arXiv subjects

K. Kleidis

Publications and source records attributed to K. Kleidis.

At least 19 recordsLinked to original sources

arXiv2019

A Study of an Einstein Gauss-Bonnet Quintessential Inflationary Model

In this paper we study a class of quintessential Einstein Gauss-Bonnet models, focusing on their early and late-time phenomenology. With regard to the early-time phenomenology, we formalize the slow-roll evolution of these models and we calculate in detail the spectral index of the primordial curvature perturbations and the tensor-to-scalar ratio. As we demonstrate, the resulting observational indices can be compatible with both the Planck and the BICEP2/Keck-Array observational constraints on inflation. With regard to the late-time behavior, by performing a numerical analysis we demonstrate that the class of models for which the coupling function $ξ(ϕ)$ to the Gauss-Bonnet scalar satisfies $ξ(ϕ)\sim \frac{1}{V(ϕ)}$, produce a similar pattern of evolution, which at late-times is characterized by a decelerating era until some critical redshift, at which point the Universe super-decelerates and subsequently accelerates until present time, with a decreasing rate though. The critical redshift crucially depends on the initial conditions chosen for the scalar field and for all the quintessential Einstein Gauss-Bonnet models studied, the late-time era is realized for large values of the scalar field.

gr-qc↗

arXiv2018

Autonomous Dynamical System Description of de Sitter Evolution in Scalar Assisted $f(R)-ϕ$ Gravity

In this letter we will study the cosmological dynamical system of an $f(R)$ gravity in the presence of a canonical scalar field $ϕ$ with an exponential potential, by constructing the dynamical system in a way that it is render autonomous. This feature is controlled by a single variable $m$, which when it is constant, the dynamical system is autonomous. We focus on the $m=0$ case which, as we demonstrate by using a numerical analysis approach, leads to an unstable de Sitter attractor, which occurs after $N\sim 60$ $e$-foldings. This instability can be viewed as a graceful exit from inflation, which is inherent to the dynamics of de Sitter attractors.

gr-qc↗

arXiv2018

Scalar Field Assisted $f(R)$ Gravity Inflation

In this paper we investigate the inflationary dynamics of an $f(R)$ gravity in the presence of a canonical scalar field. We specifically choose the cosmological evolution to be a quasi-de Sitter evolution and also the $f(R)$ gravity is assumed to be a modified version of the $R^2$ gravity. We investigate which scalar field potential can produce the quasi-de Sitter evolution for the choice of the $f(R)$ gravity we made, and also we study in detail the inflationary dynamics of the resulting theory. As we demonstrate, the spectral index is identical to the one corresponding to the ordinary $R^2$ gravity, while the scalar-to-tensor ratio is found to be smaller than the $R^2$ inflation one consequently, compatibility with both the Planck 2015 and BICEP2/Keck-Array data is achieved.

gr-qc↗

arXiv2018

Loop Quantum Cosmology Scalar Field Models

In this work we use the Loop Quantum Cosmology modified scalar-tensor reconstruction techniques in order to investigate how bouncing and inflationary cosmologies can be realized. With regard to the inflationary cosmologies, we shall be interested in realizing the intermediate inflation and the Type IV singular inflation, while with regard to bouncing cosmologies, we shall realize the superbounce and the symmetric bounce. In all the cases, we shall find the kinetic term of the LQC holonomy corrected scalar-tensor theory and the corresponding scalar potential. In addition, we shall include a study of the effective equation of state, emphasizing at the early and late time eras. As we demonstrate, in some cases it is possible to have a nearly de Sitter equation of state at the late-time era, a result that could be interpreted as the description of a late-time acceleration era. Also, in all cases we shall examine the dynamical stability of the LQC holonomy corrected scalar-tensor theory, and we shall confront the results with those coming from the corresponding classical dynamical stability theory. The most appealing cosmological scenario is that of a Type IV singular inflationary scenario, in which the singularity may occur at the late-time era. As we demonstrate, for this model, during the dark energy era, a transition from non-phantom to a phantom dark energy era occurs.

