Universe

33 pages, 19262 KB

Open AccessArticle

A Decade of Radial-Velocity Monitoring of ρ Leo: Moment Analysis and Periodic Variability

by Vitalii Checha, Anna Aret, Indrek Kolka, Tiina Liimets, Veronika Mitrokhina, Anni Kasikov, Tõnis Eenmäe, Sandipan P. D. Borthakur and Heleri Ramler

Universe 2026, 12(9), 263; https://doi.org/10.3390/universe12090263 - 30 Aug 2026

Abstract

We investigate the origin of long-term spectroscopic and photometric variability in the blue supergiant

ρ

Leo, with particular emphasis on distinguishing between intrinsic pulsations and variability induced by a possible companion. Our analysis is based on an 11.5-year spectroscopic time series obtained at [...] Read more.

We investigate the origin of long-term spectroscopic and photometric variability in the blue supergiant

ρ

Leo, with particular emphasis on distinguishing between intrinsic pulsations and variability induced by a possible companion. Our analysis is based on an 11.5-year spectroscopic time series obtained at Tartu Observatory, complemented by high-cadence, high-resolution spectroscopy from the Hertzsprung SONG telescope and space-based photometry from TESS. We studied line-profile variability using normalised moments of the He i

λ

6678, He i

λ

5875, and Si iii

λ

4552 lines. Periodic signals were identified using the generalised Lomb–Scargle periodogram with iterative pre-whitening, and their temporal stability was examined with the weighted wavelet Z-transform. We detect a persistent periodic signal at

P = 16.46

d in the first and third moments, present throughout the full observing interval, with a radial-velocity amplitude of 3.9 km/s. This signal is also present in the SONG data and is visible in multiple spectral lines, indicating a global origin. Photometric observations reveal a dominant variability timescale near ≈33 d, approximately twice the spectroscopic period. The stable 16.46-day period, present throughout the entire observing interval, most likely results from non-radial pulsations of the supergiant. A binary origin of the signal is not excluded, but distinguishing between these scenarios is complicated by the supergiant’s complex variability pattern. Full article

(This article belongs to the Special Issue Asteroseismology: Probing Stellar Interiors Through Oscillation Modes)

12 pages, 7793 KB

Open AccessArticle

Vortex Solutions of Ultralight BEC Dark Matter and Structures of Galactic Size: The Test Case of the Ring Galaxy ARP 147

by Carlos Tena-Contreras, Iván Álvarez-Rios and Francisco S. Guzmán

Universe 2026, 12(9), 262; https://doi.org/10.3390/universe12090262 - 29 Aug 2026

Abstract

We present a non-collisional mechanism for the formation of symmetric ring galaxies based on the dynamical relaxation of gas on top of quantized vortex configurations in Bose–Einstein Condensate Dark Matter (BECDM) solutions. For this we solve the fully coupled Gross–Pitaevskii–Poisson–Euler (GPPE) system in [...] Read more.

We present a non-collisional mechanism for the formation of symmetric ring galaxies based on the dynamical relaxation of gas on top of quantized vortex configurations in Bose–Einstein Condensate Dark Matter (BECDM) solutions. For this we solve the fully coupled Gross–Pitaevskii–Poisson–Euler (GPPE) system in 3D, allowing for complete gravitational back-reaction between the baryonic gas and the dark matter component. Initializing the luminous matter as an ideal gas with random initial conditions on top of a vortex line with a topological winding number

m = 1

, we systematically explore three dark matter self-interaction regimes, attractive, collisionless, and repulsive, for a characteristic ultra-light boson mass of

m_{b} = 10^{- 22} eV

. Our simulations reveal that the baryonic gas relaxes into stable, highly symmetric rings whose physical diameters from 11 to 16 kpc and total enclosed masses of order

10^{10} M_{⊙}

match the scales of intensely studied benchmarks like Arp 147. We use the detailed data from Arp 147 purely as a physical baseline to provide a structural proof of concept for our model. However, this self-consistent mechanism can be an interesting explanation for isolated ring galaxies-like Hoag-type objects, which lack the nearby companions or tidal debris required by standard collision models. Full article

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

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23 pages, 4010 KB

Open AccessArticle

Centrality Evolution of Transverse Momentum Spectra of Identified Light-Flavor Hadrons in p+Pb Collisions at

\sqrt{s_{n n}}

= 5.02 TeV at the LHC Within a Two-Component Model

by Khusniddin K. Olimov, Anastasiya Fedosimova, Fu-Hu Liu, Kobil A. Musaev, Igor A. Lebedev, Shokhida A. Khudoyberdieva, Azizjon Tokhirov, Sayora Ibraimova and Ekaterina Bondar

Universe 2026, 12(9), 261; https://doi.org/10.3390/universe12090261 - 28 Aug 2026

Abstract

Using the two-component Tsallis-Hagedorn Model with Transverse Flow (THMTF), this study analyzes centrality dependencies of midrapidity transverse momentum (

p_{T}

) spectra for identified light-flavor hadrons in asymmetric p+Pb collisions at

\sqrt{s_{n n}}

= 5.02 TeV, comparing them with [...] Read more.

