arXiv subjects

George Koutsoumbas

Publications and source records attributed to George Koutsoumbas.

At least 19 recordsLinked to original sources

arXiv2026

Thermal Stability and QNMs of a Hairy Black Hole in the Presence of a Monopole Field

We study the stability of a generalization of the GHS-GM black hole in the presence of a dilaton and a monopole field. We find that the thermal behaviour of system depends on the scalar charge of the dilaton field and as this parameter is decreasing the system becomes more thermally stable. We also find that, as the charge of the black hole is increasing, both the real and the imaginary parts of the quasi-normal frequencies decrease in absolute value. The overtone modes die out faster than the fundamental modes and no positive imaginary parts appear, indicating the stabilty of the system

gr-qc↗

arXiv2026

Formation of Bound States in Quintessence Alternative Theories

We study the formation and behaviour of bound states formed outside the horizon of a black hole in the presence of quintessence matter. Calculating the Regge and Wheeler potential for general metric function, we find that the presence of quintessence influences significantly the metric function and the Hawking temperature. We show that large black holes radiate less in the presence of quintessence matter and it seems to live longer, while small black holes radiate more in comparison with the model in the absence of quintessence. Bound states emerge at large enough quintessence parameter $|w|$ or angular momentum.

gr-qc↗

arXiv2024

Scalarization of the Reissner-Nordström black hole with higher derivative gauge field corrections

We discuss spontaneous scalarization of the Reissner-Nordström black hole in the presence of higher derivative gauge field corrections that arise in the context of string, as well as higher-dimensional more fundamental gravity theories. Our theory admits the Reissner-Nordström solution at the scalar vacuum of the theory ($ϕ=0$) and we find that the higher order derivative gauge field correction term results in the tachyonic instability of our system once the coupling function satisfies the condition that its second derivative is positive at the scalar vacuum in the appropriate parameter space. We find that the branches do not end with an extremal black hole, rather with a singularity as indicated by the divergence of the Kretschmann scalar. The black holes can be overcharged in the sense that they may carry larger electric charge in comparison to their mass. Finally, these solutions possess larger entropy at the event horizon radius when compared to the Reissner-Nordström black hole, as well as to scalarized black holes without the higher order derivative gauge field terms, indicating in this way the thermodynamic prefer-ability of our system, when compared to existing literature, while they respect the energy conditions.

gr-qc↗

arXiv2023

Motion of particles around a magnetically charged Euler-Heisenberg black hole with scalar hair and the Event Horizon Telescope

We study the motion of uncharged particles and photons in the background of a magnetically charged Euler-Heisenberg (EH) black hole (BH) with scalar hair. The spacetime can be asymptotically (A)dS or flat. After investigating particle motions around the BH and the behavior of the effective potential of the particle radial motion, we determine the contribution of the BH parameters to the geodesics. Photons follow null geodesics of an effective geometry induced by the corrections of the EH non-linear electrodynamics. Thus, after determining the effective geometry, we calculate the shadow of the BH. Upon comparing the theoretically calculated BH shadow with the images of the shadows of M87* and Sgr A* obtained by the Event Horizon Telescope collaboration, we impose constraints on the BH parameters, namely the scalar hair ($ν$), the magnetic charge ($Q_{m}$) and the EH parameter ($α$).

gr-qc↗

arXiv2023

Motion of particles in a magnetically charged Euler-Heisenberg black hole with scalar hair

We study the geodesic motion of uncharged particles in the background of a magnetically charged Euler-Heisenberg black hole with a scalar hair. The spacetime can be asymptotically (A)dS or flat and we find, analysing the behavior of the effective potential of the radial motion that in all cases there exist stable and unstable orbits. Performing numerical integrations we depict the motion of particles for planetary and critical orbits, as well as for radial geodesics. We discuss the effect that the scalar hair $ν$, the magnetic charge $Q_{m}$ and the Euler-Heisenberg parameter $α$ have on the particle motion.

