Universe

44 pages, 2214 KB

Open AccessReview

Breaking Free from the Swampland of Impossible Universes Through the DESI Portal

by Luis Alfredo Anchordoqui and Dieter Lüst

Universe 2026, 12(7), 216; https://doi.org/10.3390/universe12070216 - 21 Jul 2026

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 [...] Read more.

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

Λ

CDM 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. Full article

(This article belongs to the Special Issue String Theory and Particle Physics: A Themed Issue in Honor of Professor Ignatios Antoniadis on the Occasion of His 70th Birthday)

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17 pages, 596 KB

Open AccessArticle

On Dark-Sector Scalar Field Theories Driven by Cosmological c-Fields

by Alex E. Bernardini and O. Bertolami

Universe 2026, 12(7), 215; https://doi.org/10.3390/universe12070215 - 17 Jul 2026

Abstract

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 [...] Read more.

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

Λ

CDM 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. Full article

(This article belongs to the Section Cosmology)

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25 pages, 1319 KB

Open AccessArticle

Testing [O ii] λ3727 as a Star Formation Rate Tracer in Quasar Host Galaxies

by Xiaotong Feng, Xue-Bing Wu, Yuming Fu, Yuxuan Pang, Rui Zhu and Huimei Wang

Universe 2026, 12(7), 214; https://doi.org/10.3390/universe12070214 - 16 Jul 2026

Abstract

The [O ii]

λ 3727

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 [...] Read more.

The [O ii]

λ 3727

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 \pm 0.72

dex for the full analysis sample and

- 0.17 \pm 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 \pm 0.69

and

0.12 \pm 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. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

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42 pages, 3818 KB

Open AccessArticle

Far-Infrared Star Formation Rates of Quasar Host Galaxies from Multiwavelength Spectral Energy Distribution Decomposition

by Xiaotong Feng, Xue-Bing Wu, Yuming Fu, Yuxuan Pang, Rui Zhu and Huimei Wang

Universe 2026, 12(7), 213; https://doi.org/10.3390/universe12070213 - 16 Jul 2026

Abstract

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

, [...] Read more.

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 ({SFR}_{FIR} / 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

μ

m. 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

L_{tor} / L_{Edd}

proxy. The minimum radiation field intensity in the dust model,

U_{\min}

, increases with bolometric luminosity and dust temperature. WISE W2 (4.6

μ

m) and W3 (12

μ

m) combined with Herschel bands can also provide useful empirical indicators of

f_{AGN}

. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

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21 pages, 1342 KB

Open AccessArticle

Cosmological Parameter Estimation Using Particle Swarm Optimization

by Daniel Morales Hernández, Gabriela Garcia-Arroyo and J. Alberto Vazquez

Universe 2026, 12(7), 212; https://doi.org/10.3390/universe12070212 - 16 Jul 2026

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Section Cosmology)

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22 pages, 3651 KB

Open AccessArticle

Research on Physics-Informed Transformer for Space Object Shape Classification

by Mengci Li, Laixian Zhang, Rong Li, Yingchun Li, Shiyu Deng, Huichao Guo, Haijing Zheng, Huaili Zhang, Yang Zhao, Zhen Deng and Rui Zhu

Universe 2026, 12(7), 211; https://doi.org/10.3390/universe12070211 - 15 Jul 2026

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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10 pages, 261 KB

Open AccessEssay

Inflationary Reheating to Preheating—A Personal Account

by Robert Brandenberger and Jennie Traschen

Universe 2026, 12(7), 210; https://doi.org/10.3390/universe12070210 - 13 Jul 2026

Abstract

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. Full article

(This article belongs to the Section Cosmology)

30 pages, 9760 KB

Open AccessArticle

Observations of Crab Pulsar Giant Pulses with the Murriyang Ultra-Wideband Low-Frequency (UWL) Receiver

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

Universe 2026, 12(7), 209; https://doi.org/10.3390/universe12070209 - 11 Jul 2026

Abstract

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 [...] Read more.

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 3

σ

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. Full article

(This article belongs to the Section Compact Objects)

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30 pages, 7655 KB

Open AccessReview

Extracting Phase Structure and Stability of the Magnetic Dual Chiral Density Wave from a Ginzburg–Landau Expansion

by William Gyory

Universe 2026, 12(7), 208; https://doi.org/10.3390/universe12070208 - 11 Jul 2026

Abstract

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 [...] Read more.

