Universe

18 pages, 6104 KB

Open AccessReview

Heavy-Ion Physics at the CERN SPS H2: NA35, NA49 and NA61/SHINE (With Personal Recollections)

by Marek Gazdzicki

Universe 2026, 12(2), 58; https://doi.org/10.3390/universe12020058 - 20 Feb 2026

Viewed by 438

This review presents a unified account of the NA35, NA49, and NA61/SHINE experiments, which together form a continuous programme of heavy-ion studies conducted at the H2 beamline of the CERN North Area using the SPS accelerator. The programme, spanning about 40 years, was [...] Read more.

This review presents a unified account of the NA35, NA49, and NA61/SHINE experiments, which together form a continuous programme of heavy-ion studies conducted at the H2 beamline of the CERN North Area using the SPS accelerator. The programme, spanning about 40 years, was driven by the search for a high-density state of strongly interacting matter—the quark–gluon plasma (QGP)—and the transitions leading to it. The review focuses on this primary line of research. The highlights of the programme include the observation of the first signal of QGP creation at the top SPS energy in S+S collisions by NA35, evidence for the onset of deconfinement at low SPS energies by NA49, and the establishment by NA61/SHINE of the diagram of high-energy nuclear collisions, featuring transitions between hadron-, string-, and QGP-dominated regimes. This predominantly scientific review is complemented by brief personal recollections related to the discussed topics. Full article

(This article belongs to the Special Issue 10th Anniversary of Universe: Studying the Strongly Interacting Matter in Nuclear Reactions from Intermediate to Ultra-Relativistic Energies)

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18 pages, 2527 KB

Open AccessArticle

Chaotic Motion of Strings in a Quantum-Corrected AdS Reissner–Nordström Black Hole

by Kai Li, Da-Zhu Ma and Zhen-Meng Xu

Universe 2026, 12(2), 57; https://doi.org/10.3390/universe12020057 - 20 Feb 2026

Viewed by 449

Abstract

It has been reported that quantum correction modifies the topological charges of Anti-de-Sitter Reissner–Nordström (AdS-RN) black holes in Kiselev spacetime, yielding new perspectives on topological classification. This leads us to focus on how quantum corrections and other parameters collectively influence the long-term dynamic [...] Read more.

It has been reported that quantum correction modifies the topological charges of Anti-de-Sitter Reissner–Nordström (AdS-RN) black holes in Kiselev spacetime, yielding new perspectives on topological classification. This leads us to focus on how quantum corrections and other parameters collectively influence the long-term dynamic evolution of strings. First, we analytically examine whether the strings’ motion violates the Maldacena–Shenker–Stanford (MSS) bound. Then, we employ numerical integration to study the influence of various parameters on string chaotic dynamics. Our results demonstrate that the quantum-correction parameter a, the normalization factor c, and black-hole charge Q significantly influence chaotic behavior and the violation of the MSS bound. In particular, as a increases, the system undergoes an order–chaos–order transition, whereas an increase in c or a decrease in Q drives the system from order to chaos. Full article

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

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18 pages, 674 KB

Open AccessArticle

Scaling Properties of Two-Particle–Two-Hole Responses in Asymmetric Nuclei for Neutrino Scattering Within the Relativistic Mean-Field Framework

by Victor L. Martinez-Consentino, Jose E. Amaro and Jorge Segovia

Universe 2026, 12(2), 56; https://doi.org/10.3390/universe12020056 - 20 Feb 2026

Viewed by 448

Abstract

We perform a systematic analysis of the nuclear dependence of two-particle–two-hole meson-exchange current contributions to inclusive lepton-nucleus scattering within the relativistic mean-field framework. We present microscopic calculations of nuclear responses for a set of 17 nuclei, ranging from helium to uranium, using a [...] Read more.

We perform a systematic analysis of the nuclear dependence of two-particle–two-hole meson-exchange current contributions to inclusive lepton-nucleus scattering within the relativistic mean-field framework. We present microscopic calculations of nuclear responses for a set of 17 nuclei, ranging from helium to uranium, using a model with different Fermi momenta for protons and neutrons. We propose a novel scaling prescription based on the two-particle phase space and key nuclear parameters. The resulting description is accurate over a wide range of nuclear targets, with typical deviations below 10%, and allows for a separate treatment of the different emission channels. In addition, a consistent benchmark against electron-scattering data is provided. The parametrization presented provides a practical framework for extending the responses to different nuclear targets in neutrino event generators. Full article

(This article belongs to the Special Issue Neutrino Insights: Peering into the Subatomic Universe)

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12 pages, 263 KB

Open AccessArticle

The Geometric Proca Field in Weyl Gauge-Invariant Theory

by M. Duarte, T. A. T. Sanomiya, F. Dahia and C. Romero

Universe 2026, 12(2), 55; https://doi.org/10.3390/universe12020055 - 18 Feb 2026

Viewed by 445

Abstract

We present a detailed study on the geometrization of the Proca field in the so-called Weyl Gauge-Invariant Theory, shedding new light on the physical interpretation of the Weyl field. We first describe the field equations of the theory. We then obtain a solution [...] Read more.

We present a detailed study on the geometrization of the Proca field in the so-called Weyl Gauge-Invariant Theory, shedding new light on the physical interpretation of the Weyl field. We first describe the field equations of the theory. We then obtain a solution for the weak field using a spherically symmetric and static approximate metric. Our analysis revealed that the Weyl field, in the weak field approximation, exhibits a behavior identical to the Yukawa potential, similar to the Proca field. Furthermore, the obtained metric solution is equivalent to the Einstein–Proca case, demonstrating that the description of the Weyl field in the Weyl Gauge-Invariant Theory is consistent with Proca theory in the context of General Relativity. Finally, we conclude that the Weyl field can be formally interpreted as a Proca field of geometrical nature. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology: Insights and Advances)

26 pages, 4116 KB

Open AccessArticle

U-Net Based Forecasting of Storm-Time Total Electron Content over North Africa Using Assimilation of GNSS Observation into Global Ionospheric Maps

by Adel Fathy, Ahmed. I. Saad Farid, Daniel Okoh, Patrick Mungufeni, Ayman Mahrous, Mohamed Nassar, Yuichi Otsuka, Weizheng Fu, John Bosco Habarulema, Haitham El-Husseiny and Ahmed Arafa

Universe 2026, 12(2), 54; https://doi.org/10.3390/universe12020054 - 18 Feb 2026

Cited by 1 | Viewed by 653

Abstract

This study presents U-Net deep learning of total electron content (TEC) obtained from Global Ionosphere Maps (GIMs) to forecast ionospheric TEC over the African 0–40° N latitude sector during geomagnetic storms which have occurred between 2011 and 2024. Before being utilized in the [...] Read more.

