arXiv subjects

Emmanuel N. Saridakis

Publications and source records attributed to Emmanuel N. Saridakis.

At least 19 recordsLinked to original sources

arXiv2026

Entropy applications in cosmology: spacetime thermodynamics, holographic dark energy, entropic gravity and beyond - a review

Entropy has emerged as a key concept at the intersection of gravity, quantum theory, and cosmology, revealing deep connections between geometry, information, and thermodynamics. The realization that black holes are thermodynamic systems, characterized by an entropy proportional to their horizon area, has provided evidence that spacetime itself may possess a microscopic structure. Since then, entropy has acquired a central role across quantum field theory, holography, quantum gravity, and cosmology. In this review, we present a unified account of entropy in gravitational and cosmological contexts. We first survey entropy in gravitational systems, focusing on black-hole thermodynamics, entropy bounds, the holographic principle, entanglement entropy, and the area law, together with the main corrections and generalizations arising from quantum field theory, generalized uncertainty principles, modified statistics, and extended theories of gravity. We then discuss three conceptually distinct ways in which entropy is employed in cosmology. The first is through the spacetime-thermodynamics conjecture, in which gravitational dynamics emerge from thermodynamic relations applied to cosmological horizons, and generalized entropy forms consequently lead to modified cosmological evolution. The second is the holographic approach, in which entropy bounds constrain the vacuum energy of the Universe, giving rise to the holographic dark energy scenario and its various extensions. The third is entropic gravity, where gravity is interpreted as an emergent phenomenon of entropic origin, leading to interesting cosmological phenomenology. By reviewing the theoretical foundations and cosmological implications of these entropic frameworks, we aim to clarify the role of entropy, discuss open issues and outline directions toward a deeper thermodynamic and informational understanding of gravity and cosmology.

astro-ph.CO↗

arXiv2026

Topological dark energy from spacetime foam: A challenge for $Λ$CDM

Using only the standard considerations of spacetime foam and the Euclidean Quantum Gravity techniques known long ago, we result to a model of Topological Dark Energy (TDE) that competes the standard ΛCDM paradigm with regard to data fitting efficiency, while providing a microphysical mechanism for Dark Energy. Specifically, it is known that at the foam level, topologically non-trivial solutions such as instantons appear. In the particular case of Einstein-Gauss-Bonnet gravity, we obtain an effective dynamical Dark Energy term proportional to the instanton density, and the latter can be easily calculated through standard techniques. Hence, we can immediately extract the differential equation that determines the evolution of the topologically induced effective dark energy density. Significantly, this TDE scenario allows for changing sign of Dark Energy during the cosmic evolution and also exhibits Dark Energy interaction with Dark Matter. We confront the TDE scenario, in both flat and non-flat cases, with Pantheon+ Supernovae Type Ia (SNIa), Baryonic Acoustic Oscillations (BAO), and Cosmic Chronometers (CC) datasets. By applying standard model selection methods, we find the TDE scenario to be statistically equivalent with ΛCDM. Finally, we show that the TDE scenario passes constraints from Big Bang Nucleosynthesis (BBN) and thus does not spoil the thermal history of the Universe.

gr-qc↗

arXiv2026

Phenomenological Rotating Extension of Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity

The Event Horizon Telescope (EHT) image of M87* provides a direct test of strong-field gravity, measuring an angular shadow diameter $θ_{d}=42\pm 3~μ\mathrm{as}$ and a circularity deviation $ΔC\leq 0.1$. Such observations allow quantitative tests of the Kerr paradigm and of possible deviations from the no-hair theorem. In scalar-tensor extensions of gravity, black holes may possess primary scalar hair, introducing an additional independent parameter beyond mass and spin. In this work, we construct a rotating configuration inspired by black hole solutions with primary scalar hair in beyond Horndeski gravity and analyze their photon regions and shadow formation. We show that the scalar hair parameter $Q$ induces characteristic modifications of the shadow, and in particular negative $Q$ enlarges the shadow and reduces its oblateness, while positive $Q$ shrinks and enhances its distortion. Adopting M87* as a representative case within this framework and imposing the EHT bounds on $θ_{d}$ and $ΔC$, we identify the viable $(a,Q)$ parameter space. We find that current observations do not exclude rotating black holes with primary scalar hair, although the allowed region is significantly restricted for $Q>0$. Finally, the scalar-hair-induced deviations are of order $\mathcal{O}(μ\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging.

