Universe

38 pages, 1115 KB

Open AccessReview

Linear Dilaton in Cosmology and Particle Physics

by Eugenio Megías and Mariano Quirós

Universe 2026, 12(9), 287; https://doi.org/10.3390/universe12090287 - 20 Sep 2026

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A warped extra dimension in a five-dimensional (5D) anti-de Sitter (AdS) background was introduced in 1999 by Lisa Randall and Raman Sundrum to solve the gauge hierarchy problem in particle physics. As a bonus, a holographic interpretation in terms of four-dimensional (4D) conformal [...] Read more.

A warped extra dimension in a five-dimensional (5D) anti-de Sitter (AdS) background was introduced in 1999 by Lisa Randall and Raman Sundrum to solve the gauge hierarchy problem in particle physics. As a bonus, a holographic interpretation in terms of four-dimensional (4D) conformal field theories (CFTs) was found. Interestingly enough, another 5D background, linear dilaton (LD), was found to have a holographic interpretation in terms of Little String Theory. In this review, we will show how a set of 5D backgrounds, parametrized in terms of a real parameter

ν

, generalizes both theories and gives rise, in particular, to AdS for

ν = 0

and to LD for

ν = 1

. Furthermore, working in the 5D theory, we will consider applications of the LD background to: (i) particle physics, so that the 5D Planck scale can be lowered to sub-Planckian values, and (ii) Brane World Cosmology (BWC), based on the appearance of an extra vacuum characterized by a 5D black hole. In all cases, we find a gapped continuum for bulk propagating fields, which makes a connection with unparticles. In the case of BWC, we also point out the existence of a pressureless holographic fluid that could play the role of dark matter (DM), with feeble (gravitational) interactions with the Standard Model (SM), decoupled from the thermal SM bath, and generated by a freeze-in mechanism after inflation. We also point out the additional possibility of identifying DM with a long-lived, feebly interacting massive graviton, as an isolated resonance generated by radiative corrections to the continuum graviton propagator self-energy. 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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30 pages, 634 KB

Open AccessArticle

The Linear Response of the CMB-Inferred Hubble Constant to Perturbations in the Expansion History

by Olga Avsajanishvili and Øyvind G. Grøn

Universe 2026, 12(9), 286; https://doi.org/10.3390/universe12090286 - 19 Sep 2026

Viewed by 206

Abstract

We develop a model-independent response-window formalism that describes how small perturbations of the cosmological expansion history around that of the standard

Λ

CDM model affect the Hubble constant,

H_{0}

, inferred from the cosmic microwave background (CMB) acoustic-scale constraint. The formalism is [...] Read more.

We develop a model-independent response-window formalism that describes how small perturbations of the cosmological expansion history around that of the standard

Λ

CDM model affect the Hubble constant,

H_{0}

, inferred from the cosmic microwave background (CMB) acoustic-scale constraint. The formalism is independent of the physical origin of the perturbation, which may, for example, arise from a modification of the dark-energy contribution to the expansion history. By considering perturbations at fixed physical matter and radiation densities, we derive a first-order functional relation in which the response is determined by a single projection of the expansion-history perturbation onto a signed response kernel. The kernel is normalized by the difference between the pre-recombination sound-horizon and post-recombination distance contributions, with the latter dominating for the fiducial cosmology. We show that the resulting response window is strongly localized at late times, with an effective redshift

z_{eff} ≃ 0.46

and a dominant contribution concentrated near the present epoch. Numerical tests demonstrate substantial sensitivity to the temporal profile of smooth perturbations, while perturbations with identical kernel projections are linearly equivalent in their effect on the inferred

H_{0}

. The formalism provides a model-independent framework for quantifying and comparing the CMB response to modifications of the late-time expansion history. Full article

(This article belongs to the Special Issue Celebrating the 110th Anniversary of General Relativity: Advances, Challenges and Perspectives)

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16 pages, 2302 KB

Open AccessArticle

From Observation to Ground Truth: Reconstruction Losses of Galaxy Imaging Under Heteroscedastic Noise

by Renhao Ye, Shiyin Shen and Quanfeng Xu

Universe 2026, 12(9), 285; https://doi.org/10.3390/universe12090285 - 19 Sep 2026

Viewed by 215

Abstract

Deep learning is widely used to analyze galaxy images, and the reconstruction loss determines which image features a model prioritizes during training. Mean squared error (MSE) and mean absolute error (MAE) correspond to homoscedastic Gaussian and Laplace likelihoods, respectively, whereas the inverse-variance-weighted [...] Read more.