This study presents U-Net deep learning of total electron content (TEC) obtained from Global Ionosphere Maps (GIMs) to forecast ionospheric TEC over the African 0–40° N latitude sector during geomagnetic storms which have occurred between 2011 and 2024. Before being utilized in the deep learning procedure, the GIM-TEC data were improved by assimilating ground-based vertical TEC (VTEC) observations from available Global Navigation Satellite System (GNSS) receiver stations. The U-Net one-hour-ahead prediction of TEC was examined during the intense geomagnetic storm of May 2024. Additionally, the model’s accuracy and reliability were evaluated through quantitative comparison with established climatological models, including IRI-2020 and AfriTEC storm time models. The results indicate that the integration of data assimilation with the deep learning framework yields TEC estimates that closely agree with observations, achieving a RMSE of approximately 5 TECU. On the other hand, the IRI-2020 model exhibits substantially larger errors, with RMSE ~10–17 TECU, while the AfriTEC model shows the poorest performance, with RMSE reaching approximately 15–22 TECU. Further, the U-Net was validated using two equatorial and mid-latitude GNSS stations whose data were excluded from the assimilation process, achieving RMSE values of 4.44 and 6.75 TECU and correlation coefficients of 0.93 and 0.97, confirming the model forecasting capability for reproducing ionospheric TEC variability. These results establish the model as a precise, robust tool for TEC prediction in regions with sparse GPS coverage that is crucial for ionospheric monitoring and space weather applications. Full article

(This article belongs to the Topic Techniques and Science Exploitations for Earth Observation and Planetary Exploration-2nd Edition)

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17 pages, 3265 KB

Open AccessArticle

Archival Gravitational-Wave Optical Transient Observer Photometry and Zwicky Transient Facility Localization of Galactic Novae: Quiescent Constraints and Improved Coordinates

by Sutharut Khamrat, Farung Surina, Kanthanakorn Noysena, Kendall Ackley, Martin J. Dyer, Joe Lyman, Krzysztof Ulaczyk, Sergey Belkin, Duncan K. Galloway, Vik S. Dhillon, Paul O’Brien, Gavin Ramsay, Rubina Kotak, Rene P. Breton, Laura K. Nuttall, Ben Gompertz, Jorge Casares, Paul Chote, Ashley Chrimes, Deanne Coppejans, Rob Eyles-Ferris, Ben Godson, Dan Jarvis, Lisa Kelsey, Mark Kennedy, Tom Killestein, Andrew Levan, Soheb Mandhai, Seppo Mattila, Kangming Pu, Anwesha Sahu, Elizabeth Stanway, Rhaana Starling and Yuzhu Sun Show full author list Hide full author list

Universe 2026, 12(2), 53; https://doi.org/10.3390/universe12020053 - 16 Feb 2026

Viewed by 675

Abstract

We present archival photometry from the Gravitational-wave Optical Transient Observer (GOTO) for four Galactic novae discovered between 2017 and 2024, spanning some of the faintest ZTF24aaomlxy and PGIR22akgylf (at a marginal near-limit level consistent with the practical limiting magnitude of calibrated L to [...] Read more.

We present archival photometry from the Gravitational-wave Optical Transient Observer (GOTO) for four Galactic novae discovered between 2017 and 2024, spanning some of the faintest ZTF24aaomlxy and PGIR22akgylf (at a marginal near-limit level consistent with the practical limiting magnitude of calibrated L to the brightest V1405 Cas and V1674 Her recent eruptions. For each object, we extract GOTO measurements obtained at or near the pre-eruption state, excluding data points with observational uncertainties exceeding 0.5 mag (except for the faintest PGIR22akgylf). The resulting light curves show that GOTO can detect nova progenitors close to its observable limiting depth at calibrated L magnitudes approaching the survey’s practical limiting magnitude, providing meaningful constraints on quiescent brightness, possibly for systems that were only sparsely monitored using surveys such as ZTF and PGIR. These detections demonstrate that wide-field imaging originally designed for gravitational-wave follow-up can yield meaningful limits on both faint and fast-evolving nova progenitors. Simultaneously, we improve the sky positions of five Galactic novae—ZTF24aaomlxy, V3732 Oph, V2000 Aql, V3666 Oph, and V659 Sct—whose published coordinates are affected by crowding or limited precision. Using high-cadence photometry from ZTF and AAVSO, we identify the actual eruption source in each field and obtain revised coordinates that differ by several arcseconds. These findings highlight the importance of time-domain archives for identifying faint nova progenitors and improving astrometric accuracy across the Galactic nova population. Full article

(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)

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16 pages, 5410 KB

Open AccessArticle

Dynamics of Coronal Streamer Deflection Under the Impact of a CME-Driven Shock

by Yuhu Miao

Universe 2026, 12(2), 52; https://doi.org/10.3390/universe12020052 - 14 Feb 2026

Viewed by 403

Abstract

We present a stereoscopic analysis of coronal streamer deflection induced by a CME-driven shock, utilizing multi-viewpoint coronagraph observations from STEREO-Ahead, STEREO-Behind, and SOHO. Driven by the continuous impact of the shock, the streamer deflection propagates outward, exhibiting distinct morphological variations [...] Read more.

We present a stereoscopic analysis of coronal streamer deflection induced by a CME-driven shock, utilizing multi-viewpoint coronagraph observations from STEREO-Ahead, STEREO-Behind, and SOHO. Driven by the continuous impact of the shock, the streamer deflection propagates outward, exhibiting distinct morphological variations across the three different lines of sight. Our analysis reveals that speeds derived directly from two-dimensional (2D) images differ significantly from those obtained via three-dimensional (3D) reconstruction. Specifically, the 2D projected speeds measured from STEREO-Ahead, STEREO-Behind, and SOHO are 445, 476, and 336 km s⁻¹, respectively. Furthermore, while 2D measurements suggest a constant propagation speed, the 3D reconstruction reveals a pronounced deceleration of approximately −36 m s⁻². Significant discrepancies are also noted in the deflection amplitude between the 2D and 3D results. Since the propagating streamer deflection effectively traces the shock’s movement, we propose that measuring the deflection speed offers a robust alternative for deriving actual shock velocities in the outer corona, where direct white-light detection remains challenging. Full article

(This article belongs to the Special Issue Oscillations and Instabilities of Solar Filaments)

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24 pages, 17605 KB

Open AccessArticle

Constraining the Location of γ-Ray Flares in the Flat Spectrum Radio Quasar B2 1633+382 at GeV Energies

by Yang Liu, Zhenzhen He, Jing Fan, Xiongfei Geng, Yehui Yang, Ting Xu, Gang Cao, Xiongbang Yang, Xienan Zheng, Yingtao Miao, Songhao Pei, Zihao Zhang, Tao Dong, Haijun Lin, Fan Wu and Nan Ding

Universe 2026, 12(2), 51; https://doi.org/10.3390/universe12020051 - 13 Feb 2026

Viewed by 323

Abstract

In this study, we extract a 7-day binned

γ

-ray light curve from 2008 August to 2019 March in the energy range 0.1–300 GeV and identify four outburst periods with peak flux of >8.0

\times 10^{- 7}

ph [...] Read more.

