Universe doi: 10.3390/universe12090278

Authors: Robert C. Crew Emma C. I. Paterson Maxim Goryachev Eugene N. Ivanov Pashupati Dhakal Tugrul Talha Ersoz Michael E. Tobar Jeremy F. Bourhill

We propose a superconducting single-mode microwave haloscope based on chiral cavityresonators for the detection of ultralight dark matter axions over the mass range4 × 10<sup>−19</sup>– 4 × 10<sup>−14</sup> eV. Building on the single-mode chiral-cavity haloscope for detectingultra light dark matter (ULDM) axions we develop a resonator geometry compatible withsubtractive manufacturing from high-purity bulk niobium, taking advantage of the substantiallylower surface resistance achievable relative to the additively manufactured Möbiuscavity proposed in the earlier work. An inverse-design framework is then used to maximisea figure of merit derived to minimise the measurement time required to achieve a fixedexperimental sensitivity. The resulting optimised bulk-niobium design achieves a figure ofmerit more than three orders of magnitude larger than the additively manufactured Möbiusbenchmark. An experimentally informed microwave interferometric readout model incorporatingmeasured electronics noise and active suppression of pump amplitude noise isused to project the sensitivity of the proposed experiment. For an acquisition time of threemonths, the haloscope is projected to reach gaγγ < 10<sup>−11</sup> GeV<sup>−1</sup> across more than fourorders of magnitude in axion mass. The projected sensitivity extends approximately oneorder of magnitude below the current exclusion limits set by CAST, providing a practicalpathway towards a high-sensitivity direct search for ultralight dark matter axions.

Universe doi: 10.3390/universe12090277

Authors: Sonali Borah Asoke K. Sen

Direct kinematic measurements of cosmic acceleration via the cosmological redshift drift (the Sandage–Loeb effect) provide a powerful, model-independent avenue to map the expansion history of the Universe. In this work, which extends the forecasting framework developed in a recent study, we evaluate the capability of the Square Kilometre Array (SKA) to constrain three physically motivated, quantum-gravity inspired holographic dark energy (HDE) paradigms: Standard HDE (with a future event horizon cutoff), Tsallis HDE (THDE), and Rényi HDE (RHDE). Utilizing simulated neutral hydrogen (HI) 21 cm emission lines from a census of over 109 galaxies up to z=1.0 and 21 cm absorption lines in 1.8×103 damped Lyman-α (DLA) systems, we simulate mock velocity drift observations (v˙) at high spectral resolutions (Δν=0.001 Hz and 0.002 Hz) over a semi-annual (Δt=0.5 year) integration baseline. We contrast these forecasts with current real-world constraints obtained from the joint Markov chain Monte Carlo (MCMC) likelihood analysis of the Pantheon SNe Ia and DESI DR2 BAO compilations. Our results reveal a profound limitation of integrated geometric probes: while current SNe Ia and BAO data suffer from severe parameter degeneracies—leaving the non-additive entropy scaling exponents (δ and α) completely unconstrained as open vertical bands—the simulated SKA redshift drift successfully breaks these degeneracies. From emission-line observations at 0.001 Hz spectral resolution, we obtain marginalized 1σ constraints of σc=±0.021 for the standard holographic parameter, σδ=±0.035 for the Tsallis entropy index, and σα=±0.007 for the Rényi parameter. Crucially, we identify and resolve a critical duplicate-plotting mathematical error present in the existing redshift-drift literature by providing the mathematically correct physical scaling and peak structures for both the dimensionless redshift drift (Sz) and the physical velocity drift (Sv). We conclude that the SKA will serve as a premier instrument for testing the holographic principle and non-extensive thermodynamics at cosmological scales.

Universe doi: 10.3390/universe12090276

Authors: Jun Li

The Andromeda galaxy (M31) is the nearest massive spiral galaxy and is expected to reside in an extended dark matter (DM) halo, making it a useful target for indirect dark matter searches with γ- rays. In this study, we perform a template-based Fermi-LAT search for emission from annihilating and decaying dark matter in M31 using Pass 8 data over 17 years. The analysis considers both the bb¯ and τ+τ− final states for DM masses between 10GeV and 10TeV. We compare three representative M31 halo models, denoted MIN, MED, and MAX, that bracket uncertainties in the smooth halo profile and in the substructure contribution, and we quote the MED model as our reference. No DM signal is claimed; instead, we derive 95% confidence-level bounds: upper limits on ⟨σv⟩ and lower limits on the decay lifetime. For the bb¯ channel, the annihilation limits span 6.5×10−26–3.4×10−23cm3s−1 and the decay-lifetime lower limits span 1.8×1026–5.6×1026s; for the τ+τ− channel the corresponding ranges are 9.6×10−26–3.3×10−22cm3s−1 and 7.6×1025–2.5×1027s. We compare our results with existing experimental constraints. The results are complementary to high-mass M31 searches with HAWC and illustrate that the spatial morphology of the halo, not only the total astrophysical factor, is an important systematic in M31 DM constraints.

Universe doi: 10.3390/universe12090275

Authors: Ru-Rong Chen Yan Zhu Hai-Yan Zhang

We propose a method to measure the time delay of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) using baseband data and periodic noise injection. By applying a sliding time window and calculating the average standard deviation ratio over 1000 noise cycles, high-precision measurement of the noise-induced time delay is achieved. The proposed method offers several advantages, including a short recording duration, high measurement accuracy, and strong immunity to electromagnetic interference. It enables effective monitoring of hardware state changes in the signal chain, and provides technical support for the future participation of the FAST in radio interferometry.

Universe doi: 10.3390/universe12090274

Authors: Carlos Eduardo Gabarrete Daniela Massiel Montoya Roger Javier Raudales Rodriguez

In this article, we derive the components of the entropy covector field for a relativistic kinetic gas composed of collisionless, spinless, massive, and uncharged particles following bound orbits in a curved spacetime background. By assuming a dependence on the inclination angle of the particle orbits, we consider two distinct models that describe a rotating and a non-rotating relativistic kinetic gas around a Schwarzschild black hole. We analyze the behavior of key macroscopic observables (including the anisotropy parameter and the kinetic temperature) which are constructed from the particle density, energy density, and principal pressures. We aim to characterize and compare the morphology of the resulting configurations, thereby extending and complementing a previous work. The results reveal significant differences between the rotating and non-rotating cases, particularly in the asymptotic behavior of the anisotropy parameter, kinetic temperature, and average pressure, highlighting the role of angular momentum in shaping the macroscopic properties of collisionless gases in strong gravitational fields.

Universe doi: 10.3390/universe12090273

Authors: Christian Spiering

Neutrino astronomy at high energies is an emerging field but still in a state of infancy. It is barely a dozen years ago that a diffuse flux of extraterrestrial neutrinos with energies in the TeV and PeV ranges was detected and even fewer that first individual sources could be identified. A new window to the universe has been opened after four decades of efforts to realize the gigantic instruments that made that breakthrough possible. This review describes the road towards the present, reviews the actual status of the field and sketches future developments.

Universe doi: 10.3390/universe12090272

Authors: Michael Bishop Daniel Hooker Douglas Singleton

Many phenomenological models of quantum gravity predict a minimal observable length. A common implementation is the Generalized Uncertainty Principle (GUP), which modifies the canonical commutator between position and momentum. In many formulations, the modified position operator is not symmetric with respect to the standard quantum-mechanical inner product, requiring a modified Hilbert-space measure that breaks the natural isometry between momentum and position space. We review an alternative formulation in which the position operator is symmetrized instead of the inner product. This preserves the standard Fourier relationship between momentum and position space while retaining the same generalized uncertainty relation and minimal length. The resulting position eigenstates contain an intrinsic suppression of large momenta, suggesting a possible route toward constructing GUP-modified quantum field theories with improved ultraviolet behavior.

Universe doi: 10.3390/universe12090271

Authors: Tiberiu Harko Taoufik Ouali Ahmed Errahmani Amine Bouali

This Special Issue, The 13th Bolyai–Gauss–Lobachevsky Conference on Non-Euclidean Geometry and Its Applications, brings together seven peer-reviewed contributions originating from presentations delivered at the 13th Bolyai–Gauss–Lobachevsky (BGL2025) Conference, held in Saïdia, Morocco, from 26 to 29 May 2025 [...]

Universe doi: 10.3390/universe12090269

Authors: Nektarios Vlahakis

The minimalist approach for linear stability analysis is applied to fluids and magnetized ideal plasmas in cylindrical geometry. In this approach, the dispersion relation is obtained by integrating a single first-order differential equation—referred to as the principal equation—subject to appropriate boundary conditions. We first derive the principal equation for a general unperturbed state with radially varying density and pressure, axial and azimuthal components of both the velocity and magnetic field, and a radially directed gravitational field. We then use this formulation to analyze rotating flows with axial magnetic fields, addressing both wall-bounded and interface-driven axisymmetric instabilities. In addition to exact results for selected unperturbed states, we obtain approximate dispersion relations using the WKBJ method in the incompressible and compressible limits. The analysis encompasses centrifugal, magnetorotational, and buoyancy-driven instabilities as special cases, and it clarifies how compressibility modifies their stability properties.

Universe doi: 10.3390/universe12090270

Authors: Xiao-Yan Chew Yun Soo Myung

We study scalarized extremal black holes in the Einstein–Maxwell-scalar (EMS) theory with two different scalar couplings to two U(1) fields. This theory is inspired by the dyonic EMS theory. Two scalarized extremal black holes are found with constant scalar hair. We confirm that these are precisely obtained from the extremal scalarization and entropy function approaches. This may imply that it is not easy to find extremal black holes with primary scalar hair.

Universe doi: 10.3390/universe12090268

Authors: Xueli Lin Zihan Qian Cunshi Wang Yuyang Li

The booming data volume of modern time-domain surveys demands fast, robust early classification of sparsely sampled light curves, as newly discovered transients typically have only a handful of observations. We compare classifiers for extremely sparse light curves (3–30 observations) on a benchmark of approximately 1.72 million segments spanning seven astrophysical classes from ZTF and ATLAS. Methods include handcrafted-feature approaches (XGBoost, feature-based Transformers) and end-to-end LSTM and Transformer models. A pre-trained end-to-end Transformer achieves test accuracy of 0.946 (macro F1 0.950), exceeding 90% accuracy with only seven observations, but falls to 0.513 without pre-training. XGBoost-Reduced (38 features, excluding LS descriptors) reaches 0.922, while XGBoost-Full (56 features) reaches 0.913. Reliability diagnostics confirm LS periods and false-alarm probabilities are unreliable on 3–30-point segments; restricting training and evaluation to ≥15 points does not reverse the full-scale preference for the Reduced catalog. On CPU, XGBoost runtime is dominated by feature extraction (ratio ≈ 16:1); adding LS descriptors increases total processing time by ∼7.8% (feature extraction by ∼7.0%) without a commensurate accuracy gain. A lightweight LSTM attains 0.847 accuracy with 0.2 M parameters. These results offer practical guidance for model selection in real-time survey pipelines.

Universe doi: 10.3390/universe12090267

Authors: David S. Pereira Francisco S. N. Lobo José Pedro Mimoso

We demonstrate that the matter sector itself can drive baryogenesis in energy-momentum squared gravity, f(R,T2) with T2≡TμνTμν. High-density matter corrections provide new time-dependent sources for the baryon asymmetry through derivative couplings to T2 and to the full combination f(R,T2). Notably, the decoupling temperature is not treated as a free parameter; instead, it is fixed by the freeze-out of the Weinberg B−L-violating operator, directly linking the asymmetry to the light-neutrino mass scale and to the modified expansion history. Analyzing representative powers n=1/4, n=1/2, n=5/8, and n=1, we find that entropy evolution sharply distinguishes the models. The n=1/4 branch cannot serve as a self-contained radiation-era model, the n=1/2 branch is entropy-safe but too weak to reproduce the observed asymmetry, and the n=1 branch survives only after entropy dilution. Remarkably, the n=5/8 branch provides the cleanest viable realization, generating the observed baryon-to-entropy ratio with controlled effective-field-theory hierarchies.

