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G. Panotopoulos

Publications and source records attributed to G. Panotopoulos.

At least 19 recordsLinked to original sources
arXiv2026

Evolution of matter perturbations in the context of cosmic slowing down

We investigate dynamical, time-evolving dark-energy models exhibiting phantom behaviour within Einstein gravity by analysing the evolution of matter perturbations. In particular, we consider five dark-energy parameterisations that predict a slowing down of the current cosmic acceleration. We examine the evolution of the growth index and its first derivative, as well as the combination parameter and the statefinder diagnostics, and compare their behaviour with that of the concordance $\Lambda$CDM model. For the parameter choices considered, the present-day values of the growth index and its first derivative are found to be compatible with ranges reported in previous phenomenological studies. The combination parameter is also found to differ from its $\Lambda$CDM value, while the statefinder parameters provide additional means of distinguishing the considered dynamical dark-energy scenarios from the concordance model. Our results illustrate the phenomenological signatures associated with these models and identify observables that may be useful for future observational tests. We stress, however, that the present analysis does not constitute a statistical model comparison or establish an observational preference for any of the considered dynamical dark-energy models over $\Lambda$CDM.

gr-qc↗
arXiv2026

Radial oscillations of quark stars in light of current astrophysical constraints: A comparative study

We investigate the structural and oscillatory properties of isotropic strange quark stars within General Relativity, focusing on three physically motivated equations of state: the color flavor locked (CFL) phase, an interacting quark matter model, and a linear (causal) equation of state. By numerically solving the Tolman Oppenheimer Volkoff and radial perturbation equations, we construct equilibrium stellar sequences and compute oscillation spectra across three representative masses (0.77, 1.40, and 2.00 solar masses). Our analysis is focused on two diagnostics: (i) mass to radius profiles and (ii) radial mode eigenfrequencies with large frequency separations. We compare theoretical predictions against multimessenger constraints from NICER X ray timing of key pulsars, the massive pulsars at two solar masses, and the low mass compact object in HESS J1731--347. All three equations of state yield maximum masses exceeding 2 solar masses with canonical mass radii of (10--12) km, satisfying current observational bounds. Fundamental mode frequencies span (4--7) kHz, with asymptotic large separations differing among the models. These elevated frequencies lie within the detection band of current and next generation gravitational-wave observatories, offering potential asteroseismic signatures for distinguishing strange quark stars from hadronic neutron stars in post merger emission. Our results demonstrate that self bound quark matter naturally accommodates the sub solar mass configuration of HESS J1731--347, reinforcing the viability of strange quark star interpretations.

gr-qc↗
arXiv2026

Radial oscillations of pulsating neutron stars: The UCIa equation-of-state case

Radial oscillations provide a clean dynamical test of the high-density stiffness of neutron-star equations of state. We study spherically symmetric pulsations of nonrotating relativistic stars built from cold, charge-neutral, $\beta$-equilibrated pure nucleonic matter described within relativistic mean-field theory. As a baseline we adopt the UCIa parameter set [Astron. Astro-phys. 689, A242 (2024)], and we implement high-density stiffening via the $\sigma$-cut scheme by adding a regulator potential $U_{\rm cut}(\sigma)$ [Phys. Rev. C 92, no.5, 052801 (2015), Phys. Rev. C 106, no.5, 055806 (2022)]. For representative choices $f_s=0$ (no cutoff) and $f_s=0.58$ (stiffened), we solve the Tolman-Oppenheimer-Volkoff and tidal perturbation equations to obtain equilibrium sequences, mass-radius relations, and tidal deformabilities. We then derive and solve the linear general-relativistic radial pulsation equations to compute the eigenfrequencies and eigenfunctions of the fundamental and overtone modes. The $\sigma$-cutoff suppresses the growth of the scalar field at supranuclear density, increases the pressure, and shifts the maximum mass, radii, and $\Lambda_{1.4}$ accordingly, while systematically raising the radial-mode frequencies at fixed mass. Using the sign change of $\omega_0^2$ as a stability criterion, we identify stiffened models that remain radially stable up to the observed $\sim 2M_\odot$ mass scale and are consistent with current multimessenger constraints, demonstrating how radial spectra complement static EoS tests.

nucl-th↗
arXiv2025

Stellar modeling within regularized 4D Einstein-Gauss-Bonnet gravity in light of current astrophysical constraints