Deep learning is widely used to analyze galaxy images, and the reconstruction loss determines which image features a model prioritizes during training. Mean squared error (MSE) and mean absolute error (MAE) correspond to homoscedastic Gaussian and Laplace likelihoods, respectively, whereas the inverse-variance-weighted

χ^{2}

loss accounts for the spatially varying uncertainties of heteroscedastic astronomical noise. Comparing how closely these objectives recover the underlying signal requires a ground truth (GT), which real observations cannot provide. We therefore generate noise-free galaxy images with IllustrisTNG, SKIRT, and GalaxyGenius, convolve them with a point-spread function (PSF) to define the GT, and construct noisy simulated observations. We train otherwise identical variational autoencoders (VAEs) with the three reconstruction losses and evaluate their outputs against the GT. Over most of the evaluated signal-to-noise ratio (SNR) range, the

χ^{2}

-trained model yields lower relative reconstruction errors than the MSE- and MAE-trained models, indicating that inverse-variance weighting improves galaxy-image reconstruction under the heteroscedastic noise considered here. Full article

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

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24 pages, 5321 KB

Open AccessArticle

Periodic Variable Star Classification in Imbalanced Data: A Two-Stage Decoupled Approach with Multimodal Contrastive Learning

by Qingyu Lu, Feng Zhang, Zhiqiang Zou, Tianyu Su, Xiaohang Zhang, A-Li Luo and Xiao Kong

Universe 2026, 12(9), 284; https://doi.org/10.3390/universe12090284 - 19 Sep 2026

Viewed by 216

Abstract

Large photometric surveys require classification methods that retain sensitivity to sparsely represented periodic variable-star classes. We present a two-stage decoupled multimodal network (TDMN) for imbalanced periodic-variable-star classification using phase-folded light curves, three-channel morphology images, and global features. Stage 1 organizes the three modalities [...] Read more.

Large photometric surveys require classification methods that retain sensitivity to sparsely represented periodic variable-star classes. We present a two-stage decoupled multimodal network (TDMN) for imbalanced periodic-variable-star classification using phase-folded light curves, three-channel morphology images, and global features. Stage 1 organizes the three modalities using proxy contrastive objectives and real-observation class prototypes. Stage 2 retains low-level morphology extractors while adapting higher-level representations and combining class-balanced, logit-adjusted, and cosine-focal experts. Across three training runs on fixed survey-specific splits, TDMN achieves mean macro F1-scores of 0.85 on CRTS, 0.83 on the OGLE-IV 15-subclass task, and 0.87 on the OGLE-IV 6-superclass task. A retrained UPSILoN-feature random forest evaluated on the same test objects yields corresponding mean scores of 0.66, 0.78, and 0.88. Thus, TDMN has higher mean macro F1 on CRTS and the OGLE-IV subclass task, but not on the superclass task. Rotational variables remain difficult, and increased Type II Cepheid recall with TDMN is accompanied by lower precision. The class-wise recovery and purity estimates can support the construction of known periodic-variable samples and the prioritization of candidates for follow-up observations. The study remains a catalogue-based, closed-set classification analysis and assumes an externally supplied period for TDMN; period discovery, unknown-class recognition, and nonperiodic transients lie outside its scope. Full article

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

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16 pages, 2221 KB

Open AccessArticle

Reconstruction and Identification of High-Q² Elastic ep Events at EicC

by Wan Chang, Yutie Liang and Weizhi Xiong

Universe 2026, 12(9), 283; https://doi.org/10.3390/universe12090283 - 17 Sep 2026

Viewed by 157

Abstract

Understanding the internal structure of a proton remains a central objective of modern nuclear physics. Measurements of unpolarized elastic electron–proton scattering at high four-momentum transfer,

Q^{2}

, provide essential input for determining proton electromagnetic form factors. The proposed Electron–Ion Collider in China [...] Read more.

