arXiv subjects

A. Paliathanasis

Publications and source records attributed to A. Paliathanasis.

At least 19 recordsLinked to original sources

arXiv2026

Asymptotic Solutions of Radiating Stars

We investigate the evolution of the surface of radiating stars by studying the asymptotic behaviour of exact solutions initiated via the stationary boundary condition. This boundary condition leads to a master equation in the form of a second-order nonlinear differential equation that describes the evolution of the scale factor. We examine this master equation by introducing a set of dimensionless dynamical variables, motivated by similar approaches in cosmological settings. We derive the stationary points of the system in the presence of charge and a cosmological constant. Furthermore, we construct criteria for the initial conditions in order that the asymptotic limit approaches a static geometry.

gr-qc↗

arXiv2026

Hints Beyond $\Lambda$CDM from Barrow and Tsallis Holographic Dark Energy with GO cutoff

Barrow and Tsallis Holographic Dark Energy (HDE) are two recent extensions of the standard HDE framework, obtained by introducing generalized entropy corrections through the Barrow and Tsallis formalisms. In this work, we examine the cosmological consequences of Barrow and Tsallis HDE implemented with the Granda-Oliveros (GO) infrared (IR) cutoff. After deriving the modified Friedmann equations within the thermodynamic-gravity conjecture, we study the background evolution in both non-interacting and interacting dark sector scenarios, emphasizing the role of the entropic parameter in shaping late-time dynamics. We then confront the model with state-of-the-art observations, including PantheonPlus and Union3 Type Ia supernovae, Cosmic Chronometers and DESI DR2 BAO measurements. Using Bayesian MCMC methods, we constrain the model parameters and compare the performance of BHDE with that of $\Lambda$CDM. Our results show that BHDE is compatible with current data and can exhibit a mild statistical preference over the concordance model for certain dataset combinations. Overall, the analysis underscores the relevance of generalized entropy frameworks in late-time cosmology and identifies Barrow-Tsallis holography with the GO cutoff as a competitive alternative to $\Lambda$CDM.

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arXiv2025

Observational constraints on the modified cosmology inspired by string T-duality

We explore the cosmological consequences of a modified cosmology inspired by string T-duality. We incorporate the zero-point length correction, $l_0$, into the gravitational potential and derive the modified Friedmann equations via thermodynamic approach at the apparent horizon of a Friedmann-Robertson-Walker (FRW) universe. The resulting framework introduces a dimensionless coupling parameter $\beta\sim l_0^2H_0^2$ quantifying deviations from the standard $\Lambda$CDM model. Using Bayesian inference with \textsc{Cobaya} and MCMC sampling, we constrain the model parameter against late-time observations, including PantheonPlus and Union3 Type~Ia supernovae, cosmic chronometers, DESI~DR2 BAO measurements, and Amati-calibrated GRBs. The joint analysis yields an upper bound $\beta \lesssim \mathcal{O}(10^{-3})$ (68\% C.L.), implying that departures from $\Lambda$CDM are extremely small within current precision. Model comparison through the Akaike Information Criterion shows that the $\Lambda$CDM and T-duality models provide statistically equivalent fits to the data, exhibiting only a marginal preference for $\Lambda$CDM. These results provide the first quantitative observational constraints on string T-duality inspired modified cosmology and underscore the potential of future high-precision surveys to test quantum-gravity induced corrections in a late-time universe.

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arXiv2025

Noncoincidence $f(Q)$-Cosmology with Dark Matter Coupled to Gravity

We investigate FLRW cosmology in the framework of symmetric teleparallel $f(Q)$ gravity with a nonminimal coupling between dark matter and the gravitational field. In the noncoincidence gauge, the field equations admit an equivalent multi-scalar field representation, which we investigate the phase-space using the Hubble-normalization approach. We classify all stationary points for arbitrary function $f(Q)$ and we discuss the physical properties of the asymptotic solutions. For the power-law theory, we perform a detailed stability analysis and show that the de Sitter solution is the unique future attractor, while the matter-dominated point appears as a saddle point. Moreover, there exist a family of scaling solutions that can be related to inflationary dynamics. In contrast with uncoupled $f(Q)$ models, the presence of the coupling introduces a viable matter-dominated era alongside late-time accelerated expansion. Our study shows that the coupling function plays a crucial role in cosmological dynamics in $f(Q)$ gravity.