Universe doi: 10.3390/universe12090266

Authors: Vyaas Ramakrishnan Shantanu Desai

In recent years, NICER data have been extensively used to determine neutron star masses and radii via pulse profile modeling. Pulse profile modeling is implemented with the X-PSI package, and the best-fit parameters are typically obtained using Bayesian inference. Using simulated data, we demonstrate the first ever application of frequentist inference to determine the neutron star radius, where the nuisance parameters are treated using profile likelihood. We find that the profile likelihood technique can recover the true radius to <1σ. The uncertainty in the estimated radius is also comparable to that obtained from Bayesian analysis while being computationally much faster. Therefore, this work serves as a proof-of-principle application of frequentist inference to estimate neutron star radii using pulse profile modeling and complements the Bayesian inference technique currently used. We have also made our analysis codes for frequentist inference using X-PSI publicly available.

Universe doi: 10.3390/universe12090265

Authors: Loriano Bonora

This paper is a bottom-up attempt to incorporate the standard model and general relativity in a unique quantum field theory. In particular, the tentative model presented here is free of chiral gauge and gravitational anomalies that appear in the divergence of currents and in the divergence and trace of the energy–momentum tensor when SM matter couples to gravity. The fermion spectrum is composed of two multiplets: the SM (left) multiplet and a mirror copy (right) with opposite handedness. The right multiplet is interpreted as describing the dark matter world. The natural symmetry of the theory is enlarged to incorporate Weyl invariance by introducing one or more dilaton fields. After the cosmological and theoretical motivations, the necessary formalism is introduced for algebraic renormalization: gauge fixings, ghosts, propagators, vertices and their interplay in guaranteeing the conditions for convergence of the subtracted amplitudes according to the BPHZL scheme, the Slavnov–Taylor identity, and the relevant enlarged BRST symmetry. The corresponding (conformal) cohomology is analyzed and found to be trivial: there are no non-trivial even trace anomalies in theories with dilatons, but there are plenty of trivial ones that require corresponding counterterms in the effective action. It is shown that such counterterms can play an important role in freeing the theory from unphysical particles.

Universe doi: 10.3390/universe12090263

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

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 λ6678, He i λ5875, and Si iii λ4552 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.

Universe doi: 10.3390/universe12090264

Authors: Istvan Horvath Zsolt Bagoly Lajos G. Balazs Jon Hakkila Janos Horvath Sandor Pinter Istvan I. Racz Peter Veres Bendegúz Koncz

Gamma-ray bursts (GRBs) are bright transient sources that can be observed at high redshift. They can therefore be used as tracers of the distant large-scale structure, although the GRB redshift sample is sparse and affected by strong selection effects. We analyze the three-dimensional distribution of 542 GRBs with spectroscopic redshifts. The method extends our earlier spherical-window search by replacing spherical counting volumes with axisymmetric spheroids. The angular positions of the observed GRBs are kept fixed in the Monte-Carlo null samples, while the redshifts are shuffled within each Galactic hemisphere. In the Northern Galactic hemisphere, the overdensity associated with the Hercules–Corona Borealis Great Wall remains significant for a range of spheroidal shapes. This supports the stability of the previously reported signal and shows that it is not only a consequence of using spherical counting volumes. In the Southern Galactic hemisphere, the same method finds no structure with comparable significance. A small candidate grouping is present, but its significance is only marginal, and it is sensitive to the small number of events. We conclude that spheroidal resampling is a useful check of GRB overdensity searches, but the physical nature of any candidate structure still requires confirmation with independent tracers such as galaxy or quasar samples.

Universe doi: 10.3390/universe12090262

Authors: Carlos Tena-Contreras Iván Álvarez-Rios Francisco S. Guzmán

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 mb=10−22eV. 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 1010M⊙ 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.

Universe doi: 10.3390/universe12090261

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

Using the two-component Tsallis-Hagedorn Model with Transverse Flow (THMTF), this study analyzes centrality dependencies of midrapidity transverse momentum (pT) spectra for identified light-flavor hadrons in asymmetric p+Pb collisions at snn  = 5.02 TeV, comparing them with symmetric Pb+Pb collision data at snn  = 2.76 and 5.02 TeV at the LHC. The analysis successfully describes ALICE experimental data up to pT = 20 GeV/c across seven multiplicity classes in p+Pb collisions, estimating that collective flow emerges at ⟨Npart⟩ ≈ 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 ⟨Npart⟩ 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 pT region between p+Pb and Pb+Pb collision systems.

Universe doi: 10.3390/universe12090260

Authors: Francisco X. Azeredo Dyana C. Duarte Ricardo L. S. Farias Bruno S. Lopes João A. R. S. Prado William R. Tavares

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.

Universe doi: 10.3390/universe12090259

Authors: Tsutomu T. Takeuchi Karin T. Sakuragi Ryusei R. Kano Sena A. Matsui

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.

Universe doi: 10.3390/universe12090258

Authors: Elke Pilat-Lohinger Maximilian Zimmermann Birgit Loibnegger

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.

Universe doi: 10.3390/universe12090257

Authors: Princy Ranaivomanana Murat Uzundag

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.

Universe doi: 10.3390/universe12090256

Authors: Dengke Zhou Yi Feng Jiaying Xu Chenyuan Xu Jianhua Fang

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 σRM—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 σRM, 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 1.3radm−2, with approximately 70% of the sources showing σRM≳1radm−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 σRM≳5radm−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 σRM measurements, and substantially deepen our understanding of FRB environments.

Universe doi: 10.3390/universe12090255

Authors: Haoning He

The origin of ultra-high-energy cosmic rays (UHECRs), with energies above ∼1

Universe doi: 10.3390/universe12090254

Authors: Xiaowan Zhang Songpeng Pei Xiaoqin Ren Mingyue Li

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±0.09 to −1.19±0.08 over 5<f<120d−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.

Universe doi: 10.3390/universe12090253

Authors: Shuo Zhang Rui Wang

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.

Universe doi: 10.3390/universe12080252

Authors: Mauro D’Onofrio

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 [...]

Universe doi: 10.3390/universe12080251

Authors: Moses Onyemaechi Asogwa Seblu Humne Negu Gemechu Muleta Kumssa Innocent Okwudili Eya

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 NH3 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 Rcl−1.30±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±0.08, suggesting possible stellar initial mass function incompleteness. The dense-gas star formation timescale follows τSF,dense∝Mcl−0.77±0.04, with a median value of 0.54Myr. Assuming a relatively uniform timescale of 0.50Myr could overestimate and underestimate star formation rates in low-mass and massive clumps by factors of ∼32 and ∼25, respectively.

Universe doi: 10.3390/universe12080250

Authors: 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 Denis Bastieri

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.

Universe doi: 10.3390/universe12080249

Authors: Rogério Monteiro-Oliveira

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.

Universe doi: 10.3390/universe12080248

Authors: Mani Khurana Kunal Rawat Krishna Kumar Singh Rusa Mandal Pawan Kumar Netrakanti Kuldeep Kumar Yadav

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).

Universe doi: 10.3390/universe12080247

Authors: Viktor K. Dubrovich Yury N. Eroshenko Stanislav I. Shirokov

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∼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.

Universe doi: 10.3390/universe12080246

Authors: Maxwell R. Siebersma Basie Seibert Samuel Shuman David A. Craig

“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.

Universe doi: 10.3390/universe12080245

Authors: Alexandra P. Klipfel David I. Kaiser

Primordial black holes (PBHs) in the asteroid-mass range, with typical masses 1017g≲M≲1023g, 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(0.1AU) 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(0.1pc) and O(0.5pc), respectively. We conclude by specifically considering potential companion electromagnetic signatures in the case of a PBH explosion about 103AU 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.

Universe doi: 10.3390/universe12080244

Authors: Bence Husvéth Krisztina É. Gabányi Tao An Sándor Frey Jun Yang Iván Agudo Yingkang Zhang

High-frequency very long baseline interferometry (VLBI) polarimetry probes synchrotron-emitting plasma closer to the central engines of radio-loud active galactic nuclei (AGNs), but observations above 43 GHz are technically demanding. We present 22-GHz European VLBI Network observations of the z=4.31 quasar J1510+5702 and J1606+3124, whose published spectroscopic redshift, z=4.56, is uncertain; a photometric estimate gives zphot=0.9±0.1. For the published z>4 redshifts, 22 GHz corresponds to rest-frame frequencies above 118 GHz. Polarized emission is detected in J1510+5702, and a low-level polarized signal is recovered from the brightest feature of J1606+3124. Adopting z=4.56, that feature has a brightness temperature of Tb,VLBI=(7.4±0.8)·1010 K, allowing a mildly Doppler-boosted interpretation, while the young compact-source scenario also remains viable. The core of J1510+5702 has Tb,VLBI=(1.08±0.15)·1012 K, implying a Doppler factor of ∼22 under the equipartition assumption. This component has a ∼3.5% fractional polarization. These observations show that cm-wavelength VLBI can access rest-frame millimeter-band polarization in bright z>4 jets.

Universe doi: 10.3390/universe12080243

Authors: Heather R. Pearson Anna Molodtsova Sage C. Weisrock Sherlock Tingrui Zhao Jason E. Stalnaker

We describe an experimental investigation of a nuclear spin oscillator based on 3He nuclei as a possible detector to search for exotic spin couplings. A magnetically shielded vapor cell comprising an alkali atom mixture (95% potassium and 5% rubidium) and 3He gas is polarized via laser light resonant with the D1 transition in rubidium in the presence of a dc magnetic field. The potassium atoms and 3He nuclei are polarized via spin-exchange collisions with the polarized rubidium atoms. The nuclear spins are tipped with a magnetic field applied perpendicular to the dc magnetic field. The resulting Larmor precession of the 3He nuclear spins is monitored via Faraday rotation of laser light near resonant with the D1 transition in potassium. The Faraday rotation signal is filtered, amplified, and used to apply a magnetic field in a direction perpendicular to the dc magnetic field, resulting in a self-sustained oscillation of the nuclear spins at a frequency that is directly proportional to the dc magnetic field. We demonstrate a sensitivity to exotic spin couplings that is ≈5 times higher than the alkali atom magnetometers that have been used in the Global Network of Optical Magnetometers to Search for Exotic Physics collaboration.

Universe doi: 10.3390/universe12080242

Authors: Zhizhuo Zhang Xishun Li Haihua Qiao Yuanwei Wu Baoqi Sun Hui Lei Haiyan Yang Qiaoli Kong Shuaimin Wang Yangyang Cui Xuan Cheng Xuhai Yang

Universal Time (UT1) is a core component of the Earth orientation parameters (EOP). High-precision UT1 predictions are essential for satellite navigation, deep-space exploration, and the maintenance of national standard time. Although effective angular momentum (EAM) information can improve UT1 predictions, the impacts of different angular momentum combinations on prediction performance have not yet been systematically investigated. To improve the prediction accuracy of the National Time Service Center (NTSC) UT1 products, we constructed four prediction schemes: Case 1 uses only atmospheric angular momentum (AAM) data; Case 2 uses AAM + oceanic angular momentum (OAM) data; Case 3 uses AAM + OAM + hydrological angular momentum (HAM) data; and Case 4 uses the full EAM datasets combining AAM, OAM, HAM, and sea-level angular momentum (SLAM) data. The input UT1 series is from the NTSC EOP products, and the 10-day angular momentum forecasts are provided by the German Research Centre for Geosciences (GFZ). The rolling forecast evaluation was conducted from June 2024 to September 2025. The results show that Case 2 performs best for short-term UT1 predictions over 1–12 days, improving the mean prediction accuracy by 10.7%, 10.0%, and 52.0% relative to the predictions using Case 4, IERS finals.daily, and the original NTSC predictions, respectively. For medium- and long-term UT1 predictions over 13–90 days, Case 1 performs best, with corresponding mean improvements of 9.8%, 50.7%, and 61.3%, respectively. These results indicate that incorporating more angular momentum components does not necessarily lead to better UT1 predictions, i.e., Case 2 is preferable for short-term UT1 predictions, whereas Case 1 is more suitable for medium- and long-term UT1 predictions. These findings provide empirical evidence and practical guidance for optimizing UT1 prediction models.