Using the two-component Tsallis-Hagedorn Model with Transverse Flow (THMTF), this study analyzes centrality dependencies of midrapidity transverse momentum (

p_{T}

) spectra for identified light-flavor hadrons in asymmetric p+Pb collisions at

\sqrt{s_{n n}}

= 5.02 TeV, comparing them with symmetric Pb+Pb collision data at

\sqrt{s_{n n}}

= 2.76 and 5.02 TeV at the LHC. The analysis successfully describes ALICE experimental data up to

p_{T}

= 20 GeV/c across seven multiplicity classes in p+Pb collisions, estimating that collective flow emerges at

⟨ N_{p a r t} ⟩

≈ 4.0 ± 0.5 along with simultaneous transitioning of kinetic freeze-out temperature to a nearly plateau region. The kinetic freeze-out temperature parameter shows distinct, system-dependent behavior before stabilization with an increase in

⟨ N_{p a r t} ⟩

in p+Pb and Pb+Pb collisions at the LHC. Furthermore, the findings suggest that collective flow and particle production correlate with increased thermalization, with spectral analysis indicating different dynamics at hard

p_{T}

region between p+Pb and Pb+Pb collision systems. Full article

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

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35 pages, 875 KB

Open AccessReview

Dense and Cold Magnetized Quark Matter: A Review of Magnetic-Field-Independent Regularization and the Medium Separation Scheme

by Francisco X. Azeredo, Dyana C. Duarte, Ricardo L. S. Farias, Bruno S. Lopes, João A. R. S. Prado and William R. Tavares

Universe 2026, 12(9), 260; https://doi.org/10.3390/universe12090260 - 27 Aug 2026

Abstract

We present a comprehensive review of regularization schemes for magnetized dense quark matter within effective models of quantum chromodynamics, focusing on the Magnetic-Field-Independent Regularization (MFIR) and the Medium Separation Scheme (MSS) at finite chemical potential and magnetic field. In nonrenormalizable frameworks such as [...] Read more.

We present a comprehensive review of regularization schemes for magnetized dense quark matter within effective models of quantum chromodynamics, focusing on the Magnetic-Field-Independent Regularization (MFIR) and the Medium Separation Scheme (MSS) at finite chemical potential and magnetic field. In nonrenormalizable frameworks such as the Nambu–Jona-Lasinio model, the treatment of ultraviolet divergences is crucial, particularly in magnetized and dense environments where conventional regularization procedures may introduce unphysical artifacts. We show that MFIR consistently isolates divergent vacuum contributions from finite magnetic-field-dependent terms, while MSS extends this separation to the medium sector, ensuring that only vacuum quantities are regularized. Within this unified framework, we analyze the thermodynamics of cold and dense quark matter, including color-superconducting phases, and demonstrate that the superconducting gap remains finite at large chemical potentials, even in the presence of strong magnetic fields. In contrast to results obtained with traditional regularization schemes, we find no evidence for a transition to a normal phase at zero temperature, highlighting the importance of a proper separation between vacuum and medium contributions. These results eliminate spurious oscillations and other nonphysical artifacts, leading to a more robust and physically consistent description of strongly interacting matter under extreme conditions relevant to compact stars and heavy-ion collisions. Full article

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

60 pages, 8195 KB

Open AccessReview

Integrated Galactic Archaeology: An Inverse-Problem Framework for Galaxy Evolution

by Tsutomu T. Takeuchi, Karin T. Sakuragi, Ryusei R. Kano and Sena A. Matsui

Universe 2026, 12(9), 259; https://doi.org/10.3390/universe12090259 - 27 Aug 2026

Abstract

Integrated-light spectral energy distribution modeling is widely used to infer the star-formation and assembly histories of galaxies that cannot be resolved into individual stars. However, existing approaches are often discussed primarily in terms of particular fitting codes, star-formation-history parameterizations, or inference algorithms. In [...] Read more.

