Universe

13 pages, 569 KB

Open AccessArticle

Hawking Atmosphere of Anti-de Sitter Black Holes

by A. F. Cardona and C. Molina

Universe 2026, 12(5), 141; https://doi.org/10.3390/universe12050141 - 9 May 2026

This work investigates the semiclassical evolution of the Hawking atmosphere surrounding evaporating, spherically symmetric anti-de Sitter (adS) black holes. We model the evaporation process within a dynamical framework, treating the emission of Hawking radiation as a quantum tunneling process through the black-hole horizon. [...] Read more.

This work investigates the semiclassical evolution of the Hawking atmosphere surrounding evaporating, spherically symmetric anti-de Sitter (adS) black holes. We model the evaporation process within a dynamical framework, treating the emission of Hawking radiation as a quantum tunneling process through the black-hole horizon. Using the Parikh–Wilczek tunneling method, we incorporate backreaction effects, with the emission probability being linked to the resulting change in the Bekenstein–Hawking entropy of the black hole. This probability is then used to compute the time-dependent luminosity of the system, revealing significant deviations from ideal blackbody behavior, particularly for small adS black holes. For these objects, the luminosity does not increase with temperature due to strong mass variations during evaporation. To complement this microscopic approach, we compute the renormalized energy–momentum tensor for a quantum field propagating in the Vaidya-adS geometry modelling the evaporation process. Together, these approaches clarify the interplay between geometry, quantum fields, and thermodynamics in shaping the Hawking atmosphere and the evaporation dynamics of black holes in asymptotically adS spacetimes. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity, 2nd Edition)

15 pages, 716 KB

Open AccessArticle

Simulation of the Spin Evolution of Some Selected Exoplanets and Inferences on Their Climate

by Salvatore Camposeo, Francesco De Paolis, Vincenzo Orofino, Francesco Strafella and Leonardo Di Venere

Universe 2026, 12(5), 140; https://doi.org/10.3390/universe12050140 - 8 May 2026

Abstract

In this work, using the simulator VPLanet, we analyze the spin evolution of some selected exoplanets due to the tidal interaction with their host star. For a rocky planet, two spin “conditions” are possible, the “trapped” rotation and the “fast” rotation, referring [...] Read more.

In this work, using the simulator VPLanet, we analyze the spin evolution of some selected exoplanets due to the tidal interaction with their host star. For a rocky planet, two spin “conditions” are possible, the “trapped” rotation and the “fast” rotation, referring to the cases of achieved and non-achieved tidal trapping, respectively. We focus on planets whose spin condition is not obvious, because no study is needed for planets which are undoubtedly fast rotators or undoubtedly trapped rotators; moreover, we consider only exoplanets that are interesting from an astrobiological perspective. The current spin conditions of the considered planets are hypothesized, taking into account the age of the host star. Inferences regarding planetary climate and habitability—which is defined by the possibility of stably sustaining the liquid water on the surface—are also discussed. Results of this work show that Kepler-62f, Kepler-1126c, and Kepler-1544b are expected to be fast rotators regardless of the orbital eccentricity; the spin condition of Kepler-186f, Kepler-62e, and Kepler-442b cannot be determined without constraints on the eccentricity, which are currently unavailable; Kepler-440b is expected to be tidally trapped. Full article

(This article belongs to the Section Planetary Sciences)

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19 pages, 1413 KB

Open AccessArticle

Solar Type III Radio Burst Identification Using Few-Shot Object Detection

by Haoxiang Jiang, Shoulin Wei, Linjie Chen, Bo Liang, Wei Dai, Zhijian Zhang and Heng Zhang

Universe 2026, 12(5), 139; https://doi.org/10.3390/universe12050139 - 8 May 2026

Abstract

Solar radio bursts at very low frequencies are key phenomena in the Sun–Earth space environment, providing crucial diagnostics of the acceleration and propagation of solar wind, coronal mass ejection (CME), and non-thermal energetic particles and serving as important indicators for space weather forecasting. [...] Read more.