Universe doi: 10.3390/universe12080241

Authors: Adailton A. Araújo Filho Narges Heidari Iarley P. Lobo

This work aims to explore the gravitational consequences of a recently proposed black hole solution previously introduced, which incorporates quantum gravitational corrections of General Relativity. We initiate our analyses by taking into account the horizon structure, focusing on both the event and Cauchy horizons. Subsequently, we examine the quasinormal modes by considering all types of perturbations—scalar, vector, tensor, and spinorial. To strengthen these results, we also compute the time domain for each perturbation. Next, we turn to the study of optical properties of the black hole. In particular, we investigate null geodesics, the photon sphere and its stability, and the corresponding black hole shadows. Following this, we analyze gravitational lensing phenomena in two regimes: the weak-field limit, utilizing the Gauss–Bonnet theorem, and the strong deflection limit, employing Tsukamoto’s approach. In addition, we address the lensing observables with Event Horizon Telescope (EHT) data for SgrA* and M87*. Finally, constraints on the parameter ξ—which is introduced by higher-order curvature–scalar gravity, thereby differing from the Schwarzschild solution—are estimated using Solar System measurements such as the precession of Mercury’s orbit, gravitational light bending, and time delay (or the Shapiro effect).

Universe doi: 10.3390/universe12080239

Authors: Cristofher Z. Vargas William C. Algoner Angel E. Obispo Andrés G. Jirón

Although the standard model of cosmology (ΛCDM) has been successful in explaining a wide range of observational phenomena, it still faces significant limitations, particularly regarding the nature and properties of the dark sector of the universe. These challenges motivate the exploration of alternative approaches that can deepen our understanding of cosmic evolution. This study proposes an extension of the ΛCDM model by incorporating irreversible processes through a viscous fluid with out of equilibrium pressure. This fluid interacts dynamically with the dark sector components, offering new possibilities for describing the expansion of the universe. To constrain the proposed scenarios, we perform a Bayesian statistical analysis using the Pantheon+ Type Ia supernova compilation, Cosmic Chronometers, and DESI DR2 Baryon Acoustic Oscillation measurements. The observational results show that all three interaction models successfully reproduce the late-time expansion history while preserving a dark matter barotropic index close to the pressureless limit expected in the standard cosmological scenario. In contrast, the dark energy barotropic index consistently favors positive values, suggesting a mild departure from the cosmological constant description. The incorporation of Cosmic Chronometers and, particularly, BAO measurements substantially improves the constraints on the matter sector, leading to a significant reduction in the uncertainty of the present dark matter density parameter, whereas the bulk-viscosity parameter and the asymptotic ratio remain only weakly constrained. Finally, a Bayesian model comparison based on the Bayes factor shows that the interacting dissipative models remain observationally compatible with the reference ΛCDM cosmology. According to the Jeffreys scale, the resulting Bayesian evidence is inconclusive, indicating that current observations neither favor nor disfavor these extensions with respect to the standard cosmological model.

Universe doi: 10.3390/universe12080240

Authors: Yangyang Cui Xishun Li Yuanwei Wu Haihua Qiao Dang Yao Zewen Zhang Zhizhuo Zhang Xuhai Yang

In the original publication [...]

Universe doi: 10.3390/universe12080238

Authors: Jorge Alda Jacobo Asorey Alejandro Mir Siannah Peñaranda

Global fits in high-energy physics and cosmology often face the challenge of exploring high-dimensional parameter spaces with computationally expensive or topologically complex likelihood functions. In this work, we present a machine-learning framework designed to emulate complex, often non-Gaussian likelihood landscapes using gradient-boosted regression trees (XGBoost). We discuss the advantages of the machine-learning approach in terms of computational efficiency and the resolution of confidence regions, particularly in scenarios with complex correlations or “curved” degeneracies. We validate this methodology by applying it to a recent analysis of flavour anomalies in semileptonic B meson decays and discussing the adaptability of the framework to other phenomenological systems, such as axion-like particles and global fits in cosmology. Finally, we utilise SHAP (Shapley Additive exPlanations) values to provide a transparent analysis of feature importance, ensuring that the machine-learning predictions remain physically interpretable and consistent with the underlying physics.

Universe doi: 10.3390/universe12080237

Authors: Xiao Xiao De-Kai Kong

We pose and study the inverse problem of the chiral low-energy constants (LECs): with our re-implemented and checked Yang large-Nc forward operator (reproducing all eight LECs of Yang’s Table I within 30%, four within 15%), we use the experimental LECs of Pan et al. and the lattice quark mass function of Oliveira et al. as data to invert for the quark self-energy ∑(p2). (1) Treating ∑ as a free function results in a multimodal posterior; two independent analyses locate the LEC information window at ∼0.5–1.75GeV and distinguish the profile-sensitive L1–L3, L5 from the comparatively profile-rigid L7, L8 under the tested changes in ∑. (2) Parametrizing ∑ through the kernel of the gap equation (Maris–Tandy/Qin–Chang) collapses the inversion to three kernel parameters (D,ω,m): the resulting posterior is unimodal, the lattice data and LECs are fitted simultaneously, and the uncertainties in ∑ at representative momenta are reduced by factors of 2–4. (3) The fitted result contains four apparent deviations from Pan/FLAG. A proper-time regulator scan examines their response to the forward prescription. The apparent tensions in L5, L7 and the condensate are not particularly stable under the checks performed, whereas L8 remains above the Pan central value throughout the scan. It also varies little across the two kernel realizations and the exploratory M=B/A profile proxy. At the representative Λreg=2GeV point, 103L8=0.99 differs by 2.6σ from the quoted Pan value 103L8Pan=0.60±0.15; the condensate-based B0 comparison and the scalar resonance-saturation estimate show the same tendency. These results support a possible tension associated with the scalar-channel LEC L8 within the tested leading-Nc rainbow construction.

Universe doi: 10.3390/universe12080236

Authors: Saarik Kalia Ibrahim A. Sulai

Ultralight bosonic dark matter (UBDM) that couples to electromagnetism can generate an oscillating magnetic-field signal at the Earth’s surface. This is referred to as the “Earth transducer” effect, as the Earth converts UBDM into a detectable magnetic field. Similar DM-induced fields in laboratory experiments typically scale with the size L of the experiment. Because the Earth transducer signal instead scales with the large radius of the Earth, R, it is one of the most powerful direct probes of UBDM with masses mDM≲1/R∼3×10−14eV. It has many other favorable properties, such as high spatial and temporal coherence and robustness to atmospheric modeling. In this review, we derive the Earth transducer effect and its properties for multiple UBDM models, and we discuss current and future prospects to detect it.

Universe doi: 10.3390/universe12080235

Authors: Indranil Banik José Antonio Nájera Harry Desmond

We constrain the difference in gravitational potential between our location and sources at z≳0.3 using datasets at those redshifts. Our motivation is that the Hubble tension might be caused by a local void, as suggested by galaxy number counts. This would increase the redshift through outflow and gravitational redshift (GR). Only the latter is important at high redshift, where a void contributes a fixed additional GR contribution of z0 due to our location on a potential hill. This z0 model has various subtle effects that were not previously considered, including a hotter CMB and reduced BAO scale rd. We test whether z0 can have the previously expected value of 0.84%, which was based on the fitting of void parameters to galaxy number counts and local H0 measurements. Combining BBN, CMB, BAO, and CC datasets at z>0.5, we find that z0 = −0.4−0.9+0.8%, which rises to −0.1±0.7% when extending our analysis down to z>0.29. Although the results prefer the standard value of z0=0, the best-fitting model with z0=0.84% fits the data almost as well as ΛCDM, with Δχ2<2. We find that ΛCDM faces a 3.07σ BAO anomaly in the standard (H0rd,Ωm) parameter space, where different regions are preferred by BAO and non-BAO datasets from z>0.29. Fixing z0=0.84% reduces this to 2.79σ. This suggests that a local void large enough to solve the Hubble tension cannot be ruled out by higher-redshift datasets, despite its novel impacts on them.

Universe doi: 10.3390/universe12080234

Authors: Predrag Jovanović Vesna Borka Jovanović Duško Borka

In this paper, we investigated the capability of Yukawa gravity to explain the Schwarzschild precession of S-stars as a probe for General Relativity and to map the allowed parameter space of a potential fifth force. We simulated the S38 star orbit in a Yukawa gravity model and fitted it into the observed astronomical data of the S38 star using the Markov Chain Monte Carlo method in order to constrain the parameters (strength δ and range λ) of the Yukawa interaction. Comparing these findings with previous results for the S2 star reveals that the best-fit values for λ are remarkably close, while the magnitudes of δ are slightly smaller for S38. These results map the joint statistical boundaries of a fifth force at the Galactic Center. Although the General Relativity limit (δ=0) falls well within the reported 1σ uncertainties, rendering potential nominal deviations statistically insignificant, the interaction range λ exhibits a highly stable spatial scale across different stellar orbits. This cross-consistency between independent datasets, with stable clusters near 360, 1900, and 7000 AU, keeps a viable physical window open for a fifth force. This agreement demonstrates that the interaction scale is robust against individual single-orbit systematic errors, ensuring that this parameter domain remains a prime target for future high-precision astrometric observations. Ultimately, analyzing S-star kinematics within Yukawa gravity provides a powerful independent tool for testing General Relativity and bounding non-standard gravitational effects at the Galactic Center.

Universe doi: 10.3390/universe12080233

Authors: Arijit Das Prashanth Jaikumar Tanumoy Mandal

We review the properties of strongly interacting matter under the influence of an external magnetic field, with emphasis on results obtained using the Nambu–Jona–Lasinio model and its variants. Strong magnetic fields, relevant for heavy-ion collisions and magnetars, introduce modifications to the phase structure of quantum chromodynamics. In the first half of the review, we discuss the Nambu–Jona–Lasinio model and its major variants, including Polyakov loop extended model, Kobayashi–Masakawa–’tHooft extended model, entangled model, and nonlocal formulations, highlighting their successes and limitations. Particular attention is given to magnetic catalysis and inverse magnetic catalysis and the tension between model predictions and lattice simulation results. We further review the emergence of phases at high density and low temperature such as the magnetic color–flavor locked phase, the magnetic two-flavor superconducting phase, the crystalline color superconductor and magnetized quarkyonic matter, along with possible mesonic π and ρ meson condensation channels. Implications for strongly magnetized neutron stars are discussed wherever relevant. This review aims to provide a focused overview of the capabilities and limitations of approaches within the Nambu–Jona–Lasinio model and to outline open challenges in understanding strongly interacting magnetized matter at intermediate densities.

Universe doi: 10.3390/universe12080232

Authors: Luigi Tedesco

The Hubble tension is usually formulated as a disagreement between two determinations of a single scalar parameter, H0, within an exactly isotropic FLRW model. We develop a quantitative framework in which the tension is instead treated as a consistency test of the scalar FLRW compression of cosmological data in a homogeneous but anisotropically expanding Bianchi type I background. Beyond synthesizing established results on Bianchi I kinematics, null geodesics and optical propagation, the original contribution is a worked weak-shear, axisymmetric calculation that maps a specified shear history into a low-redshift luminosity-distance quadrupole. The calculation explicitly separates the direction-dependent redshift–affine-parameter mapping from the Jacobi-focusing contribution and then propagates the resulting distance quadrupole through an analytic polar-cap toy window. For freely decaying shear we obtain AD(z)=−BH0+(2q0−1)BH0z/2+(5−q0−18q02+6j0)BH0z2/12+O(z3,BH02), where BH0=(H‖0−H⊥0)/H0 and j0 is the mean jerk parameter. A representative BBN limit, Ωσ0≲10−23, implies |BH0|≲9.5×10−12 and a distance-modulus quadrupole below approximately 2.4×10−11 mag at z=0.15. The early-Universe bound used in this comparison is adopted from previous work and is not itself a new result of the present analysis; the novelty is its propagation through the derived direction-dependent redshift and Sachs–Jacobi mapping into quantitative limits on the luminosity-distance quadrupole and on the catalog-window bias of an isotropic H0 fit. By contrast, even a maximally aligned one-percent directional shift requires Ωσ0≃2.5×10−5, while a shift comparable with the Planck 2018–SH0ES 2022 benchmark separation requires Ωσ0≃1.8×10−3. Thus the minimal shear-only model cannot resolve the tension, although the framework supplies a falsifiable program for testing sustained late-time anisotropy with supernovae, BAO, distance-ladder measurements and future standard sirens.