gr-qc↗

arXiv2017

Loop Quantum Cosmology Corrected Gauss-Bonnet Singular Cosmology

In this work we investigate which Loop Quantum Cosmology corrected Gauss-Bonnet $F(\mathcal{G})$ gravity can realize two singular cosmological scenarios, the intermediate inflation and the singular bounce scenarios. The intermediate inflation scenario has a Type III sudden singularity at $t=0$, while the singular bounce has a soft Type IV singularity. By using perturbative techniques, we find the holonomy corrected $F(\mathcal{G})$ gravities that generate at leading order the aforementioned cosmologies and we also argue that the effect of the holonomy corrections is minor to the power spectrum of the primordial curvature perturbations of the classical theory.

gr-qc↗

arXiv2016

Effects of Finite-time Singularities on Gravitational Waves

We analyze the impact of finite-time singularities on gravitational waves, in the context of $F(R)$ gravity. We investigate which singularities are allowed to occur during the inflationary era, when gravitational waves are considered, and we discuss the quantitative implications of each allowed singularity. As we show, only a pressure singularity, the so-called Type II and also a Type IV singularity are allowed to occur during the inflationary era. In the case of a Type II, the resulting amplitude of the gravitational wave is zero or almost zero, hence this pressure singularity has a significant impact on the primordial gravitational waves. The case of a Type IV singularity is more interesting since as we show, the singularity has no effect on the amplitude of the gravitational waves. Therefore, this result combined with the fact that the Type IV singularity affects only the dynamics of inflation, leads to the conclusion that the Universe passes smoothly through a Type IV singularity.

gr-qc↗

arXiv2015

Generating Solutions to the Einstein - Maxwell Equations

The Einstein-Maxwell (E-M) equations in a curved spacetime that admits at least one Killing vector are derived, from a Lagrangian density adapted to symmetries. In this context, an auxiliary space of potentials is introduced, in which, the set of potentials associated to an original (seed) solution of the E-M equations are transformed to a new set, either by continuous transformations or by discrete transformations. In this article, continuous transformations are considered. Accordingly, originating from the so-called $γ_A$-metric, other exact solutions to the E-M equations are recovered and discussed.

gr-qc↗

arXiv2015

Generating Solutions to the Einstein Field Equations

Exact solutions to the Einstein field equations may be generated from already existing ones (seed solutions), that admit at least one Killing vector. In this framework, a space of potentials is introduced. By the use of symmetries in this space, the set of potentials associated to a known solution are transformed into a new set, either by continuous transformations or by discrete transformations. In view of this method, and upon consideration of continuous transformations, we arrive at some exact, stationary axisymmetric solutions to the Einstein field equations in vacuum, that may be of geometrical or/and physical interest.

gr-qc↗

arXiv2014

Polytropic dark matter flows illuminate dark energy and accelerated expansion

Currently, a large amount of data implies that the matter constituents of the cosmological dark sector might be collisional. An attractive feature of such a possibility is that, it can reconcile dark matter (DM) and dark energy (DE) in terms of a single component, accommodated in the context of a polytropic-DM fluid. Accordingly, we explore the time evolution and the dynamical characteristics of a spatially-flat cosmological model, in which, in principle, there is no DE at all. Instead, in this model, the DM itself possesses some sort of fluid-like properties, i.e., the fundamental units of the Universe matter-energy content are the volume elements of a DM fluid, performing polytropic flows. In this case, the energy of this fluid's internal motions is also taken into account as a source of the universal gravitational field. This form of energy can compensate for the extra energy needed to compromise spatial flatness, namely, to justify that, today, the total-energy density parameter is exactly unity. The polytropic cosmological model, depends on only one free parameter, the corresponding exponent, Γ. What makes this model particularly interesting, is that, for Γ< 0.541, the (conventional) pressure becomes negative enough, so that the Universe accelerates its expansion at cosmological redshifts below a transition value. Several physical reasons impose further constraints on the value of Γ, which, eventually, is settled down to the range -0.089 < Γ< 0. Such a cosmological model does not suffer either from the age problem or from the coincidence problem. At the same time, this model reproduces to high accuracy the distance measurements performed with the aid of the supernovae Type Ia standard candles, and most naturally interprets, not only when, but also, why the Universe transits from deceleration to acceleration, thus arising as a mighty contestant for a DE model.