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arXiv2025

Anisotropic Spacetimes in $f(G)$-gravity: Bianchi I, Bianchi III and Kantowski-Sachs Cosmologies

We investigate the evolution of cosmological anisotropies within the framework of $f\left(G\right)$-gravity. Specifically, we consider a locally rotationally symmetric geometry in four-dimensional spacetime that describes the Bianchi I, Bianchi III, and the Kantowski-Sachs spacetimes. Within this context, we introduce a Lagrange multiplier which allows us to reformulate the geometric degrees of freedom in terms of a scalar field. The resulting theory is dynamically equivalent to an Einstein-Gauss-Bonnet scalar field model. We normalize the field equations by introducing dimensionless variables. The dynamics of our system is then explored by solving the resulting nonlinear differential equations numerically for various sets of initial conditions. Our analysis reveals the existence of two finite attractors: the Minkowski universe and an isotropic, spatially flat solution capable of describing accelerated expansion. Although de Sitter expansion may be recovered, it appears only as an unstable solution. In addition, the theory suffers from the existence of Big Rip singularities.

gr-qc↗

arXiv2024

Kantowski-Sachs and Bianchi III dynamics in $f\left(Q\right)$-gravity

We explore the phase-space of homogeneous and anisotropic spacetimes within symmetric teleparallel $f(Q)$-gravity. Specifically, we consider the Kantowski-Sachs and locally rotational Bianchi III geometries to describe the physical space. By analyzing the phase-space, we reconstruct the cosmological history dictated by $f(Q)$-gravity and comment about the theory's viability. Our findings suggest that the free parameters of the connection must be constrained to eliminate nonlinear terms in the field equations. Consequently, new stationary points emerge, rendering the theory cosmologically viable. We identify the existence of anisotropic accelerated universes, which may correspond to the pre-inflationary epoch.

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arXiv2023

Exploring Quantum Cosmology within the Framework of Teleparallel $f(T)$-gravity

We investigate quantum cosmology in teleparallel $f(T)$-gravity. We delve extensively into the minisuperspace description within the context of teleparallelism. The $f(T)$-theory constitutes a second-order theory of gravity, whose cosmological counterpart is delineated by a degenerate point-like Lagrangian. To formulate the Hamiltonian function encompassing all constraints and degrees of freedom inherent to $f(T)$ cosmology, we employ the Dirac-Bergmann algorithm. Subsequently, we determine the wave function of the universe and introduce a ``probabilistic'' interpretation. We perform comparisons to some classical solutions to see to what extent the quantum approach can cure classical singularities.

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arXiv2023

Minisuperspace description of $f(Q)$-cosmology

We investigate the existence of minisuperspace description for the homogeneous cosmological field equations within the framework of symmetric teleparallel $f(Q)$-gravity. We consider the background space to be described by the isotropic Friedmann--Lema\^ıtre--Robertson--Walker geometry, the anisotropic Kantowski-Sachs and the anisotropic Bianchi III geometries. Across all these models, we establish that the field equations in $f(Q)$-cosmology exhibit second-order characteristics in the coincident gauge and those of a sixth-order theory in the non-coincident gauge. Specifically, within the latter scenario, the dynamic degrees of freedom are attributed to two scalar fields. Finally, as an example of integrability, we derive a vacuum cosmological solution within the non-coincident gauge.

gr-qc↗

arXiv2023

Anisotropic Solutions in Symmetric Teleparallel $f\left( Q\right)$-theory: Kantowski-Sachs and Bianchi III LRS Cosmologies

We investigate the existence of anisotropic self-similar exact solutions in symmetric teleparallel $f\left( Q\right)$-theory. For the background geometry we consider the Kantowski-Sachs and the Locally Rotationally Symmetric Bianchi type III geometries. These two anisotropic spacetimes are of special interest because in the limit of isotropy they are related to the closed and open Friedmann--Lema\^ıtre--Robertson--Walker cosmologies respectively. For each spacetime there exist two distinct families of flat, symmetric connections, which share the symmetries of the spacetime. We present the field equations, and from them, we determine the functional form of the $f\left( Q\right)$ Lagrangian which yields self-similar solutions. We initially consider the vacuum case and subsequently we introduce a matter source in terms of a perfect fluid. Last but not least, we report some self-similar solutions corresponding to static spherically symmetric spacetimes.

gr-qc↗

arXiv2022

Self-similar Cosmological Solutions in Symmetric Teleparallel theory: Friedmann-Lemaître-Robertson-Walker spacetimes

The existence of self-similar solutions is discussed in symmetric teleparallel $f(Q)$-theory for a Friedmann-Lemaître-Robertson-Walker background geometry with zero and non-zero spatial curvature. For the four distinct families of connections which describe the specific cosmology in symmetric teleparallel gravity, the functional form of $f(Q)$ is reconstructed. Finally, to see if the analogy with General Relativity holds, we discuss the relation of the self-similar solutions with the asymptotic behaviour of more general $f(Q)$ functions.

gr-qc↗

arXiv2022

FLRW solutions in $f(Q)$ theory: the effect of using different connections

We study a Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) space-time in the theory of $f(Q)$-gravity, where $Q$ denotes the non-metricity scalar. It has been previously shown in the literature, that there exist four distinct families of connections, which are compatible with the isometries of the FLRW metric; three for the spatially flat case and one when the spatial curvature is present. In the spatially flat case, one connection is dynamically irrelevant and yields the dynamics of the coincident gauge in the Cartesian coordinates. For this, we obtain the general solution of an arbitrary $f(Q)$ theory with a perfect fluid matter content, and present various examples for specific choices of the $f(Q)$ function. We proceed by studying the effect of the rest of the connections, which are dynamical and affect the equations of the motion. We concentrate in scenarios that depart from the $Q=$const. case, which just reproduces General Relativity with a cosmological constant, and derive novel vacuum solutions for a power-law $f(Q)$ function.