Integrated-light spectral energy distribution modeling is widely used to infer the star-formation and assembly histories of galaxies that cannot be resolved into individual stars. However, existing approaches are often discussed primarily in terms of particular fitting codes, star-formation-history parameterizations, or inference algorithms. In this Review, we formulate the recovery of galaxy evolution histories from integrated spectral energy distributions as a unified inverse problem. We separate the physical spectral-generation operator from the observational operator and examine the resulting information loss through non-identifiability, singular-value structure, null directions, effective resolution, regularization, and model discrepancy. We then classify parametric and nonparametric star-formation histories, PCA, MOPED, VESPA, non-negative matrix factorization, deep learning, and simulation-based inference within a common five-component framework consisting of the representation space, forward operator, physical or statistical constraints, inference method, and uncertainty assessment. On this basis, we introduce information-driven adaptive representation as a general design principle in which the complexity of the recovered history is matched to the information supported by the observations. Finally, we extend the framework from star-formation histories to coupled galaxy-evolution states involving chemical enrichment, dust evolution, interstellar-medium conditions, and radiative transfer, and outline a three-layer research program linking controlled mock experiments, inverse-problem theory, and physical forward modeling. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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16 pages, 2369 KB

Open AccessArticle

Oort Cloud Comets: Perturbations Due to the Passage of Gliese 710

by Elke Pilat-Lohinger, Maximilian Zimmermann and Birgit Loibnegger

Universe 2026, 12(9), 258; https://doi.org/10.3390/universe12090258 - 27 Aug 2026

Abstract

The outermost region of the Solar System is called the Oort cloud, which is the Solar System’s reservoir of long-period comets that extends to distances up to 200,000 au from the Sun. Long-period comets can be injected towards the inner Solar System due [...] Read more.

The outermost region of the Solar System is called the Oort cloud, which is the Solar System’s reservoir of long-period comets that extends to distances up to 200,000 au from the Sun. Long-period comets can be injected towards the inner Solar System due to galactic tides and passing stars. While galactic tides are effective on long time scales, perturbations of stellar flybys are short-term effects that have not only occurred in the past. Gaia observations confirmed that the Solar System will experience a close flyby of the K-type star Gliese 710 in about 1.29 million years. This star will probably pass at a distance of about 10,500 au to the Sun. When crossing the Oort cloud, this 0.6 solar mass star will perturb the long-period comets, especially those objects that are close to Gliese 710’s trajectory. In this numerical investigation, we use our recently developed GPU-based N-body code GANBISS and study the orbits of 100 million test-comets for the time when the star enters the Oort cloud until its closest approach to the Sun, which takes about 32,000 years. The simulations show that the stellar passage generates cometary streams, which either transport comets into the inner Solar System or scatter a huge number of comets into interstellar space. Of the 100 million comets, about one third were scattered into interstellar space, and only 63,453 comets were directed toward the Sun, of which 1662 entered the inner Solar System within 2 au. Comet streams into this region would be possible over a period of 45 million years. After Gliese 710’s flyby, we investigated the influence of giant planets. This showed that the giant planets protect us from massive comet stremas. The first comet with a perihelion distance close to the Earth’s orbit will take about 1.505 million years to reach its perihelion after Gliese 710’s flyby. Full article

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

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20 pages, 2484 KB

Open AccessArticle

Exploring Late Stellar Evolution in the Era of Large Surveys: Machine Learning Prospects for Hot Subdwarfs and White Dwarfs

by Princy Ranaivomanana and Murat Uzundag

Universe 2026, 12(9), 257; https://doi.org/10.3390/universe12090257 - 26 Aug 2026

Abstract

The rapid growth of large-scale astronomical surveys and advances in data-driven analysis techniques have transformed the study of late-stage stellar evolution. Modern facilities are producing large volumes of photometric, spectroscopic, and astrometric data, enabling systematic investigations of compact stellar populations across the Milky [...] Read more.

