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Andronikos Paliathanasis

Publications and source records attributed to Andronikos Paliathanasis.

At least 19 recordsLinked to original sources
arXiv2026

Solutions for neutron stars in General Relativity from a complexity structure scalar boundary condition

A new condition involving the complexity factor, a structure scalar arising from the orthogonal splitting of the Riemann tensor, is generated at the boundary of a spherically symmetric anisotropic fluid. This condition leads to the generation of a model, without invoking any further constraints on geometry or matter variables. The model generated is physically reasonable and suggests that there is a non-zero, positive minimum constraint on the complexity scalar value evaluated at the surface. This suggests that vanishing complexity is a globalized constraint which does not apply at the boundary, specifically for compact objects in the strong-gravity regime.

gr-qc↗
arXiv2026

Inhomogeneous Cosmologies with Bumpy Structure from Superfluid Pionic Vortices and the propagation of electromagnetic field

In this work we introduce exact cosmological solutions in General Relativity in (3+1) dimensions which take into account both the usual fluid component as well as a superfluid component describing superfluid Pionic vortices. Such superfluid vortices are source of inhomogeneities which can be treated exactly, no perturbation theory is required. These analytic solutions are relevant to obtain a non-perturbative description of inhomogeneities in cosmology. We discuss the peculiar effects of these hadronic inhomogeneities on the propagation of electromagnetic fields on such cosmological backgrounds analyzing possible observable effects. Finally, the properties that the effective superfluid satisfy for such solutions to exist are discussed, together with possible extensions of this family of solutions.

gr-qc↗
arXiv2026

Beyond dynamical dark energy: the role of dark sector interactions after DESI DR2

Recent DESI DR2 observations have renewed interest in extensions of the $\Lambda$CDM cosmological model, particularly through indications of a time-varying dark energy equation of state. In this work, we investigate whether such deviations may also involve interactions within the dark sector. We consider an interacting dark energy scenario in which the dark matter density evolves as $\rho_{\rm dm}\propto a^{-3+\delta}$, with the constant $\delta$ quantifying the interaction strength, and allow the dark energy equation of state to be either constant but different from $-1$, or dynamically evolving through the CPL parametrization. The models are constrained using Planck CMB data, DESI DR2 BAO measurements, and three Type Ia supernova compilations: PantheonPlus, Union3, and DES-Dovekie. For the constant equation-of-state case, the inclusion of DESI and supernova data leads to a preference for a small negative interaction parameter, with a significance above $2\sigma$. When dynamical dark energy is allowed, the evidence for interaction becomes weak, while the data favor a quintessence-like evolving dark energy component. In both scenarios, Bayesian model comparison still favors $\Lambda$CDM. Our results show that the inferred role of dark-sector interactions depends strongly on the nature of dark energy, highlighting the importance of jointly testing dark energy dynamics and interactions in the DESI era.

astro-ph.CO↗
arXiv2026

Reconstructing the generalized Barrow holographic dark energy with physics-informed neural networks

Barrow holographic dark energy connects cosmic acceleration with possible quantum-gravitational deformations of horizon entropy, encoded in the Barrow exponent $\Delta$. If such effects are scale dependent, however, there is no fundamental reason for $\Delta$ to remain constant throughout cosmic history. In this work we reconstruct $\Delta(z)$ directly from observations, without assuming any particular functional form, using the Cosmo-PINN physics-informed neural-network framework. The generalized Barrow holographic evolution equation is incorporated into the training, while PantheonPlus supernovae, DESI DR2 baryon acoustic oscillations and cosmic chronometers constrain the reconstruction. We find a mild and smooth redshift evolution, with the posterior mean favoring negative $\Delta$ and this tendency becoming stronger when the Cepheid calibration is included. Nevertheless, $\Delta=0$ and constant negative values remain compatible with current uncertainties. The reconstructed cosmology yields a viable late-time evolution, with $w_{\rm DE}$ close to $-1$ and the expected transition to accelerated expansion. Our results demonstrate that cosmological observations can directly probe the functional behavior of a quantity entering the underlying entropy law itself.

physics.gen-ph↗
arXiv2026

Observational constraints on Barrow holographic dark energy coupled with a non-cold dark matter component from DESI DR2

