasure to see that the international participation in these meetings has been rising constantly during the last few years and NEB-14 continued this tradition. The Conference was attended by a total of 106 participants, with 62 of them from Institutions outside of Greece. We had 20 plenary session invited speakers (15 of whom were of international origin), 42 parallel session talks and 2 posters. The topics that are traditionally covered by the NEB conference series are: Cosmology (Dark Energy, Dark Matter, CMB etc) Gravitational Waves Alternative Theories of Gravity Relativistic Astrophysics Mathematical Relativity Quantum Gravity During the present conference, the following variety of topics were addressed through the participants' talks and posters: mainstream and alternative models of dark energy and dark matter, construction and testing of new cosmological models, alternative (either 4-dimensional or higher-dimensional) theories of gravity and their predictions, the interplay of gravity with gauge theories, properties of astrophysical and miniature black holes and other massive astrophysical objects, cosmological perturbation spectra, modeling of astrophysical and cosmological observables via alternative theories, attempts to build a consistent quantum gravity theory. We chose to place the emphasis in this meeting on subjects related to Observational and Theoretical Cosmology due to the rapid recent development of these research areas. Our goal was to provide a stimulating environment for the presentation and discussion of the most recent cutting-edge research results in cosmology and classical and quantum gravity. We believe that this goal was achieved and we hope that the next meeting that will take place at Chania (Crete) in 2012 (NEB-15) will further raise the standards of this series. Special thanks are due to the invited and keynote speakers for providing exciting talks and to all the participants for presenting interesting contributions and initiating fruitful discussions. We would also like to thank the Organizing Committee and the Scientific Committee for their valuable contributions to the organization of the meeting. Finally we would like to thank our main sponsors - the University of Ioannina, the Academy of Athens, the National Bank of Greece and the 'Prokos' Bookstore - for providing financial support and making this meeting possible. Last but not least, we are grateful to the European Research and Training Network "UniverseNet" (MRTN-CT-2006035863-1) for providing the funds for the publication of the proceedings of our meeting. The EditorsLeandros PerivolaropoulosPanagiota Kanti The Organizing Committee:L Perivolaropoulos (Ioannina) (Chair)P Kanti (Ioannina) (Co-Chair)C Kolasis (Ioannina)N Stergioulas (Thessaloniki)K Kokkotas (Thessaloniki, Tuebingen)D Papadopoulos (Thessaloniki)M Plionis (Athens)S Basilakos (Athens)E Vagenas (Athens)S Nesseris (Niels Bohr)N Pappas (Ioannina) The Scientific Committee:A Ashtekar (Penn-State)N Batakis (Ioannina)D Christodoulou (ETH)G Contopoulos (Academy of Athens)C Frenk (University of Durham)V Frolov (Alberta) The Plenary Invited Speakers:Prof. J A Font (University of Valencia)Prof. V Frolov (University of Alberta)Prof. R Gregory (University of Durham)Prof. O Lahav (UCL London)Prof. D Psaltis (University of Arizona )Prof. S Sarkar (University of Oxford)Prof. J Sola (Barcelona University)Prof. R Woodard (University of Florida) Keynote speakers:Prof. I Bakas (University of Patras)Prof. K Dimopoulos (University of Lancaster)Prof. D Giannios (Princeton University)Prof. N Mavromatos (Kings College, London)Prof. D Polarski (University Montpellier II)Prof. M Sakellariadou (Kings College, London)Prof. T Sotiriou (University of Cambridge)Prof. N Tetradis (University of Athens)Prof. C Tsagas (University of Thessaloniki)
Physical Review D, 2010
We perform a comprehensive study of the emission of massive scalar fields by a higher-dimensional... more We perform a comprehensive study of the emission of massive scalar fields by a higher-dimensional, simply rotating black hole both in the bulk and on the brane. We derive approximate, analytic results as well as exact numerical ones for the absorption probability, and demonstrate that the two sets agree very well in the low and intermediate-energy regime for scalar fields with mass m_\Phi < 1 TeV in the bulk and m_\Phi < 0.5 TeV on the brane. The numerical values of the absorption probability are then used to derive the Hawking radiation power emission spectra in terms of the number of extra dimensions, angular-momentum of the black hole and mass of the emitted field. We compute the total emissivities in the bulk and on the brane, and demonstrate that, although the brane channel remains the dominant one, the bulk-over-brane energy ratio is considerably increased (up to 33%) when the mass of the emitted field is taken into account.
Physical Review D, 1999
We consider a string-inspired, gravitational theory of scalar and electromagnetic fields and we i... more We consider a string-inspired, gravitational theory of scalar and electromagnetic fields and we investigate the existence of axially-symmetric, G\"{o}del-type cosmological solutions. The neutral case is studied first and an "extreme" G\"{o}del-type rotating solution, that respects the causality, is determined. The charged case is considered next and two new configurations for the, minimally-coupled to gravity, electromagnetic field are presented. Another configuration motivated by the expected distribution of currents and charges in a rotating universe is studied and shown to lead to a G\"{o}del-type solution for a space-dependent coupling function. Finally, we investigate the existence of G\"{o}del-type cosmological solutions in the framework of the one-loop corrected superstring effective action and we determine the sole configuration of the electromagnetic field that leads to such a solution. It turns out that, in all the charged cases considered, Closed Timelike Curves do appear and the causality is always violated.
Physics Letters B, 2009
Recent works have included the effect of rotation on simulations of black hole events at the LHC,... more Recent works have included the effect of rotation on simulations of black hole events at the LHC, showing that the angular momentum of the black hole cannot be ignored and it makes a non-trivial contribution for most of the lifetime of the black hole. A key consequence of the rotation of the black hole is that the Hawking radiation is no longer isotropic, making it more difficult to infer space–time parameters from measurements of the emitted particles. In this Letter we study the angular distribution of the Hawking emission of non-zero spin particles with specific helicity on the brane. We argue that the shape of the distribution could be used as a measure of the angular momentum of the black hole.
Physical Review D, 2009
In this work, we study the emission of tensor-type gravitational degrees of freedom from a higher... more In this work, we study the emission of tensor-type gravitational degrees of freedom from a higher-dimensional, simply rotating black hole in the bulk. The decoupled radial part of the corresponding field equation is first solved analytically in the limit of low-energy emitted particles and low-angular momentum of the black hole in order to derive the absorption probability. Both the angular and radial equations are then solved numerically, and the comparison of the analytical and numerical results show a very good agreement in the low and intermediate energy regimes. By using our exact, numerical results we compute the energy and angular momentum emission rates and their dependence on the spacetime parameters such as the number of additional spacelike dimensions and the angular momentum of the black hole. Particular care is given to the convergence of our results in terms of the number of modes taken into account in the calculation, and the multiplicity of graviton tensor modes that correspond to the same angular-momentum numbers.
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Papers by Panagiota Kanti