Universe doi: 10.3390/universe12080231

Authors: Wei Lu Yang Zhong Sheng-Qin Feng

We study the causality and stability of first-order relativistic spin hydrodynamics with particle-number conservation. By deriving the complete dispersion relations of linear perturbations around a global equilibrium state, we find that conserved-charge dynamics modifies the sound sector and introduces additional non-hydrodynamic modes absent in the charge-neutral theory, while the structure of spin relaxation modes remains unchanged. Moreover, the stability conditions acquire new contributions from charge diffusion and thermodynamic susceptibilities. More importantly, a particle-number-induced mode is shown to violate the causality condition in the short-wavelength limit. We further demonstrate that particle-number conservation does not remove the instability inherent in first-order spin hydrodynamics. These results reveal a nontrivial interplay between spin and conserved-charge dynamics and provide important constraints on relativistic spin hydrodynamic theories at finite density.

Universe doi: 10.3390/universe12080230

Authors: Yubo Su

Despite a decade’s worth of gravitational wave observations, the origin of the binary black hole (BBH) mergers detected by the LIGO–Virgo–KAGRA (LVK) collaboration remains an open question. Towards assessing the feasibility and prevalence of the many proposed BBH formation channels, the spin properties of the merging black holes (BHs) hold significant promise, particularly their orientations. Moderate preferential alignment of BH spins with their orbit normals, as well as an apparent anti-correlation between the BBH effective spin parameters χeff and their mass ratios, would appear to disfavor purely dynamical BBH formation mechanisms, as they lack a preferred orientation to the system. However, alternatives are filled with physical and modeling uncertainties. In this paper, we highlight a dynamical route to easily characterizable spin evolution that results in analytically predictable spin distributions. We show that, when a stellar binary forms a BBH through two phases of stable mass transfer, and the BBH is subsequently driven to merger by the gravitational perturbation of a distant massive object (such as a supermassive black hole), the resulting χeff may be anti-correlated with the binary mass ratio. While the mechanism as proposed only operates in a modest region of parameter space, it also predicts significantly tighter correlations than are seen in the GWTC-4.0 systems. We discuss avenues for future work that may significantly expand the parameter space of our mechanism while still remaining broadly consistent with observations.

Universe doi: 10.3390/universe12080229

Authors: Roberta M. Humphreys

The properties of luminous evolved stars in nearby resolved galaxies are briefly reviewed with emphasis on recent results for the low-metallicity dwarf irregulars. Highlights and recent discoveries of some of the more extreme examples with high rates of mass loss such as the yellow and red hypergiants, LBVs, and giant eruptions are summarized. Some of the most interesting new results are the evidence for high-luminosity stars at high redshift and at very low metallicities with mass loss. The presence of an empirical upper luminosity limit at very low metallicities similar to what we observe in the Milky Way and Local Group galaxies is presented. Its implications for the first stars in the early Universe and mass loss mechanisms at very low metallicity are discussed.

Universe doi: 10.3390/universe12080228

Authors: Yunning Zhao Yulong Gao Wei Zhang Fuzhen Cheng Ming Yang Shiming Wen Shichao Han Hong Wu

The observed ratio of Wolf–Rayet (WR) star subtypes (e.g., WC/WN) in M31 is significantly higher than the predictions of standard stellar evolution models, yet this key diagnostic discrepancy remains unresolved. This is partly due to incomplete census and the challenge of resolving individual stars in compact clusters with ground-based telescopes. In this work, we report a young cluster with WR features with the (R.A., Dec) = (00:43:39.36, +41:10:08.7), discovered with LAMOST spectra. We use the high spatial resolution of the HST imaging data to resolve the cluster members, leading to the identification of four WR star candidates, four OB stars, and one yellow supergiant. Based on the current known WR sample in M31 observed in the HST F275W filter, about one-third of WRs are contaminated by other sources within a 1.″1×1.″1 aperture, while this fraction can reach up to two-thirds for the LAMOST fiber size of 3.″3. Applying the approach demonstrated in this work to a larger sample of crowded regions in M31 will yield a more complete census and allow for the measurement of a definitive WC/WN ratio, corrected for contamination. Whether this refined ratio increases or decreases the current tension with models remains to be seen, but it will provide the essential observational constraint needed to advance our understanding.

Universe doi: 10.3390/universe12080227

Authors: Wei-Xi Kong Jin-Wen Kang Sa Wang Yao Li Meng-Quan Yang Ben-Wei Zhang En-Ke Wang

We present the first study of leading subjet (LSJ) radial distributions as a novel jet substructure observable in high-energy nuclear collisions using Pythia8 for p + p collisions and the Linear Boltzmann Transport (LBT) model for Pb + Pb collisions. In p + p collisions, the LSJ is concentrated near the jet core, with multi-subjet events exhibiting enhanced probability compared to single-subjet events except in the innermost core of the jet. In Pb + Pb collisions, the inclusive sample exhibits pronounced suppression without significant radial broadening, while single-subjet events display weak modification throughout the jet cone. Multi-subjet events exhibit suppression at small radii followed by enhancement at large radii, indicating a medium-induced redistribution of energy toward the jet periphery. This broadening is masked in the inclusive distribution due to the dominance of single-subjet events. The increased transverse momentum balance between the LSJ and subleading subjet (SLSJ) in two subjet events drives the jet axis away from the LSJ, providing a mechanistic explanation for the observed radial broadening. These results establish the LSJ radial distribution as an independent and complementary probe of medium-induced jet modifications.

Universe doi: 10.3390/universe12080226

Authors: Adnan Malik Ramy M. Hafez M. Waqas Aslam Ahdab K. Althukair Wenbin Lin

This article investigates wormhole geometry in modified f(R,ϕ,X) gravity, where R, ϕ, and X denote the Ricci scalar, scalar potential, and kinetic term, respectively. We also introduce charge to examine its influence on the wormhole geometry. In the present scenario, the Karmarkar condition is employed to determine the metric potentials. A key feature of this work is the derivation of the modified Tolman–Oppenheimer–Volkoff equation, which is used to analyze the stability of the wormhole geometry. We have analyzed the behavior of the matter-plus-charge energy conditions and found that the null energy condition is satisfied for the considered model, while the strong energy condition is violated. In contrast to general relativity, where the matter NEC must be violated to support a traversable wormhole, the present modified-gravity model fulfils the flare-out requirement through the effective energy–momentum tensor generated by the curvature–scalar–kinetic sector. Hence the required exoticity is shifted from ordinary matter to the effective modified-gravity sector. We also discuss the behavior of the equation-of-state parameters and anisotropy. Moreover, we perform an additional stability analysis based on the adiabatic index. Our analysis reveals that the wormhole solutions are stable for the considered model in modified f(R,ϕ,X) gravity.

Universe doi: 10.3390/universe12080225

Authors: Mehmet Dağ Necla Çakmak

In this paper, microscopic calculations of the β-decay properties of selected Xe, I, and Te isotopes in the mass region A = 120–146 were performed within the framework of the proton–neutron quasiparticle random-phase approximation (pn-QRPA). The investigated nuclei extend from the vicinity of the valley of stability toward the neutron-rich region. Allowed Gamow–Teller (GT) transitions were calculated within the Schematic Model (SM) by including both particle–particle (pp) and particle–hole (ph) residual interactions, whereas first-forbidden (FF) transitions were treated within the schematic model by considering the particle–hole (ph) channel. Woods–Saxon single-particle energies were adopted as the mean-field basis, and all nuclei were assumed to be spherical. The calculated total β-decay half-lives, logft values, and transition strengths were systematically compared with the available experimental data and previous theoretical calculations. Overall, satisfactory agreement was obtained for both allowed and first-forbidden transitions. The microscopic structure of the calculated GT and FF resonance states was further analysed using their dominant proton–neutron two-quasiparticle configurations and the corresponding pn-QRPA amplitudes, thereby providing additional insight into the origin of the transition strengths. The fulfillment of the Ikeda sum rule confirms the internal consistency of the calculations. This paper also provides theoretical predictions for several transition channels for which experimental information is currently unavailable, offering useful benchmark data for future experimental investigations and nuclear-structure and astrophysical applications.

Universe doi: 10.3390/universe12080224

Authors: Shantong Yang Chengming Liu Jinbin Cao Yangyang Liu Xining Xing

Plasma jets are important carriers of mass and magnetic flux and are favorable regions for development of various waves and instabilities in Earth’s magnetotail. Here we conduct a comprehensive investigation of electron temperature anisotropy constraints from electron microinstabilities inside magnetotail jets. We find that data distributions (temperature anisotropy vs. electron parallel beta) are well constrained by electron microinstability thresholds, with stronger magnetic fluctuations near the instability thresholds, providing direct evidence for the operation of microinstability instabilities. We reveal that the jets’ leading and inner regions typically host stronger magnetic fluctuations than their trailing regions, indicating that the microinstabilities are typically developed in association with jets’ interaction with ambient plasma. We find that instability-related magnetic fluctuations generally become weaker during the jets’ Earthward propagation, indicating gradual relaxation of the jets’ free energy. These results suggest that electron microinstabilities play an important role in controlling electron thermodynamics in plasma jets.

Universe doi: 10.3390/universe12080223

Authors: Xin Zhong Shuping Shan Jianshan Wang

Rare charm decays offer a clean arena for visible massive dark photon searches through narrow dielectron resonances. In the process D0→γA′→γe+e−, the observable signal rate is controlled simultaneously by the flavor changing charm transition, the radiative D0→γ hadronic matrix element, the visible decay probability of A′, and the experimental response of the reconstructed mass spectrum. We introduce a visible coupling formulation that combines these ingredients into a single search parameter while keeping the form factor and the A′→e+e− branching fraction as explicit external inputs. This provides a direct bridge between branching fraction sensitivity and coupling reach, without assuming a specific experimental background model. The central novelty of this approach is that it separates the particle physics information entering the signal rate from the analysis-dependent ingredients that determine the observable mass spectrum. We apply this construction to the D0→γe+e− spectrum, including effective exposure, reconstruction efficiency, mass resolution, and the treatment of the ρ/ω and ϕ regions. The ρ/ω and ϕ dominated intervals are kept outside the coupling definition and left to experiment specific resonance treatment. This formulation gives a practical and reusable basis for comparing visible massive dark photon sensitivity across current and future charm data sets.

Universe doi: 10.3390/universe12080222

Authors: Luiz L. Lopes

In this second part, I discuss how to introduce non-atomic degrees of freedom in neutron stars’ core, as well as the role they play, using the formalism developed in Part I.

Universe doi: 10.3390/universe12080221

Authors: Saad Mohammed Alshehri Nazhatulshima Ahmad

Nova eruptions exhibit diverse photometric and spectroscopic properties traditionally classified using empirical light-curve morphology and spectral taxonomy. However, these observational classifications do not uniquely constrain the underlying eruption physics. This review examines the coupling between light-curve evolution, spectral development, ejecta dynamics, and multi-wavelength emission in classical and recurrent novae, with particular emphasis on the role of shocks and multi-phase outflows. Recent observations across optical, radio, X-ray, and gamma-ray wavelengths demonstrate that nova ejecta are intrinsically structured, anisotropic, and dynamically interacting systems, challenging the traditional interpretation of novae as spherically symmetric thermonuclear explosions. We synthesise observational and theoretical studies that link decline timescales, spectral transitions, expansion velocities, and high-energy emission to fundamental physical parameters, including white dwarf mass, accretion rate, ejecta geometry, and shock energetics. Using a compiled multi-parameter dataset of classical, recurrent, and symbiotic novae, we demonstrate that many commonly used observational diagnostics are intrinsically degenerate, with similar observable properties arising from different physical conditions. We argue that nova diversity is better understood within a continuous multi-dimensional parameter space rather than through purely empirical classifications. The implications of this framework for mass retention efficiency and the evolution of recurrent novae toward Type Ia supernova progenitors are discussed. Finally, we outline a predictive observational framework integrating photometric, spectroscopic, and high-energy diagnostics for future nova studies.

Universe doi: 10.3390/universe12080220

Authors: Claudio Furtado Antonio L. A. Moreira Jose R. Nascimento Albert Yu. Petrov Paulo J. Porfírio

We study gravitational lensing by k−n generalized black-bounce space-times both in regimes of weak and strong field approximations. These metrics interpolate between regular black holes and one-way or traversable wormholes. First, we investigate the light-like geodesic trajectories and derive an analytical expression for the deflection angle in terms of the bounce parameter in the weak-field gravitational regime. We then turn to the strong-field gravitational regime and display the behavior of the bending angle as a function of both the impact parameter and the bounce parameter. Next, using the lens equations, we analyze how the observables for Sagittarius A* behave concerning the bounce parameter. We obtain the shadow’s radii for some black-bounce metrics and plot the graph of their sizes, comparing them with the Schwarzschild one.