gr-qc↗

arXiv2026

Reconstructing $f(R)$ gravity from generalized entropies: exact Lagrangians

Generalized horizon entropies are widely used as theoretical modifications of the Bekenstein-Hawking area law, but through the Wald construction they may also encode modifications of the underlying gravitational dynamics. We reconstruct metric $f(R)$ gravity from prescribed entropy-area relations and show that the procedure is intrinsically branch dependent through the required area-curvature map. On the maximally symmetric branch, where $A=48π/R$ exactly, the reconstruction reduces to a single quadrature and can be performed non-perturbatively. We obtain closed-form Lagrangians for several generalized entropies and show that an entropy term $a~S_{BH}^{q}$ generates a curvature term proportional to $R^{2-q}$. In particular, Kaniadakis entropy produces a $1/R$ correction, while logarithmic entropy generates an $R^2\ln R$ term. We further derive a branch-independent criterion, $\partial_{R}^{2} f=(ds/dR)d(S/s)/ds$, relating Dolgov-Kawasaki stability directly to the entropy functional, together with $m_{\rm sc}^2=S'(s)/(3f_{RR})$ on the maximally symmetric branch. Comparison with the fixed-mass Schwarzschild-de Sitter branch reveals different reconstructed Lagrangians and reversed stability properties. Finally, the weak-isolated-horizon boost charge reproduces the original generalized entropy. These results establish a direct non-perturbative link between generalized horizon thermodynamics and modified gravitational dynamics.

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 $Λ$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 $ρ_{\rm dm}\propto a^{-3+δ}$, with the constant $δ$ 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 $2σ$. 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 $Λ$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

Polytropic wormholes

Traversable wormholes in general relativity require non-standard matter sources, making the identification of physically motivated equations of state particularly important. We investigate wormholes supported by a polytropic equation of state, considering homogeneous and inhomogeneous configurations within a unified framework. We derive the corresponding solutions and analyze the effects of the polytropic parameters on the geometry and energy conditions. In the homogeneous case, the polytropic construction yields a consistent wormhole interior whose geometry and matter content are governed by the constant polytropic parameters. For the inhomogeneous case, we obtain a general analytical expression showing that the geometry is completely determined by the radial polytropic coefficient $ω(r)$. For positive $ω(r)$, the requirement for physically meaningful solutions naturally restricts the polytropic exponent to odd integer values. Using a power-law profile, we construct explicit classes of solutions exhibiting distinct parameter regimes and finite radial support. Interestingly enough, for an exponent $α=2γ-3$, a generalized absurdly benign traversable wormhole-like configuration emerges naturally. Although the flare-out condition implies null-energy-condition violation at the throat, the inhomogeneous framework allows its radial distribution to be controlled. Our results establish a systematic connection between polytropic matter and wormhole geometry, providing a flexible framework for constructing compact wormholes with localized exotic matter.

gr-qc↗

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 $Δ$. If such effects are scale dependent, however, there is no fundamental reason for $Δ$ to remain constant throughout cosmic history. In this work we reconstruct $Δ(z)$ 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 $Δ$ and this tendency becoming stronger when the Cepheid calibration is included. Nevertheless, $Δ=0$ 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

Area-product universality in multi-horizon black hole entropy

Black-hole entropy and multi-horizon area-product relations represent two apparently distinct manifestations of universality, arising respectively from microscopic statistics and classical geometry. We show that these structures are unexpectedly connected. Extending a minimum-assumptions statistical discretization to multiple Killing horizons, with distinct horizon sectors treated as statistically independent, we derive a general area-product rule. In particular, while the leading Bekenstein-Hawking entropy probes the sum of the horizon areas, the logarithmic correction probes their product. Consequently, whenever the classical area product is mass independent, the logarithmic entropy automatically inherits this universality. We demonstrate this for Reissner-Nordström and, under an area-based extension to rotating horizons, Kerr-Newman black holes. However, Reissner-Nordström-de Sitter provides a crucial three-horizon counterexample, for which the area-product rule survives while mass independence is generically lost for the physical horizons. Thus, statistical area-product selection and classical universality are distinct properties. Our results uncover a structural connection between microscopic entropy counting and classical multi-horizon geometry that is invisible at the level of the leading area law, suggesting that logarithmic corrections encode distinctive information about the global horizon structure.