Understanding the internal structure of a proton remains a central objective of modern nuclear physics. Measurements of unpolarized elastic electron–proton scattering at high four-momentum transfer,

Q^{2}

, provide essential input for determining proton electromagnetic form factors. The proposed Electron–Ion Collider in China (EicC), with its broad kinematic coverage and advanced detector capabilities, offers a promising opportunity to study elastic scattering in this kinematic region. In this work, we present a feasibility study of high-

Q^{2}

unpolarized elastic electron–proton scattering at EicC. Particular attention is given to the reconstruction of elastic events and the suppression of inelastic backgrounds using kinematic constraints, with the aim of assessing whether a clean elastic event sample can be obtained. Full article

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

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11 pages, 357 KB

Open AccessCommunication

Revisiting Clusters of PBHs as Dark Matter Candidates

by Konstantin Belotsky, Maxim Khlopov and Maxim Krasnov

Universe 2026, 12(9), 282; https://doi.org/10.3390/universe12090282 - 16 Sep 2026

Viewed by 237

Abstract

In this paper we revisit Primordial Black Hole (PBH) clusters as Dark Matter (DM) candidates. Here, we present a continuation of our previous work, extending the analysis. The analysis is devoted to re-consideration of the constraint on PBH abundance from stellar cluster observation [...] Read more.

In this paper we revisit Primordial Black Hole (PBH) clusters as Dark Matter (DM) candidates. Here, we present a continuation of our previous work, extending the analysis. The analysis is devoted to re-consideration of the constraint on PBH abundance from stellar cluster observation in dwarf galaxy Eridanus II. We take into account the non-trivial density profile of PBH clusters in order to re-estimate the energy exchange between PBHs and stars. Here we keep momentum approximation to strengthen the constraint, making it robust. The impact of the density profile concludes in the form of the Coulomb logarithm, making analysis trivial, but showing important results. It is shown that density profile matters and energy exchange can be suppressed by up to three times due to it, making clusters of PBHs more viable DM candidates compared to single PBHs, if their radius is sufficiently large. Full article

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

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13 pages, 871 KB

Open AccessArticle

JW-FD: A 15-Year Multimodal Dataset for Solar Flare Forecasting

by Mingfu Shao, Jiaben Lin, Hui Wang, Liyue Tong, Chen Yang, Yin Zhang and Yuyang Li

Universe 2026, 12(9), 281; https://doi.org/10.3390/universe12090281 - 16 Sep 2026

Viewed by 242

Abstract

Solar flares drive severe space weather hazards, and forecasting their occurrence remains a central challenge for both heliophysics and operational space weather services. Data-driven methods require long-horizon datasets in which images, magnetic features, and flare labels are coregistered in space and time. We [...] Read more.

Solar flares drive severe space weather hazards, and forecasting their occurrence remains a central challenge for both heliophysics and operational space weather services. Data-driven methods require long-horizon datasets in which images, magnetic features, and flare labels are coregistered in space and time. We present JW-FD (JinWu-Flare Dataset), a 15-year multimodal release spanning 1 January 2011 through 31 December 2025, constructed from SDO/HMI line-of-sight magnetograms, NOAA Solar Region Summary reports, and NOAA X-ray flare event lists. The dataset comprises 3,064 independent active regions and 1,991,247 coregistered magnetogram crops, together with FITS, PNG, CSV, and MP4 modalities. Each sample provides 29 magnetic features linked to configurable flare labels under a strict pre-eruption window spanning seven forecast horizons and four GOES intensity thresholds. An 8:1:1 split at the active region level is adopted to prevent temporal leakage between partitions. PNG branches are released at ten magnetic saturation thresholds, and internal Transformer and four-model 24 h experiments on ≥C1.0 and ≥M1.0 forecasting suggest