Universe doi: 10.3390/universe12080219

Authors: Spyridon Talaganis

Finite-dimensional closed-system gate synthesis is a geometric optimal-control problem on a compact Lie group, but global solutions require careful treatment of logarithm branches, determinant-one constraints, drift, anisotropic penalties, and amplitude limits. This paper assembles and extends a self-contained family of exactly solved benchmarks under explicit hypotheses. For a positive right-invariant quadratic metric, smooth stationary curves obey the Euler–Arnold equation and possess Lax invariants. Under fully actuated isotropic control on SU(n), the fixed-time minimum action is the squared Hilbert–Schmidt distance divided by twice the gate time and is generated by a minimum-norm skew-Hermitian logarithm. A finite eigenphase-unwrapping rule computes that logarithm, while the affine spectral-width cut wall and a branch-gap criterion distinguish stable selection from set-valued behavior. Drift is removed isometrically whenever the metric is invariant under the drift adjoint action. Weighted action and box-constrained time on maximal tori and arbitrary closed commuting subtori reduce to explicit period-lattice problems; the latter gives distinct strict determinant-one and projective period lattices for an ideal fSim sector. Numerical validation uses 1506 Haar-random targets through dimension 128, a complete cost-bounded dynamic-program cross-check for every target, 300 direct Cartesian branch enumerations, 120 random-conjugation tests with analytically prescribed spectra, 240 independently cross-checked random weighted-torus instances, a separate SU(256) stress target, large-cluster selector regressions, repeated timing trials, GRAPE–L-BFGS, a first-order sequential Krotov-type update with exact discrete propagation and Fréchet derivatives, randomized and drift-adapted CRAB–Powell bases, and cut-locus tests that exercise the implemented Schur/logarithm solver. When sparse actuation, decoherence, leakage, or uncertainty invalidates the ideal hypotheses, the exact results are positioned as ideal-model reference values, branch-aware seeds, and regression tests rather than as certificates for the enlarged objective.

Universe doi: 10.3390/universe12080218

Authors: Koushiki Włodzimierz Piechocki Grzegorz Plewa

We examine the existence of an apparent horizon in the collapse of an isolated dust cloud in spherically symmetric spacetime. The interior geometry is expressed in the Lemaître–Tolman–Bondi coordinates, while the outside geometry is the standard Schwarzschild. The novelty of the approach lies in using the Lemaître coordinates for the Schwarzschild part of the geometry when discussing junction conditions. We provide general formulas for expansions in the marginally bound cloud case and use them to determine the position of the apparent horizon. Our results indicate that in the region of spacetime far from the gravitational singularity, the system under consideration has a horizon, and thus the singularity is not naked. The results are expected to be correct away from the singularity, where quantum effects can be neglected. Near the singularity, general relativity breaks down, and quantum gravity would be of primary importance. The classical formulas for expansion scalars can serve as a starting point for more detailed quantum analysis.

Universe doi: 10.3390/universe12070217

Authors: Alexandre Winant Viviane Pierrard Edith Botek

When high-energy particles of cosmic origin penetrate the atmosphere, they collide with atmospheric atoms and molecules, triggering cascades of secondary particles that can propagate down to the surface. These energetic particles constitute a radiation background in the atmosphere with direct implications for aviation exposure safety and influence atmospheric chemistry through the ionization of ambient air. For these reasons, the development of secondary particle showers has been extensively studied, and considerable effort has been devoted to Monte Carlo-based models capable of computing secondary particle fluxes in the atmosphere. However, the atmospheric models in which particle transport is simulated are generally standard atmospheric models, which can differ substantially from actual atmospheric conditions. In this work, the Atmospheric Radiation Interaction Simulator (AtRIS) is used in conjunction with the NRLMSIS-2.1 atmospheric model to investigate the effects of varying density, pressure, and temperature profiles on secondary particle fluxes at different atmospheric levels. For a given incident radiation spectrum, the AtRIS simulations reveal substantial differences between atmospheric profiles when a uniform vertical cutoff rigidity is prescribed across all latitudes, effectively isolating atmospheric effects from geomagnetic shielding. Below ∼20 km, secondary particle fluxes are consistently higher in the polar atmosphere than at the equator. At ∼7 km, the difference between the equatorial and southern polar winter profiles reaches ∼40% for protons, photons, electrons, and positrons, and ∼30% for neutrons and muons.

Universe doi: 10.3390/universe12070216

Authors: Luis Alfredo Anchordoqui Dieter Lüst

The persistent challenge of creating stable de Sitter vacua within string theory undermines the observational validity of the Λ cold dark matter (CDM) model. This difficulty suggests that the concordance model of cosmology, characterized by a constant dark energy Λ, may reside in the swampland of inconsistent quantum gravity theories rather than the string landscape of consistent ones. Recent observational data, particularly from the Dark Energy Spectroscopic Instrument (DESI), have significantly challenged ΛCDM cosmology. Specifically, the combination of DESI baryon acoustic oscillation measurements with cosmological surveys seems to indicate a preference for a dynamic, time-evolving dark energy rather than a constant, with roughly 10% reduction in density over the last several billion years. This review summarizes significant advancements made over the past two years in linking DESI findings to string-inspired scenarios.

Universe doi: 10.3390/universe12070215

Authors: Alex E. Bernardini O. Bertolami

The interplay of Hoyle–Narlikar (HN) creation field cosmology and scalar field models for the dark sector, including the Generalized Chaplygin gas (GCG) model, is investigated. Though originating from distinct theoretical frameworks, both the inclusion and the non-inclusion of the creation field degree of freedom (DoF) involve a scalar DoF, which addresses some of the limitations of the standard ΛCDM model. Using a Lagrangian scalar field formulation and the first-order Hamiltonian reconstruction method, the HN c-field dynamics are shown to be encompassed by the GCG equation of state through an equivalent modified scalar field theory. Late-time acceleration and stability of linear perturbations are derived within this unified description. Our results suggest that creation field cosmologies may be embedded in a broader class of scalar field models which encompasses subtle modifications to the Hubble expansion rate and related physical observables.

Universe doi: 10.3390/universe12070214

Authors: Xiaotong Feng Xue-Bing Wu Yuming Fu Yuxuan Pang Rui Zhu Huimei Wang

The [O ii] λ3727 emission line is a widely used star formation rate (SFR) tracer. However, its application to type I quasars is not straightforward, because the line can be affected by dust extinction, metallicity and contamination from the AGN narrow-line region (NLR). We test the reliability of [O ii] SFRs using a sample of 202 SDSS and PG quasars, by comparing [O ii] SFRs and reference far-infrared (FIR) SFRs derived from multiwavelength SED decomposition. We measure [O ii], [O iii], and narrow Balmer emission lines by spectral fitting. Then, we calculate [O ii] SFRs after correcting dust extinction and metallicity. We then compare these SFRs with the FIR SFRs, both with and without subtracting the AGN contribution estimated from [O iii]. After this correction, the median offset between [O ii] and FIR SFRs is −0.20±0.72 dex for the full analysis sample and −0.17±0.69 dex for sources with S/N > 5 in both [O ii] and [O iii]. Without subtracting the AGN contribution, the corresponding offsets are 0.00±0.69 and 0.12±0.66 dex. We conclude that [O ii] is useful as a statistical SFR tracer for quasar host galaxies, but individual objects still require careful treatment of AGN contamination, extinction, metallicity, aperture effects, and redshift-dependent systematics.

Universe doi: 10.3390/universe12070213

Authors: Xiaotong Feng Xue-Bing Wu Yuming Fu Yuxuan Pang Rui Zhu Huimei Wang

Reliable star formation rates (SFRs) are essential for studying the connection between black hole growth and quasar host galaxies. We study the far-infrared (FIR) SFRs and the host galaxy properties of 202 SDSS and PG quasars at 0.02<z≲0.8, spanning log(SFRFIR/M⊙yr−1)≃−0.45–2.76, using multiwavelength spectral energy distribution (SED) decomposition. The photometry covers wavelengths from the optical to the FIR and is supplemented by JCMT/SCUBA-2 observations at 450 and 850 μm. We model the SEDs with CIGALE and AGNfitter and adopt multiple cold dust templates to quantify systematic uncertainties. The median model-dependent scatter among the five FIR SFR estimates is 0.14 dex, and AGNfitter gives FIR SFRs lower than the mean CIGALE estimate by a median of 0.09 dex. For the 58 quasars with SCUBA-2 coverage, including SCUBA-2 data changes the adopted FIR SFR by only ∼0.01 dex on average but can affect individual sources with limited Herschel coverage or radio-loud emission. Within our FIR-constrained sample, many quasar hosts lie on or above the star-forming main sequence, but the redshift-dependent FIR selection of the SDSS subsample limits conclusions about the full quasar-host population. We find no clear correlation between the main-sequence (MS) offset and the direct Eddington ratio, while the offset is positively related to the infrared-based Ltor/LEdd proxy. The minimum radiation field intensity in the dust model, Umin, increases with bolometric luminosity and dust temperature. WISE W2 (4.6 μm) and W3 (12 μm) combined with Herschel bands can also provide useful empirical indicators of fAGN.

Universe doi: 10.3390/universe12070212

Authors: Daniel Morales Hernández Gabriela Garcia-Arroyo J. Alberto Vazquez

The quest for a theoretical framework and ingredients that capture our current understanding of the cosmos has motivated the design of a large number of highly informative experiments, generating an abundant flow of data. Given this quantity of data and the need for thorough analysis, the main aim of this work is to present and assess the Particle Swarm Optimization (PSO) algorithm as a complementary tool to conventional cosmological data analysis techniques. PSO is one of the most representative bio-inspired algorithms, offering good robustness for high-dimensional or complex problems while remaining relatively simple to implement and requiring only a few hyperparameters. In this study, we employ two standard variants of the canonical PSO algorithm—global best and local best—to investigate dark energy models using measurements of Type Ia Supernovae and Baryon Acoustic Oscillations, focusing in particular on the DESI and DESI + Union3 datasets. Our findings demonstrate that PSO effectively recovers the best-fit parameters from observational data and show that, under suitable conditions, PSO can achieve results comparable to those of traditional MCMC techniques, but in a significantly reduced computation time. Moreover, the solutions obtained with PSO can be used as high-quality initial conditions for MCMC analyses, thereby accelerating their convergence.

Universe doi: 10.3390/universe12070211

Authors: Mengci Li Laixian Zhang Rong Li Yingchun Li Shiyu Deng Huichao Guo Haijing Zheng Huaili Zhang Yang Zhao Zhen Deng Rui Zhu

Estimating the shape of space objects helps infer key characteristics such as object type, mass, and potential operational status, thereby providing critical decision-making support for space threat assessment. This paper proposes a Physics-Informed Transformer deep learning model for space object shape classification based on photometric time series data. The model achieves a classification accuracy of 89.22% on a hybrid dataset combining simulated and measured data, outperforming eight classical models with an average accuracy improvement of 3.61%. Ablation experiments demonstrate that the introduction of physical gating yields an average accuracy improvement of 1.43%. Using evaluation metrics including the confusion matrix, PR curve, ROC curve, and confidence distribution histogram, we demonstrate that the model possesses high accuracy, strong robustness, and good interpretability.

Universe doi: 10.3390/universe12070210

Authors: Robert Brandenberger Jennie Traschen

This is a personal account of the early work that led to what is now known as the preheating stage of inflationary cosmology. The broader applicability of the underlying instability mechanisms in cosmology are indicated.

