Congratulations to Prof. Roger Penrose, Advisory Board member of Universe, for receiving the Nobel Prize in Physics 2020.

Journal Description

Universe

Universe is a peer-reviewed, open access journal focused on theoretical, experimental, and observational progress in fundamental and applied physics, from circumterrestrial space to cosmological scenarios, and is published monthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within Scopus, SCIE (Web of Science), Astrophysics Data System, INSPIRE, CAPlus / SciFinder, Inspec, and other databases.
Journal Rank: JCR - Q2 (Astronomy and Astrophysics) / CiteScore - Q2 (General Physics and Astronomy)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 21.8 days after submission; acceptance to publication is undertaken in 3.6 days (median values for papers published in this journal in the second half of 2025).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.
Journal Cluster of Gravitation, Cosmology and Astrophysics: Universe, Galaxies, Particles and Astronomy.

Impact Factor: 2.6 (2024); 5-Year Impact Factor: 2.6 (2024)

Imprint Information Journal Flyer Open Access ISSN: 2218-1997

Latest Articles

32 pages, 3182 KB

Open AccessArticle

TD-DFT Investigation of Sulfur and Chlorine Species as Potential Contributors to Venusian Unknown UV Absorber

by Parmanand Pandey, Pravi Mishra, Rachana Singh, Manisha Yadav, Shivani, Aftab Ahamad, Alka Misra, Poonam Tandon and Amritanshu Shukla

Universe 2026, 12(5), 151; https://doi.org/10.3390/universe12050151 - 21 May 2026

Abstract

The identification of the chemical species responsible for the anomalous near-ultraviolet (UV) opacity in the Venusian cloud for “unknown absorber” remains a paramount challenge in planetary science. This study presents a comprehensive quantum chemical investigation into a broad suite of candidate molecules, including isomers of thiosulfeno (S₂O₂), the hydroxysulfonyl radical (HSO₃), disulfur monoxide (S₂O), disulfur dichloride (S₂Cl₂), iron(III) chloride (FeCl₃), phosphine (PH₃), and structural isomers of polysulfur oxides (S₃O). Utilizing Time-Dependent Density Functional Theory (TD-DFT) at the CAM-B3LYP/def2-TZVPP level of theory, we systematically mapped electronic transitions across three distinct environmental phases: gas-phase (without solvent), supercritical CO₂, and concentrated H₂SO₄ aerosols. To establish confidence in the predicted results, our TD-DFT approach was rigorously benchmarked against high-level theoretical methods (CCSD(T), EOM-CCSD, and MRCI+Q) from recent literature. All these electronic transitions were modeled via the Solvation Model based on Density (SMD). Our results demonstrate a profound topological and environmental dependence on spectral signatures. Among the candidates, trans-OSSO (t-OSSO) emerged as the most viable near-UV absorber candidate, exhibiting a highly allowed π → π* transition at 379.37 nm (f = 0.1140) in H₂SO₄, providing a near-perfect alignment with the observed 365 nm planetary albedo drop. Conversely, the polysulfur oxide cis-S₃O was acknowledged as a primary visible-light chromophore, with an intense absorption at 436.31 nm (f = 0.1280) responsible for the characteristic yellow tint of the planet. Additionally, the photochemically maintained SSCl₂ isomer was identified as a critical broadband near-UV absorber. Species such as S₂O and planar S₃O were found to function as critical mid-UV shields (270–300 nm). This work establishes a multi-chromophore model of the Venusian atmosphere, where a chemically stratified network of sulfur-oxygen chains and chlorine-sulfur reservoirs, tuned by the acidic aerosol matrix, collectively governs radiative balance and atmospheric super-rotation of the planet. Furthermore, to account for massive continuum tailing into the visible region (>400 nm), we employed a semi-classical Reflection Principle approach to model 1D vibronic broadening. This analysis revealed that while standard solvent effects induce minor solvatochromic shifts, ground-state structural fluxionality in the OSSO isomers drives intense, symmetry-allowed transitions deep into the visible spectrum, an effect absent in structurally constrained or rigid control species. Full article