gr-qc↗

arXiv2026

The generalized second law as a thermodynamic selection criterion for dynamical dark energy

The growing number of generalized horizon entropy proposals has led to a wide variety of modified cosmological models, yet there is currently no general physical principle capable of discriminating among them. We show that the generalized second law (GSL) of thermodynamics provides such a criterion. Considering a general modified Friedmann framework described by an arbitrary function $f(H)$ of the Hubble parameter, and allowing the apparent horizon and the cosmic fluid to evolve out of thermal equilibrium, we derive model-independent constraints on the asymptotic scaling of the horizon entropy, $S_A\propto A^k$. We find that phantom evolution requires $k\ge(3ω-1)/(2ω)$, with $ω$ the total fluid equation of state, whereas quintessence imposes the complementary upper bound. Additionally, in the exact thermal equilibrium limit, the dynamical coupling strictly enforces $k \le 2$, recovering the non-equilibrium quintessence bound. The two bounds converge to the unique value $k=2$ as the phantom divide is approached, indicating that a smooth crossing of the phantom divide is thermodynamically associated with a quadratic entropy-area scaling, where the effective theory degenerates into a logarithmic gravity framework. Applying this criterion to representative generalized entropy models shows that many commonly used proposals are constrained by the generalized second law, whereas multiparameter constructions are naturally compatible with the required asymptotic behavior. Finally, we demonstrate that the same thermodynamic selection principle extends to derivative-dependent cosmologies described by $f(H,\dot H)$, highlighting its robustness beyond entropy functionals that depend only on the horizon area.

gr-qc↗

arXiv2026

Unified dark sector and Hubble-tension alleviation in scalar-vector-tensor gravity

We investigate a scalar-vector-tensor theory in which matter is minimally coupled to a Jordan-frame metric $\tilde g_{μν}=(1+Ξ)g_{μν}$, while a massive vector sector interacts with the baryonic current. We show that the conformal scalar coupling modifies the physical expansion rate measured by matter observers, leading to an enhancement of the Hubble constant inferred at low redshift. We stress, however, that the Hubble rate is not a conformal invariant, whereas the acoustic angular scale $θ_s$ is, and we derive the exact integral condition that the scalar field evolution must satisfy. We show that a single-signed scalar velocity cannot satisfy it, and we construct instead a two-epoch phenomenological evolution which matches $θ_s$ exactly while retaining the late-time enhancement, at the cost of a small pre-recombination shift of the effective gravitational coupling. Importantly, the recombination temperature is unmodified, since particle masses are constant in the Jordan frame, only the expansion rate at that epoch being altered. The scalar potential naturally acts as a dynamical dark-energy sector, while the vector sector provides two distinct contributions. The temporal component, determined algebraically by the baryon current, yields an apparent matter-like term in the background expansion that is not a true fluid but rather a manifestation of the interaction energy. The propagating spatial modes, on the other hand, form a vector condensate that behaves as a collisionless pressureless component and can play the cosmological role of cold dark matter. Hence, the framework connects scalar dynamics, effective dark-energy evolution, and the $H_0$ tension within a single setup.

gr-qc↗

arXiv2026

Primordial black hole dark matter from ultra-slow-roll inflation in Horndeski gravity