The rapid growth of large-scale astronomical surveys and advances in data-driven analysis techniques have transformed the study of late-stage stellar evolution. Modern facilities are producing large volumes of photometric, spectroscopic, and astrometric data, enabling systematic investigations of compact stellar populations across the Milky Way. Among the most important tracers of these advanced evolutionary phases are hot subdwarfs and white dwarfs: hot subdwarfs are core-helium-burning tracers of late, binary-driven stellar evolution, while white dwarfs represent the final evolutionary endpoint of low- and intermediate-mass stars. These compact objects provide important laboratories for studying stellar interiors, binary evolution, and the long-term fate of planetary systems. This paper explores how recent advances in machine learning are being applied to the detection, characterization, and, when combined with follow-up spectroscopy and modeling, the physical interpretation of hot subdwarfs and white dwarfs. By combining photometric, spectroscopic, and time-domain observations with these computational tools, it is now possible to efficiently discover rare objects, detect stellar variability, and probe the internal structure and evolutionary pathways of compact stars. Ultimately, these developments highlight the growing role of advanced algorithms in supporting the study of the final stages of stellar evolution, provided their outputs are validated against physical observables. Full article

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

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10 pages, 692 KB

Open AccessArticle

Spectro-Polarimetric Properties of CHIME FRB Sources

by Dengke Zhou, Yi Feng, Jiaying Xu, Chenyuan Xu and Jianhua Fang

Universe 2026, 12(9), 256; https://doi.org/10.3390/universe12090256 - 26 Aug 2026

Abstract

Fast radio bursts (FRBs) are enigmatic millisecond-duration radio transients whose polarization properties offer crucial insights into their origins and environments. In particular, low-frequency depolarization—quantified by the parameter

σ_{RM}

—probes the complex magneto-ionic medium surrounding the progenitor, and has been observed across a [...] Read more.

Fast radio bursts (FRBs) are enigmatic millisecond-duration radio transients whose polarization properties offer crucial insights into their origins and environments. In particular, low-frequency depolarization—quantified by the parameter

σ_{RM}

—probes the complex magneto-ionic medium surrounding the progenitor, and has been observed across a population of repeating FRBs. We present a systematic spectro-polarimetric analysis of repeating and non-repeating FRBs using observations from the Canadian Hydrogen Intensity Mapping Experiment (CHIME). For 28 repeating FRBs, we measure

σ_{RM}

, expanding the known sample from 14 to 36 sources (an increase by a factor of 2.6). The kernel density estimate (KDE) of the repeating population peaks at

1.3 rad m^{- 2}

, with approximately 70% of the sources showing

σ_{RM} ≳ 1 rad m^{- 2}

, implying that most reside in complex magneto-ionic environments. For 70 non-repeating FRBs, we investigate four spectro-polarimetric models; no source exhibits significant depolarization with

σ_{RM} ≳ 5 rad m^{- 2}

. Roughly half of the non-repeaters are consistent with a constant linear polarization fraction across frequency. We caution, however, that these results may be affected by the limited frequency coverage of CHIME. Future ultra-wideband polarimetry, spanning widely separated frequencies, will overcome current observational biases, enable precise

σ_{RM}

measurements, and substantially deepen our understanding of FRB environments. Full article

(This article belongs to the Special Issue Fast Radio Bursts in the Era of Multi-Messenger Astrophysics)

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3 pages, 168 KB

Open AccessEditorial

Editorial for the Special Issue “Ultra-High-Energy Cosmic Rays”

by Haoning He

Universe 2026, 12(9), 255; https://doi.org/10.3390/universe12090255 - 25 Aug 2026

Abstract

The origin of ultra-high-energy cosmic rays (UHECRs), with energies above ∼

1

Full article

(This article belongs to the Special Issue Ultra-High-Energy Cosmic Rays)

21 pages, 3585 KB

Open AccessArticle

State-Dependent Optical Flickering in the Recurrent Nova T Coronae Borealis from TESS Photometry

by Xiaowan Zhang, Songpeng Pei, Xiaoqin Ren and Mingyue Li

Universe 2026, 12(9), 254; https://doi.org/10.3390/universe12090254 - 24 Aug 2026

Abstract

T Coronae Borealis (T CrB) is a symbiotic recurrent nova that is approaching its next thermonuclear eruption after a long super-active accretion state and a pre-eruption dip state. Flickering has been observed in T CrB for decades, but its amplitude in the red-optical [...] Read more.

