arXiv subjects

Andronikos Paliathanasis

Publications and source records attributed to Andronikos Paliathanasis.

At least 55 records · Page 3Linked to original sources

arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Slow-Fast Evolution of Scalar Fields in Higher-Order Cosmological Gravity: Dynamics Inspired by the Pais--Uhlenbeck Oscillator

We investigate the cosmological dynamics of scalar fields governed by higher-order gravity, with particular emphasis on models inspired by the Pais-Uhlenbeck oscillator--a prototypical fourth-order system known for its connection to ghost-free formulations. By recasting the field equations into a slow-fast dynamical system, we analyze phase space evolution across exponential and power-law coupling regimes. Our approach integrates numerical simulations and geometric methods to visualize trajectories, stream flows, and asymptotic behavior under varying potential parameters. The underlying system admits singular surfaces and non-smooth transitions, revealing intricate dynamical structures. We examine the stability of de Sitter solutions, the crossing of the phantom divide, and the emergence of cyclic behavior through multiple-scale analysis. The inclusion of radiation and dust fluids enables the creation of realistic cosmological scenarios, including a transient matter-dominated era and a late-time accelerated expansion. Our results highlight the viability of Pais-Uhlenbeck scalar models in accounting for inflationary dynamics and dark energy, offering diagnostic tools for characterizing attractors and bifurcation phenomena in higher-derivative cosmology.

gr-qc↗

arXiv2025

Avoiding Big Rip Singularities in Phantom Scalar Field theory with Gauss-Bonnet term

We consider a phantom scalar field coupled to the Gauss-Bonnet scalar within a spatially flat FLRW geometry. Moreover, we assume a nonzero interaction between the scalar field and the matter term. We perform a detailed phase space analysis using two sets of dimensionless variables. Specifically, we introduce dimensionless variables based on the Hubble normalization approach and a new set based on the matter-scalar field normalization. These two sets of variables allow for a comprehensive phase space analysis. This model supports inflationary solutions without the Big Rip or Big Crunch singularities appearing as asymptotic solutions. This outcome is attributed to the presence of the Gauss-Bonnet scalar. The result remains valid even in the absence of the interaction term.

gr-qc↗

arXiv2025

Eisenhart-Duval lift, Nonlocal Conservation laws and Painlevé Analysis in Scalar field Cosmology

We investigate the existence of nonlocal conservation laws for the gravitational field equations of scalar field cosmology in an FLRW background with a dust fluid source. We perform such analysis by using a novel approach for the Eisenhart-Duval lift. It follows that the scalar field potential $V\left( ϕ\right) =α\left( e^{λϕ}+β\right) $ admits nontrivial conservation laws. Furthermore, we employ the Painlevé analysis to examine the integrability of the field equations. For the quintessence model, we establish that the cosmological field equations possess the Painlevé property and are integrable for $λ^{2}>6$. In contrast, for the phantom scalar field, the cosmological field equations exhibit the Painlevé property for any value of the parameter $% λ$. We present analytic solutions expressed in terms of Right Laurent expansions for various values of the parameter $λ$. Finally, we discuss the qualitative evolution of the effective equation of state parameter for these analytic solutions.

gr-qc↗

arXiv2025

Can an Extra Degree of Freedom in Scalar-Tensor Non-Metricity Gravity Account for the Evolution of the Universe?

We investigate whether the extra scalar degree of freedom that arises in the second connection class of scalar-tensor non-metricity gravity can accurately replicate and potentially enrich the cosmic expansion history. Focusing on a spatially flat FLRW background, we introduce Hubble-normalized variables and recast the field equations into an autonomous dynamical system. Four representative scenarios are analyzed comprehensively. Phase-space research reveals a rich hierarchy of critical points: matter-dominated, stiff-fluid, and de Sitter solutions, together with asymptotic trajectories leading to Big-Crunch/Rip singularities and transient, unstable matter epochs. With suitable parameter choices, the standard $Λ$CDM sequence is reinstated; however, novel late-time and high-curvature regimes arise exclusively from the non-metricity sector. A systematic comparison of metric scalar-tensor and teleparallel scalar-torsion theories reveals unique stability characteristics and potential observational discriminants. Our findings indicate that the additional time-dependent function inherent to scalar-tensor non-metricity gravity can effectively explain the Universe's evolution while providing new phenomenology that can be tested by upcoming surveys.