Solar radio bursts at very low frequencies are key phenomena in the Sun–Earth space environment, providing crucial diagnostics of the acceleration and propagation of solar wind, coronal mass ejection (CME), and non-thermal energetic particles and serving as important indicators for space weather forecasting. To meet the demand for rapid screening of burst events in large-scale observational datasets, we present an end-to-end automatic detection and evaluation framework tailored for Type III bursts, built upon long-term radio dynamic spectra from STEREO-A/SWAVES. We formulate radio burst detection as a one-dimensional interval localization task along the time axis and, in view of the scarcity of annotated samples, cast it as a few-shot object detection task. Building upon the Faster R-CNN architecture with a ResNet50-FPN backbone, we propose the Meta-FSOD framework, which adopts an episodic training paradigm to construct support–query episode pairs. The framework incorporates a metric-guided prototype learning branch to semantically align and calibrate region-of-interest (RoI) features via class prototypes, and integrates a dynamic Beta-Gating mechanism coupled with Soft-NMS to effectively suppress false positives while preserving high-recall performance. Experimental results demonstrate that, despite being trained on a significantly smaller dataset than comparable studies, Meta-FSOD achieves competitive performance, closely matching that of conventional supervised model. The proposed framework exhibits strong cross-temporal generalization capabilities and holds considerable potential for engineering applications in deep space exploration missions. Full article

(This article belongs to the Special Issue Astroinformatics and Big Data in Astronomy)

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22 pages, 328 KB

Open AccessArticle

Spontaneous BRST Symmetry Breaking in Infrared QCD

by Angelo Raffaele Fazio and Adam Smetana

Universe 2026, 12(5), 138; https://doi.org/10.3390/universe12050138 - 7 May 2026

Abstract

We present a novel proposal for the effective Lagrangian of the low-energy Yang–Mills quantum field theory. The proposed effective Lagrangian exhibits the spontaneous BRST symmetry breaking. We build on the Fujikawa model that we couple to the Yang–Mills elementary field sector, motivated by [...] Read more.

We present a novel proposal for the effective Lagrangian of the low-energy Yang–Mills quantum field theory. The proposed effective Lagrangian exhibits the spontaneous BRST symmetry breaking. We build on the Fujikawa model that we couple to the Yang–Mills elementary field sector, motivated by the analogy with Chiral Quark Model. We interpret the Fujikawa fields as effective fields, composites of the elementary gluon and ghost fields. In order to justify the existence of two massless Nambu–Goldstone modes among the Fujikawa fields, we require not only the BRST but also the anti-BRST invariance of the effective Lagrangian, with both being spontaneously broken. The most striking consequence of that is the emergence of the effective gluon and ghost masses. We reproduce the Curci–Ferrari model as a special case of our effective model upon the spontaneous BRST symmetry breaking. In order to reproduce also the non-nilpotent modified-BRST symmetry, characteristic for the Curci–Ferrari model, we modify our effective Lagrangian to be invariant with respect to the extended-BRST symmetry, which mixes the elementary and Fujikawa field sectors, and which is nilpotent. The Curci–Ferrari model is reproduced by the elementary field sector of the resulting Lagrangian. The remaining Fujikawa-field-dependent terms guarantee the underlying nilpotent extended-BRST symmetry, which is now hidden in the sense of the spontaneous symmetry breaking. Full article

(This article belongs to the Section Field Theory)

17 pages, 874 KB

Open AccessArticle

Comparison of JUNO and DUNE Sensitivities to Cosmic-Ray- Produced Dark Mesons

by Zirong Chen, Jinmian Li, Junle Pei and Feng Yang

Universe 2026, 12(5), 137; https://doi.org/10.3390/universe12050137 - 7 May 2026

Abstract

We study the projected sensitivities of the Jiangmen Underground Neutrino Observatory (JUNO) and the Deep Underground Neutrino Experiment (DUNE) to cosmic-ray-produced dark mesons in a confining dark sector with a leptophobic vector portal. Using the same atmospheric dark meson flux framework as in [...] Read more.