In this study, we extract a 7-day binned

γ

-ray light curve from 2008 August to 2019 March in the energy range 0.1–300 GeV and identify four outburst periods with peak flux of >8.0

\times 10^{- 7}

ph

{cm}^{- 2} s^{- 1}

. Four active states in the optical are also marked during this period. The fastest variability timescale suggests the emission region radius is R ∼ 2.4

\times 10^{16}

cm, and the observed emission region lies within <0.7 pc distance from the central engine. The majority of short-timescale flares exhibit a symmetric temporal profile, implying that the rise and decay timescales are dominated by disturbances caused by dense plasma blobs passing through the standing shock front in the jet region. To understand the properties of the source jets, we employ a standard one-zone leptonic scenario to model the broadband spectral energy distributions (SEDs) of flaring periods and determine that the

γ

-ray spectrum is better reproduced when the dissipation region of the jet is located within the molecular torus (MT). The

γ

-ray spectra from the outburst phases show an obvious spectral break with a break energy between 3.00 and 7.08 GeV, which may be attributed to an intrinsic break in the energy distribution of radiating particles. The studies of the survival time of a sheet before being destroyed by the turbulent motions of plasma (

τ_{c s} \sim 2.9 \times 10^{4}

s), the shock acceleration time (

t_{a c c} \sim

4.3 \times 10^{4}

s), and the minimum interaction height (

Z_{m i n}

≈ 2.57–4.55

\times 10^{17}

cm >

R_{B L R}

∼ 1.0

\times 10^{17}

cm) suggest that the

γ

-ray flaring event maybe caused by a magnetic reconnection mechanism, but we cannot completely rule out the shock-in-jet model. Full article

(This article belongs to the Special Issue HE/VHE Gamma-Ray in Extragalactic-Astrophysics: Observations, Models, and Cosmic Implications)

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14 pages, 7000 KB

Open AccessArticle

A Two-Stage Machine Learning Framework for Predicting Sporadic E Occurrence and Intensity

by Licheng Liu and Ding Yang

Universe 2026, 12(2), 50; https://doi.org/10.3390/universe12020050 - 12 Feb 2026

Viewed by 420

Abstract

Sporadic E (Es) layers exhibit strong intermittency and highly skewed intensity distributions, exerting significant impacts on high-frequency communication and navigation systems and posing challenges for data-driven prediction. Conventional single-stage regression models are often dominated by abundant non-event samples and therefore tend to underestimate [...] Read more.

Sporadic E (Es) layers exhibit strong intermittency and highly skewed intensity distributions, exerting significant impacts on high-frequency communication and navigation systems and posing challenges for data-driven prediction. Conventional single-stage regression models are often dominated by abundant non-event samples and therefore tend to underestimate Es intensity during occurrence periods. To address this issue, this study proposes a unified two-stage neural network framework that decouples the prediction of Es occurrence probability from the estimation of Es intensity. The model is trained using multi-station ionosonde observations, incorporating cyclic representations of seasonal and local time variations together with solar and geomagnetic indices and station-aware encoding to enable unified learning across multiple stations. Results show that the proposed two-stage framework achieves event-only MAE values of 0.53–0.76 MHz and RMSE values of approximately 1.0–1.4 MHz at most mid- and low-latitude stations, with larger errors at the high-latitude Casey station (MAE ≈ 1.45 MHz and RMSE ≈ 2.31 MHz). The consistently bounded MRE values (≈0.18–0.23) observed across multiple stations demonstrate that the framework effectively mitigates severe data imbalance and suppresses spurious high-intensity estimates under non-Es conditions. Full article

(This article belongs to the Special Issue Applications of Artificial Intelligence in Modern Astronomy)

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13 pages, 349 KB

Open AccessArticle

Quasibound States of Massive Charged Scalars Around Dilaton Black Holes in

2 + 1

Dimensions: Exact Frequencies

by Horacio Santana Vieira

Universe 2026, 12(2), 49; https://doi.org/10.3390/universe12020049 - 12 Feb 2026

Viewed by 504

Abstract

In this work, we investigate massive charged scalar perturbations in the background of three-dimensional dilaton black holes with a cosmological constant. We demonstrate that the wave equations governing the dynamics of these perturbations are exactly solvable, with the radial part expressible in terms [...] Read more.

In this work, we investigate massive charged scalar perturbations in the background of three-dimensional dilaton black holes with a cosmological constant. We demonstrate that the wave equations governing the dynamics of these perturbations are exactly solvable, with the radial part expressible in terms of confluent Heun functions. The quasibound state frequencies are computed analytically, and we examine their dependence on the scalar field’s mass and charge, as well as on the black hole’s mass and electric charge. Our analysis also underscores the crucial role played by the cosmological constant in shaping the behavior of these perturbations. This specific black hole metric arises as a solution to the low-energy effective action of string theory in

2 + 1

dimensions, and it holds potential for experimental realization in analog gravity systems due to the similarity between its surface gravity and that of acoustic analogs. Moreover, the analytic tractability of this system offers a valuable testing ground for exploring aspects of black hole spectroscopy, stability, and quantum field theory in curved spacetime. The exact solvability facilitates deeper insights into the interplay between geometry and matter fields in lower-dimensional gravity, where quantum gravitational effects can be more pronounced. Such studies not only enrich our understanding of dilaton gravity and its string-theoretic implications but also pave the way for potential applications in simulating black hole phenomena in laboratory settings using analog models. Full article

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

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29 pages, 767 KB

Open AccessReview

The Impact of Dark Matter on Gravitational Wave Detection by Space-Based Interferometers

by Yuezhe Chen, Pan-Pan Wang, Bo Wang, Rui Luo and Cheng-Gang Shao

Universe 2026, 12(2), 48; https://doi.org/10.3390/universe12020048 - 11 Feb 2026

Cited by 1 | Viewed by 1223

Abstract

The existence of dark matter is supported by multiple astrophysical observations, yet its particle nature remains unknown. The development of gravitational wave astronomy, especially with future space-based detectors such as LISA, provides new opportunities to study the interactions between dark matter and compact-object [...] Read more.