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20 pages, 1873 KB

Open AccessArticle

Geometric Bias and Centrality Dependence of Jet Quenching in High-Energy Nuclear Collisions

by Changle Sun, Yichao Dang and Shanshan Cao

Universe 2026, 12(5), 150; https://doi.org/10.3390/universe12050150 - 21 May 2026

Abstract

Jet quenching provides a valuable measure of the opacity of the quark–gluon plasma (QGP) produced in high-energy heavy-ion collisions. However, substantial suppression of charged hadron spectra is observed in highly peripheral collisions, despite the expectation of negligible jet–QGP interactions in this regime. To address this, we develop a HIJING-based initial condition model that accounts for the impact parameter dependence of both inelastic nucleon–nucleon (NN) collisions and the number of hard partonic scatterings per inelastic NN collision. This dependence introduces a geometric bias effect on the jet yield within a given centrality class of nucleus–nucleus (AA) collisions, suppressing the high-

p_{T}

hadron spectrum in peripheral collisions due to dilute nucleon overlap at large AA impact parameters. By combining this improved initial condition model with a linear Boltzmann transport model for jet–QGP interactions, we obtain a satisfactory description of the centrality dependence of charged hadron suppression in Pb+Pb collisions at

\sqrt{s_{NN}} = 5.02

TeV. Full article

(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)

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24 pages, 1307 KB

Open AccessArticle

Finite-Size Effects on the Critical End Point of Magnetized Quark Matter in the Nonlocal PNJL Model

by G. Lugones, S. A. Ferraris and A. G. Grunfeld

Universe 2026, 12(5), 149; https://doi.org/10.3390/universe12050149 - 20 May 2026

Abstract

We investigate finite-size effects in the T–

μ

We investigate finite-size effects in the T–

μ

phase diagram of magnetized quark matter within the framework of a nonlocal extension of the Polyakov–Nambu–Jona-Lasinio (PNJL) model. Finite-size corrections are incorporated through the multiple reflection expansion (MRE) formalism, which describes a spherical quark droplet of radius R and modifies the density of states by including surface and curvature contributions. We consider two-flavor quark matter at finite temperature and chemical potential in the presence of a uniform magnetic field with strengths ranging from

e B = 0

to 1

{GeV}^{2}

, and droplet radii from

R = 3

fm to the bulk limit. The nonlocal PNJL (nlPNJL) model naturally reproduces both magnetic catalysis at low temperatures and inverse magnetic catalysis near the chiral transition, in agreement with lattice QCD results. We analyze the chiral condensate, the traced Polyakov loop, the normalized quark condensate, and the corresponding susceptibilities. We find that finite-size effects do not modify the overall structure of the phase diagram, and that the coincidence of the chiral restoration and deconfinement transitions persists for all magnetic field strengths and system sizes explored, within the present implementation in which finite-size corrections are restricted to the fermionic sector. However, the critical end point (CEP) is notably shifted as a function of both magnetic field strength and system size: It moves toward higher chemical potentials and lower temperatures as system size decreases, an effect that is significantly amplified by strong magnetic fields. Our results have potential implications for the physics of phase conversion in compact stars and for the interpretation of relativistic heavy-ion collision experiments. 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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12 pages, 817 KB

Open AccessArticle

Probing ALP-Photon Oscillations with Fermi-LAT Observation of the Andromeda Galaxy

by Jun Li

Universe 2026, 12(5), 148; https://doi.org/10.3390/universe12050148 - 20 May 2026

Abstract

Axion-like particles (ALPs) can induce energy-dependent irregularities in gamma-ray spectra through ALP-photon oscillation in astrophysical magnetic fields. In this study, we investigate the impact of this effect on the

γ

-ray emission from the Andromeda Galaxy (M31). We employ the

{CL}_{s}

method [...] Read more.