We study the projected sensitivities of the Jiangmen Underground Neutrino Observatory (JUNO) and the Deep Underground Neutrino Experiment (DUNE) to cosmic-ray-produced dark mesons in a confining dark sector with a leptophobic vector portal. Using the same atmospheric dark meson flux framework as in our previous JUNO study, which includes proton bremsstrahlung, Standard Model meson decays, and Drell–Yan production followed by dark hadronization described by a modified Quark Combination Model, we perform a controlled comparison between JUNO and DUNE within a common source-side setup. Our results indicate that JUNO achieves stronger overall sensitivity across most of the parameter space, primarily because its inclusive event-level visible-energy criterion efficiently retains soft elastic recoils. In contrast, DUNE demonstrates systematically larger visible effective cross sections in the deep-inelastic scattering (DIS) channel, where energetic final states readily exceed its particle-level hadronic thresholds. Moreover, kinematic hardening of elastic recoils at heavier mediator masses (

m_{Z^{'}} ≳ 1

GeV) and higher incident energies (

E_{K_{D}} ≳ 1

GeV) further enhances DUNE’s elastic acceptance. Nevertheless, over most of the benchmark parameter space considered here, JUNO yields a larger total signal rate because the incident dark meson flux peaks sharply at low energies, favoring the soft elastic regime. Consequently, this interplay between flux distribution and detector thresholds causes the sensitivity gap between JUNO and DUNE to narrow significantly in the heavy-mediator regime. Full article

(This article belongs to the Special Issue Search for New Physics Through Combined Approaches)

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13 pages, 671 KB

Open AccessArticle

Causal Structure of Black Holes Immersed in a Chaplygin-like Dark Fluid Environment: Horizons and Singularities

by Rodrigo Dal Bosco Fontana and Jeferson de Oliveira

Universe 2026, 12(5), 136; https://doi.org/10.3390/universe12050136 - 6 May 2026

Abstract

In the present work, we study the causal structure of spherically symmetric black holes immersed in a Chaplygin-like dark fluid, emphasizing the impact of the fluid parameters on curvature and horizon formation. We show that the spacetime curvature is significantly stronger than in [...] Read more.

In the present work, we study the causal structure of spherically symmetric black holes immersed in a Chaplygin-like dark fluid, emphasizing the impact of the fluid parameters on curvature and horizon formation. We show that the spacetime curvature is significantly stronger than in its similar counterpart, the Reissner–Nordström–de Sitter geometry with the same mass and charge, leading to modifications of the internal causal structure. For the presence of horizons, the Chaplygin black hole possesses an upper bound

Q \approx 0.556219 M

, which is much smaller than that for Reissner–Nordström spacetime

Q_{c r i t i c a l} = M

or for the Reissner–Nordström–de Sitter case

Q_{c r i t i c a l} = 3 M / 2 \sqrt{2}

, indicating that the black holes immersed in a Chaplygin-like dark fluid reach the extremal regime more easily. We derive a second critical condition for the Chaplygin cosmological parameter B,

B_{c} Q_{c} = 4 / 3^{9}

, setting an upper bound on B for a multi-horizon solution. Full article

(This article belongs to the Section Gravitation)

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3 pages, 956 KB

Open AccessCorrection

Correction: Yusupova et al. Accretion Flow onto Ellis–Bronnikov Wormhole. Universe 2021, 7, 177

by Rosaliya M. Yusupova, Ramis Kh. Karimov, Ramil N. Izmailov and Kamal K. Nandi

Universe 2026, 12(5), 135; https://doi.org/10.3390/universe12050135 - 6 May 2026

Abstract

When deriving the equations for the velocity, density, pressure, and accretion rate in our paper [1], an unintentional error in taking the square root of

\sqrt{{(1 + ω)}^{2}}

by

(1 + ω)

(the correct value would be

|1 + ω|

) led [...] Read more.