We investigate the cosmological viability of Barrow holographic dark energy in a spatially flat Friedmann--Lema\^{\i}tre--Robertson--Walker universe in which the dark matter component is allowed to have a non-zero pressure, characterized by a constant equation-of-state parameter $w_{m}$. By considering the future event horizon as the infrared cutoff, we derive the master equation, which describes the cosmological dynamics for the background space. We constrain the model against late-time data, combining the baryon acoustic oscillation from DESI DR2 with three different catalogues for the Type Ia Supernova measurements and the Cosmic Chronometers. The dark matter equation of state is constrained to $w_{m}=0.033_{-0.030}^{+0.045}$, $0.009_{-0.041}^{+0.047}$, and $0.033_{-0.029}^{+0.042}$, for the PantheonPlus, the Union3.0 and the DES-Dovekie supernova datasets respectively. Therefore, the pressureless limit is recovered within the $2\sigma$ regime. On the other hand, the Barrow exponent is weakly constrained, due to the $\Delta-w_{m}$ degeneracy. The dark energy equation of state remains above the phantom divide throughout the redshift range probed. Finally, in the comparison of the statistical parameters with that of $\Lambda$CDM, it follows $\Delta\mathrm{AIC}=+2.23$, $+0.75$, and $+1.38$, $\ $while for the Bayesian evidence we find $\Delta\ln Z=-0.63$, $+0.47$, and $-0.13$, which suggest that the datasets considered in this analysis do not have a preferred model. Finally the relation with the corresponding Tsalis holographic dark energy model is discussed.

physics.gen-ph↗
arXiv2026

Early- and late-time constraints on Wald-Gauss-Bonnet topological dark energy and implications for the $H_0$ and $S_8$ tensions

The persistent $H_0$ and $S_8$ tensions motivate the search for new dark-energy mechanisms capable of modifying the late-time expansion history while preserving the successful early-Universe predictions of $\Lambda$CDM scenario. Wald-Gauss-Bonnet (WGB) topological dark energy provides a physically motivated realization of this possibility, where the effective dark-energy sector emerges from cosmic horizon thermodynamics and the black-hole formation and merger history. We present the first early- and late-Universe analysis of WGB cosmology, implementing the model as an effective fluid in a modified \texttt{CLASS} solver and constraining it against CMB data from \textit{Planck}, ACT~DR6 and SPT-3G, DESI~DR2 BAO, and Pantheon+ supernovae. While late-time data alone are consistent with $\Lambda$CDM, the full dataset prefers a non-zero WGB contribution, $\Cn=0.435^{+0.150}_{-0.132}$, corresponding to a $\sim3\sigma$ phantom-like deviation and an improved fit. The preferred solution raises $H_0$ from $68.5$ to $69.8~\kmsmpc$, reducing the Hubble tension by $\sim0.9\sigma$, at the cost of a mild increase in $S_8$. The reconstructed cosmological observables show that WGB leaves the primary CMB almost unchanged while enhancing lensing and small-scale clustering, revealing a characteristic $H_0$-$S_8$ trade-off. WGB dark energy therefore emerges as a physically motivated and observationally viable late-time mechanism for partially alleviating the Hubble tension without introducing new early-Universe physics.

gr-qc↗
arXiv2026

Unifying the Dark Sector with the New Generalized Chaplygin Gas: Observational Constraints

In light of recent cosmological observations, we examine a generalized Chaplygin gas model with a redshift-dependent exponent as a framework for describing the dark energy and dark matter content of the Universe. Specifically, we treat this fluid as a single unified component and test it against late-time background observational data. We employ Type Ia supernova data, cosmic chronometers, and baryon acoustic oscillations from the second data release of the Dark Energy Spectroscopic Instrument. We perform a Bayesian analysis for parameter estimation and compare the model with $\Lambda$CDM. We find that the generalized Chaplygin gas provides systematically higher values of the combined likelihood; nevertheless, once the larger number of free parameters is taken into account, both the Bayesian evidence and the Akaike Information Criterion suggest that the model is statistically indistinguishable from $\Lambda$CDM.