T Coronae Borealis (T CrB) is a symbiotic recurrent nova that is approaching its next thermonuclear eruption after a long super-active accretion state and a pre-eruption dip state. Flickering has been observed in T CrB for decades, but its amplitude in the red-optical TESS band and its dependence on accretion state have not yet been characterized using a homogeneous set of short-cadence space photometry. We analyze TESS Science Processing Operations Center light curves of T CrB obtained in Sectors 24, 25, 51, and 78. The primary measurements use the 120 s PDCSAP_FLUX products; agreement with independently detrended SAP_FLUX, 20 s products, and target-pixel checks demonstrates that the rapid signal is not created by Presearch Data Conditioning (PDC) processing or aperture contamination. After normalizing each light curve by its median flux, we remove slow sector-scale variability with a 0.5 d segment-wise smooth trend and measure the excess fractional rms variability, binned rms amplitudes, residual distributions, and power spectral densities. TESS detects millimagnitude-level stochastic variability in all four sectors. The total-light excess fractional rms values are

{0.273}_{- 0.014}^{+ 0.013}

,

{0.276}_{- 0.010}^{+ 0.010}

,

{0.318}_{- 0.012}^{+ 0.013}

, and

{0.215}_{- 0.017}^{+ 0.013}

per cent in Sectors 24, 25, 51, and 78, respectively, equivalent to 2.96, 3.00, 3.45, and 2.33 mmag. The power spectral densities are red-noise-like, with slopes from

α = - 1.56 \pm 0.09

to

- 1.19 \pm 0.08

over

5 < f < 120 d^{- 1}

. The largest TESS-band flickering amplitude occurs in Sector 51, while the lowest occurs in Sector 78, observed in May 2024 after the end of the 2015–2023 super-active state and after the main pre-eruption dip. The persistence of flickering in Sector 78 shows that rapidly variable accretion continued, but the reduced amplitude indicates that the red-optical variable component was weaker, more diluted by the red giant, or geometrically altered. Because the TESS passband is dominated by the cool giant, the intrinsic fractional variability of the accretion component must be larger than the observed total-light values. Full article

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

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21 pages, 1195 KB

Open AccessArticle

KAN-Payne: A Controlled Evaluation of Kolmogorov–Arnold Networks for Stellar Spectral Emulation and Label Recovery

by Shuo Zhang and Rui Wang

Universe 2026, 12(9), 253; https://doi.org/10.3390/universe12090253 - 23 Aug 2026

Abstract

Kolmogorov–Arnold networks (KANs) replace the fixed activations of a multilayer perceptron (MLP) with learnable univariate edge functions. We evaluate KANs as Payne-style label-to-flux emulators using the public 1000-spectrum Kurucz grid released with The Payne. Two capacity-matched KAN–MLP pairs (approximately 2.3 and 18–19 million [...] Read more.

Kolmogorov–Arnold networks (KANs) replace the fixed activations of a multilayer perceptron (MLP) with learnable univariate edge functions. We evaluate KANs as Payne-style label-to-flux emulators using the public 1000-spectrum Kurucz grid released with The Payne. Two capacity-matched KAN–MLP pairs (approximately 2.3 and 18–19 million parameters) were tuned separately, trained for the same 120,000-update budget across three seeds, and evaluated on a fixed 200-spectrum holdout. At the smaller capacity, the KAN did not outperform the tuned MLP. At the larger capacity, KAN-L achieved the lowest flux errors among the models trained from scratch: its mean absolute error was 35% below the best small model, all three large-MLP layouts remained less accurate, and its residual tails were lighter. In injection–recovery tests, KAN-L improved the recovery of effective temperature, surface gravity, and metallicity at all the tested signal-to-noise ratios relative to the small matched MLP; the best large-MLP control was not included in this test. The learned KAN response shapes were reproducible across seeds. These gains required substantially greater training and inference costs. In an APOGEE Data Release 17 deployment, surface-gravity offsets decreased by approximately 0.09 dex for all the tested emulators when calibrated APOGEE Stellar Parameter and Chemical Abundances Pipeline (ASPCAP) values were replaced by the raw spectroscopic scale, indicating a common reference-scale contribution. KANs are therefore not efficient drop-in replacements for MLP emulators, but the large KAN tested here improved spectral emulation accuracy and residual-tail robustness. Full article

(This article belongs to the Special Issue New Discoveries in Astronomical Data (II))

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6 pages, 188 KB

Open AccessEditorial

Editorial for the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters"

by Mauro D’Onofrio

Universe 2026, 12(8), 252; https://doi.org/10.3390/universe12080252 - 20 Aug 2026

Abstract

Galaxies and galaxy clusters represent the fundamental visible components of the cosmic web and provide unique laboratories for investigating the physical processes that shape the Universe across a vast range of spatial and temporal scales [...] Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

31 pages, 1546 KB

Open AccessArticle

Star Formation Efficiency and Class I Protostellar Timescales in ATLASGAL Dense Clumps

by Moses Onyemaechi Asogwa, Seblu Humne Negu, Gemechu Muleta Kumssa and Innocent Okwudili Eya

Universe 2026, 12(8), 251; https://doi.org/10.3390/universe12080251 - 18 Aug 2026

Abstract

Star formation in galactic dense clumps is commonly interpreted using nearly uniform protostellar evolutionary timescales, yet the extent to which such assumptions obscure variations in star formation efficiency remains uncertain. Using 60 ATLASGAL dense clumps associated with MIPSGAL Class I protostars and [...] Read more.