In this study we obtain interior solutions and investigate structural properties of isotropic compact stars in the framework of four-dimensional regularized Einstein-Gauss-Bonnet (4DEGB) gravity. For stellar matter content, we adopt a widely used quark-matter model that approximates a realistic equation of state (EoS). By numerically integrating the modified Tolman-Oppenheimer-Volkoff equations, we obtain interior solutions for static, spherically symmetric fluid spheres. The resulting sequences are compared directly with the predictions of General Relativity (GR). Our analysis focuses on three diagnostic indicators: (i) the mass-radius profiles under GR and three representative choices of the Gauss-Bonnet coupling; (ii) the stellar compactness factor, $C \equiv M/R$; and (iii) the relation between stellar mass and central energy density. Recent observational studies suggest that the maximum masses inferred from the mass-radius relation may be larger than previously expected. To address this, we include a comparative set of constraints from multi-messenger astrophysical observations, including gravitational-wave event GW190814, as well as X-ray measurements from NICER for PSR~J0740+6620 and PSR~J0030+0451. These data provide stringent, astrophysically grounded tests of the viability of the models discussed here. Our results indicate that compact stars within 4DEGB gravity are systematically less compact and achieve moderately higher maximum masses compared to the GR case. This trend is consistent with recent theoretical analyses of compact stars in higher-curvature gravity theories and with constraints from multi-messenger astrophysics. Together, these findings suggest that regularized Gauss-Bonnet corrections constitute a plausible extension of GR in the strong-field regime.

gr-qc↗
arXiv2025

Exploring the Structural Properties of Anisotropic Dark Matter-Admixed Quark Stars

We investigate anisotropic compact stars comprising two non-interacting fluids: quark matter and condensed dark matter. Using the MIT Bag model equation of state for quark matter and Bose-Einstein Condensate equation of state for dark matter, we numerically compute interior solutions for those two-fluid component spherical configurations. Varying the initial central density ratio of dark matter to quark matter, we examine how different proportions of these components influence the mass-radius profile, the factor of compactness as well as the quark mass fraction. Recent studies suggest that quark matter may exist in the cores of massive neutron stars, significantly affecting their structure and stability. We calculate the factor of compactness for both negative and positive anisotropy cases explored in this article. Our findings demonstrate that dark matter-admixed quark stars are more compact yet less massive compared to pure quark matter stars, aligning with recent theoretical predictions and gravitational wave observations.

gr-qc↗
arXiv2025

Universal relations for non-rotating objects made of dark energy

We obtain universal relations for fluid spheres without rotation made of dark energy assuming the Extended-Chaplygin Gas equation-of-state. After integrating the relevant differential equations, we make a fit to obtain the unknown coefficients of the functions a) normalized moment of inertia versus dimensionless deformability and b) normalized moment of inertia versus factor of compactness. We find that the form of the functions does not depend on the details of the underlying equation-of-state.

gr-qc↗
arXiv2024

Anisotropic Dark Energy Stars within Vanishing Complexity Factor Formalism: Hydrostatic Equilibrium, Radial Oscillations, and Observational Implications

We investigate the structure and radial oscillations of anisotropic compact stars composed of dark energy, using the vanishing complexity factor formalism within general relativity. This novel approach establishes a direct link between the energy density and anisotropic factor, providing a robust framework for studying these exotic stellar objects. Employing an Extended Chaplygin Gas equation of state, we numerically compute interior solutions for both isotropic and anisotropic stars, revealing distinct differences in their properties. Additionally, we examine the oscillation modes and frequencies of these stars, highlighting the impact of anisotropy on their pulsational behavior. Our results reveal distinct differences in the stellar properties, such as the metric potentials, pressure, speed of sound, and relativistic adiabatic index, between the two cases. Furthermore, we calculate the large frequency separation for the fundamental and first excited modes, offering insights relevant for future asteroseismology studies. Our findings shed light on the complex interplay of gravity, matter, and anisotropy in compact stars, providing a new perspective on dark energy's role in their structure and dynamics.

gr-qc↗
arXiv2023

Novel charged black hole solutions of Born-Infeld type: General properties, Smarr formula and Quasinormal frequencies

We investigate two novel models of charged black holes in the framework of non-linear electrodynamics of Born-Infeld type. In particular, starting from two concrete Lagrangian densities, the corresponding metric potentials, the electric field, the Smarr formula and finally, the (scalar) quasinormal modes are computed for each model. Our findings show that although the models look very similar, their quasinormal spectra are characterized by certain differences.

gr-qc↗
arXiv2023

Correlation of structure growth index with current cosmic acceleration: constraints on dark energy models

We study dynamical dark energy models within Einstein's theory by means of matter perturbations and the growth index $γ$. Within four-dimensional General Relativity, we assume that dark energy does not cluster, and we adopt a linear ansatz for the growth index to investigate its impact on the deceleration parameter, $q$, and on the dark energy equation-of-state parameter, $w$. Following this approach, we identify a relationship between $q_0$ (today's value of $q$) and $γ$, which to the best of our knowledge is new. For $w(z)$, we find that in most of the cases considered it crosses the -1 line (quintom) ending at a present day value $w_0 > -1$. Furthermore, we show that an analytic expression for $w(z)$ may be obtained in the form of order (4,4) (or higher) Pad{é} parameterizations.