Universe doi: 10.3390/universe12070209

Authors: Lanqin Wang Rushuang Zhao Hui Liu Zefeng Tu Ruwen Tian Hongwei Xu Quan Zhou Dongyang Yan Yi Zhou Kun Yang Junjie Feng

The Crab pulsar produces extremely intense, short-duration radio bursts known as giant pulses (GPs). We introduce a cumulative-energy diagnostic to quantify the apparent spectral extent of individual Crab giant pulses across the ultra-wideband low-frequency (UWL) receiver band, aiming to build a reproducible method for describing the frequency-domain concentration of emission and to characterize the observed spectral diversity of Crab GPs. Using UWL receiver on the Murriyang (Parkes) radio telescope, we present a systematic study of GPs from the Crab pulsar (PSR J0534+2200). We introduce an empirical classification scheme based on the cumulative distribution function of the pulse energy as a function of observing frequency. We use this diagnostic to separate events with apparent spectral concentration from events with broader spectral coverage. Under this empirical classification scheme, most detected events show apparent spectral concentration within a limited frequency range. Events classified as apparently spectrally concentrated contain most of their measured relative energy within limited frequency ranges, whereas broadband events show more extended spectral coverage. We emphasize that this classification describes the observed spectral extent and should not by itself be interpreted as proof of intrinsically narrow-band emission. Spectral fitting shows that most apparently spectrally concentrated (ASC) GPs have negative spectral indices, while a few events exhibit positive slopes, indicating substantial spectral diversity within the sample. The 3σ widths of ASC main pulse GPs appear to cluster around two characteristic ranges, although this feature should be interpreted with caution given the finite time resolution of the data. The energy distribution of ASC main pulse GPs is broadly consistent with a log-normal functional form at low-to-intermediate energies and resembles a power-law-like tail at the high-energy end. The waiting-time distribution can be described by a Weibull function, while a sliding-window comparison with Monte Carlo realizations of a Poisson distribution shows no statistically significant deviation from temporal independence over the present 18.9-min observing span. The CDF-based classification method developed here is transferable to other wideband receiver data, provided that careful consideration is given to the instrumental bandpass, frequency-dependent sensitivity, RFI masking, and signal-to-noise thresholds. These results provide observational constraints on the phenomenology of Crab GPs and may be useful for future studies of pulsar coherent emission and related radio transients.

Universe doi: 10.3390/universe12070208

Authors: William Gyory

We review some recent findings on thermal properties of the magnetic dual chiral density wave (MDCDW) condensate in the Nambu–Jona-Lasinio (NJL) model of dense quark matter, as well as a convenient method for investigating this phase with a high-order Ginzburg–Landau (GL) expansion. We show how a recently discovered formula for the GL coefficients can be used to compute key physical properties of the condensate, such as its ground state order parameters and critical temperature in the mean-field approximation and its stability against thermal phonon fluctuations. We find that magnetic fields of order 1018 G significantly increase the condensate magnitude and critical temperature, eventually making the condensate favored up to temperatures a few times 10 MeV over the entire range of densities in the model. At much smaller fields, the condensate is still preferred and thermally stable over a range of densities relevant to cold neutron stars. We emphasize how the topological features of MDCDW are encoded in certain terms of the GL expansion, which can be used to show that the preceding effects have a topological origin. Finally, we present a new result on the convergence properties of the GL expansion, proving that it converges when |m|+|b|<μ2+(πT)2, where m and b are order parameters proportional to the condensate magnitude and spatial modulation, respectively. This condition holds over a large region of parameter space, including the region of interest for neutron star applications.

Universe doi: 10.3390/universe12070207

Authors: Arpine Piloyan Sergey Pavluchenko

The current paper addresses the possibility of Dark Energy scalar field potential reconstruction from SNe Ia data and the problems arising during the process. We describe the method and test its limits and features with use of synthetic data, as well as discuss several issues connected to error propagation. We conclude that the chosen smoothing method—binning of the data—introduces immense uncertainty amplification which limits the practical application of this method, leaving us with other alternatives. We also address the “instability of the reconstruction”, an important effect when a false scalar field—real or phantom—could be reconstructed if the Ωm and H0 parameters are wrongly estimated; a similar effect could be expected in other Dark Energy models as well.

Universe doi: 10.3390/universe12070206

Authors: Aurimas Vitkus Simonas Draukšas Thomas Gajdosik

We consider an extension of the Standard Model by an additional Higgs doublet and a Majorana neutrino, which we call the Grimus–Neufeld Model (GNM). For certain parameter choices the GNM can be compared to the Inert Doublet Model (IDM), which has a scalar dark matter candidate. This motivates that the scalars of the GNM could possibly contribute to dark matter. To check this, we present the tree level two-body decays of the heavy scalars of the GNM and compute the lifetime of the pseudoscalar in the IDM limit.

Universe doi: 10.3390/universe12070205

Authors: Tejinder P. Singh

We present a spectral-action framework for connecting generalized trace dynamics (GTD) to the structural form of the low-energy action of the observed Universe. The fundamental single-STM-atom Lagrangian is decomposed exactly into a purely bosonic sector, boson–fermion cross terms, and bifermionic terms. This sectorwise decomposition furnishes a dictionary to almost-commutative spectral geometry: the bosonic sector supplies a quadratic GTD Dirac functional built from the six split-biquaternionic differential directions together with octonionic vector/gauge fluctuations; the cross-sector supplies, under an explicit localization hypothesis, a sesquilinear fermionic pairing; and the bifermionic sector supplies the scalar/internal channel that is bosonized into the Higgs bridge field. We also record the principal-symbol link between the SO(3,3) BF variables and the four-dimensional leafwise Dirac operator. The two four-dimensional leaves of the six-dimensional base overlap in two common directions; from the observed (gravitational) leaf, the two nonintersecting directions of the complementary leaf are internal, so the second leaf is reinterpreted as the weak-interaction sector rather than as an independent spacetime—a reinterpretation stated here as an explicit hypothesis. Under stated assumptions—spontaneous localization, Euclidean continuation, six- to four-dimensional BF reduction, and a candidate observed-leaf finite geometry compatible with the E6/J3(OC) inputs—the bosonic heat-kernel expansion yields the structural low-energy classes of terms: Einstein–Hilbert gravity, Yang–Mills kinetic terms, and scalar kinetic and potential terms. In addition, we provide a candidate finite spectral triple with explicit finite trace invariants, verify that the localization map respects the one-generation lepton/quark representation split, identify visible color-singlet scalar channels with electroweak quantum numbers (1,2,±1/2), and exhibit a smooth regulator family with explicit cutoff moments (f0,f2,f4). Conversely, the assembled low-energy spectral action admits a natural inverse bilinear lift back to split bioctonionic trace dynamics. Every arrow of the construction is classified as an exact algebraic identity, an imported result, a working hypothesis, or an open problem. Under this classification, the paper offers a possible architecture for obtaining low-energy gauge–gravity physics from GTD, with conditional consistency checks and reductions; it is not a completed first-principles derivation of the Standard Model coupled to gravity.

Universe doi: 10.3390/universe12070204

Authors: Sebastian S. Chicaiza-Medina Juan Carlos Degollado

We examined the Einstein–Klein–Gordon system coupled to a nonlinear electrodynamics framework that asymptotically supports a Hayward spacetime. We explored the solution space of a static, spherically symmetric, complex scalar field minimally coupled to the gravitational field, known as Hayward boson stars. We constructed families of Hayward boson stars in the ground state, for different values of the charge parameter Q and different values of the central scalar field. One of the main results of our analysis is the fact that the existence of boson stars is guaranteed once the condition βQ>1.49661, where β is a parameter of the electrodynamic theory, is fulfilled. When this condition is not satisfied, the electrovacuum spacetime contains at least one horizon, and the spacetime can not support a scalar field configuration.

Universe doi: 10.3390/universe12070203

Authors: Tamás Kovács

The trans-Neptunian region (30–50 AU) is a dynamically structured reservoir of icy planetesimals whose orbital architecture reflects resonant dynamics, chaotic transport, and long-term gravitational sculpting by the giant planets. This review synthesizes recent developments in the dynamical investigation of trans-Neptunian objects (TNOs), with an emphasis on mean-motion and secular resonances, as well as chaotic diffusion, in a system whose growing observational census makes it an ideal testbed for chaos detection methods. Classical indicators, including Lyapunov exponents, MEGNO, SALI/GALI, and frequency map analysis, provide the quantitative backbone for mapping TNO phase space and are complemented by modern approaches such as Lagrangian descriptors, the FAIR resonance identification method, entropy-based chaos indicators, and recurrence plot divergence methods. An anomalous diffusion framework, in which mean squared displacement scales as a power law in time, further enables classification of sub- and superdiffusive orbital transport. Machine learning has emerged as a powerful complement to traditional dynamical methods: surrogate classifiers, deep neural network solvers, and hybrid physics–data-driven frameworks together extend reliable prediction horizons in chaotic regimes and open new routes for Bayesian inference of migration scenarios. The review concludes that the most promising path forward lies in hybrid dynamical–statistical frameworks anchored to Hamiltonian dynamics, enabling efficient exploration of high-dimensional parameter spaces informed by the expanding body of trans-Neptunian observations.

Universe doi: 10.3390/universe12070202

Authors: S. Mallik F. Gulminelli C. Ciampi D. Gruyer

The density dependence of nuclear symmetry energy remains one of the key uncertainties in contemporary nuclear physics, with significant implications for the structure of exotic nuclei, the dynamics of heavy-ion collisions, and the properties of astrophysical objects such as neutron stars and core-collapse supernovae. However, extracting robust constraints requires observables that are minimally affected by final-state interactions and are reliably predicted by transport models. This review synthesizes recent theoretical and experimental advancements in constraining the symmetry energy by leveraging isospin diffusion in heavy-ion reactions within the Fermi energy domain. Recent results from the INDRA-FAZIA collaboration, including isospin transport ratio data and Boltzmann–Uehling–Uhlenbeck (BUU) transport model calculations, are highlighted. Confidence regions for the symmetry energy are extracted from isospin transport ratios and isospin diffusion currents by utilizing state-of-the-art nuclear functionals, including both ab initio and phenomenological approaches, with a particular focus on the density regions probed by these experiments. The resulting constraints will aid future Bayesian studies of the nuclear equation of state and contribute to a more unified understanding of dense matter in both terrestrial experiments and astrophysical environments.

Universe doi: 10.3390/universe12070201

Authors: Pablo Reig

Be/X-ray binaries constitute the largest and most diverse subgroup of neutron star high-mass X-ray binaries. These systems feature a rapidly rotating Be star surrounded by a circumstellar decretion disk that serves as the primary reservoir of accreted matter onto a strongly magnetized neutron star. While a few Be/X-ray binaries remain persistently active, the majority manifest as hard X-ray transient sources, becoming detectable only during X-ray outbursts. This review synthesizes current understanding of their rich phenomenology across optical and X-ray wavelengths, focusing on variability ocurring over timescales that span from seconds to years.

Universe doi: 10.3390/universe12070200

Authors: Sanjib Dey Andreas Fring

We develop a Bohmian analysis of a two-dimensional ghost Hamiltonian and its mapping to the degenerate Pais-Uhlenbeck model. Using Gaussian wavepackets, we derive the corresponding guidance equations, the centre and width evolution, and the quantum potential. We use these quantities to characterise bounded, quasi-semiclassical, spiral, and runaway regimes. The Bohmian trajectories provide a direct dynamical diagnostic of coherence, packet deformation, and quantum–classical separation. We then compare a bi-Hamiltonian pair consisting of the ghost Hamiltonian and a classically equivalent alternative formulation. While the two descriptions produce identical classical trajectories, they lead to different Bohmian trajectories and different quantum potentials evaluated along those trajectories. This demonstrates that classical equivalence need not extend to Bohmian quantum dynamics and identifies a concrete quantum ambiguity in the degenerate higher-derivative system.

Universe doi: 10.3390/universe12070199

Authors: Jinhao Fan Zhenyang Huang Chengze Li Cunshi Wang Yu Zhang Ying Wu Yuyang Li

Nighttime ground-based astronomical observations are often hindered by unpredictable cloud cover, which significantly reduces observing efficiency and complicates manual schedule adjustments. We present a deep learning-based method that automatically reorders observation sequences to avoid cloud-obscured sky regions. A semantic segmentation dataset of all-sky fisheye images is constructed, and a DeepLabV3-MobileNetV3 model is trained to classify “observable” versus “unobservable” areas in real time. Through astrometric calibration, each pixel is precisely mapped to altitude–azimuth coordinates, enabling the system to check whether a scheduled target falls into an observable region. When a target is predicted to be obstructed, a rule-based replanning module dynamically selects a suitable alternative from the remaining targets, respecting altitude and Moon-separation constraints. The method is validated on real observation sequences from the Nearby Galaxy Supernova Survey at Xinglong Observatory. Replanned sequences achieve observable rates above 90% under partially cloudy conditions, compared to original rates often below 10%. This work demonstrates that integrating all-sky camera semantic segmentation with astrometric calibration and intelligent rescheduling can robustly mitigate cloud-induced downtime, paving the way toward fully autonomous observatory operations and embodied intelligent telescopes.