B_{th} = 1000

G as a preliminary default, although skill remains modest and the optimal saturation is model- and task-dependent. The open-source construction pipeline and dataset are publicly available. Full article

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

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13 pages, 696 KB

Open AccessArticle

Study of Supernova Neutrinos at ESSnuSB

by J. Aguilar, M. Anastasopoulos, D. Barčot, E. Baussan, A. K. Bhattacharyya, A. Bignami, M. Blennow, M. Bogomilov, B. Bolling, E. Bouquerel, F. Bramati, A. Branca, G. Brunetti, I. Bustinduy, C. J. Carlile, J. Cederkall, T. W. Choi, S. Choubey, P. Christiansen, E. Cristaldo Morales, P. Cupiał, D. D’Ago, H. Danared, J. P. A. M. de André, M. Dracos, I. Efthymiopoulos, T. Ekelöf, M. Eshraqi, G. Fanourakis, A. Farricker, E. Fasoula, T. Fukuda, S. Gago, N. Gazis, Th. Geralis, M. Ghosh, A. Giarnetti, G. Gokbulut, C. Hagner, L. Halić, J. Hiegel, M. Hooft, K. E. Iversen, N. Jachowicz, M. Jakkapu, M. Jensen, I. Karakoulias, E. Kasimi, A. Kayis Topaksu, B. Kliček, K. Kordas, B. Kovač, A. Leisos, A. Longhin, M. López, C. Maiano, S. Marangoni, J. G. Marcos, C. Marrelli, D. Meloni, M. Mezzetto, N. Milas, R. Mohanta, J. L. Muñoz, K. Niewczas, M. Oglakci, T. Ohlsson, M. Olvegård, P. Panda, M. Pari, D. Patrzalek, G. Petkov, Ch. Petridou, P. Poussot, A. Psallidas, F. Pupilli, D. Saiang, D. Sampsonidis, A. Scanu, C. Schwab, F. Sordo, G. Stavropoulos, M. Stipčević, R. Tarkeshian, F. Terranova, T. Tolba, M. Topp-Mugglestone, E. Trachanas, R. Tsenov, A. Tsirigotis, S. E. Tzamarias, M. Vanderpoorten, G. Vankova-Kirilova, N. Vassilopoulos, S. Vihonen, J. Wurtz, V. Zeter and O. Zormpa Show full author list Hide full author list

Universe 2026, 12(9), 280; https://doi.org/10.3390/universe12090280 - 14 Sep 2026

Viewed by 412

Abstract

In this paper, we have studied the sensitivity of the ESSnuSB far detector to supernova neutrinos. ESSnuSB is a proposed long-baseline neutrino experiment in Sweden, which will use a 538 kt water Cherenkov detector to probe the leptonic phase

δ_{CP}

by studying [...] Read more.

In this paper, we have studied the sensitivity of the ESSnuSB far detector to supernova neutrinos. ESSnuSB is a proposed long-baseline neutrino experiment in Sweden, which will use a 538 kt water Cherenkov detector to probe the leptonic phase

δ_{CP}

by studying the second oscillation maximum. However, given the very large detector volume, it will have an excellent sensitivity to supernova neutrinos if a supernova explosion occurs during the run-time of ESSnuSB. Motivated by this, we first estimate the expected event rates at the ESSnuSB far detector for three different supernova flux models and then we probe its capability to distinguish these flux models. Additionally, we also investigate the impact of systematic errors and detector efficiency. Our results show that depending on the model of the supernova neutrinos, the expected number of events detected at Earth varies significantly. Our results also show that the ESSnuSB far detector may have excellent potential in distinguishing these flux models depending upon the distance of the supernova explosion, systematic errors and detector efficiency. Full article

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

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20 pages, 338 KB

Open AccessArticle

Complexity and Structure Scalars of Generalized Vaidya Spacetime

by Samarjit Chakraborty, Rituparno Goswami and Sunil D. Maharaj

Universe 2026, 12(9), 279; https://doi.org/10.3390/universe12090279 - 14 Sep 2026

Viewed by 254

Abstract

We study gravitational complexity in the framework of Generalized Vaidya geometry for the first time. Adopting a general semi-tetrad covariant approach, we find the structure scalars of a Type II fluid in this spacetime and establish the relationship between the [...] Read more.