γ

-ray emission from the Andromeda Galaxy (M31). We employ the

{CL}_{s}

method to set constraints on the ALP parameters. We find that ALP-photon oscillation can produce characteristic oscillatory features in the gamma-ray spectrum within the mass range

m_{a} \sim 10^{- 9}

–

10^{- 7} eV

. No significant deviation from the standard astrophysical model is observed, allowing us to place constraints on the ALP parameter space. The resulting limits probe a region complementary to existing constraints from other astrophysical observations. Full article

(This article belongs to the Special Issue Fuzzy Dark Matter Paradigm and Ultralight Axions)

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17 pages, 952 KB

Open AccessPerspective

Magnetized Matter in Neutron Star Dynamics: Challenges Ahead

by J. E. Horvath

Universe 2026, 12(5), 147; https://doi.org/10.3390/universe12050147 - 18 May 2026

Abstract

Matter at ultra-high densities finds a physical realization inside neutron stars. It is generally acknowledged that huge magnetic fields are present in these stellar objects, and if not for the presence of the magnetic fields, neutron stars would be much more “silent” and practically invisible. However, a series of questions still remain concerning the role of magnetic fields in the neutron star structure and internal dynamics. We present an overview of these topics, pointing out the importance of a new set of observations (glitches and related events, and precession) and old questions that must be accommodated by a more complete theory of neutron star physics. 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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11 pages, 2277 KB

Open AccessArticle

On the Origin of Sodium Atoms in the Comae and Trails of Comets

by Marco Fulle, Paolo Molaro and Ilya Ilyin

Universe 2026, 12(5), 146; https://doi.org/10.3390/universe12050146 - 18 May 2026

Abstract

Background: The observed abundance ratios of alkali species in ground-based spectra of comets deviate from solar composition, suggesting alkali ejection from phenoxides reacting with carbon dioxide at the nucleus surface (alkali-phenoxide carbonylation). Methods: Here, we search for the alkali emissions in spectra of the coma and of the trail of Comet C/2023 A3 (Tsuchinshan-ATLAS) exploiting the double-fiber entrance of the high-resolution PEPSI spectrograph at the 8.4 m Large Binocular Telescope. Results: Spectra sampling the nucleus yield

N a / K

ratios 3.6 times higher than the chondritic value, and even higher ratios sampling the trail. This fact excludes photodesorption as the main sodium source, leaving sodium-phenoxide carbonylation at the surface of the main nucleus and the trail mininuclei as the primary sodium source. Conclusions: The nucleus temperature and the faint KI line exclude potassium-phenoxide carbonylation. For the first time, KI is detected in the trail of an Oort cloud comet, suggesting potassium photodesorbed from the trail mininuclei. Sodium-phenoxide carbonylation is at least six times more efficient than sodium photodesorption if the

N a / K

ratio in the C/2023 A3 nuclei is chondritic. Trails composed of sub-km-sized mininuclei may be common features of Oort cloud comets. Full article

(This article belongs to the Special Issue The Hidden Stories of Small Planetary Bodies)

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12 pages, 552 KB

Open AccessArticle

Acceptance Effects on the Extracted Spin Alignment of K*⁰ Mesons in Relativistic Heavy-Ion Collisions

by Shaowei Lan, Qiuhua Liu and Pengfei Ji

Universe 2026, 12(5), 145; https://doi.org/10.3390/universe12050145 - 16 May 2026

Abstract

The spin alignment of vector mesons, characterized by the spin-density-matrix element

ρ_{00}

, is an important observable for studying spin dynamics in relativistic heavy-ion collisions. Experimental measurements have reported deviations of

ρ_{00}

from the isotropic expectation of

1 / 3

, [...] Read more.