The existence of dark matter is supported by multiple astrophysical observations, yet its particle nature remains unknown. The development of gravitational wave astronomy, especially with future space-based detectors such as LISA, provides new opportunities to study the interactions between dark matter and compact-object systems. This review summarizes the main dark matter candidates and their macroscopic distributions, and highlights three mechanisms through which dark matter can affect gravitational wave observations: (1) modifications to compact-object orbits and the dynamics of systems such as extreme mass-ratio inspirals, including dark matter spikes, dynamical friction, and potential perturbations; (2) gravitational lensing effects induced by the spatial distribution of dark matter, altering waveform amplitudes and phases; and (3) direct couplings between ultralight dark matter fields and detectors. As low-frequency gravitational wave detection techniques are proposed and continue to develop, these effects may offer a novel avenue for probing the properties of dark matter, and combining precise waveform modeling with multi-messenger observations could reveal insights into its microscopic structure. Full article

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

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12 pages, 384 KB

Open AccessArticle

Effects of Curvature–Scalar Coupling on Vacuum Energy in Flat (3+1)-Dimensional Space-Time

by Volodymyr Gorkavenko, Oleh Barabash, Pavlo Nakaznyi, Mariia Tsarenkova, Nazar Yakovenko and Andrii Zaporozhchenko

Universe 2026, 12(2), 47; https://doi.org/10.3390/universe12020047 - 11 Feb 2026

Viewed by 398

Abstract

We investigated how a magnetic topological defect affects the vacuum polarization of a charged massive scalar field in a flat

(3 + 1)

-dimensional space-time. The defect was modeled as an impenetrable-to-matter-field, finite-thickness tube with magnetic flux inside. We implemented the [...] Read more.

We investigated how a magnetic topological defect affects the vacuum polarization of a charged massive scalar field in a flat

(3 + 1)

-dimensional space-time. The defect was modeled as an impenetrable-to-matter-field, finite-thickness tube with magnetic flux inside. We implemented the most general form of the Robin boundary condition on the surface of the magnetic tube, which enables a fully general analysis of the problem. We found that in flat space-time, the total vacuum energy generated by a magnetic topological defect depends on the curvature (

ξ

), except for special cases corresponding to the Dirichlet and Neumann boundary conditions. By contrast, when Robin’s general boundary conditions are imposed, the induced vacuum energy acquires an explicit dependence on the curvature coupling (

ξ

), which is significant even in flat space-time. A detailed study of the dependence of the effect on the boundary-condition parameter was carried out. The obtained results highlight the nontrivial role played by boundary conditions in vacuum polarization phenomena. Full article

(This article belongs to the Special Issue BGL2025: The 13th Bolyai–Gauss–Lobachevsky Conference on Non-Euclidean Geometry and Its Applications)

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8 pages, 261 KB

Open AccessCommunication

Scale-Independent Relations Between Neutrino Mass Parameters

by Mu-Chun Chen, Shaheed Perez and Michael Ratz

Universe 2026, 12(2), 46; https://doi.org/10.3390/universe12020046 - 11 Feb 2026

Viewed by 442

Abstract

Theories of flavor operate at various scales. Recently it has been pointed out that in the context of modular flavor symmetries, certain combinations of observables are highly constrained, or even uniquely fixed, by modular invariance and holomorphicity. We find that even in the [...] Read more.

Theories of flavor operate at various scales. Recently it has been pointed out that in the context of modular flavor symmetries, certain combinations of observables are highly constrained, or even uniquely fixed, by modular invariance and holomorphicity. We find that even in the absence of supersymmetry, these combinations are surprisingly immune against quantum corrections. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

16 pages, 579 KB

Open AccessArticle

Thermodynamic Interpretation of the Kompanneets–Chernov–Kantowski–Sachs Solutions

by Salvador Mengual and Joan Josep Ferrando

Universe 2026, 12(2), 45; https://doi.org/10.3390/universe12020045 - 10 Feb 2026

Viewed by 362

Abstract

The spatially homogeneous perfect fluid solutions by Kompanneets–Chernov–Kantowski–Sachs are interpreted as a thermodynamic perfect fluid in isentropic evolution, namely, the isentropic limit of their non-homogeneous generalizations, the T-models. Some specific solutions that model a generic ideal gas are examined, and the associated thermodynamic [...] Read more.

The spatially homogeneous perfect fluid solutions by Kompanneets–Chernov–Kantowski–Sachs are interpreted as a thermodynamic perfect fluid in isentropic evolution, namely, the isentropic limit of their non-homogeneous generalizations, the T-models. Some specific solutions that model a generic ideal gas are examined, and the associated thermodynamic variables are obtained. We show that the necessary macroscopic conditions for physical reality are fulfilled in wide spacetime domains. The field equations for a classical ideal gas are established, and the behavior of the solution is analyzed. The models fulfilling a relativistic

γ

-law are also examined, and the solutions for some particular cases are obtained. Full article

(This article belongs to the Section Gravitation)

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34 pages, 489 KB

Open AccessArticle

Gauge-Invariant Gravitational Wave Polarization in Metric f(R) Gravity with Cosmological Implications

by Ramesh Radhakrishnan, David McNutt, Delaram Mirfendereski, Alejandro Pinero, Eric Davis, William Julius and Gerald Cleaver

Universe 2026, 12(2), 44; https://doi.org/10.3390/universe12020044 - 5 Feb 2026

Viewed by 1070

Abstract

We develop a fully gauge-invariant analysis of gravitational-wave polarizations in metric

f (R)

gravity with a particular focus on the modified Starobinsky model

f (R) = R + α R^{2} - 2 Λ

, whose constant-curvature solution [...] Read more.