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arXiv2022

Interacting dark energy in curved FLRW spacetime from Weyl Integrable Spacetime

In the present article, we show that a simple modification to the Einstein-Hilbert action can explain the possibility of mutual interaction between the cosmic fluids. That is achieved considering the Weyl Integrable Spacetime in the background of a nonflat Friedmann-Lemnaître-Robertson-Walker geometry for the universe. We show that widely-known phenomenological interacting cosmological scenarios can naturally appear in this context. We then performed the dynamical system analysis of the underlying cosmological scenario and explored many possibilities extracted from this gravitational theory.

gr-qc↗

arXiv2022

Lie symmetry classification and qualitative analysis for the fourth-order Schrödinger equation

The Lie symmetry analysis for the study of a $1+n~$fourth-order Schrödinger equation inspired by the modification of the deformation algebra in the presence of a minimum length is applied. Specifically, we perform a detailed classification for the scalar field potential function where non-trivial Lie symmetries exist and simplify the Schrödinger equation. Then, a qualitative analysis allows for the reduced ordinary differential equation to be analyzed to understand the asymptotic dynamics.

math-ph↗

arXiv2022

New analytic solutions in $f\left( R\right) $-Cosmology from Painlev\'{e} analysis

Using the singularity analysis, we investigate the integrability properties and existence of analytic solutions in $f\left( R\right)$-cosmology. Specifically, for some power-law $f\left( R\right) $-theories of particular interest, we apply the ARS algorithm to prove if the field equations possess the Painlev\'{e} property. Constraints for the free parameters of the power-law models are derived, and new analytic solutions are derived, expressed in terms of Laurent expansions.

gr-qc↗

arXiv2021

Anisotropic Spacetimes in Chiral Scalar Field Cosmology

Study the behaviour and the evolution of the cosmological field equations in an homogeneous and anisotropic spacetime with two scalar fields coupled in the kinetic term. Specifically, the kinetic energy for the scalar field Lagrangian is that of the Chiral model and defines a two-dimensional maximally symmetric space with negative curvature. For the background space we assume the locally rotational spacetime which describes the Bianchi I, the Bianchi III and the Kantowski-Sachs anisotropic spaces. We work on the $H$% -normalization and we investigate the stationary points and their stability. For the exponential potential we find a new exact solution which describes an anisotropic inflationary solution. The anisotropic inflation is always unstable, while future attractors are the scaling inflationary solution or the hyperbolic inflation. For scalar field potential different from the exponential, the de Sitter universe exists.

gr-qc↗

arXiv2021

Quantum Cosmology in $f(Q)$ theory

We use Dirac's method for the quantization of constrained systems in order to quantize a spatially flat Friedmann-Lemaître-Robertson-Walker spacetime in the context of $f(Q)$ cosmology. When the coincident gauge is considered, the resulting minisuperspace system possesses second class constraints. This distinguishes the quantization process from the typical Wheeler-DeWitt quantization, which is applied for cosmological models where only first class constraints are present (e.g. for models in General Relativity or in $f(R)$ gravity). We introduce the Dirac brackets, find appropriate canonical coordinates and then apply the canonical quantization procedure. We perform this method both in vacuum and in the presence of matter: a minimally coupled scalar field and a perfect fluid with a linear equation of state. We demonstrate that the matter content changes significantly the quantization procedure, with the perfect fluid even requiring to put in use the theory of fractional Quantum Mechanics in which the power of the momentum in the Hamiltonian is associated with the fractal dimension of a Lévy flight. The results of this analysis can be applied in $f(T)$ teleparallel cosmology, since $f(Q)$ and $f(T)$ theories have the same degrees of freedom and same dynamical constraints in cosmological studies.

gr-qc↗

arXiv2021

Quantization of Einstein-aether Scalar field Cosmology

We present, for the first time, the quantization process for the Einstein-aether scalar field cosmology. We consider a cosmological theory proposed as a Lorentz violating inflationary model, where the aether and scalar fields interact through the assumption that the aether action constants are ultra-local functions of the scalar field. For this specific theory there is a valid minisuperspace description which we use to quantize. For a particular relation between the two free functions entering the reduced Lagrangian the solution to the Wheeler-DeWitt equation as also the generic classical solution are presented for any given arbitrary potential function.

gr-qc↗

arXiv2020

Dynamics in Varying vacuum Finsler-Randers Cosmology

In the context of Finsler-Randers theory we consider, for a first time, the cosmological scenario of the varying vacuum. In particular, we assume the existence of a cosmological fluid source described by an ideal fluid and the varying vacuum terms. We determine the cosmological history of this model by performing a detailed study on the dynamics of the field equations. We determine the limit of General Relativity, while we find new eras in the cosmological history provided by the geometrodynamical terms provided by the Finsler-Randers theory.

gr-qc↗