gr-qc↗

arXiv2025

Observational Constraints on Scalar Field--Matter Interaction in Weyl Integrable Spacetime

We test an analytic cosmological solution within the framework of Weyl Integrable Spacetime using current observational data. In this model, dark energy is described by a pressureless fluid, while a scalar field arises naturally through the definition of the connection. This gravitational theory reveals a Chameleon Mechanism leading to a nonzero interaction between the scalar field and the matter sector. This model extends the standard $Λ$CDM cosmology by introducing one additional degree of freedom, allowing for deviations from $Λ$CDM dynamics. For the observational constraints we consider the Supernova data of Pantheon+ collaboration, the Cosmic Chronometers, the Baryonic Acoustic Oscillators of DESI DR2 collaboration and the gamma-ray bursts. We find that Weyl Integrable Spacetime fits the data in a better way than the $Λ$CDM. When all datasets are considered, the statistical comparison indicates a weak to moderate preference for Weyl Integrable Spacetime according to the Akaike Information Criterion and Jeffrey's scale for the Bayesian evidence.

gr-qc↗

arXiv2025

Testing Non-Coincident $f(Q)$-gravity with DESI DR2 BAO and GRBs

We consider the $f\left( Q\right) $-theory for the description of dark energy with a non-trivial connection defined in the non-coincident gauge. The resulting field equations form a two-scalar-field, quintom-like gravitational model. For the power-law model $f\left( Q\right) \simeq Q^{\frac{n}{n-1}}$, we construct an analytic expression for the dynamical evolution of dark energy, which depends on the parameter $n$. We constrain this dark energy model using the the baryon acoustic oscillations from DESI DR2, and gamma-ray bursts. The cosmological data provides $n\simeq0.33$. The $f\left( Q\right) $-model challenges the $Λ$CDM by providing a smaller value for $χ_{\min}^{2}$. Although the Akaike Information Criterion indicates that the two cosmological models fit the data from the Pantheon+, the cosmic chronometers and the baryonic acoustic oscillators in a similar way, when the gamma-ray bursts are introduced there is weak evidence in favor of the $f\left( Q\right) $-theory.

gr-qc↗

arXiv2025

Equivalence of $f(Q)$ cosmology with quintom-like scenario: the phantom field as effective realization of the non-trivial connection

We show that $f(Q)$ cosmology with a non-trivial connection, namely the Connection II of the literature, is dynamically equivalent with a quintom-like model. In particular, we show that the scalar field arising from the non-linear $f(Q)$ form, and the scalar field associated to the non-trivial connection, are combined to provide one canonical and one phantom field in the minisuperspace Lagrangian. Hence, the combination of two well-behaved fields appears effectively as a phantom field. This is fundamentally different from the usual approach in phantom and quintom cosmology, in which the phantom field is introduced ad hoc, and thus it may open a new way to handle the phantom fields, namely as effective realizations of the connection-related canonical fields.

gr-qc↗

arXiv2025

Constraints on Barrow and Tsallis Holographic Dark Energy from DESI DR2 BAO data

Barrow and Tsallis Holographic Dark Energy (HDE) are two recently proposed extensions of the standard HDE framework, incorporating generalized corrections to horizon entropy through the use of Barrow and Tsallis entropies. Tsallis entropy arises from non-extensive statistical phenomena which account for long-range correlations and deviations from additivity, while Barrow entropy emerges from quantum-gravitational effects on the horizon geometry, associated with fractal modifications and deformations. At the cosmological level, both scenarios lead to the same equations, nevertheless the involved parameters obey different theoretical bounds. In this work, we use observational data from Supernova Type Ia (SNIa), Cosmic Chronometers (CC) and Baryonic acoustic oscillations (BAO), including the recently released DESI DR2 dataset, to place constraints on both scenaria. We show that both can be in agreement with observations, although they cannot alleviate the $H_0$ tension. However, applying information criteria we deduce that both of them are not favoured comparing to $Λ$CDM concordance cosmological paradigm.