astro-ph.CO↗
arXiv2026

Cosmo-PINN: A Physics-Informed Neural Network for Cosmological Reconstruction

We introduce Cosmo-PINN, a Physics-Informed Neural Network for reconstruction of the cosmological theory. In this work we demonstrate the application of the Cosmo-PINN in the reconstruction of the dark energy equation of state parameter $w_{DE}\left( z\right) $ directly from late-time cosmological observations. This framework overcomes the main limitation shared by Gaussian Process and Artificial Neural Network reconstruction approaches, where the recovered solution is driven by the data and it is not necessarily true that it is physically consistent, by embedding the cosmological constraints directly into the loss function as hard constraints, ensuring that the reconstructed quantities satisfy the physical laws at every point during the training. For the training of the network, we employed background data, and specifically the Baryon Acoustic Oscillation from DESI DR2, the Cosmic Chronometers and three different Supernova compilations, while we simultaneously introduce the cosmological parameters $H_{0},~\Omega _{m0}$ and $r_{\mathrm{drag}}$ as trained parameters. The reconstruction shows that the trained $w_{DE}\left( z\right) $ crosses the phantom divide within the redshift range $z=0.27-0.42$ in agreement with the value obtained by the Chevallier-Polarski-Linder model. In the quintessence scenario, for large redshifts the dark energy $\Omega _{DE}\left( z\right) $ provides a pressureless nonzero contribution to the cosmological fluid suggesting a unified scenario. Finally, we demonstrate the significance of imposing the physical constraints within the loss function by comparing the Cosmo-PINN reconstruction against a purely data-driven neural network with the same architecture.

astro-ph.CO↗
arXiv2026

Compact Stars in Symmetric Teleparallel Scalar-Tensor Gravity

We investigate the existence of static, spherically symmetric compact objects within the framework of symmetric teleparallel scalar-tensor gravity. This theory extends the Brans-Dicke and scalar-tensor models within the symmetric teleparallel formalism. We consider a nontrivial connection that allows for genuinely nontrivial solutions in the limit of General Relativity. The field equations admit a minisuperspace description and by applying the method of variational symmetries we construct the corresponding conservation laws in vacuum. The application of these conservation laws enables the reconstruction of analytic black-hole solutions. Finally, we study the interior structure of compact objects matched to an extremal Reissner-Nordström exterior and show that the symmetric teleparallel scalar-tensor theory supports the existence of viable astrophysical objects.

gr-qc↗
arXiv2026

Saturation Mechanisms in the Interacting Dark Sector

We introduce a family of phenomenological cosmological models featuring an interacting dark sector modulated by a sparseness scale parameter, in order to describe the late-time accelerated expansion of the universe. The sparseness scale, inspired by well-established saturation mechanisms in ecology and biology, is introduced in the interaction as a half-saturation constant that bounds the energy exchange between dark matter and dark energy, controls the dynamical behaviour of the physical variables and can prevent the phantom crossing. We consider three nonlinear interacting models, where two of them recover the linear interacting scenarios when the sparsity parameter vanishes. We examine the phase-space of the cosmological field equations by using the Hubble normalization approach. We determine the stationary points and their stability properties in order to reconstruct the asymptotics behaviour of the field equations. Such an analysis allows us to demonstrate the effects of the sparseness scale on the background dynamics. We test the interacting models with observational data. Specifically, we employ Supernovae catalogues, cosmic chronometers, Baryon Acoustic Oscillation measurements from DESI DR2, and redshift-space distortion measurements of the growth of large-scale structure through the $f$ and $fσ_8$ observables. The Bayesian analysis suggests that, for two of the three models, a vanishing sparsity parameter is disfavoured at more than the 95\% confidence interval, providing observational support for a nonzero sparseness scale in the dark sector interaction.