We study gravitational complexity in the framework of Generalized Vaidya geometry for the first time. Adopting a general semi-tetrad covariant approach, we find the structure scalars of a Type II fluid in this spacetime and establish the relationship between the

1 + 1 + 2

covariant quantities and these structure scalars. By calculating the complexity factor in terms of the Misner–Sharp mass and the matter variables, we obtain a non-trivial class of spacetimes with vanishing complexity. Furthermore, we show that the Vaidya spacetime with a pure Type II matter field yields a negative complexity factor, highlighting the fundamental difference between the complexity of Type I and Type II matter fields. We also compute the propagation and evolution equations of the structure scalars to showcase their interdependency through the kinematical variables. Finally, we study the wave equation of the gravitational mass of the 2-shell and analyze its dependence on these structure scalars. Full article

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

28 pages, 2428 KB

Open AccessArticle

UPLOAD-HELIX: A High-Helicity Single-Mode Microwave Haloscope with Low-Noise Interferometric Readout for Ultralight Axion Dark Matter

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

Universe 2026, 12(9), 278; https://doi.org/10.3390/universe12090278 - 11 Sep 2026

Viewed by 245

Abstract

We propose a superconducting single-mode microwave haloscope based on chiral cavity resonators for the detection of ultralight dark matter axions over the mass range 4 × 10⁻¹⁹–4 × 10⁻¹⁴ eV. Building on the single-mode chiral-cavity haloscope for detecting ultra light [...] Read more.

We propose a superconducting single-mode microwave haloscope based on chiral cavity resonators for the detection of ultralight dark matter axions over the mass range 4 × 10⁻¹⁹–4 × 10⁻¹⁴ eV. Building on the single-mode chiral-cavity haloscope for detecting ultra light dark matter (ULDM) axions we develop a resonator geometry compatible with subtractive manufacturing from high-purity bulk niobium, taking advantage of the substantially lower surface resistance achievable relative to the additively manufactured Möbius cavity proposed in the earlier work. An inverse-design framework is then used to maximise a figure of merit derived to minimise the measurement time required to achieve a fixed experimental sensitivity. The resulting optimised bulk-niobium design achieves a figure of merit more than three orders of magnitude larger than the additively manufactured Möbius benchmark. An experimentally informed microwave interferometric readout model incorporating measured electronics noise and active suppression of pump amplitude noise is used to project the sensitivity of the proposed experiment. For an acquisition time of three months, the haloscope is projected to reach g_aγγ < 10⁻¹¹ GeV⁻¹ across more than four orders of magnitude in axion mass. The projected sensitivity extends approximately one order of magnitude below the current exclusion limits set by CAST, providing a practical pathway towards a high-sensitivity direct search for ultralight dark matter axions. Full article

(This article belongs to the Special Issue Ultralight Bosonic Dark Matter: Theoretical Developments and Experimental Searches)

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20 pages, 1806 KB

Open AccessArticle

Forecasting Constraints on Entropic Holographic Dark Energy via SKA 21 cm Redshift Drift: A Joint Analysis of Galaxy Emission and Damped Lyman-Alpha Systems

by Sonali Borah and Asoke K. Sen

Universe 2026, 12(9), 277; https://doi.org/10.3390/universe12090277 - 11 Sep 2026

Viewed by 294

Abstract

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 [...] Read more.