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

10^{18}

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 | < \sqrt{μ^{2} + {(π T)}^{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. Full article

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

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16 pages, 637 KB

Open AccessArticle

On the Stability of Dark Energy Scalar Field Reconstruction from SNe Ia Data

by Arpine Piloyan and Sergey Pavluchenko

Universe 2026, 12(7), 207; https://doi.org/10.3390/universe12070207 - 9 Jul 2026

Abstract

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 [...] Read more.

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

Ω_{m}

and

H_{0}

parameters are wrongly estimated; a similar effect could be expected in other Dark Energy models as well. Full article

(This article belongs to the Special Issue Tensions, Anomalies, and Challenges in the Standard Cosmological Model)

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18 pages, 441 KB

Open AccessArticle

Decays of Heavy Scalars in the Grimus–Neufeld Model

by Aurimas Vitkus, Simonas Draukšas and Thomas Gajdosik

Universe 2026, 12(7), 206; https://doi.org/10.3390/universe12070206 - 9 Jul 2026

Abstract

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 [...] Read more.

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. Full article

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

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70 pages, 728 KB

Open AccessArticle

Towards Deriving the Standard Model Coupled to Gravity from Generalized Trace Dynamics via the Spectral Action Principle

by Tejinder P. Singh

Universe 2026, 12(7), 205; https://doi.org/10.3390/universe12070205 - 8 Jul 2026

Abstract

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 [...] Read more.

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

E_{6} / J_{3} (O_{C})

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, \pm 1 / 2)

, and exhibit a smooth regulator family with explicit cutoff moments

(f_{0}, f_{2}, f_{4})

. 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. Full article

(This article belongs to the Section Gravitation)

15 pages, 773 KB

Open AccessArticle

Hayward Boson Stars

by Sebastian S. Chicaiza-Medina and Juan Carlos Degollado

Universe 2026, 12(7), 204; https://doi.org/10.3390/universe12070204 - 8 Jul 2026

Abstract

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 [...] Read more.

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

\frac{\sqrt{β}}{Q} > 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. Full article

(This article belongs to the Special Issue Hairy Black Holes: Insights and Advances)

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33 pages, 14758 KB

Open AccessReview

Advanced Techniques in Stability Analysis of Trans-Neptunian Objects

by Tamás Kovács

Universe 2026, 12(7), 203; https://doi.org/10.3390/universe12070203 - 7 Jul 2026

Abstract

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 [...] Read more.

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. Full article

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

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24 pages, 3937 KB

Open AccessArticle

Probing the Density Dependence of Nuclear Symmetry Energy Through Isospin Transport in Heavy-Ion Reactions

by S. Mallik, F. Gulminelli, C. Ciampi and D. Gruyer

Universe 2026, 12(7), 202; https://doi.org/10.3390/universe12070202 - 6 Jul 2026

Abstract

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 [...] Read more.

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. Full article

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

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22 pages, 639 KB

Open AccessReview

Be/X-Ray Binaries: Phenomenology, Variability, and Accretion Dynamics

by Pablo Reig

Universe 2026, 12(7), 201; https://doi.org/10.3390/universe12070201 - 6 Jul 2026

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue X-Ray Binary Transients: Insights and Discoveries)

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17 pages, 1205 KB

Open AccessArticle

Quantum–Classical Diagnostics and Bohmian Inequivalence for Higher Time-Derivative Hamiltonians

by Sanjib Dey and Andreas Fring

Universe 2026, 12(7), 200; https://doi.org/10.3390/universe12070200 - 6 Jul 2026

Abstract

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, [...] Read more.

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. Full article

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

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15 pages, 14966 KB

Open AccessArticle

Cloud-Adaptive Observation Replanning with All-Sky Camera Semantic Segmentation at Xinglong Observatory

by Jinhao Fan, Zhenyang Huang, Chengze Li, Cunshi Wang, Yu Zhang, Ying Wu and Yuyang Li

Universe 2026, 12(7), 199; https://doi.org/10.3390/universe12070199 - 3 Jul 2026

Abstract

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 [...] Read more.

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. Full article

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

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6 pages, 243 KB

Open AccessCommunication

Signatures of an Entangled Graviton Duet

by Preston Jones, Logan Finke and Joseph Ribaudo

Universe 2026, 12(7), 198; https://doi.org/10.3390/universe12070198 - 3 Jul 2026

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Selected Papers from the 3rd International Online Conference on Universe (IOCU 2026))

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Journal Description

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