We develop a fully gauge-invariant analysis of gravitational-wave polarizations in metric

f (R)

gravity with a particular focus on the modified Starobinsky model

f (R) = R + α R^{2} - 2 Λ

, whose constant-curvature solution

R_{d} = 4 Λ

provides a natural de Sitter background for both early- and late-time cosmology. Linearizing the field equations around this background, we derive the Klein–Gordon equation for the curvature perturbation

δ R

and show that the scalar propagating mode acquires a mass

m_{ψ}^{2} = 1 / (6 α)

, highlighting how the same scalar degree of freedom governs inflationary dynamics at high curvature and the propagation of gravitational waves in the current accelerating Universe. Using the scalar–vector–tensor decomposition and a decomposition of the perturbed Ricci tensor, we obtain a set of fully gauge-invariant propagation equations that isolate the contributions of the scalar, vector, and tensor modes in the presence of matter. We find that the tensor sector retains the two transverse–traceless polarizations of General Relativity, while the scalar sector contains an additional massive scalar propagating degree of freedom, which manifests through breathing and longitudinal tidal responses depending on the wave regime and detector frame. Through the geodesic deviation equation—computed both in a local Minkowski patch and in fully covariant de Sitter form—we independently recover the same polarization content and identify its tidal signatures. The resulting framework connects the extra scalar polarization to cosmological observables: the massive scalar propagating mode sets the range of the fifth force, influences the time evolution of gravitational potentials, and affects the propagation and dispersion of gravitational waves on cosmological scales. This provides a unified, gauge-invariant link between gravitational-wave phenomenology and the cosmological implications of metric

f (R)

gravity. Full article

(This article belongs to the Section Gravitation)

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13 pages, 5709 KB

Open AccessArticle

Utilizing Structured Lasers for High-Precision Alignment in Astronomical Telescopes

by Han Gao, Chunyan Wang, Hao Yang, Zhixu Wu and Zhengyang Li

Universe 2026, 12(2), 43; https://doi.org/10.3390/universe12020043 - 5 Feb 2026

Viewed by 379

Abstract

To address image degradation in optical telescopes with fast focal ratios—a problem caused by the misalignment of optical elements during assembly and observation—this study proposes a high-precision calibration method for image quality detection and correction. The method substitutes parallel laser beams for starlight [...] Read more.

To address image degradation in optical telescopes with fast focal ratios—a problem caused by the misalignment of optical elements during assembly and observation—this study proposes a high-precision calibration method for image quality detection and correction. The method substitutes parallel laser beams for starlight to generate the incident wavefront required for calibration. Low-order aberrations resulting from system misalignment are calculated from the centroid coordinate offsets of laser spots on defocused planes, thereby enabling feedback-controlled alignment adjustments. Simulations and experiments were conducted on a single parabolic mirror system with a diameter (D) of 500 mm and a focal ratio of

F / 3

. The results indicate that for mirror tilt misalignments ranging from

- 2^{\circ}

to

+ 2^{\circ}

, the estimated error for the Zernike coefficients

Z_{4}

–

Z_{6}

is below

0.1 λ

(

λ = 650

nm). This accuracy meets the alignment requirements for telescopes with fast focal ratios and eliminates the need for large flat mirrors and clear night skies, which are traditionally required for outdoor calibration. Consequently, the method provides a low-cost, high-precision solution for the real-time calibration of telescopes at remote sites, such as those in Antarctica. Full article

(This article belongs to the Section Space Science)

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17 pages, 6027 KB

Open AccessArticle

Has Kronos Devoured Planet Nine and Its Epigones?

by Lorenzo Iorio

Universe 2026, 12(2), 42; https://doi.org/10.3390/universe12020042 - 31 Jan 2026

Viewed by 1002

Abstract

The Planet Nine hypothesis encompasses a body of about 5–8 Earth’s masses whose orbital plane would be inclined to the ecliptic by one or two tens of degrees and whose perihelion distance would be as large as about 240–385 astronomical units. Recently, a [...] Read more.

The Planet Nine hypothesis encompasses a body of about 5–8 Earth’s masses whose orbital plane would be inclined to the ecliptic by one or two tens of degrees and whose perihelion distance would be as large as about 240–385 astronomical units. Recently, a couple of his epigones have appeared: Planet X and Planet Y. The former is similar to a minor version of Planet Nine in that all its physical and orbital parameters would be smaller. Instead, the latter would have a mass ranging from that of Mercury to Earth’s and a semimajor axis within 100–200 astronomical units. By using realistic upper bounds for the orbital precessions of Saturn, one can obtain insights on their position which, for Planet Nine, appears approximately confined around its aphelion. Planet Y can only be a Mercury-sized object at no less than about 125 astronomical units, while Planet X appears to be ruled out. Dedicated data reductions by modeling such perturber(s) are required to check the present conclusions, to be intended as hints of what might be detectable should planetary ephemerides include them. A probe on the same route of Voyager 1 would be perturbed by Planet Nine by about 20–40 km after some decades. Full article

(This article belongs to the Section Planetary Sciences)

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19 pages, 5415 KB

Open AccessArticle

Real-Time Detection of the Ground Level Enhancement 74 (GLE74) Event on 11 May 2024 by the A.Ne.Mo.S. GLE Alert++ System

by Maria Gerontidou, Norma B. Crosby, Helen Mavromichalaki, Maria-Christina Papailiou, Pavlos Paschalis and Mark Dierckxsens

Universe 2026, 12(2), 41; https://doi.org/10.3390/universe12020041 - 31 Jan 2026

Viewed by 1043

Abstract

During a period of intense solar activity and highly disturbed geomagnetic conditions, a large Forbush decrease began on 10 May 2024 accompanied by a historic geomagnetic storm that lasted for four days. This extreme geomagnetic disturbance classified as G5 according to “NOAA Space [...] Read more.

During a period of intense solar activity and highly disturbed geomagnetic conditions, a large Forbush decrease began on 10 May 2024 accompanied by a historic geomagnetic storm that lasted for four days. This extreme geomagnetic disturbance classified as G5 according to “NOAA Space Weather Scale for Geomagnetic Storms” is referred to in the literature as the Mother’s Day Storm. This resulted from multiple, at least seven, Coronal Mass Ejections (CMEs) that had been occurring since 7 May. In addition, on 11 May, a powerful X5.8 class solar flare, reaching its maximum at 01:32 UT, was followed by an abrupt increase in proton flux with energies > 100 MeV (with onset on 11 May at 01:45 UT and peaking at 02:45 UT), as recorded by GOES satellites. This resulted in a Ground Level Enhancement (GLE), identified as GLE74, occurring on 11 May 2024 during the recovery phase of the deep Forbush decrease (~15%). This Solar Energetic Particle (SEP) event consisted of both impulsive and gradual components, where the high-energy tail of the gradual component was recorded by several stations of the worldwide ground-based neutron monitor network. Approximately 15 minutes after the onset of the SEP event and 40 minutes prior to its peak, an alert was issued by the GLE Alert++ system of the Athens Neutron Monitor Station of the National and Kapodistrian University of Athens (NKUA), available as a federated product on the ESA SWE Portal under the Space Radiation Expert Service Centre. In this paper, a description of the solar activity, i.e., solar flares and CMEs, occurring during this time period is given. Moreover, recordings of cosmic ray data obtained by ground-based neutron monitors are used to perform a detailed analysis of GLE74. Finally, the response of the NKUA GLE Alert++ system to GLE74 is thoroughly presented. Full article

(This article belongs to the Section Space Science)

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50 pages, 17330 KB

Open AccessArticle

The Scaling Relations of Galaxies with Different Morphology: Comparison Among WINGS, MANGA and Illustris Data Samples

by Mauro D’Onofrio, Francesco Brevi, Cesare Chiosi and Paola Marziani

Universe 2026, 12(2), 40; https://doi.org/10.3390/universe12020040 - 30 Jan 2026

Viewed by 481

Abstract

We present a panoramic view of several scaling relations (ScRs) of galaxies of different morphology. The ScRs are obtained from the data of two large surveys (WINGS and MANGA). We analyze the distribution (parameterized by the percent over the total) of galaxies in [...] Read more.