astro-ph.CO↗

arXiv2014

Extended Supersymmetric Quantum Mechanics Algebras in Scattering States of Fermions off Domain Walls

We study the underlying extended supersymmetric structure in a system composed of fermions scattered off an infinitely extended static domain wall in the $xz$-plane. As we shall demonstrate, the fermionic scattered states are associated to two $N=2$ one dimensional supersymmetric quantum mechanical algebras with zero central charge. These two symmetries are combined to form a non-trivial one dimensional $N=4$ superalgebra with various central charges. In addition, we form higher dimensional irreducible representations of the two $N=2$ algebras. Moreover, we study how the Witten index behaves under compact odd and even perturbations, coming from a background magnetic field and some non-renormalizable Yukawa mass terms for the fermions. As we shall demonstrate, the Witten index is invariant only when the magnetic field is taken into account and particularly when only the $z$-component of the field is taken into account. Finally, we study the impact of this supersymmetric structures on the Hilbert space of the fermionic states and also we present a deformed extension of the $N=2$ supersymmetric structure.

hep-th↗

arXiv2014

Central Charge Extended Supersymmetric Structures for Fundamental Fermions Around non-Abelian Vortices

Fermionic zero modes around non-abelian vortices are shown that they constitute two $N=2$, $d=1$ supersymmetric quantum mechanics algebras. These two algebras can be combined under certain circumstances to form a central charge extended $N=4$ supersymmetric quantum algebra. We thoroughly discuss the implications of the existence of supersymmetric quantum mechanics algebras, in the quantum Hilbert space of the fermionic zero modes.

hep-th↗

arXiv2011

Testing extra dimensions with boundaries using Newton's law modifications

Extra dimensions with boundaries are often used in the literature, to provide phenomenological models that mimic the standard model. In this context, we explore possible modifications to Newton's law due to the existence of an extra-dimensional space, at the boundary of which the gravitational field obeys Dirichlet, Neumann or mixed boundary conditions. We focus on two types of extra space, namely, the disk and the interval. As we prove, in order to have a consistent Newton's law modification (i.e., of the Yukawa-type), some of the extra-dimensional spaces that have been used in the literature, must be ruled out.

hep-th↗

arXiv2009

Charged cosmic strings interacting with gravitational and electromagnetic waves

Under a particular choice of the Ernst potential, we solve analytically the Einstein-Maxwell equations to derive a new exact solution depending on five parameters: the mass, the angular-momentum (per unit mass), the electromagnetic-field strength, k, the parameter-p and the Kerr-NUT parameter, l. This (Petrov Type D) solution is cylindrically-symmetric and represents the curved background around a charged, rotating cosmic string, surrounded by gravitational and electromagnetic waves, under the influence of the Kerr-NUT parameter. A C-energy study in the radiation zone suggests that both the incoming and the outgoing radiation is gravitational, strongly focused around the null direction and preserving its profile. In this case, the absence of the k-parameter from the C-energy implies that, away from the linear defect the electromagnetic field is too weak to contribute to the energy-content of the cylindrically-symmetric space-time under consideration. In order to explain this result, we have evaluated the Weyl and the Maxwell scalars near the axis of the linear defect and at the spatial infinity. Accordingly, we have found that the electromagnetic field is concentrated (mainly) in the vicinity of the axis, while falling-off prominently at large radial distances. However, as long as k differs from unity, the non-zero Kerr-NUT parameter enhances those scalars, both near the axis and at the spatial infinity, introducing some sort of gravitomagnetic contribution.