gr-qc↗

arXiv2025

Fractional Time-Delayed differential equations: Applications in Cosmological Studies

Fractional differential equations model processes with memory effects, providing a realistic perspective on complex systems. We examine time-delayed differential equations, discussing first-order and fractional Caputo time-delayed differential equations. We derive their characteristic equations and solve them using the Laplace transform. We derive a modified evolution equation for the Hubble parameter incorporating a viscosity term modeled as a function of the delayed Hubble parameter within Eckart's theory. We extend this equation using the last-step method of fractional calculus, resulting in Caputo's time-delayed fractional differential equation. This equation accounts for the finite response times of cosmic fluids, resulting in a comprehensive model of the Universe's behavior. We then solve this equation analytically. Due to the complexity of the analytical solution, we also provide a numerical representation. Our solution reaches the de Sitter equilibrium point. Additionally, we present some generalizations.

gr-qc↗

arXiv2025

Observational Constraints on Dark Energy Models with $Λ$ as an Equilibrium Point

We investigate a dynamical reconstruction of the dark energy equation of state parameter by assuming that it satisfies a law of motion described by an autonomous second-order differential equation, with the limit of the cosmological constant as an equilibrium point. We determine the asymptotic solutions of this equation and use them to construct two families of parametric dark energy models, employing both linear and logarithmic parametrization with respect to the scale factor. We perform observational constraints by using the Supernova, the Cosmic Chronometers and the Baryon Acoustic Oscillations of DESI DR2. The constraint parameters are directly related with the initial value problem for the law of motion and its algebraic properties. The analysis shows that most of the models fit the observational data well with a preference to the models of the logarithmic parametrization. Furthermore, we introduce a new class of models as generalizations of the CPL model, for which the equilibrium point is a constant value rather than the cosmological constant. These models fit the data in a similar or better way to the CPL and the $Λ$CDM cosmological models.

astro-ph.CO↗

arXiv2025

Cosmological phase-space analysis of $f(G)$-theories of gravity

The impact of topological terms that modify the Hilbert-Einstein action is here explored by virtue of a further $f(G)$ contribution. In particular, we investigate the phase-space stability and critical points of an equivalent scalar field representation that makes use of a massive field, whose potential is function of the topological correction. To do so, we introduce to the gravitational Action Integral a Lagrange multiplier and model the modified Friedmann equations by virtue of new non-dimensional variables. We single out dimensionless variables that permit \emph{a priori} the Hubble rate change of sign, enabling regions in which the Hubble parameter either vanishes or becomes negative. In this respect, we thus analyze the various possibilities associated with a universe characterized by such topological contributions and find the attractors, saddle points and regions of stability, in general. The overall analysis is carried out considering the exponential potential first and then shifting to more complicated cases, where the underlying variables do not simplify. We compute the eigenvalues of our coupled differential equations and, accordingly, the stability of the system, in both a spatially-flat and non-flat universe. Quite remarkably, regardless of the spatial curvature, we show that a stable de Sitter-like phase that can model current time appears only a small fraction of the entire phase-space, suggesting that the model under exam is unlikely in describing the whole universe dynamics, i.e., the topological terms appear disfavored in framing the entire evolution of the universe.

gr-qc↗

arXiv2025

Stability analysis of dilaton-inspired scalar field within the geometrical trinity of gravity