We present a panoramic view of several scaling relations (ScRs) of galaxies of different morphology. The ScRs are obtained from the data of two large surveys (WINGS and MANGA). We analyze the distribution (parameterized by the percent over the total) of galaxies in each region of the diagnostic planes that are set up by means of suitable physical quantities. In addition to this, we discuss the origin of the differences observed in the ScRs between the two samples. Finally, we compare the observational data with the theoretical ones taken from two subsets of the Illustris large-scale simulations (TNG50 and TNG100), and we discuss how the comparison should be performed for a correct statistical answer. Full article

(This article belongs to the Section Galaxies and Clusters)

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13 pages, 493 KB

Open AccessArticle

The Vortex State of Ultralight Dark Matter and the Fornax Timing Problem

by Volodymyr Gorkavenko, Oleh Barabash, Tetiana Gorkavenko, Kateryna Korshynska, Olena Teslyk, Andrii Zaporozhchenko and Eduard Gorbar

Universe 2026, 12(2), 39; https://doi.org/10.3390/universe12020039 - 30 Jan 2026

Viewed by 474

Abstract

We investigate the impact of the vortex state of ultralight dark matter (ULDM) on the dynamical friction acting on moving globular clusters. Comparing this force with that for the solitonic ground state, it is shown that the internal structure and rotation of the [...] Read more.

We investigate the impact of the vortex state of ultralight dark matter (ULDM) on the dynamical friction acting on moving globular clusters. Comparing this force with that for the solitonic ground state, it is shown that the internal structure and rotation of the ULDM core strongly affect the orbital decay of globular clusters. In particular, co-directional rotation in a vortex state can lead to significant suppression of dynamic friction at certain distances where globular clusters and ULDM velocities match. Applying these findings to the Fornax dwarf galaxy, it is found that the Fornax timing problem is naturally alleviated. Full article

(This article belongs to the Special Issue BGL2025: The 13th Bolyai–Gauss–Lobachevsky Conference on Non-Euclidean Geometry and Its Applications)

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16 pages, 831 KB

Open AccessArticle

Properties of Polarized Radio Sources in the Wide Chandra Deep Field South from 2 to 4 GHz

by Samantha Adams, Mark Lacy, Preshanth Jagannathan, Jose Afonso, William Nielsen Brandt, B. M. Gaensler, Evanthia Hatziminaoglou, Anna Kapinska, Josh Marvil, Hugo Messias, Steve Myers, Ray Norris, Kristina Nyland, Wiphu Rujopakarn, Nick Seymour, Mattia Vaccari and Rick White

Universe 2026, 12(2), 38; https://doi.org/10.3390/universe12020038 - 28 Jan 2026

Viewed by 407

Abstract

We present a study of the linear polarization properties of radio sources within the 10 deg². Wide Chandra Deep Field South (W-CDFS) in S-band (2–4 GHz). Our W-CDFS image has an angular resolution of 15 arcsec and a 1

σ

RMS [...] Read more.

We present a study of the linear polarization properties of radio sources within the 10 deg². Wide Chandra Deep Field South (W-CDFS) in S-band (2–4 GHz). Our W-CDFS image has an angular resolution of 15 arcsec and a 1

σ

RMS in Stokes I of ≈50

μ

Jy/beam. We detect 1920 distinct source components in Stokes I and 175 in linear polarization. We examine the polarized source counts, Faraday Rotation measures, and fractional polarization of the sources in the survey. We show that sources with a total intensity above ≈10 mJy have a mean fractional polarization value of ≈3% from modeling the polarized counts. We also calculate an estimate for the limit on the fractional polarization level of sources with a total intensity below 1 mJy (mostly star-forming galaxies) of ≲3% using stacking. The mean Faraday Rotation we measure is consistent with that due to the Milky Way. We also show that fractional polarization is correlated with in-band spectral index, consistent with a lower mean fractional polarization for the flat-spectrum population. In addition to characterizing the S-band polarization properties of sources in the W-CDFS, this study will be used to validate the shallower, but higher angular resolution S-band polarimetric information that the VLA Sky Survey will provide for the whole sky above Declination −40 degrees over the next few years. Full article

(This article belongs to the Section Galaxies and Clusters)

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13 pages, 4275 KB

Open AccessArticle

Fluctuations of Temperature in the Polyakov Loop-Extended Nambu–Jona-Lasinio Model

by He Liu, Peng Wu, Hong-Ming Liu and Peng-Cheng Chu

Universe 2026, 12(2), 37; https://doi.org/10.3390/universe12020037 - 28 Jan 2026

Viewed by 362

Abstract

In this study, we investigate temperature fluctuations in hot QCD matter using a three-flavor Polyakov loop-extended Nambu–Jona-Lasinio (PNJL) model. The high-order cumulant ratios

R_{n 2}

(

n > 2

) exhibit non-monotonic variations across the chiral phase transition, characterized by slight fluctuations [...] Read more.

In this study, we investigate temperature fluctuations in hot QCD matter using a three-flavor Polyakov loop-extended Nambu–Jona-Lasinio (PNJL) model. The high-order cumulant ratios

R_{n 2}

(

n > 2

) exhibit non-monotonic variations across the chiral phase transition, characterized by slight fluctuations in the chiral crossover region and significant oscillations around the critical point. In contrast, distinct peak and dip structures are observed in the cumulant ratios at low-baryon chemical potential. These structures gradually weaken and eventually vanish at high chemical potential as they compete with the sharpening of the chiral phase transition, particularly near the critical point and the first-order phase transition. Our results indicate that these non-monotonic peak and dip structures in high-order cumulant ratios are associated with the deconfinement phase transition. This study quantitatively analyzes temperature fluctuation behavior across different phase transition regions, and the findings are expected to be observed and validated in heavy-ion collision experiments through measurements of event-by-event mean transverse momentum fluctuations. Full article

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

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12 pages, 506 KB

Open AccessArticle

LAMOST J064137.77+045743.8: A Newly Discovered Binary of an A7 Pulsating Subgiant and a Flaring Red Dwarf

by Yanhui Chen, Chaomi Duan and Baokun Sun

Universe 2026, 12(2), 36; https://doi.org/10.3390/universe12020036 - 27 Jan 2026

Viewed by 407

Abstract

With the progressive release of data from numerous sky surveys, humanity has entered the era of astronomical big data. Multi-wavelength, multi-method research is playing an increasingly crucial role. Binaries account for a substantial fraction of all stellar systems, and research into binaries is [...] Read more.