Star formation in galactic dense clumps is commonly interpreted using nearly uniform protostellar evolutionary timescales, yet the extent to which such assumptions obscure variations in star formation efficiency remains uncertain. Using 60 ATLASGAL dense clumps associated with MIPSGAL Class I protostars and

{NH}_{3}

velocity information, we show that compactness and dense-gas evolutionary state provide a stronger explanation of instantaneous and cumulative star formation behavior than adopting a universal Class I lifetime. By combining cumulative efficiencies with a dense-gas star formation calibration, we find that star formation proceeds with systematically mass- and density-dependent timescales, implying that a single evolutionary clock can significantly bias inferred efficiencies across the clump population. The lower-limit cumulative star formation efficiency is observed to increase with decreasing clump radius following

R_{cl}^{- 1.30 \pm 0.09}

, while no significant correlation is found with a Galactocentric radius. Upper- and lower-limit cumulative efficiencies exhibit a sublinear relation with slope

0.66 \pm 0.08

, suggesting possible stellar initial mass function incompleteness. The dense-gas star formation timescale follows

τ_{SF, dense} \propto M_{cl}^{- 0.77 \pm 0.04}

, with a median value of

0.54 Myr

. Assuming a relatively uniform timescale of

0.50 Myr

could overestimate and underestimate star formation rates in low-mass and massive clumps by factors of ∼32 and ∼25, respectively. Full article

(This article belongs to the Section Galaxies and Clusters)

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20 pages, 806 KB

Open AccessArticle

A Machine-Readable Value-Added Catalog for Uncertain-Type Blazars in the Fifth Roma-BZCAT

by Jingtian Zhu, Xinwen Shu, Jun Zhang, Junhui Fan, Hubing Xiao, Haitao Cao, Guo Li, Yang Wen, Xianyao Zhang, Aibo Liu, Yuhui Song, Fengchun Tao, Qian Wang, Tianbo Xu, Kun Ou and Denis Bastieri

Universe 2026, 12(8), 250; https://doi.org/10.3390/universe12080250 - 17 Aug 2026

Abstract

The fifth edition of the Roma-BZCAT catalog (5BZCAT) contains 227 blazars of the uncertain type (BZU), whose available data do not support an unambiguous assignment to the BL Lacertae object (BZB), flat-spectrum radio quasar (BZQ), or host-galaxy-dominated blazar-like source (BZG) categories. We construct [...] Read more.

The fifth edition of the Roma-BZCAT catalog (5BZCAT) contains 227 blazars of the uncertain type (BZU), whose available data do not support an unambiguous assignment to the BL Lacertae object (BZB), flat-spectrum radio quasar (BZQ), or host-galaxy-dominated blazar-like source (BZG) categories. We construct a machine-readable value-added catalog for these sources using 1151 BZB, 1909 BZQ, and 274 BZG catalog entries as the supervised reference sample. The compact input set contains redshift, optical magnitude, radio, X-ray, and gamma-ray measurements; missingness indicators; and spectral slopes connecting the radio, optical, and X-ray bands. Redshift and its availability are explicitly treated as optical-spectroscopy-related label proxies. Cross-validation shows that the Light Gradient Boosting Machine (LightGBM) provides the most reliable probability scores among the tested models, although the stacking ensemble gives a slightly higher balanced accuracy. The final catalog reports LightGBM probabilities and contains 61 BZB-like, 120 BZQ-like, and 46 BZG-like sources; 17 are flagged for low probability separation. These probabilities quantify similarity to existing 5BZCAT labels conditional on catalog measurements, rather than an independent physical taxonomy. Full article

(This article belongs to the Special Issue New Discoveries in Astronomical Data (II))

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36 pages, 17071 KB

Open AccessReview

Merging Galaxy Clusters and the Search for New Physics of Dark Matter: A Review

by Rogério Monteiro-Oliveira

Universe 2026, 12(8), 249; https://doi.org/10.3390/universe12080249 - 15 Aug 2026

Abstract

Merging galaxy clusters represent one of the most powerful macroscopic laboratories in the Universe for searching for new physics within the dark sector. High-velocity cosmic collisions inherently separate the dark matter and stellar components from the highly collisional, X-ray-emitting intracluster gas. These massive [...] Read more.