When deriving the equations for the velocity, density, pressure, and accretion rate in our paper [1], an unintentional error in taking the square root of

\sqrt{{(1 + ω)}^{2}}

by

(1 + ω)

(the correct value would be

|1 + ω|

) led to erroneous conclusions regarding phantom accretion onto the objects under consideration [...] Full article

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17 pages, 1183 KB

Open AccessArticle

Observational Constraints and Cosmological Dynamics of Interacting Fractional Holographic Dark Energy in Light of DESI DR2

by Qihong Huang, Hao Chen and Qingdong Wu

Universe 2026, 12(5), 134; https://doi.org/10.3390/universe12050134 - 4 May 2026

Abstract

Based on the fractional entropy originating from fractional quantum mechanics, the fractional holographic dark energy (FHDE) model has been proposed. In this paper, we consider an interaction between the pressureless matter and FHDE and analyze three different interacting FHDE models. Combining the latest [...] Read more.

Based on the fractional entropy originating from fractional quantum mechanics, the fractional holographic dark energy (FHDE) model has been proposed. In this paper, we consider an interaction between the pressureless matter and FHDE and analyze three different interacting FHDE models. Combining the latest observational data including SNIa, OHD, BAO, and CMB, we estimate the model parameters and find that the interaction forms

Q = γ H ρ_{d e}

and

Q = β H ρ_{m} + γ H ρ_{d e}

show some preference from the observational data. Using phase space analysis, we further find that only interacting FHDE model with

Q = β H ρ_{m} + γ H ρ_{d e}

can describe the full evolutionary history of the universe. The statefinder diagnostic pair reveals that this model deviates from the

Λ

CDM model but converges to the

Λ

CDM fixed point and the de Sitter expansion fixed point in the future. Finally, we analyze the evolution of cosmological parameters and demonstrate that this model can drive the late time acceleration of the universe. Full article

(This article belongs to the Topic Dark Matter, Dark Energy and Cosmological Anisotropy)

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18 pages, 317 KB

Open AccessArticle

Mechanical Equilibrium in the Magnetized Quark–Hadron Mixed Phase: A Covariant Generalization of the Gibbs Condition

by Aric Hackebill

Universe 2026, 12(5), 133; https://doi.org/10.3390/universe12050133 - 4 May 2026

Abstract

We formulate a covariant mechanical equilibrium condition for the quark–hadron mixed phase boundary in the presence of a magnetic-field-induced pressure anisotropy. Using the relativistic thin-shell formalism to describe the quark–hadron boundary, we interpret conservation of stress-energy across the interface as a set of [...] Read more.

We formulate a covariant mechanical equilibrium condition for the quark–hadron mixed phase boundary in the presence of a magnetic-field-induced pressure anisotropy. Using the relativistic thin-shell formalism to describe the quark–hadron boundary, we interpret conservation of stress-energy across the interface as a set of generalized Young–Laplace conditions which characterize the geometry of the interface. In a comoving stationary frame, this provides a covariant description of mechanical equilibrium at the interface, which serves as a replacement for the scalar pressure-balance condition used in the isotropic Gibbs construction. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

40 pages, 14424 KB

Open AccessArticle

Novel Realizations of Warp Drive Spacetimes as Solutions of General Relativity

by Thomas Buchert and Antony Frackowiak

Universe 2026, 12(5), 132; https://doi.org/10.3390/universe12050132 - 3 May 2026

Abstract

We first take a closer look at the original warp drive proposal by Alcubierre, examine its kinematics in the context of a covariant 3+1 setting, and explain some drawbacks of this construction. In this model, changes in the velocity profile are suppressed, apart [...] Read more.

We first take a closer look at the original warp drive proposal by Alcubierre, examine its kinematics in the context of a covariant 3+1 setting, and explain some drawbacks of this construction. In this model, changes in the velocity profile are suppressed, apart from an externally given amplitude. We then discuss Einstein’s equations for currently employed spacetime restrictions, and provide the governing equations for the Natário class of metrics with one-component coordinate velocity in a subcase. Following Synge’s G-method we determine the constraints on realizations for two examples: assuming the form of the solution a priori as in Alcubierre’s model, and determining the solution through an assumption imposed along geodesics. We analyze in detail the role of coordinate acceleration and coordinate vorticity, providing illustrations for both example solutions. For the second we find an expected generic instability of the warp field. We then propose a framework that allows for spatial curvature and the description of warp field dynamics within a relativistic Lagrangian perturbation approach, also including exact solutions of the Szekeres class II. These generalizations allow us to link studies on warp fields to relativistic cosmology. A direct correspondence between solutions of Newtonian gravity and general relativity is exploited. We conclude by discussing possible future paths towards physical warp drives within tilted fluid flows. Full article