Universe doi: 10.3390/universe12070198

Authors: Preston Jones Logan Finke Joseph Ribaudo

With the detection of the Higgs boson, the standard model is an almost complete theory. Missing from the model is a massless spin-two boson, the graviton. We might reasonably expect that detection of gravitational radiation would include the observation of the graviton as the quanta of the radiation. However, the connection between gravitational radiation observations and the theoretical graviton has yet to be realized due to the physical limitations on single-graviton detections. A more promising approach to the demonstration of the non-classical nature of gravitational radiation is to look at quantum entanglement in bipartite detections.

Universe doi: 10.3390/universe12070197

Authors: Da-Ding Zhang Mu-Zi Li Niu Liu

Stellar-aberration-based navigation requires angular measurements at the milliarcsecond (mas) level. While random sensor noise can be reduced by temporal integration, plate-solution uncertainty and residual geometric distortion may set a practical astrometric error floor. Here, we quantify the plate-model contribution to this error budget and examine its impact on the feasibility of stellar-aberration-based navigation. Using Gaia DR3 stars, HEALPix all-sky sampling, and covariance propagation to epoch J2026.0, we evaluate nine polynomial plate models while accounting for reference-star density and spatial distribution. We identify a bias–variance trade-off between model complexity, distortion-correction capability, and numerical stability. For the adopted ∼1° sparse-field configuration, the four-parameter linear model gives the lowest plate-constant variance, with a median of 0.95 mas and a 95th percentile of 1.7 mas. Using the first-order scaling of δv∼c δθ, this uncertainty corresponds to an approximate velocity-error scale of 0.9–2.5 m/s. These results show that plate-model errors can contribute at the meter-per-second level and must be included explicitly in StarNAV filter design.

Universe doi: 10.3390/universe12070196

Authors: Alisson P. Costa Andre L. B. Ribeiro Zhonglue L. Wen Flavio R. Morais-Neto

Galaxy clusters are key probes of cosmology and structure formation, yet their dynamical classification traditionally relies on spectroscopic redshifts, which do not scale efficiently with survey size. As large photometric surveys such as LSST become available, photometric redshifts offer an attractive alternative, but their impact on velocity-based diagnostics remains poorly constrained. We quantify the sensitivity of two Gaussianity diagnostics—the Anderson–Darling (AD) test and Gaussian mixture modeling (Mclust)—to different photometric redshift error prescriptions. By propagating Gaussian and Student-t uncertainties through velocity distributions constructed from SDSS photometric redshifts, we assess how the choice of error model affects the recovery of cluster dynamical states established by the independent Γ morphological proxy. Using 1672 SDSS clusters with pre-existing relaxation parameters (Γ), we perform Monte Carlo resampling under Gaussian and Student-t error models, the latter used to mimic heavy-tailed uncertainties and catastrophic outliers. We also conduct a spectroscopic control experiment in which mock photometric redshifts are generated from spectroscopic measurements. Under Gaussian errors, relaxed clusters are recovered in ∼95% of realizations, whereas unrelaxed systems are detected in only ∼5%, revealing a strong bias toward relaxed classifications. Student-t errors reduce relaxed recovery to ∼60–70% and increase unrelaxed recovery to ∼30–45%, although this remains incomplete. Paired Wilcoxon tests confirm that these differences are statistically significant. This limitation has direct implications for large photometric surveys, suggesting that dynamical studies based primarily on photometric data may significantly underestimate the fraction of disturbed clusters unless supported by robust spectroscopic calibration, catastrophic-outlier mitigation, and validation with realistic survey mock catalogs.

Universe doi: 10.3390/universe12070195

Authors: Rojin Bayati Paolo Salucci

We exploit the halo densities of three dwarf spheroidals recently obtained to investigate their dark matter distribution. We address the well-known DM halo core-cusp issue by finding that the NFW ΛCDM halo profile is in strong disagreement with these inferred halo densities, which instead result very well-reproduced by the Burkert halo cored profile with the central density ρ0 and the core radius r0. These structural parameters result connected among themselves and with the length scale of the stellar matter component in a way similar to that present in disk systems. By analyzing the properties of dark halo densities in their innermost regions, we find no trace of a central intermediate-mass black hole or of a nuclear stellar cluster.

Universe doi: 10.3390/universe12070194

Authors: Chen Zhang Wen-Biao Han

The effective-one-body (EOB) formalism accurately describes the conservative dynamics of general binary orbits, but current implementations of radiation reaction remain largely limited to quasi-circular inspirals. Extending EOB to eccentric orbits currently requires the corresponding post-Newtonian (PN) energy fluxes in EOB coordinates, which are only known to 1PN order. In this paper, we compute the instantaneous gravitational-wave energy flux in EOB coordinates to 2PN accuracy using a systematic coordinate transformation between the Arnowitt–Deser–Misner (ADM) and EOB phase-space variables. We derive the 2PN-accurate transformation laws for the relative coordinates and velocities between the two coordinate systems and re-express the 2PN instantaneous energy flux entirely in EOB variables. Working within the EOB test-particle framework (with finite mass ratio ν) for an equatorial elliptic orbit, we adopt a Keplerian reparameterization in terms of the semilatus rectum p, eccentricity e, and two phase variables (ξ,ϕ) associated with the radial and azimuthal motion, which makes the calculations more transparent and facilitates the subsequent computation of gravitational waveforms. Using the conservative orbital angular frequency, we compute the orbit-averaged energy flux. In addition to the energy flux, we also compute the corresponding 2PN angular momentum flux in EOB coordinates using the same transformation method. Our results, expressed in gauge-invariant variables x=(Mω)2/3 and eccentricity et, agree with known PN results and show improved accuracy compared to the 1PN EOB fluxes of Hinderer and Babak.

Universe doi: 10.3390/universe12070193

Authors: Rajendra P. Gupta Nikolaos Samaras

We investigate whether Big Bang nucleosynthesis (BBN) remains compatible with the covarying coupling constants plus tired light (CCC+TL) cosmology. In this framework, only quantities with explicit length dimensionality covary through a universal scaling function f(z), while dimensionless constants and dimensionless ratios remain invariant. At the redshifts z relevant to BBN, f(z) approaches a constant plateau fmax≃3, and the tired light contribution is negligible, so the early-time dynamics reduce to a global rescaling of dimensioned quantities. In particular, the Hubble expansion rate H at fixed temperature T satisfies HCTLT=fmax−1HΛCDM(T), implying a longer cooling time Δt between weak freeze-out and the onset of nucleosynthesis by the same factor (CCC+TL labelled as CTL). We find that BBN predictions are preserved, provided the relevant interaction rates Γ and decay rates governing the neutron lifetime τn share the same plateau scaling as H, so that governing combinations such as Γ/H and exp (−Δtτn) remain invariant. Implementing these plateau rescalings in the Kawano/NUC123 network (via a single control parameter fctl ≡fmax) yields identical light-element abundances for fctl =1 (ΛCDM) and fctl =3(CCC+TL) to within 10−3−10−4 level, consistent with numerical rounding. We also illustrate that adopting the lower late-time CCC+TL baryon density from Pantheon+ data fit can reduce the 7Li discrepancy but simultaneously increases D/H, implying that BBN alone does not select between the late-time baryon density inferences considered here.

Universe doi: 10.3390/universe12070192

Authors: Alexander S. Sakharov Rostislav Konoplich Merab Gogberashvili Jack Simoni

We propose an infrared mechanism for alleviating the Hubble constant tension, based on a small departure from entanglement equilibrium at the cosmological apparent horizon. If the horizon entanglement entropy falls slightly below the Bekenstein–Hawking value, we parametrize the shortfall by a fractional deficit δ(a) evolving with the FLRW scale factor a. The associated equipartition deficit at the Gibbons–Hawking temperature then sources a smooth, homogeneous component whose density scales as H2/G, with a dimensionless coefficient ce2(a) of order unity times δ(a). Because this component tracks H2, it is negligible at early times but can activate at redshifts z≲1, raising the late-time expansion rate by a few percent without affecting recombination or the sound horizon. We present a minimal three-parameter activation model for ce2(a) and derive its impact on the background expansion, the effective equation of state, and linear growth for a smooth entanglement sector. The framework predicts a small boost in H(z), a mild suppression of fσ8(z), and a corresponding modification of the low–z distance–redshift relation. We test these predictions against current low–redshift data sets, including SN Ia distance moduli, baryon acoustic oscillation distance measurements, cosmic chronometer H(z) data, and redshift space distortion constraints. We then discuss whether the H0 tension can be consistently interpreted as a late–time, horizon–scale information deficit rather than an early universe modification.

Universe doi: 10.3390/universe12070191

Authors: Richard Dvorsky

This paper presents a further step in the development of scale invariant fully conformal cosmology (FCC), formulated in our previous study. Whereas the previous paper focused mainly on the global cosmological consequences of the fully conformal metric and their confrontation with selected astrophysical data, here we analyze its local gravitational and background consequences. On the background of the fully conformal metric we formulate an effective generalization of the weak Schwarzschild field in the corresponding FCC global coordinates and derive from it the associated modified intensity of the Newtonian central field. We further derive the cosmological state/constitutive equation p = − ε/3 as a direct consequence of the fully conformal metric rather than as an ad hoc additional postulate. Likewise, within the fully conformal metric, spatial flatness and the critical density ρcrit are understood as direct consequences of this metric structure rather than as independently postulated inputs. From the condition of global equilibrium between negative cosmological pressure and the gravitational cohesive pressure of homogeneously distributed matter, the effective particulate fraction is obtained as β ≈ 0.45 of the total critical density ρcrit. For the relatively well-confirmed baryonic matter fraction Ω¯bar ≈0.05, this stable-equilibrium condition then leads to the corresponding particulate fraction of collisionless dark matter Ω¯FCCdm ≈0.40, which is in principle determined by the global cosmological equilibrium within this framework. Because direct identification of the entire dark fraction with standard collisionless cold dark matter would very probably be incompatible with the main structural observables, we discuss an effective phenomenological decomposition into a structuring cold dark matter component (cdm) and an almost homogeneous residual warm-dark-matter-like component (wdm). In this interpretation, the paper preserves the previously introduced global FCC framework while simultaneously providing a concrete background prediction for the matter content and a physically motivated basis for further testing of structure formation within scale invariant fully conformal cosmology.

Universe doi: 10.3390/universe12070190

Authors: Luiz Augusto Stuani Pereira

Fast radio bursts (FRBs) are millisecond-duration radio transients of extragalactic origin, while ultra-high-energy cosmic rays (UHECRs; E≳1018 eV) remain among the most important unresolved problems in astroparticle physics. This review examines the viability of FRBs and their central engines as sources of UHECRs within a comprehensive multi-messenger framework. We summarize the observational constraints on UHECR source populations imposed by the energy spectrum, nuclear composition, anisotropy measurements, diffuse γ-ray background, and high-energy neutrino observations, which, together, favor source classes capable of accelerating heavy nuclei with hard injection spectra, modest cosmological evolution, and sufficiently high source densities. We then review the current landscape of FRB progenitor and engine models, including magnetars, supramassive neutron stars, compact-object mergers, and accretion-powered systems, emphasizing their energetics, environments, and particle-acceleration capabilities through relativistic shocks, magnetic reconnection, magnetar wind nebulae, and direct electromagnetic acceleration by ultra-relativistic FRB pulses. We discuss how these scenarios are constrained by neutrino and γ-ray observations from IceCube, KM3NeT, and Fermi-LAT, as well as by large-scale UHECR anisotropy measurements from the Pierre Auger Observatory and Telescope Array. Finally, we examine the observational tests that will become possible in the coming decade through large samples of localized FRBs, composition-resolved UHECR measurements, next-generation neutrino observatories, and wide-field γ-ray facilities. We emphasize that FRB dispersion and rotation measures provide unique probes of the baryonic and magnetic environments relevant for UHECR acceleration and propagation, enabling a new form of multi-messenger tomography of cosmic-ray source environments and allowing the FRB–UHECR connection to become a quantitatively testable astrophysical framework.