The spin alignment of vector mesons, characterized by the spin-density-matrix element

ρ_{00}

, is an important observable for studying spin dynamics in relativistic heavy-ion collisions. Experimental measurements have reported deviations of

ρ_{00}

from the isotropic expectation of

1 / 3

, motivating careful evaluation of possible acceptance effects. In this work, we investigate the influence of finite experimental coverage on the extracted

ρ_{00}

K^{* 0}

mesons using a toy model constrained by realistic kinematic distributions from the AMPT model. The reconstructed

ρ_{00}

is examined as a function of pseudorapidity (

η

) and transverse momentum (

p_{T}

) within typical experimental acceptance ranges. We find that limited pseudorapidity coverage can lead to reconstructed

ρ_{00}

values above

1 / 3

, even when the input distribution is isotropic. This behavior originates from the selective removal of decay daughters outside the

η

window, which modifies the

cos θ^{*}

distribution. A dependence on transverse momentum is also observed, particularly at low

p_{T}

where daughter particles are more sensitive to longitudinal acceptance constraints. Comparisons with STAR measurements are presented for reference, without attempting to reinterpret the experimental results. Overall, this study provides a systematic examination of acceptance-induced effects and may serve as a useful reference for future measurements of vector-meson spin alignment. Full article

(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)

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4 pages, 181 KB

Open AccessEditorial

Observational Strategies

by Jeremy Mould

Universe 2026, 12(5), 144; https://doi.org/10.3390/universe12050144 - 14 May 2026

Abstract

It is fifty years since Stephen Hawking laid out the physics of primordial black holes (PBHs) and fifty years since the cold dark matter paradigm became the standard model for the formation of structure in the universe [...] Full article

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

10 pages, 277 KB

Open AccessArticle

Field Theory Vacuum and Entropic Dark Energy Models

by Michael Maziashvili

Universe 2026, 12(5), 143; https://doi.org/10.3390/universe12050143 - 14 May 2026

Abstract

While, in standard quantization, the energy spectrum of an oscillator does not depend on its mass, in Planck length deformed quantization, the energy spectrum becomes mass-dependent. That means that the field oscillator masses will source a gravitational field through the Nullpunktsenergie as long as we follow this scheme of quantization. Admitting these masses are tangible, their gravitational effect will manifest itself even within the framework of standard field theory. We shall consider the possible gravitational implications based on this approach. If the mass scale for field oscillators is set by the inverse size of the box containing the field and the three-momentum cutoff dictated by the black hole energy bound is exploited, one finds that the number of Fourier modes saturates the black hole entropy bound. Following certain “holographic” reasoning, one can derive various kinds of dark energy models that may be interesting for further study. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

29 pages, 1844 KB

Open AccessArticle

GRMHD Simulations of Magnetized Accretion Disk/Jet: Variabilities of Black Holes and Spectral Energy Distributions in Magnetic States

by Rohan Raha, Banibrata Mukhopadhyay and Koushik Chatterjee

Universe 2026, 12(5), 142; https://doi.org/10.3390/universe12050142 - 12 May 2026

Abstract

We perform three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of a near-maximally spinning black hole (spin parameter

a = 0.998

We perform three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of a near-maximally spinning black hole (spin parameter

a = 0.998

) with varying initial magnetic field geometries, systematically exploring the parameter space connecting magnetically arrested disk (MAD), intermediate (INT), and standard and normal evolution (SANE) accretion states. The magnetic flux threading the black hole horizon emerges as the fundamental state variable controlling jet efficiency, flow magnetization, and radiative output across all three states. We introduce complementary diagnostics—broadband spectral energy distributions spanning radio through hard X-ray frequencies and time-resolved X-ray light curves—that together connect simulation dynamics directly to multiwavelength observables. The radiative output follows a clear MAD > INT > SANE hierarchy in time-averaged luminosity, mean X-ray emission, as well as variability. Furthermore, MAD exhibits the highest fractional variability through quasi-periodic magnetic flux eruption events, and INT and SANE show moderate variability driven by episodic reconnection and stochastic MRI turbulence, respectively. Scaling to GRS 1915+105, Cyg X-1, and HLX-1, we demonstrate that all twelve temporal classes of GRS 1915+105 map naturally onto our three magnetic states, Cyg X-1’s persistent hard state is reproduced by a sustained INT configuration, and HLX-1’s extreme luminosities arise through efficient Blandford–Znajek extraction in MAD states scaled to higher black hole mass. 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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13 pages, 578 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

Abstract

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)

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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 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. 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 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 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 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}}

(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}}

(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.

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, 316 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 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, 14415 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 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)

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