gr-qc↗

arXiv2007

Dynamo effects in magnetized ideal-plasma cosmologies

The excitation of cosmological perturbations in an anisotropic cosmological model and in the presence of a homogeneous magnetic field has been studied, using the ideal magnetohydrodynamic (MHD) equations. In this case, the system of partial differential equations which governs the evolution of the magnetized cosmological perturbations can be solved analytically. Our results verify that fast-magnetosonic modes propagating normal to the magnetic field, are excited. But, what's most important, is that, at late times, the magnetic-induction contrast grows, resulting in the enhancement of the ambient magnetic field. This process can be particularly favored by condensations, formed within the plasma fluid due to gravitational instabilities.

astro-ph↗

arXiv2005

Magnetohydrodynamics and Plasma Cosmology

We study the linear magnetohydrodynamic (MHD) equations, both in the Newtonian and the general-relativistic limit, as regards a viscous magnetized fluid of finite conductivity and discuss instability criteria. In addition, we explore the excitation of cosmological perturbations in anisotropic spacetimes, in the presence of an ambient magnetic field. Acoustic, electromagnetic (e/m) and fast-magnetosonic modes, propagating normal to the magnetic field, can be excited, resulting in several implications of cosmological significance.

gr-qc↗

arXiv2002

Interactive Quadratic Gravity

A quadratic semiclassical theory, regarding the interaction of gravity with a massive scalar quantum field, is considered in view of the renormalizable energy-momentum tensor in a multi-dimensional curved spacetime. According to it, a self-consistent coupling between the square curvature term R^{2} and the quantum field Φshould be introduced in order to yield the "correct" renormalizable energy-momentum tensor in quadratic gravity theories. The subsequent interaction discards any higher-order derivative terms from the gravitational field equations, but, in the expence, it introduces a geometric source term in the wave equation for the quantum field. Unlike the conformal coupling case (RΦ^{2}), this term does not represent an additional "mass" and, therefore, the quantum field interacts with gravity not only through its mass (or energy) content (~Φ^{2}), but also, in a more generic way (R^{2}Φ). Within this context, we propose a general method to obtain mode-solutions for the quantum field, by means of the associated Green's function in an anisotropic six-dimensional background.

hep-th↗

arXiv1999

Geodesic motions versus hydrodynamic flows in a gravitating perfect fluid: Dynamical equivalence and consequences

Stimulated by the methods applied for the observational determination of masses in the central regions of the AGNs, we examine the conditions under which, in the interior of a gravitating perfect fluid source, the geodesic motions and the general relativistic hydrodynamic flows are dynamically equivalent to each other. Dynamical equivalence rests on the functional similarity between the corresponding (covariantly expressed) differential equations of motion and is obtained by conformal transformations. In this case, the spaces of the solutions of these two kinds of motion are isomorphic. In other words, given a solution to the problem "hydrodynamic flow in a perfect fluid", one can always construct a solution formally equivalent to the problem "geodesic motion of a fluid element" and vice versa. Accordingly, we show that, the observationally determined nuclear mass of the AGNs is being overestimated with respect to the real, physical one. We evaluate the corresponding mass-excess and show that it is not always negligible with respect to the mass ofthe central dark object, while, under circumstances, can be even larger than the rest-mass of the circumnuclear gas involved.

gr-qc↗

arXiv1999

Parametric resonant acceleration of particles by gravitational waves

We study the resonant interaction of charged particles with a gravitational wave propagating in the non-empty interstellar space in the presence of a uniform magnetic field. It is found that this interaction can be cast in the form of a parametric resonance problem which, besides the main resonance, allows for the existence of many secondary ones. Each of them is associated with a non-zero resonant width, depending on the amplitude of the wave and the energy density of the interstellar plasma. Numerical estimates of the particles' energisation and the ensuing damping of the wave are given.

gr-qc↗