Primordial black holes (PBHs) provide a well-motivated non-particle candidate for dark matter, requiring an enhancement of curvature perturbations on small inflationary scales consistent with observational constraints. In this work we study PBH production within Horndeski gravity, accounting for compatibility with the GW170817 constraint on the gravitational-wave (GW) speed and imposing a constant coupling to the Ricci scalar. Under these conditions, and assuming an inflaton field characterised by a canonical kinetic term and a smooth potential, the inflationary dynamics is controlled by the cubic Horndeski interaction. By investigating standard functional forms of the latter we identify the specific kinetic structure that allows enhancement of the effective friction on the inflaton, thereby inducing a transient ultra-slow-roll phase embedded within a standard slow-roll evolution. For representative parameter choices we find that pronounced amplifications in the scalar power spectrum are generated, leading to the formation of asteroid-mass PBHs with masses of order $\mathcal{O}(10^{-16})\,M_\odot$, which can account for a substantial fraction of the dark matter abundance, reaching $f_{\rm PBH}\simeq 0.9$, while satisfying current observational constraints. The resulting characteristic features in the scalar power spectrum also imply potentially observable scalar-induced gravitational-wave (SIGW) signatures.

gr-qc↗

arXiv2026

Dark energy perturbations and the robustness of cosmological neutrino-mass constraints

Cosmological observations are placing increasingly stringent bounds on the sum of neutrino masses, approaching the lower limits implied by neutrino oscillation experiments. Recent studies have suggested that dynamical dark energy may alleviate this apparent tension. However, these conclusions generally rely on the assumption that dark energy remains smooth, neglecting its perturbations. In this work we investigate the robustness of cosmological neutrino-mass constraints by consistently incorporating dark-energy perturbations. Using CMB, BAO, RSD, and supernova data, we show that the commonly reported alleviation of the neutrino-mass tension in dynamical dark-energy models is not generic. While smooth dark energy substantially relaxes the neutrino-mass bounds, allowing dark energy to cluster shifts the preferred neutrino mass toward smaller, and even more negative, effective values. We demonstrate that this behavior originates from a degeneracy between neutrino free-streaming and dark-energy perturbations in structure-growth observables. Different combinations of neutrino mass and dark-energy clustering can provide similarly good fits to current data while yielding significantly different neutrino-mass constraints. Our results show that cosmological neutrino-mass measurements are inherently model dependent and that reliable neutrino-mass inference requires a consistent treatment of dark-energy perturbations.

astro-ph.CO↗

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 $Λ$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 $Λ$CDM, the full dataset prefers a non-zero WGB contribution, $\Cn=0.435^{+0.150}_{-0.132}$, corresponding to a $\sim3σ$ 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 $\sim0.9σ$, 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

Atomic clocks and gravitational waves as probes of non-metricity

Non-metricity provides a natural extension of Riemannian geometry, yet its experimental signatures remain largely unexplored. In this work, we investigate how spacetime non-metricity can be probed through high-precision observations, focusing on atomic clocks and gravitational waves as complementary tools. Working within Weyl geometry as a minimal realization of vectorial non-metricity, we formulate observable effects in a gauge-invariant manner and show that they are associated with path-dependent length transport governed by the Weyl field strength. We derive constraints from atomic-clock experiments and demonstrate that, although gravitational waves do not directly source the Weyl field at linear order, its dynamical contribution induces a backreaction on gravitational-wave propagation, leading to an anomalous strain. As a result, the absence of deviations from General Relativity in current gravitational-wave observations already places meaningful and strong constraints on dynamical non-metric degrees of freedom, within the phenomenological classical framework considered.

gr-qc↗

arXiv2026

Higgs-like inflation in scalar-torsion $f(T,ϕ)$ gravity in light of ACT-SPT-DESI constraints

We study Higgs-like inflation in the framework of scalar-torsion gravity, focusing on the general class of $f(T,ϕ)$ theories in which gravitation is mediated by torsion rather than curvature. Motivated by the increasing precision of cosmic microwave background and large-scale-structure observations, we examine whether Higgs-like inflation remains compatible with current data in this extended gravitational setting. Working within the slow-roll approximation, we analyze the inflationary dynamics both analytically and numerically. In the dominant-coupling regime we derive closed-form expressions for the scalar spectral index and the tensor-to-scalar ratio as functions of the number of e-folds, and we subsequently relax this assumption by numerically solving the slow-roll equations. Confrontation with the latest constraints from Planck 2018, ACT DR6, DESI DR1, and BICEP/Keck shows that Higgs-like inflation in $f(T,ϕ)$ gravity is fully consistent with current bounds, naturally accommodating the preferred shift in the scalar spectral index and leading to distinctive tensor-sector signatures.