Merging galaxy clusters represent one of the most powerful macroscopic laboratories in the Universe for searching for new physics within the dark sector. High-velocity cosmic collisions inherently separate the dark matter and stellar components from the highly collisional, X-ray-emitting intracluster gas. These massive systems provide an ideal environment to probe the fundamental nature of dark matter, specifically testing whether it behaves as a strictly collisionless particle or exhibits non-zero self-interactions. While pioneering systems like the Bullet Cluster historically demonstrated the macroscopic decoupling of dark and ordinary matter, the field has evolved into a sophisticated discipline driven by multi-disciplinary methodologies. This review synthesizes recent theoretical and empirical advances in interpreting post-collision dynamics. It examines how the synergy of combined approaches—integrating multi-wavelength observations from gravitational lensing and X-ray mapping with high-fidelity N-body hydrodynamical simulations—allows the translation of macroscopic spatial observables into stringent constraints on microscopic particle properties. Through this synthesis, the work evaluates how leveraging heterogeneous merger ensembles can reliably advance the ongoing search for physics beyond the standard cosmological model. Full article

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

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14 pages, 12837 KB

Open AccessArticle

Role of Branching in High-Energy γ-Ray Emission from Dark Matter Annihilation: An Example of the Inert Doublet Model

by Mani Khurana, Kunal Rawat, Krishna Kumar Singh, Rusa Mandal, Pawan Kumar Netrakanti and Kuldeep Kumar Yadav

Universe 2026, 12(8), 248; https://doi.org/10.3390/universe12080248 - 15 Aug 2026

Abstract

Understanding the nature of dark matter (DM) and its detection remains one of the most significant unsolved problems in astroparticle physics and cosmology. Considerable efforts have been devoted to the detection of DM through both direct and indirect approaches. Gamma-ray observations offer a [...] Read more.

Understanding the nature of dark matter (DM) and its detection remains one of the most significant unsolved problems in astroparticle physics and cosmology. Considerable efforts have been devoted to the detection of DM through both direct and indirect approaches. Gamma-ray observations offer a powerful probe for the indirect detection of DM. In particular, the spectral features of gamma rays produced through DM interactions are strongly dependent on the underlying annihilation channels into the Standard Model (SM) particles. In this work, we investigate the role of annihilation branching fractions in determining the

γ

-ray emission from dark matter within the framework of the Inert Doublet Model (IDM). The lightest neutral inert scalar, which serves as a viable DM candidate, can annihilate into various SM particles, including fermions, gauge bosons, and Higgs bosons, depending on DM mass and the model parameters. We analyze how the branching fractions into these final states influence the resulting

γ

-ray spectra and fluxes. Our study demonstrates that different dominant annihilation channels produce distinct spectral features, significantly affecting the predicted high-energy

γ

-ray signals. By examining the dependence of

γ

-ray emission on the branching behavior of DM within IDM, we identify regions of parameter space that can provide better experimental constraints while remaining consistent with the relic density requirements. The results highlight the importance of annihilation branching fractions in interpreting indirect detection signals and provide insights into distinguishing DM within IDM from other weakly interacting massive particles (WIMPs) scenarios. This work underscores the potential of

γ

-ray observations as a sensitive probe of the IDM parameter space and its underlying annihilation dynamics. Variations in the branching ratios directly influence the resulting

γ

-ray spectra, thereby affecting the prospects for indirect search of DM with the ground-based

γ

-ray telescopes such as the Major Atmospheric Cherenkov Experiment (MACE). Full article

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

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16 pages, 338 KB

Open AccessArticle

Observational Manifestations of Primordial Objects in the Early Universe Through the Hydrogen Subordinate Lines

by Viktor K. Dubrovich, Yury N. Eroshenko and Stanislav I. Shirokov

Universe 2026, 12(8), 247; https://doi.org/10.3390/universe12080247 - 14 Aug 2026

Abstract

A new mechanism for the formation of spectral–spatial distortions in cosmic microwave background radiation near primordial massive compact objects (such as primordial black holes) at redshifts from

z \sim 1000

to ∼100 is proposed. After hydrogen recombination, the radiation from these objects leads [...] Read more.