Universe doi: 10.3390/universe12070189

Authors: Eric Bergshoeff Kevin Grosvenor Luca Romano Ziqi Yan

We review the recent developments on the decoupling limit zooming in on a BPS string state in heterotic string theory. In this limit, the target-space geometry lacks any ten-dimensional metric description and acquires various non-Lorentzian features. We present the supersymmetric worldsheet sigma model in the decoupling limit. This sigma model describes the heterotic version of non-relativistic string theory, whose second quantization is related to heterotic matrix string theory. We introduce bookkeeping to separate the Yang–Mills gauge and gravitational anomaly analyses. We review the Yang–Mills gauge anomaly analysis in the sigma models before and after the decoupling limit is performed and provide further details on how the quantum calculation commutes with the decoupling limit. We comment on how the analogous calculation can be performed for the gravitational anomaly. This contribution is based on a talk at the University of Science and Technology of China.

Universe doi: 10.3390/universe12070188

Authors: Spiros Cotsakis

We develop a physics-facing version of the persistence/transition-variety framework for scalar-field cosmology, which is tailored to inflationary dynamics. The guiding idea is that observationally viable inflationary models are often best understood not as single asymptotic phases but rather as concatenations of persistent regimes separated by universal transition episodes. In this picture, slow roll appears as a robust persistent balance, ultra-slow roll as a bottleneck passage near a nonhyperbolic organising set, and oscillatory post-inflationary behaviour as a recurrent exit sector. Using the exponential model as a reference regime atlas and the massive case as a dynamical realisation of slope drift, we show how such histories may be organised and read geometrically. The resulting framework makes explicit that the relevant regime transitions are organised precisely where hyperbolicity is lost or the spectrum crosses the imaginary axis, and they are therefore invisible to a purely hyperbolic or asymptotic treatment.

Universe doi: 10.3390/universe12070187

Authors: Shou-Hang Wang Xue-Rui Ouyang Ming-Xuan Lu Yun-Feng Liang

We investigate the correlations between IceCube high-energy neutrinos and Fermi-LAT γ-ray sources using an unbinned likelihood analysis. In previous analyses of the same IceCube public dataset, only the spatial information of neutrino events was utilized, while the energy term in the probability density functions (PDFs) was neglected, limiting the achievable sensitivity. In this work, we incorporate both spatial and energy terms into the likelihood, with the energy PDFs constructed from the effective areas and smearing matrices. We focus on the Third Catalog of Hard Fermi-LAT Sources (3FHL) and the Fourth LAT AGN Catalog (4LAC-DR2). To account for the significant difference in IceCube’s sensitivity between the two hemispheres, we perform stacking analyses for the all-sky, Northern hemisphere, and Southern hemisphere source subsets separately, under both equal weighting and flux weighting schemes. No statistically significant neutrino excess is found in any configuration. We therefore derive 95% confidence level upper limits on the total neutrino flux contributed by these source populations. For a spectral index of Γ=−2.5, the all-sky stacking analysis indicates that the 3FHL and 4LAC-DR2 populations contribute at most 3.12% and 2.83% (equal weighting), and 4.45% and 3.49% (flux weighting) of the IceCube diffuse neutrino flux, respectively. Compared to the spatial-only analysis, the inclusion of the energy term improves the constraints on hard-spectrum emission by over one order of magnitude. Our results further demonstrate that the 3FHL and 4LAC-DR2 sources are subdominant contributors to the diffuse astrophysical neutrino flux observed by IceCube.

Universe doi: 10.3390/universe12060186

Authors: Zewei Chen Qiong Chen

This paper addresses the imaging crosstalk problem in 2D spectra from the LAMOST Phase II upgrade, caused by increased fiber density. We propose HyperDecouple_Net, a hypernetwork-based decoupling algorithm designed to overcome key limitations of existing deep learning models, including overlapping-layer collapse and structural distortion. The method integrates an adaptive overlapping-layer enhancement module, a dual-scale hypernetwork differential decoupling module, and a linear consistency constraint module. Additionally, we introduce LAMOST-SD-2026, a public dataset comprising 15,500 linearly superimposed spectral samples with ground-truth labels, derived from real LAMOST Phase I observations. Experimental results on this dataset show that HyperDecouple_Net achieves superior performance, with a PSNR_A of 12.71 dB, PSNR_B of 10.87 dB, SSIM_B of 0.3895, and SAM of 0.4841, outperforming both traditional methods (e.g., NMF, ICA) and recent deep learning approaches. The proposed method can be directly integrated into the LAMOST Phase II preprocessing pipeline, offering a robust solution for high-precision spectral decoupling and supporting the scientific output of the survey.

Universe doi: 10.3390/universe12060185

Authors: Vedant Mokal Shantanu Desai

Testing the validity of the cosmological principle’s assumption of large-scale isotropy remains crucial for modern cosmology. We investigate the angular distributions of gamma-ray bursts using the GRB catalog from Neil Gehrels Swift Observatory (Swift) for an independent probe of isotropy. Using the HEALPix spherical harmonic decomposition, we estimate the dipole and quadrupole amplitudes and compare them against the null hypothesis obtained from 500 isotropic Monte Carlo realizations. Our results show 2.9σ dipole and 7.2σ quadrupole amplitude when applied to the raw data. To account for observational biases, we then create an exposure map using the pointing history, roll angle, and the partial coding fraction of the Swift Telescope. Reevaluating the null hypothesis using this map reduces the significance of these anisotropies to less than 1σ. Therefore, our findings confirm statistical isotropy of the GRB sky using the Swift data, consistent with previous studies. We have also made the Swift exposure map publicly available.

Universe doi: 10.3390/universe12060184

Authors: Rositsa Miteva Susan W. Samwel Momchil Dechev

This study presents the relationship between the typical parameters of the solar energetic protons (SEPs) and their solar origin—solar flares (SFs) and coronal mass ejections (CMEs) in solar cycles (SCs) 23 and 24 (1996–2019). In this study, the calculated onset-to-peak SEP and reported SF fluences are preferred over the peak SEP intensity and SF class, respectively. The energy dependence of the proton fluence is quantitatively assessed in terms of correlation analyses (Pearson and partial) with the parameters of the solar origin, i.e., SF fluence, CME speed and angular width. The energy trends of the results are investigated as a function of the SC (SC23 vs. SC24), helio-longitude (Eastern vs. Western), SEP magnitude (high vs. low) and SEP profile type (fast vs. slow-rising). The possible applications to space weather research are discussed.

Universe doi: 10.3390/universe12060183

Authors: Rostislav D. Nikandrov Sergei B. Popov

Robust age measurements for isolated neutron stars (NSs) are not easily available. That is why the characteristic age τch=P/2P˙ is often used as a proxy. Here, P is the spin period of the NS and P˙ is the time derivative of P. Additional assumptions related to the initial properties and spin-down evolution are made to derive τch. As a result, it is expected that τch is an upper limit for the real age τreal. Recently, Chrimes et al. presented measurements of kinematic ages τkin for several magnetars. Surprisingly, for the majority of these sources, τkin>τch. We present a simple model that includes a realistic approximation for magnetic field decay in magnetars and a simple phenomenological description of field re-emergence following fallback after the birth of an NS. We demonstrate that this simple model can explain the observed relation τkin>τch for a realistic set of parameters.

Universe doi: 10.3390/universe12060182

Authors: Meir Shimon

On background cosmological scales, after subtraction of peculiar velocities and local bound-system motions, observed cosmological signals are redshifted rather than blueshifted. Yet, redshift alone does not distinguish the past lightcone of an expanding Universe from the future lightcone of a contracting one. In practice, the identification of the observed redshifted branch with the observational past is set primarily by electromagnetic radiation, whose retarded character is independently established in controlled physics, albeit over non-cosmological scales. From that perspective, the observed cosmological arrow is not separable from the causal/radiative prescription used to interpret the signals. This effective entanglement between the cosmological and the radiative arrows should nevertheless be distinguished from the notion of arrow used in the present work. Here instead, the relevant arrow is not thermodynamic but kinematic; it is defined by the symmetry or asymmetry of background lightcone observables under ξ↔−ξ, where ξ≡ln(1+z) and z is the redshift—a criterion motivated directly by the time-reversal-symmetric special-relativistic longitudinal Doppler shift. Equivalently, the arrow considered here is the observed redshift/blueshift asymmetry of cosmological lightcone signals; retarded observations of an expanding FRW Universe are in the redshifted branch, whereas the opposite rapidity orientation would correspond to the blueshifted branch. This naturally suggests using rapidity-reversal symmetry as the redshift-space no-arrow condition when passing from special relativity (SR) to Friedmann–Robertson–Walker (FRW) cosmology, where the empty Milne Universe is a bridging borderline case. In fact, the viewpoint advocated here is that ξ-symmetry/asymmetry is practically more fundamental than t-symmetry/asymmetry simply because the former is more readily related to cosmological observables. It is shown here that generic non-empty FRW Universes possess an intrinsic ξ-asymmetry already at the background level, independently of entropy, coarse-graining, structure growth, or a Past Hypothesis.

Universe doi: 10.3390/universe12060181

Authors: Wenjun Zhang Anhua Zhou Lei Yuan Yuchen Liang Yihan Song Zhenping Yi

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has collected tens of millions of spectra, providing an unprecedented resource for large-scale spectroscopic studies. Efficient retrieval techniques are therefore essential for exploring such massive datasets. Existing approaches often rely on predefined templates or manually labeled training samples, which can limit their applicability in large and diverse spectral archives. In this work, we present a general similarity-retrieval framework that combines self-supervised contrastive learning based on a convolutional neural network with Facebook AI Similarity Search (FAISS) for efficient large-scale spectral retrieval. The framework learns spectral representations directly from unlabeled data and enables flexible retrieval from user-defined wavelength regions based on feature similarity. We evaluate the framework on several stellar populations in LAMOST DR8. For late-type M8-star retrieval, 90.5% of the top 1000 retrieved spectra are later than M6. For M0–M5 giants, the mean retrieval accuracy across six subtypes reaches 94.8%. Using a C-H star spectrum as the query spectrum, 90.8% of the top 1000 retrieved candidates are classified as carbon stars by the LAMOST pipeline. Cross-matching with SIMBAD further confirms 255 C-H stars and 47 C-R stars among the retrieved candidates. These results demonstrate that the proposed framework can efficiently identify spectrally similar objects across large spectroscopic databases and can serve as a useful tool for searching for rare or spectrally distinctive stellar populations.

Universe doi: 10.3390/universe12060180

Authors: İzzet Sakallı

We present a path-based curvature representation of the gravitational bending of light in black-hole (BH) spacetimes. The bending angle is written as a one-dimensional line integral of the optical Gaussian curvature Kopt along the photon trajectory, weighted by a geometric kernel W(r,b). This representation sits within the Gibbons–Werner Gauss–Bonnet (GB) optical-geometry family rather than alongside it: the kernel is fixed by a co-area reduction of the GB surface integral along an undeflected reference path, and the single new computational object is the resulting radial integral together with its cumulative, directly plottable reading of how the deflection builds up along the ray. With the lever-arm choice W=r2−b2, the integral reproduces α^=4M/b for every static, asymptotically flat metric (Theorem 1) and evaluates in closed form for Schwarzschild, Reissner–Nordström (RN), and equatorial Kerr. The representation becomes reliable at a large impact parameter; at the small impact parameters relevant to horizon-scale imaging, it is not numerically competitive with the standard expansions, a limitation we quantify. Beyond leading order the kernel must import information from the bent geodesic, after which the scheme reconstructs the known perturbative series; the second-order mismatch in the lever-arm result therefore measures, rather than hides, the deformation of the photon path away from the straight-line reference. Finite source–observer distances enter through the Ono–Ishihara–Asada (OIA) construction, and a winding-sum continuation outlines the route toward the strong-deflection regime, whose closed-form reduction is left to future work.

Universe doi: 10.3390/universe12060179

Authors: Sergei M. Kuzenko

We review the general formalism of duality rotations for N-extended (super)conformal gauge multiplets of arbitrary (super)spin in four dimensions, with N≥0. Self-dual models for a vector field (N=0) and for N=1 and N=2 vector supermultiplets are naturally formulated on general (super)gravity backgrounds. For all other (super)spin values, the corresponding self-dual systems are realised on arbitrary conformally flat backgrounds. Every U(1) duality-invariant model is demonstrated to be self-dual with respect to a Legendre transformation. Methods are described to generate such self-dual models, including superconformal ones. We show that every model for self-dual nonlinear electrodynamics admits a higher-spin extension. Throughout this review, we make use of the formalism of conformal (super)space, which is the geometric setting to describe the gauge theory of the (super)conformal group.