With the progressive release of data from numerous sky surveys, humanity has entered the era of astronomical big data. Multi-wavelength, multi-method research is playing an increasingly crucial role. Binaries account for a substantial fraction of all stellar systems, and research into binaries is of fundamental importance. The low-resolution spectra from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) suggest that LAMOST J064137.77+045743.8 is a binary consisting of an A7-type subgiant star and a cool red dwarf star. LAMOST J064137.77+045743.8 has not yet been recorded in the SIMBAD astronomical database. We conducted a comprehensive analysis of the binary based on multi-wavelength and multi-method research. The spectral analysis suggests that the A7-type subgiant primary star has parameters of

T_{eff}

∼ 7500 K and log g ∼ 3.9, and the red dwarf companion star is cool. Additional flux observations in the infrared bands further corroborate the presence of the red dwarf companion, and the near-infrared color index indicates a K4-type red dwarf. Astrometric data from Gaia support the binary speculation with a Renormalized Unit Weight Error metric value of 1.9. The i-band flare detected by the Zwicky Transient Facility (ZTF) photometric observations bolsters the interpretation of the M- or K-type red dwarf companion. Both the radial velocity variations in the H

α

lines from LAMOST medium-resolution spectra and the light curves from ZTF support the classification of the A7 subgiant as a pulsating star. No clear evidence of binary eclipses was detected in 1789 days of photometric observations from the ZTF. Future asteroseismology studies will enable us to further probe the internal physics of the A7 subgiant primary star. Full article

(This article belongs to the Section Solar and Stellar Physics)

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20 pages, 358 KB

Open AccessArticle

A Note on the Existence of Equal-Time Correlators

by Bruno Bucciotti

Universe 2026, 12(2), 35; https://doi.org/10.3390/universe12020035 - 27 Jan 2026

Viewed by 316

Abstract

We study the conditions under which momentum-space equal-time correlators of scalar fields are finite in flat space. We identify cases where these correlators can be divergent even after renormalization, and we provide sufficient criteria for their existence. Concrete examples are discussed, including the [...] Read more.

We study the conditions under which momentum-space equal-time correlators of scalar fields are finite in flat space. We identify cases where these correlators can be divergent even after renormalization, and we provide sufficient criteria for their existence. Concrete examples are discussed, including the well-known

λ ϕ^{4}

model, composite operators, and effective field theories. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

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22 pages, 31479 KB

Open AccessArticle

Bayesian Inference of Primordial Magnetic Field Parameters from CMB with Spherical Graph Neural Networks

by Juan Alejandro Pinto Castro, Héctor J. Hortúa, Jorge Enrique García-Farieta and Roger Anderson Hurtado

Universe 2026, 12(2), 34; https://doi.org/10.3390/universe12020034 - 26 Jan 2026

Viewed by 639

Abstract

Deep learning has emerged as a transformative methodology in modern cosmology, providing powerful tools to extract meaningful physical information from complex astronomical data. This paper implements a novel Bayesian graph deep learning framework for estimating key cosmological parameters in a primordial magnetic field [...] Read more.

Deep learning has emerged as a transformative methodology in modern cosmology, providing powerful tools to extract meaningful physical information from complex astronomical data. This paper implements a novel Bayesian graph deep learning framework for estimating key cosmological parameters in a primordial magnetic field (PMF) cosmology from simulated Cosmic Microwave Background (CMB) maps. Our methodology utilizes DeepSphere, a spherical convolutional neural network architecture specifically designed to respect the spherical geometry of CMB data through HEALPix pixelization. To advance beyond deterministic point estimates and enable robust uncertainty quantification, we integrate Bayesian Neural Networks (BNNs) into the framework, capturing aleatoric and epistemic uncertainties that reflect the model confidence in its predictions. The proposed approach demonstrates exceptional performance, achieving

R^{2}

scores exceeding 89% for the magnetic parameter estimation. We further obtain well-calibrated uncertainty estimates through post hoc training techniques including Variance Scaling and GPNormal. This integrated DeepSphere-BNNs framework delivers accurate parameter estimation from CMB maps with PMF contributions while providing reliable uncertainty quantification, enabling robust cosmological inference in the era of precision cosmology. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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18 pages, 387 KB

Open AccessArticle

Transfer of Quantum Information and Genesis of Superfluid Vacuum in the Pre-Inflationary Universe

by Konstantin G. Zloshchastiev

Universe 2026, 12(2), 33; https://doi.org/10.3390/universe12020033 - 26 Jan 2026

Viewed by 1163

Abstract

We conjecture that during the time period preceding the inflationary epoch, the background matter was initially a condensate formed from a many-body system of indistinguishable particles whose states were in a quantum superposition. This resulted in the occurrence of a statistical ensemble of [...] Read more.

We conjecture that during the time period preceding the inflationary epoch, the background matter was initially a condensate formed from a many-body system of indistinguishable particles whose states were in a quantum superposition. This resulted in the occurrence of a statistical ensemble of spacetimes, thus causing the probabilistic uncertainty in the spacetime geometry of the pre-inflationary multiverse. Then, at a certain moment in time, a measurement event occurred, which broke the linear superposition and reduced the primordial geometrical multiverse to a single state. This process can be described as a quantum Shannon information transfer, which induces logarithmic nonlinearity in the evolution equations of the background system. The latter, therefore, transformed into a logarithmic quantum liquid of a superfluid type and formed the physical vacuum. This measurement also generated the primary mass energy necessary for the Universe’s further evolution into the inflationary epoch, followed by the contemporary “dark energy” era. Full article

(This article belongs to the Section Cosmology)

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18 pages, 331 KB

Open AccessEssay

Novikov Coordinates and the Physical Description of Gravitational Collapse

by Jaume de Haro

Universe 2026, 12(2), 32; https://doi.org/10.3390/universe12020032 - 26 Jan 2026

Viewed by 426

Abstract

We show that the Novikov coordinates can be obtained in a direct and physically transparent way from the radial geodesics of massive particles with negative energy in the Schwarzschild spacetime. These geodesics form a complete congruence that covers the entire spacetime. By rectifying [...] Read more.