(This article belongs to the Section Gravitation)

23 pages, 377 KB

Open AccessArticle

Arrow of Time in Gravitational Collapse

by Samarjit Chakraborty, Sunil D. Maharaj, Rituparno Goswami and Sarbari Guha

Universe 2026, 12(5), 131; https://doi.org/10.3390/universe12050131 - 30 Apr 2026

Abstract

We investigate the arrow of time problem in the context of gravitational collapse of radiating stars in higher dimensions for both neutral and charged matter. The interior spacetime is described by a shear-free, isotropic spherically symmetric metric filled with a dissipative fluid. The [...] Read more.

We investigate the arrow of time problem in the context of gravitational collapse of radiating stars in higher dimensions for both neutral and charged matter. The interior spacetime is described by a shear-free, isotropic spherically symmetric metric filled with a dissipative fluid. The exterior spacetime of the radiating star is taken as the higher dimensional Vaidya metric. We establish that the arrow of time, measured by the epoch function, is opposite to the thermodynamic arrow of time for all dimensions in such spacetimes. The physical consequences of our results are considered. Our results conform with previous studies on shear-free spherical collapse, which suggests avoidance of the naked singularity as the end state results in a wrong arrow of time, indicating a fundamental problem with the local application of the epoch functions to test the Weyl curvature hypothesis, which we have demonstrated in the context of shear-free, pressure-isotropic subclass of radiating spherical collapse for dimension four and beyond. Full article

(This article belongs to the Section Gravitation)

22 pages, 8682 KB

Open AccessReview

Anisotropic Compact Stars: Theory and Simulation from Microphysical Models to Macroscopic Structure and Observables

by Zenia Zuraiq, Mayusree Das, Debabrata Deb, Surajit Kalita, Fridolin Weber and Banibrata Mukhopadhyay

Universe 2026, 12(5), 130; https://doi.org/10.3390/universe12050130 - 30 Apr 2026

Abstract

Strong magnetic fields and anisotropic stresses can substantially modify the structure and observable properties of compact stars. In this review, we present a unified treatment of magnetically induced anisotropy across neutron stars, hybrid stars, and white dwarfs, connecting the microphysical equation of state [...] Read more.

Strong magnetic fields and anisotropic stresses can substantially modify the structure and observable properties of compact stars. In this review, we present a unified treatment of magnetically induced anisotropy across neutron stars, hybrid stars, and white dwarfs, connecting the microphysical equation of state effects to macroscopic structure and multimessenger observables. We demonstrate that magnetic-field geometry plays a decisive role: toroidally oriented (transverse) fields enhance the maximum mass by providing additional perpendicular pressure support, whereas radially oriented fields primarily increase central compression with comparatively small mass gain. In neutron stars, anisotropy and magnetic stresses can shift phase-transition thresholds in hybrid models and enable configurations in the lower mass gap with significantly smaller magnetic energy compared to the gravitational binding energy. We further show that continuous gravitational wave emission from magnetically deformed neutron stars provides a complementary probe of internal field geometry through ellipticity-driven strain evolution. In magnetized white dwarfs, super-Chandrasekhar masses arise from the spatial redistribution of magnetic stresses rather than from globally strong magnetic energy. Taken together, these results highlight that magnetic-field geometry and matter anisotropy are as important as field strength in determining mass–radius relations, tidal deformability, gravitational wave detectability, and the emergence of extreme compact-star configurations. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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21 pages, 546 KB

Open AccessArticle

Geometric Cosmology Models: Statistical Analysis with Observational Data

by Matías Leizerovich, Luisa G. Jaime, Susana J. Landau and Gustavo Arciniega

Universe 2026, 12(5), 129; https://doi.org/10.3390/universe12050129 - 29 Apr 2026

Abstract

Although the standard cosmological model successfully describes most current observational data, it faces several theoretical and observational challenges that motivate the exploration of alternative frameworks. In this work, we investigate a class of geometric cosmology models (GC) obtained by adding an infinite tower [...] Read more.