astro-ph.CO↗
arXiv2026

Viaggiu holographic dark energy in light of DESI DR2

We test the cosmological viability of the Viaggiu holographic dark energy (VHDE) model by using late-time observational data. In particular, we place constraints on the free parameters of the model using Type Ia supernovae from the PantheonPlus, Union3.0, and DES-Dovekie catalogues, the Cosmic Chronometers, and the Baryon Acoustic Oscillations from the DESI DR2. Our analysis suggests that the VHDE model fits the observational data better or similar to the $Λ$CDM for all dataset combinations considered. The value obtained for $H_0$ is similar to the $Λ$CDM, while the current matter density parameter is constrained around $Ω_{m0}\simeq 0.24$, smaller to that obtained by the $Λ$CDM. Moreover, the parameter introduced by the VHDE is found to have a mean value within the range $\fracπ{3} δ^2 \sim 0.27-0.33$. Finally, we used Akaike's Information Criterion (AIC) and Bayesian evidence to test the VHDE model against the $Λ$CDM scenario. The AIC demonstrates that the two models are statistically indistinguishable, while Bayesian evidence reveals that the data have a mild preference for the $Λ$CDM model for most of the dataset combinations considered. Nevertheless, the VHDE model remains consistent with current late-time cosmological observations and offers a feasible mechanism for describing the late-time accelerating scenario.

gr-qc↗
arXiv2026

Cosmological Constraints on the Generalized Uncertainty Principle from Redshift-Space Distortions

We investigate the imprints of the Generalized Uncertainty Principle on cosmological scales by using redshift-space distortion measurements in combination with background cosmological data to determine constraints on the deformation parameter $β$. We consider the modified Poisson bracket related to the existence of a minimal length, which leads to a modified Raychaudhuri equation for the standard $Λ$CDM model and gives rise to a phenomenological one-parameter dynamical dark energy scenario. Through this modification, we can reveal the effects of the minimal length on the late-time structure of the universe. We employ the $f$ and $fσ_8$ measurements of the growth rate combined with background data, including cosmic chronometers, baryon acoustic oscillations and Type Ia supernova observations. The observational constraints reveal a systematically negative value for the deformation parameter $β$, with the $Λ$CDM limit lying within the 95\% credible interval. When supernova data are included, the Akaike Information Criterion indicates weak-to-strong support in favour of the GUP-modified model depending on the SNIa catalogue, while the Bayesian evidence suggests a weak preference.

astro-ph.CO↗
arXiv2026

Observational Constraints on Noncoincident $f(Q)$-Gravity with Matter-Gravity Coupling

We investigate $f\left( Q\right) $-gravity with a matter-gravity coupling as a geometric dark energy candidate for the description of the late-time cosmic acceleration within a spatially flat Friedmann--Lema\^ıtre-Robertson-Walker geometry. We select a noncoincident connection that naturally follows from the general framework of cosmological models with nonzero spatial curvature. We present observational constraints for the simplest $f\left( Q\right) =f_{0}Q^{n}$ model using data from Supernovae, Baryon Acoustic Oscillations and Cosmic Chronometers. For different data combinations we found consistent constraints, with a best-fit value for the power-law index $n\simeq2$. A comparison with the $Λ$CDM model shows that the $f\left( Q\right) $-gravity leads to larger values for the likelihood, while Akaike's Information Criterion suggests statistical equivalence between the two models for most data combinations.

gr-qc↗
arXiv2026

Testing the Coexistence of Dark Energy and Dark Matter with Late-time Observational Data

We investigate the viability of a cosmological scenario with interacting dark sector, which can describe the coexistence between dark energy and dark matter. The model possesses an analytical solution for the Hubble function and we constrain the free parameters by applying the newly released cosmic chronometers data (31 old data and 3 new data from DESI), the Baryonic Acoustic Oscillators from the Dark Energy Spectroscopic Instrument Survey (DESI DR2 BAO), along with Gamma-ray bursts (GRBs) and Supernova catalogues (Pantheon Plus, Union3, and DES-Dovekie). We find that the coexistence model fits the data sets in a better way than the reference models - the $Λ$CDM and $w$CDM models. The analysis shows that the coexistence scenario can provide a cosmologically viable model for the description of the late-time acceleration of the universe. Nevertheless, for large redshifts, the model has a similar behaviour to that of the $w$CDM model, as the introduction of the GRB data indicates in the statistical parameters. Finally, it is worth mentioning that the coexistence model provides a statistically smaller value for the $H_{0}$ parameter.