gr-qc↗
arXiv2023

Anisotropic stars made of exotic matter within the complexity factor formalism

We investigate exotic stars composed of dark energy within the context of Einstein's General Relativity, by applying an extended Chaplygin gas equation-of-state. To account for anisotropies, we utilize a formalism based on the complexity factor to obtain numerical solutions. By applying well-established criteria, we demonstrate that the solutions are physically valid and well-behaved. In addition, a comparison with a more conventional approach is also conducted.

gr-qc↗
arXiv2021

Geodesic analysis and black hole shadows on a general non--extremal rotating black hole in five--dimensional gauged supergravity

In this work, motivated by the fact that higher-dimensional theories predict the existence of black holes which differ from their four-dimensional counterpart, we analyse the geodesics and black hole shadow cast by a general non-extremal five-dimensional black hole. The system under consideration corresponds to the Chong-Cvetič-Lü-Pope (Phys. Rev.Lett.{\bf 95}, 161301 (2005)), which has the Myers--Perry black hole as a limit.

gr-qc↗
arXiv2020

Asteroseismology: radial oscillations of neutron stars with realistic equation of state

We study radial oscillations of non-rotating neutron stars (NSs) in four-dimensional General Relativity. The interior of the NS was modelled within a recently proposed multicomponent realistic equation of state (EoS) with the induced surface tension (IST). In particular, we considered the IST EoS with two sets of model parameters, that both reproduce all the known properties of normal nuclear matter, give a high quality description of the proton flow constraint, hadron multiplicities created in nuclear-nuclear collisions, consistent with astrophysical observations and the observational data from the NS-NS merger. We computed the 12 lowest radial oscillation modes, their frequencies and corresponding eigenfunctions, as well as the large frequency separation for six selected fiducial NSs (with different radii and masses of 1.2, 1.5 and 1.9 solar masses) of the two distinct model sets. The calculated frequencies show their continuous growth with an increase of the NS central baryon density. Moreover, we found correlations between the behaviour of first eigenfunction calculated for the fundamental mode, the adiabatic index and the speed of sound profile, which could be used to probe the internal structure of NSs with the asteroseismology data.

astro-ph.HE↗
arXiv2019

Black hole shadow of a rotating polytropic black hole by the Newman--Janis algorithm without complexification

In this work, starting from a spherically symmetric polytropic black hole, a rotating solution is obtained by following the Newman--Janis algorithm without complexification. Besides studying the horizon, the static conditions and causality issues of the rotating solution, we obtain and discuss the shape of its shadow. Some other physical features as the Hawking temperature and emission rate of the rotating polytropic black hole solution are also discussed.

gr-qc↗
arXiv2017

Scalar field descriptions of two dark energy models

We give a scalar field description of two dark energy parameterizations, and we analyze in detail its cosmology both at the level of background evolution and at the level of linear perturbations. In particular, we compute the statefinder parameters and the growth index as functions of the red-shift for both dark energy parameterizations, and the comparison with the $ΛCDM$ model as well as with a few well-known geometrical dark energy models is shown. In addition, the combination parameter $A=f σ_8$ of both models is compared against current data.

astro-ph.CO↗
arXiv2012

Cosmological evolution in brane-worlds with large transverse dimensions: Inflation and dark matter

In the present work we discuss inflation, dark matter and cosmological evolution in the context of the brane-world scenario. Being string theory inspired, the brane-world models provide corrections to the General Relativity, which is considered to be the low-energy limit of string theory. We find that novel cosmologies are obtained, which potentially can provide answers to some of the longstanding problems of modern cosmology, such as the origin and nature of dark energy. At the same time the successes of the standard four-dimensional cosmology are preserved, and in some cases the treatment in the framework of brane cosmology is even more satisfactory.

hep-ph↗
arXiv2009

Absorption on horizon-wrapped branes

We compute the absorption cross section of space-time scalars on a static D2 rane, in global coordinates, wrapped on the S^2 of an AdS_2 X S^2 X CY_3 geometry. We discuss its relevance for the construction of the dual quantum mechanics of Calabi-Yau black holes.

hep-th↗
arXiv2006

Cosmological constraints on a dark energy model with a non-linear scalar field

In the present work we study a dark energy model in which a non-linear scalar field (tachyon) with a Born-Infeld type of action is responsible for the observed cosmic acceleration. The potential of the tachyon is well-motivated since it comes from open string theory and the model is subjected to various cosmological constraints with data coming from supernovae as well as from microwave background radiation. Our analysis shows that in the particular model under study the tachyon can be an excellent candidate for dark energy in the universe, as the model agrees with a series of observational data and for a wide range of the parameters of the model.

astro-ph↗