Although the standard cosmological model successfully describes most current observational data, it faces several theoretical and observational challenges that motivate the exploration of alternative frameworks. In this work, we investigate a class of geometric cosmology models (GC) obtained by adding an infinite tower of higher-order curvature invariants to the Einstein–Hilbert action. Focusing on an exponential ansatz for the characteristic function entering the modified Friedmann equations, we derive the late-time background evolution for three families of solutions within this framework, named as (i) GILA, (ii) GR-deformation, and (iii) non-GR contribution. These models are confronted with recent Cosmic Chronometer and Type Ia supernova data, as well as age estimates of the oldest globular clusters—a constraint frequently overlooked in the literature. The stiffness of the equations in certain regions of parameter space, together with technical difficulties arising from the inclusion of the globular cluster bound, motivates the development of a dedicated methodology as an alternative to standard Markov Chain Monte Carlo techniques. Our results show that two entire families of GC models (non-GR contribution and GR-deformation) are ruled out by the data, whereas some families within the GILA model can successfully account for all data sets. For these models, meaningful constraints on their free parameters can be derived from the statistical analysis. Nevertheless, model comparison criteria reveal a preference in the data for

Λ

CDM over the GILA models examined here. Although none of the proposed models provides a preferred alternative to

Λ

CDM given the specific characteristic function considered here, this work establishes a clear methodology for testing alternative cosmological models, including the globular cluster constraint, and indicates the way for future research of GILA models with alternative choices of the characteristic function. Full article

(This article belongs to the Section Cosmology)

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22 pages, 2959 KB

Open AccessArticle

Magnetic Field Effects on the Structure of Neutron Stars

by Harsh Chandrakar, Ishfaq Ahmad Rather, Prashant Thakur, Tarun Kumar Jha, Rodrigo Negreiros, Carline Biesdorf, Mariana Dutra and Odilon Lourenço

Universe 2026, 12(5), 128; https://doi.org/10.3390/universe12050128 - 28 Apr 2026

Abstract

We investigate the impact of ultrastrong magnetic fields on the structure of neutron stars within a density-dependent relativistic mean-field framework (DDME2). In the first case, we incorporate a magnetic field framework through Landau quantization of charged particles, yielding anisotropic pressure contributions and showing [...] Read more.

We investigate the impact of ultrastrong magnetic fields on the structure of neutron stars within a density-dependent relativistic mean-field framework (DDME2). In the first case, we incorporate a magnetic field framework through Landau quantization of charged particles, yielding anisotropic pressure contributions and showing that field-induced stiffening increases stellar radii, maximum masses, and tidal deformabilities. To capture anisotropic stresses and geometric distortions, we employ axisymmetric equilibrium configurations computed with the XNS 4.0 code under the extended conformally flat condition. For magnetic field strengths up to

4.5 \times 10^{17}

G, we analyze purely poloidal and toroidal geometries across a representative mass range (1.2–2.0

M_{⊙}

). Axisymmetric models reveal that purely toroidal fields induce prolate deformations reaching

| \bar{e} | \approx 0.67

for a 1.2

M_{⊙}

star, while purely poloidal fields drive oblate deformations with

\bar{e} \approx 0.24

, both diminishing with increasing stellar mass as greater gravitational binding resists magnetic reshaping. These macroscopic effects, combined with microphysical stiffening, have direct implications for gravitational-wave emission and systematic biases in radius measurements. Our study provides a systematic mapping between magnetic field strength, topology, and dense-matter stiffness, offering constraints relevant to multimessenger observations of magnetized neutron stars. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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22 pages, 1411 KB

Open AccessArticle

Late-Time Cosmic Acceleration from QCD Confinement Dynamics

by Jonathan Rincón Saucedo, Humberto Martínez-Huerta, Adolfo Huet, Alberto Hernández-Almada and Miguel A. García-Aspeitia

Universe 2026, 12(5), 127; https://doi.org/10.3390/universe12050127 - 28 Apr 2026

Abstract

We explore a phenomenological extension of the Polyakov–Nambu–Jona-Lasinio (PNJL) model by introducing a curvature-sensitive effective contribution to the Polyakov-loop potential, motivated by the hypothesis that the non-perturbative QCD vacuum in the confined phase may retain a residual sensitivity to cosmic expansion. In a [...] Read more.