astro-ph.CO↗
arXiv2026

Dirac-Bergmann algorithm and canonical quantization of $k$-essence cosmology

We develop a general canonical quantization scheme for $k$-essence cosmology in scalar-tensor theory. Utilizing the Dirac-Bergmann algorithm, we construct the Hamiltonian associated with the cosmological field equations and identify the first- and second-class constraints. The introduction of appropriate canonically conjugate variables with respect to Dirac brackets, allows for the canonical quantization of the model. In these new variables, the Hamiltonian constraint reduces to a quadratic function with no potential term. Its quantum realization leads to a Wheeler-DeWitt equation reminiscent of the massless Klein-Gordon case. As an illustrative example, we consider the action of a tachyonic field and investigate the conditions under which a phantom crossing can occur as a quantum tunneling effect. For the simplified constant potential case, we investigate the consequences of different boundary conditions on the singularity avoidance and to the mean expansion rate.

gr-qc↗
arXiv2026

Dynamics of the Bianchi~V cosmological model inspired by quintessential $α$-attractors

We investigate scalar-field cosmologies in the Bianchi V spacetime using a dynamical-systems framework. Motivated by representative $α$-attractor potentials - the E-model and T-model - we apply averaging theorems and amplitude--phase reductions to monomial potentials $\sim ϕ^{2n}$ of the scalar field, which approximate the attractor models near their minima, in the presence of matter with barotropic index $γ$. The reduced averaged system admits five generic isolated equilibria: Kasner vacua $\mathcal{K}_0^\pm$, the matter FLRW point $\mathcal{F}$, the scalar FLRW point $\mathcal{S}$, and the curvature Milne-type point $\mathcal{K}$, together with special families for tuned $(n,γ)$. We find that $\mathcal{K}_0^\pm$ are always sources, $\mathcal{F}$ is generically a saddle but can act as a sink for $γ<\min\{\tfrac{2n}{n+1},\tfrac{2}{3}\}$, $\mathcal{S}$ is a sink if $0 \tfrac{2}{3}$ and $n>\tfrac{1}{2}$. These results demonstrate that isotropic FLRW $α$-attractor models extend naturally to anisotropic Bianchi~V cosmologies: inflationary attractors remain robust, while the Milne-type curvature solution emerges as the late-time state.

gr-qc↗
arXiv2026

Scalar field coupled to boundary in non-metricity: a new avenue towards dark energy

While conformal transformations in metric scalar-tensor theories recover General Relativity, this feature is notably absent in standard non-metricity-based theories. We demonstrate that by introducing the boundary term C, a non-metricity scalar-tensor theory can recover Symmetric Teleparallel Equivalent of General Relativity (STEGR) in the Einstein frame. Motivated by this, we propose a novel gravity model where a scalar field couples nonminimally to both the non-metricity scalar Q and the boundary term C. We focus in the cosmological scenario where we present the covariant formulation and a unified autonomous system framework that treats generic affine-connection choices, including coincident and non-coincident gauges, on an equal footing. Our dynamical analysis across three connection branches reveals standard thermal histories and stable de Sitter attractors. These results show that boundary-term couplings provide a well-posed, geometrically flexible route to addressing late-time cosmic acceleration.

gr-qc↗
arXiv2026

Relativistic stars in $f(Q)$-gravity: Exact analytic solution for the power-law case $f(Q) = Q + b \: Q^ν$

We investigate static spherically symmetric spacetimes within the framework of symmetric teleparallel $f(Q)$ gravity in order to describe relativistic stars. We adopt a specific ansatz for the background geometry corresponding to a singularity-free space-time. We obtain an expression for the connection, which allows the derivation of solutions for any $f(Q)$ theory in this context. Our approach aims to address a recurring error appearing in the literature, where even when a connection compatible with spherical symmetry is adopted, the field equation for the connection is systematically omitted and not checked if it is satisfied. For the stellar configuration, we concentrate on the power-law model $f(Q)=Q+αQ_{0}\left( \frac{Q}{Q_{0}}\right) ^{ν}$. The de Sitter-Schwarzschild geometry naturally emerges as an attractor beyond a certain radius, we thus utilize it as the external solution beyond the boundary of the star. We perform a detailed investigation of the physical characteristics of the interior solution, explicitly determining the mass function, analyzing the resulting gravitational fluid properties and deriving the angular and radial speed of sound.

gr-qc↗