We investigate the dynamics of the dilaton-inspired scalar field, formally rewritten by means of a Brans-Dicke Lagrangian, within the framework of \emph{geometrical trinity of gravity}. In this respect, we perform a stability analysis by adopting a non-flat Friedmann-Robertson-Walker (FRW) metric and considering the well-established exponential potential in three distinct gravitational frameworks: general relativity, teleparallel gravity, and symmetric-teleparallel gravity. By comparing the scalar field behaviors across these theories, we highlight the role of curvature, torsion, and non-metricity in shaping cosmic evolution. Our analysis reveals that, both in general relativity and teleparallel gravity, the dilaton-inspired field can drive the accelerated expansion of the universe, effectively behaving as cosmological constant at late times. In contrast, within the symmetric teleparallel gravity scenario, performing a complete linear stability analysis is prevented by the use of the non-coincident gauge. Nevertheless, the latter paradigm introduces complexity into the autonomous system, resulting in a structurally different analysis. For general relativity and teleparallel scenarios, we remark the regions of attractor solutions and unphysical domains in which we do not expect the viability of our dilaton-inspired Lagrangian. However, within the framework of symmetric-teleparallel gravity, the stability analysis reveals no attractor points for the chosen set of free parameters. In support of these findings, physical conclusions, kinematical studies, and consequences on Friedmann dynamics are thus explored.

gr-qc↗

arXiv2025

The solution space of a five-dimensional geometry: Kundt spacetimes and cosmological time-crystals

We uncover the solution space of a five dimensional geometry which we deem it as the direct counterpart of the Bianchi Type V cosmological model. We kinematically reduce the scale factor matrix and then, with an appropriate scaling and choice of time, we cast the spatial equations into a simple "Kasner" like form; thus revealing linear integrals of motion. Their number is enough so that, along with the quadratic constraint, it suffices to scan the entire space of solutions. The latter is revealed to be quite rich, containing cosmological solutions, some of which admit dimensional reduction asymptotically to four dimensions, Kundt spacetimes with vanishing type I (polynomial) curvature scalars and solutions describing periodic universes which behave like cosmological time crystals.

gr-qc↗

arXiv2025

Compactification of Anisotropies in Einstein-Scalar-Gauss-Bonnet Cosmology

We investigate the evolution of anisotropies in Einstein-Gauss-Bonnet theory with a scalar field coupled to the Gauss-Bonnet term. Specifically, we examine the simplest scenario in which the scalar field lacks a kinetic term, and its kinetic contribution arises from an integration by parts of the Gauss-Bonnet scalar. We consider four- and five-dimensional anisotropic spacetimes, focusing on Bianchi I and extended Bianchi I geometries. Our study reveals that the asymptotic solutions correspond to locally symmetric spacetimes where at least two scale factors exhibit analogous behavior or, alternatively, to isotropic configurations where all scale factors evolve identically. Additionally, we discuss the effects of a cosmological constant, finding that the presence of the cosmological constant does not lead to an isotropic universe.

gr-qc↗

arXiv2025

Lagrangians and Newtonian analogues for Biological Systems

This study investigates the potential for biological systems to be governed by a variational principle, suggesting that such systems may evolve to minimize or optimize specific quantities. To explore this idea, we focus on identifying Lagrange functions that can effectively model the dynamics of selected population systems. These functions provide a deeper understanding of population evolution by framing their behavior in terms of energy-like variables. We present an algorithm for generating Lagrangian functions applicable to a family of population dynamics models and demonstrate the equivalence between two-dimensional population models and a one-dimensional Newtonian mechanical analog. Furthermore, we explore the existence of conservation laws for these models, utilizing Noether's theorems to investigate their implications.

q-bio.PE↗

arXiv2025

Szekeres Universes with GUP corrections

We demonstrate that introducing a deformed algebra with a minimum length modifies the field equations for an inhomogeneous spacetime, resulting in the emergence of acceleration. Specifically, we examine the analytic effects of the Generalized Uncertainty Principle on the classical field equations of the Szekeres system. Our findings show that the deformed algebra leads to a modified Szekeres system capable of describing cosmic acceleration. Moreover, the spatial curvature of the spacetime is influenced by the presence of the minimum length.

gr-qc↗

arXiv2025

Noether Symmetries in $f(Q)-$Cosmology

We apply the Noether symmetry analysis in $f\left( Q\right)$-Cosmology to determine invariant functions and conservation laws for the cosmological field equations. For the FLRW background and the four families of connections, it is found that only power-law $f\left( Q\right)$ functions admit point Noether symmetries. Finally, exact and analytic solutions are derived using the invariant functions.

gr-qc↗