A new mechanism for the formation of spectral–spatial distortions in cosmic microwave background radiation near primordial massive compact objects (such as primordial black holes) at redshifts from

z \sim 1000

to ∼100 is proposed. After hydrogen recombination, the radiation from these objects leads to a significant increase in the population of hydrogen subordinate levels in their surrounding environment. Consequently, this allows for the observation of a Fraunhofer-like absorption spectrum in the cosmic microwave background. Such a distortion is formed due to the temperature difference between matter and relic radiation at the corresponding epoch. Ultimately, we should observe circular absorption or emission features around the objects with small angular sizes. These subordinate hydrogen lines currently lie in the radio wavelength range. Estimates indicate that the effect under consideration is accessible for the observations with planned large radiotelescopes. The possibility of the proposed mechanism lies in the ability of an object (e.g., accretion disk around black hole) to emit few-eV photons that populate the

n = 2, 3, 4

and higher levels of hydrogen, enabling the required excitation. Full article

(This article belongs to the Special Issue Bouncing Cosmology and the Early Universe: From Theory to Observations)

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49 pages, 1490 KB

Open AccessArticle

Polymer Quantum Mechanics on Compact Configuration Spaces

by Maxwell R. Siebersma, Basie Seibert, Samuel Shuman and David A. Craig

Universe 2026, 12(8), 246; https://doi.org/10.3390/universe12080246 - 14 Aug 2026

Abstract

“Polymer quantum mechanics” is the name given to a quantization scheme inspired by loop quantum gravity in which the configuration space of the theory is chosen to have a discrete topology. Polymer quantization yields a representation of the canonical commutation relations that is [...] Read more.

“Polymer quantum mechanics” is the name given to a quantization scheme inspired by loop quantum gravity in which the configuration space of the theory is chosen to have a discrete topology. Polymer quantization yields a representation of the canonical commutation relations that is genuinely distinct from the conventional “Schrödinger” representation. In this paper, we summarize the main features of polymer quantum mechanics and investigate in detail the polymer quantization of systems with configuration spaces that are classically compact. We show explicitly how using the standard construction of polymer states leads to a Hilbert space of states defined on a finite graph of points. By way of example, we find the exact energy eigenvalues and eigenfunctions for a particle on a ring and a particle in a box defined on such lattices, and discuss similarities and differences from standard Schrödinger quantum mechanics. We also explore the continuum limit of states in these systems, and demonstrate in detail how the exact eigenfunctions in the position representation approach their continuum counterparts. Full article

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

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36 pages, 22486 KB

Open AccessArticle

Electromagnetic Signatures from Primordial Black Holes in the Solar System

by Alexandra P. Klipfel and David I. Kaiser

Universe 2026, 12(8), 245; https://doi.org/10.3390/universe12080245 - 14 Aug 2026

Abstract

Primordial black holes (PBHs) in the asteroid-mass range, with typical masses

10^{17} g ≲ M ≲ 10^{23} g

, have drawn significant recent attention as viable dark matter candidates. The peak frequencies of photons emitted via Hawking radiation from asteroid-mass PBHs [...] Read more.

Primordial black holes (PBHs) in the asteroid-mass range, with typical masses

10^{17} g ≲ M ≲ 10^{23} g

, have drawn significant recent attention as viable dark matter candidates. The peak frequencies of photons emitted via Hawking radiation from asteroid-mass PBHs range from infrared to

γ

-ray bands. We calculate expected local transit rates for extended PBH mass distributions that could comprise all dark matter. We evaluate prospects for detecting Hawking-radiated photons from local PBH transits through the inner Solar System and from PBH explosions in the far outer edges of the Solar System. We consider several existing and proposed ground-based and space-based instruments sensitive to photons from the radio band to ultrahigh-energy

γ

-rays. We find that the proposed instruments, such as the All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) satellite, can reliably detect PBH transits within

O (0.1 AU)

of the Earth, while the High Altitude Water Cherenkov (HAWC) observatory and Large High Altitude Air Shower Observatory (LHAASO) are both sensitive to PBH explosions out to

O (0.1 pc)

and

O (0.5 pc)

, respectively. We conclude by specifically considering potential companion electromagnetic signatures in the case of a PBH explosion about

10^{3} AU

from Earth, which has been suggested as a potential source for the ∼220 PeV ultrahigh-energy KM3-230213A neutrino event observed by the KM3NeT collaboration in 2023. Whereas we find that the recent KM3NeT event would not have yielded detectable electromagnetic signals—due to its location on the sky, proposed distance from Earth, and the offline status of the HAWC observatory at that time—we demonstrate that future PBH explosions at comparable distances could yield electromagnetic signals measurable from Earth, depending on the alignment of the PBH burst with detector fields of view. Full article

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

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