gr-qc↗

arXiv2023

Black Holes with Scalar Hair in Three Dimensions

Three - dimensional static and spinning black hole solutions of the Einstein-Klein-Gordon system are obtained for a particular scalar field configuration. At large distances, and for small scalar field, the solutions reduce to the BTZ black hole. The scalar field dresses the black hole with secondary scalar hair, since the scalar charge is related to the conserved black hole mass and is not an independent charge. A self interacting potential is included, containing a mass term that is above the Breitenlohner-Freedman bound in three dimensions. Independence of the scalar potential from the conserved black hole charges, imposes fixed mass and angular momentum to scalar charge ratios. The thermodynamic properties as well as the energy conditions of the black hole are analysed.

gr-qc↗

arXiv2022

Magnetically Charged Euler-Heisenberg Black Holes with Scalar Hair

We study the Einstein-Euler-Heisenberg theory in the presence of a self interacting scalar field, minimally coupled to gravity. We solve analytically the field equations for the magnetically charged case and we obtain novel magnetically charged hairy black holes. The scalar field dresses the black hole with a secondary scalar hair. The hairy black hole develops three horizons when Euler-Heisenberg parameter and the magnetic charge are small and the horizon radius is getting large when the scalar charge and the gravitational mass are large. The presence of matter and the magnetic field outside the horizon of the black hole increases the temperature only for small black holes. Calculating the heat capacity we show that the asymptotically AdS Euler-Heisenberg hairy black hole undergoes a second order phase transition and then it is stabilized. Also the weak energy condition is violated for the asymptotically AdS Euler-Heisenberg hairy black hole.

gr-qc↗

arXiv2022

Topological Black Holes with curvature induced scalarization in the extended scalar-tensor theories

We study the perturbative behaviour of topological black holes in the presence of a cosmological constant and a scalar field coupled to the Gauss-Bonnet term. We calculate both analytically and numerically the quasi-normal modes of scalar perturbations in the extended scalar-tensor-Gauss-Bonnet gravity. In the case of small black holes we find a phase transition of the topological black hole to a hairy configuration.

gr-qc↗

arXiv2021

Formation of Bound States of scalar fields in AdS-asymptotic Wormholes

We use the Wentzel-Kramers-Brillouin (WKB) approximation to study the formation and propagation of bound states in the vicinity of a wormhole in the non-minimal derivative coupling theory of gravity. The wormhole throat connects two Anti-de Sitter spacetimes. We show that when the scalar field lies in high orbital states, the corresponding potential has potential barriers that block the passage of a classical field. We investigate the behaviour of the bound states trapped in the potential wells and provide the flow between the two AdS regions.

gr-qc↗

arXiv2020

Neutrino oscillations in gravitational and cosmological backgrounds

We use the eikonal approximation in order to calculate the additional phase shift between two neutrino mass eigenstates during their propagation in a background of gravitational wave or scalar perturbations in the flat and the FRW spacetime metric. We comment on the dependence of the results on the characteristics of the perturbations, give some order-of-magnitude estimates, and find that, although small, the resulting phase difference persists for large redshifts, up to the validity of our approximations.

hep-ph↗

arXiv2018

Quantum Effects in Galileon Black Holes

Using the Wentzel-Kramers-Brillouin (WKB) approximation we study the formation and propagation of quantum bound states in the vicinity of a Galileon black hole. We show that for given ranges of black hole horizon radii and of the derivative coupling which appears in the metric of the Galileon black hole, a Regge-Wheeler potential containing a local well is formed. Varying the strength of the derivative coupling we investigate the behaviour of the bound states trapped in the potential well or penetrating the horizon of the Galileon black hole.