We explore a phenomenological extension of the Polyakov–Nambu–Jona-Lasinio (PNJL) model by introducing a curvature-sensitive effective contribution to the Polyakov-loop potential, motivated by the hypothesis that the non-perturbative QCD vacuum in the confined phase may retain a residual sensitivity to cosmic expansion. In a spatially flat FLRW background, this modification reduces to a term proportional to

α {(H / H_{0})}^{d} f (Φ, Φ^{*})

, which naturally vanishes in the deconfined regime and behaves as an effective dynamical vacuum component at late times, without invoking a fundamental cosmological constant. The construction provides an effective thermodynamic description of the QCD sector within an adiabatic framework and introduces a minimal phenomenological extension characterized by the exponent d and the amplitude parameter

α

. We analyze the cosmological implications at the background level and compare the model with low-redshift observations, including cosmic chronometers, Type Ia supernovae, HII galaxies, and quasars. Using Bayesian Monte Carlo techniques, we constrain the model parameters and compare its performance with the ΛCDM. Our results indicate that the modified PNJL cosmology provides a statistically competitive fit to current data while allowing small departures from the ΛCDM within observational uncertainties. We also investigate the impact of the coupling on the QCD phase diagram and the critical end point. The framework offers a tractable effective approach to connect confinement physics with late-time cosmology and suggests directions for further theoretical development in QCD under curved backgrounds. Full article

(This article belongs to the Topic Dark Matter, Dark Energy and Cosmological Anisotropy)

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21 pages, 398 KB

Open AccessArticle

Modified Gravity as Entropic Cosmology

by Shin’ichi Nojiri, Sergei D. Odintsov, Tanmoy Paul and Soumitra SenGupta

Universe 2026, 12(5), 126; https://doi.org/10.3390/universe12050126 - 27 Apr 2026

Abstract

The present work reveals a direct correspondence between modified theories of gravity (cosmology) and entropic cosmology based on the thermodynamics of apparent horizon. It turns out that due to the total differentiable property of entropy, the usual thermodynamic law (used for Einstein gravity) [...] Read more.

The present work reveals a direct correspondence between modified theories of gravity (cosmology) and entropic cosmology based on the thermodynamics of apparent horizon. It turns out that due to the total differentiable property of entropy, the usual thermodynamic law (used for Einstein gravity) needs to be generalized for modified gravity theories having more than one thermodynamic degree of freedom (d.o.f.). For the modified theories having n number of thermodynamic d.o.f., the corresponding horizon entropy is given by

S_{h} \sim S_{BH} +

terms containing the time derivatives of

S_{BH}

up to

(n - 1)

-th order, and moreover, the coefficient(s) of the derivative term(s) are proportional to the modification parameter of the gravity theory (compared to the Einstein gravity;

S_{BH}

is the Bekenstein–Hawking entropy). By identifying the independent thermodynamic variables from the first law of thermodynamics, we show that the equivalent thermodynamic description of modified gravity naturally allows the time derivative of the Bekenstein–Hawking entropy in the horizon entropy. Full article

(This article belongs to the Special Issue Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday)

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12 pages, 289 KB

Open AccessReview

Review of Strongly Coupled Regimes in Gravity with Dyson–Schwinger Approach

by Marco Frasca and Anish Ghoshal

Universe 2026, 12(5), 125; https://doi.org/10.3390/universe12050125 - 27 Apr 2026

Abstract

We analyze various gravity theories involving de Sitter, quadratic

R^{2}

and non-minimally coupled scalar backgrounds in the light of the application of the Dyson–Schwinger technique involving an exact background solution of the Green’s function. We denote specific set of solutions for the [...] Read more.