gr-qc↗

arXiv2026

Observational constraints on Luciano-Saridakis holographic dark energy

Holographic dark energy (HDE) models provide a natural framework for linking gravitational thermodynamics to the late-time accelerated expansion of the Universe. In this work, we investigate the observational viability of an extended HDE scenario arising from a recently proposed generalized entropy. For bounded systems, this entropy exhibits a generalized holographic scaling with two independent area contributions, giving rise to a modified HDE density that encompasses both standard HDE and $Λ$CDM as limiting cases. Focusing on the Hubble-horizon infrared cutoff, we constrain the model using Cosmic Chronometers, the Pantheon$^+$+SH0ES Type Ia supernova compilation, DESI DR2 baryon acoustic oscillations, and compressed Planck 2018 CMB shift parameters. We find that the model provides an excellent fit to the combined dataset and admits regions of parameter space in which the Pantheon$^+$+SH0ES and CMB constraints can be simultaneously accommodated. The preferred solutions lie close to the $Λ$CDM regime, although non-standard entropic contributions remain compatible with current observations. We further compare the complete realization of the model, containing both independent area contributions, with its reduced single-contribution limit, finding that both provide essentially equivalent descriptions of the data, with a mild preference for the latter. Our results establish generalized entropic HDE as a viable and theoretically motivated extension of the standard cosmological scenario and provide the first observational assessment of this cosmological framework.

physics.gen-ph↗

arXiv2026

Probing the sound speed and clustering of dark energy

Recent Dark Energy Spectroscopic Instrument (DESI) observations favor a dynamical dark energy component with a time-varying equation-of-state, potentially crossing the cosmological-constant boundary $w=-1$, challenging the standard $Λ$CDM paradigm. In this paper we present the first joint observational constraints on the clustering properties of such dynamical dark energy, using both the Parameterized Post-Friedmann (PPF) framework and the effective field theory (EFT) of dark energy. Combining DESI DR2 baryon acoustic oscillations with Planck 2018 cosmic microwave background data and the Union3 supernova sample, we constrain the effective sound speed $c_s^{2}$. For a time-varying equation-of-state, the degeneracy between $(1+w)$ and $c_s^{2}$ is broken, yielding the first meaningful constraint $\log_{10}c_{s}^{2}=-3.00^{+2.9}_{-0.99}$, while constant-$w$ models remain unconstrained. A complementary EFT analysis gives consistent results, favoring $c_s^{2}\sim 0.3$ or $0.4$. Our findings demonstrate that current data are now sensitive to the perturbative properties of dynamical dark energy, opening a new observational window on the nature of cosmic acceleration.

astro-ph.CO↗

arXiv2026

Chaotic imprints of dark matter in extreme mass-ratio inspirals

Extreme mass-ratio inspirals (EMRIs) are among the most powerful probes of strong-field gravity and of the environments surrounding supermassive compact objects. Motivated by the expected presence of dark matter near galactic centers, we investigate the emergence and gravitational-wave imprints of chaotic dynamics in EMRIs evolving in non-vacuum spacetimes. Within a unified dynamical framework, we analyze test-particle motion in a broad class ofdark-matter-embedded geometries, including singular black holes, regular black holes, naked singularities, and Einstein-cluster configurations. We show that environmental perturbations generically break integrability in the strong-field regime, giving rise to chaotic motion whose onset, duration, and termination depend sensitively on horizon structure, core regularization, and matter distribution. Using the numerical Kludge approach, we demonstrate that chaotic trajectories produce systematic qualitative modifications of the emitted gravitational radiation, such as irregular amplitude modulation and loss of phase coherence, in contrast to the smooth, quasi-periodic waveforms generated by regular motion. Our results establish the robustness of chaos in environmentally perturbed EMRIs and provide a clear conceptual link between nonlinear orbital dynamics, spacetime structure, and observable gravitational-wave signatures.

gr-qc↗