gr-qc↗

arXiv2018

Unification of Dark Matter - Dark Energy in Generalized Galileon Theories

We present a unified description of the dark matter and the dark energy sectors, in the framework of shift-symmetric generalized Galileon theories. Considering a particular combination of terms in the Horndeski Lagrangian in which we have not introduced a cosmological constant or a matter sector, we obtain an effective unified cosmic fluid whose equation of state $w_U$ is zero during the whole matter era, namely from redshifts $z\sim3000$ up to $z\sim2-3$. Then at smaller redshifts it starts decreasing, passing the bound $w_U=-1/3$, which marks the onset of acceleration, at around $z\sim0.5$. At present times it acquires the value $w_U=-0.7$. Finally, it tends toward a de-Sitter phase in the far future. This behaviour is in excellent agreement with observations. Additionally, confrontation with Supernovae type Ia data leads to a very efficient fit. Examining the model at the perturbative level, we show that it is free from pathologies such as ghosts and Laplacian instabilities, at both scalar and tensor sectors, at all times.

gr-qc↗

arXiv2017

Reheating predictions in Gravity Theories with Derivative Coupling

We investigate the inflationary predictions of a simple Horndeski theory where the inflaton scalar field has a non-minimal derivative coupling (NMDC) to the Einstein tensor. The NMDC is very motivated for the construction of successful models for inflation, nevertheless its inflationary predictions are not observationally distinct. We show that it is possible to probe the effects of the NMDC on the CMB observables by taking into account both the dynamics of the inflationary slow-roll phase and the subsequent reheating. We perform a comparative study between representative inflationary models with canonical fields minimally coupled to gravity and models with NMDC. We find that the inflation models with dominant NMDC generically predict a higher reheating temperature and a different range for the tilt of the scalar perturbation spectrum $n_s$ and scalar-to-tensor ratio $r$, potentially testable by current and future CMB experiments.

gr-qc↗

arXiv2017

Gravitational Collapse of a Homogeneous Scalar Field Coupled Kinematically to Einstein Tensor

We study the gravitational collapse of a homogeneous time-dependent scalar field that, besides its coupling to curvature, it is also kinematically coupled to the Einstein tensor. This coupling is a part of the Horndeski theory and we investigate its effect on the collapsing process. We find that the time required for the scalar field to collapse depends on the value of the derivative coupling and the singularity is protected by a horizon. Matching the internal solution with an external Schwarzschild- AdS metric we show that a black hole is formed, while the weak energy condition is satisfied during the collapsing process. The scalar field takes on a finite value at the singularity.

gr-qc↗

arXiv2015

Quantum phase transition of high dimensional Yang-Mills theories

We determine the critical value of the coupling where the first order quantum phase transition takes place for lattice SU(2) Yang-Mills theories in dimensions higher than four. Within a Mean-Field approach we derive an approximate law valid for any dimension d and in the context of a Monte Carlo approach, in addition to the already known d=5 case, we look at d=6,7,8.

hep-lat↗

arXiv2013

Phase Transition to a Hairy Black Hole in Asymptotically Flat Spacetime

We discuss a phase transition of a Reissner-Nordström black hole to a hairy black hole in asymptotically flat spacetime. The hair is due to a massive charged scalar field. The no-hair theorem is evaded thanks to a derivative coupling of the scalar field to the Einstein tensor. The resulting hairy configuration is spherically symmetric. We solve the equations analytically near the transition temperature and show that the hair is concentrated near the horizon decaying exponentially away from it.

hep-th↗

arXiv2013

Gravitational Particle Production in Gravity Theories with Non-minimal Derivative Couplings

We study the gravitational production of heavy X-particles of mass of the order of the inflaton mass, produced after the end of inflation. We find that, in the presence of a derivative coupling of the inflaton field or of the X-field to the Einstein tensor, the number of gravitationally produced particles is suppressed as the strength of the coupling is increased.

gr-qc↗

arXiv2012

On the large N limit of SU(N) lattice gauge theories in five dimensions

We develop the necessary tools for computing fluctuations around a mean-field background in the context of SU(N) lattice gauge theories in five dimensions. In particular, expressions for the scalar observable and the Wilson Loop are given. As an application, using these observables we compute a certain quantity k5 that can be viewed as Coulomb's constant in five dimensions. We show that this quantity becomes independent of N in the large N limit. Furthermore, the numerical value of k5 we find for SU(infinity) deviates by 17% from its value predicted by holography.

hep-th↗