We analyze various gravity theories involving de Sitter, quadratic

R^{2}

and non-minimally coupled scalar backgrounds in the light of the application of the Dyson–Schwinger technique involving an exact background solution of the Green’s function. We denote specific set of solutions for the metric to move towards a quantum analysis of the theory. This kind of solution is identified as a conformally flat metric. Such a conclusion naturally arises in the use of the Dyson–Schwinger equations in the study of the Yang–Mills theory through the mapping theorem. We show a sequence of cosmological phase transitions starting from the breaking of such conformal invariance that can be hindered by the presence of the non-minimal coupling. Full article

(This article belongs to the Special Issue General Relativity, Modified Theories of Gravity and Their Applications in Astrophysics)

23 pages, 805 KB

Open AccessArticle

CLASH-VLT: The Fifth Force in Chameleon Gravity from Joint Lensing and Kinematics Cluster Mass Profiles

by Lorenzo Pizzuti, Federico Rivano, Keiichi Umetsu and Andrea Biviano

Universe 2026, 12(5), 124; https://doi.org/10.3390/universe12050124 - 26 Apr 2026

Abstract

We present a high-precision joint gravitational-lensing and kinematic analysis of nine massive galaxy clusters from the CLASH and CLASH-VLT surveys to test chameleon screening gravity and its

f (R)

sub-class at Mpc scales. We investigate the dependence on the assumed parametrization [...] Read more.

We present a high-precision joint gravitational-lensing and kinematic analysis of nine massive galaxy clusters from the CLASH and CLASH-VLT surveys to test chameleon screening gravity and its

f (R)

sub-class at Mpc scales. We investigate the dependence on the assumed parametrization of the total cluster mass profile by adopting three models, namely Navarro–Frenk–White (NFW), Burkert, and Hernquist. When cuspy models (NFW or Hernquist) are assumed in the general chameleon framework, the combined constraints from the nine clusters are fully consistent with General Relativity (GR), excluding large regions of the modified-gravity parameter space (the coupling constant

Q

and the background chameleon field

ϕ_{\infty}

), providing one of the tightest bounds on general chameleon models with clusters to date. In contrast, adopting a Burkert profile—disfavored by lensing data—leads to a mild (∼2σ) departure from the GR expectation in joint analysis. When considering the

f (R)

sub-case, we obtain a bound on the background scalaron field of

| f_{R} | ≲ 2 - 5 \times 10^{- 5}

(95% C.L.) for NFW and Hernquist models, in agreement with current constraints at cosmological scales, and an apparent deviation from standard gravity of

{log}_{10} | f_{R} | = - 4.7 \pm 1.2

for the Burkert case. We investigate the impact of systematics in the kinematical analysis, showing that the tension is mitigated when clusters exhibiting clear dynamical disturbance are excluded from the sample. Our results show that galaxy clusters provide competitive tests of screened modified gravity at mega-parsec scales, while highlighting the critical role of accurate mass modeling and dynamical-state assessment. The upcoming generation of wide-field lensing surveys and spectroscopic follow-up programs will enable similar analyses on substantially larger samples, offering the prospect of tightening cluster-based constraints on gravity and the dark sector. Full article

(This article belongs to the Special Issue Celebrating the 110th Anniversary of General Relativity: Advances, Challenges and Perspectives)

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23 pages, 1405 KB

Open AccessReview

Stellar Microlensing Surveys as a Probe of Primordial Black Holes: Status and Prospects

by Anne M. Green

Universe 2026, 12(5), 123; https://doi.org/10.3390/universe12050123 - 25 Apr 2026

Abstract

Stellar microlensing surveys are a powerful tool for probing dark matter in the form of planetary and stellar mass compact objects (COs) in particular primordial black holes (PBHs). Under standard assumptions, current observations exclude COs in the mass range [...] Read more.

Stellar microlensing surveys are a powerful tool for probing dark matter in the form of planetary and stellar mass compact objects (COs) in particular primordial black holes (PBHs). Under standard assumptions, current observations exclude COs in the mass range

10^{- 11} ≲ M / M_{⊙} ≲ 10^{4}

making up all of the dark matter. We provide an overview, aimed at theorists working on PBHs, of the history, theory, observational status, and future prospects of the field. Full article

(This article belongs to the Special Issue Primordial Black Holes: Observational Strategies)

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