We show that the Novikov coordinates can be obtained in a direct and physically transparent way from the radial geodesics of massive particles with negative energy in the Schwarzschild spacetime. These geodesics form a complete congruence that covers the entire spacetime. By rectifying this family of trajectories using the proper time as the time coordinate, the Novikov variables naturally emerge, providing a clear dynamical interpretation of the different regions usually identified as black-hole and white-hole sectors. In Novikov coordinates, observers at fixed spatial positions follow free-fall trajectories. From their perspective, the gravitational collapse of a dust star is completed in a finite proper time, independently of their initial distance from the star. In contrast, observers described by Schwarzschild–Droste coordinates perceive the boundary of the collapsing star as taking an infinite coordinate time to reach the horizon. We emphasize that Schwarzschild–Droste observers are static with respect to the center of mass of the star and therefore cannot be in free fall. The use of these coordinates implicitly requires the presence of a force that compensates for the gravitational attraction. From this viewpoint, the apparent infinite-time collapse is not a physical effect but a coordinate artifact associated with non-inertial observers. Full article

(This article belongs to the Section Gravitation)

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27 pages, 6867 KB

Open AccessArticle

Recovering Gamma-Ray Burst Redshift Completeness Maps via Spherical Generalized Additive Models

by Zsolt Bagoly and Istvan I. Racz

Universe 2026, 12(2), 31; https://doi.org/10.3390/universe12020031 - 24 Jan 2026

Viewed by 403

Abstract

We present an advanced statistical framework for estimating the relative intensity of astrophysical event distributions (e.g., Gamma-Ray Bursts, GRBs) on the sky tofacilitate population studies and large-scale structure analysis. In contrast to the traditional approach based on the ratio of Kernel Density Estimation [...] Read more.

We present an advanced statistical framework for estimating the relative intensity of astrophysical event distributions (e.g., Gamma-Ray Bursts, GRBs) on the sky tofacilitate population studies and large-scale structure analysis. In contrast to the traditional approach based on the ratio of Kernel Density Estimation (KDE), which is characterized by numerical instability and bandwidth sensitivity, this work applies a logistic regression embedded in a Bayesian framework to directly model selection effects. It reformulates the problem as a logistic regression task within a Generalized Additive Model (GAM) framework, utilizing isotropic Splines on the Sphere (SOS) to map the conditional probability of redshift measurement. The model complexity and smoothness are objectively optimized using Restricted Maximum Likelihood (REML) and the Akaike Information Criterion (AIC), ensuring a data-driven bias-variance trade-off. We benchmark this approach against an Adaptive Kernel Density Estimator (AKDE) using von Mises–Fisher kernels and Abramson’s square root law. The comparative analysis reveals strong statistical evidence in favor of this Preconditioned (Precon) Estimator, yielding a log-likelihood improvement of

Δ L \approx 74.3

(Bayes factor

> 10^{30}

) over the adaptive method. We show that this Precon Estimator acts as a spectral bandwidth extender, effectively decoupling the wideband exposure map from the narrowband selection efficiency. This provides a tool for cosmologists to recover high-frequency structural features—such as the sharp cutoffs—that are mathematically irresolvable by direct density estimators due to the bandwidth limitation inherent in sparse samples. The methodology ensures that reconstructions of the cosmic web are stable against Poisson noise and consistent with observational constraints. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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13 pages, 1409 KB

Open AccessArticle

Revisiting a Quasar Microlensing Event Towards AGN J1249+3449

by Mario Cazzolla, Francesco De Paolis, Antonio Franco and Achille Nucita

Universe 2026, 12(2), 30; https://doi.org/10.3390/universe12020030 - 24 Jan 2026

Viewed by 458

Abstract

The gravitational wave event GW190521 seems to be the only BH merger event possibly correlated with an electromagnetic counterpart, which appeared about 34 days after the GW event. This work aims to confirm that the electromagnetic bump towards the Active Galactic Nucleus (AGN) [...] Read more.

The gravitational wave event GW190521 seems to be the only BH merger event possibly correlated with an electromagnetic counterpart, which appeared about 34 days after the GW event. This work aims to confirm that the electromagnetic bump towards the Active Galactic Nucleus (AGN) J1249+3449 can be explained within the framework of the gravitational microlensing phenomenon. In particular, considering the data of the Zwicky Transient Facility (ZTF), what emerges from a detailed analysis of the observed light curve using three fitting models (Point Source Point Lens, Finite Source Point Lens, Uniform Source Binary Lens) is that the optical bump can be explained as a microlensing event caused by a lens with mass

\sim

0.1

M_{⊙}

, lying in the host galaxy of the AGN in question. Full article

(This article belongs to the Special Issue Recent Advances in Gravitational Lensing and Galactic Dynamics)

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23 pages, 13361 KB

Open AccessArticle

Conceptual Design and Structural Assessment of a Hemispherical Two-Chamber Water Cherenkov Detector for Extensive Air-Shower Arrays

by Jasmina Isaković, Marina Manganaro and Michele Doro

Universe 2026, 12(2), 29; https://doi.org/10.3390/universe12020029 - 24 Jan 2026

Viewed by 484

Abstract

A conceptual design study is presented for a hemispherical, two-chamber water Cherenkov detector instrumented with bladder-embedded light traps. The detector consists of a rigid aluminium vessel enclosing a water volume that is divided into an outer, optically black chamber and a inner, reflective [...] Read more.

A conceptual design study is presented for a hemispherical, two-chamber water Cherenkov detector instrumented with bladder-embedded light traps. The detector consists of a rigid aluminium vessel enclosing a water volume that is divided into an outer, optically black chamber and a inner, reflective chamber lined by a flexible bladder. Arrays of light-trap modules, based on plastic scintillators with wavelength-shifting elements and thin silicon photomultipliers, are integrated into the bladder and selected inner surfaces. This geometry is intended to enhance muon tagging, increase acceptance for inclined air showers, and enable improved discrimination between electromagnetic and hadronic components. The study describes the mechanical and optical layout of the detector, the baseline aluminium housing, and the use of 3D-printed hexagonal prototypes to validate integration of the bladder and readout electronics. A first-order structural assessment based on thin-shell and plate theory is presented, indicating large safety margins for the hemispherical shells and identifying the flat base as the mechanically most loaded component. While GEANT4 simulations for detector response to extensive air showers in the atmosphere and performance measurements are left to future work, the present study establishes a mechanically validated, costed baseline design and outlines the steps needed to assess its impact in air-shower arrays. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

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Universe, Volume 12, Issue 2 (February 2026) – 31 articles

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