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

10^{9}

galaxies up to

z = 1.0

and 21 cm absorption lines in

1.8 \times 10^{3}

damped Lyman-

α

(DLA) systems, we simulate mock velocity drift observations (

\dot{v}

) at high spectral resolutions (

Δ ν = 0.001

Hz and

0.002

Hz) over a semi-annual (

Δ t = 0.5

year) cadence 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

1 σ

constraints of

σ_{c} = \pm 0.021

for the standard holographic parameter,

σ_{δ} = \pm 0.035

for the Tsallis entropy index, and

σ_{α} = \pm 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 (

S_{z}

) and the physical velocity drift (

S_{v}

). We conclude that the SKA will serve as a premier instrument for testing the holographic principle and non-extensive thermodynamics at cosmological scales. 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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13 pages, 434 KB

Open AccessArticle

17-Year Fermi-LAT Observations of the Andromeda Galaxy: Updated Constraints on WIMP Properties

by Jun Li

Universe 2026, 12(9), 276; https://doi.org/10.3390/universe12090276 - 10 Sep 2026

Viewed by 294

Abstract

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 [...] Read more.

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

b \bar{b}

and

τ^{+} τ^{-}

final states for DM masses between

10 GeV

and

10 TeV

. 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

⟨ σ v ⟩

and lower limits on the decay lifetime. For the

b \bar{b}

channel, the annihilation limits span

6.5 \times 10^{- 26}

–

3.4 \times 10^{- 23} {cm}^{3} s^{- 1}

and the decay-lifetime lower limits span

1.8 \times 10^{26}

–

5.6 \times 10^{26} s

; for the

τ^{+} τ^{-}

channel the corresponding ranges are

9.6 \times 10^{- 26}

–

3.3 \times 10^{- 22} {cm}^{3} s^{- 1}

and

7.6 \times 10^{25}

–

2.5 \times 10^{27} s

. 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. Full article

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

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9 pages, 1822 KB

Open AccessCommunication

Time Delay Measurement of FAST Using Baseband Data and Periodic Noise

by Ru-Rong Chen, Yan Zhu and Hai-Yan Zhang

Universe 2026, 12(9), 275; https://doi.org/10.3390/universe12090275 - 8 Sep 2026

Viewed by 246

Abstract

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 [...] Read more.

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. Full article

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

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24 pages, 1569 KB

Open AccessArticle

Entropy Covector Field and Macroscopic Observables for Rotating and Non-Rotating Relativistic Kinetic Gases Around a Schwarzschild Black Hole

by Carlos Eduardo Gabarrete, Daniela Massiel Montoya and Roger Javier Raudales Rodriguez

Universe 2026, 12(9), 274; https://doi.org/10.3390/universe12090274 - 8 Sep 2026

Viewed by 324

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Section Gravitation)

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50 pages, 8610 KB

Open AccessReview

Neutrino Astronomy at High Energies—An Experimental Review

by Christian Spiering

Universe 2026, 12(9), 273; https://doi.org/10.3390/universe12090273 - 7 Sep 2026

Viewed by 278

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Ultra-High Energy Cosmic Rays: Past, Present and Future)

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11 pages, 248 KB

Open AccessCommunication

Restoring the Isometry of Position and Momentum Space in Phenomenological Quantum Gravity and Implications for Quantum Field Theory

by Michael Bishop, Daniel Hooker and Douglas Singleton

Universe 2026, 12(9), 272; https://doi.org/10.3390/universe12090272 - 7 Sep 2026

Viewed by 239

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, 3rd Edition)

3 pages, 147 KB

Open AccessEditorial

Editorial: Special Issue “The 13th Bolyai–Gauss–Lobachevsky Conference on Non-Euclidean Geometry and Its Applications”

by Tiberiu Harko, Taoufik Ouali, Ahmed Errahmani and Amine Bouali

Universe 2026, 12(9), 271; https://doi.org/10.3390/universe12090271 - 6 Sep 2026

Viewed by 228

Abstract

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 [...] Full article

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

9 pages, 255 KB

Open AccessArticle

Scalarized Extremal Black Holes in the Einstein–Maxwell-Scalar Theory with Two U(1) Fields

by Xiao-Yan Chew and Yun Soo Myung

Universe 2026, 12(9), 270; https://doi.org/10.3390/universe12090270 - 4 Sep 2026

Viewed by 169

Abstract

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 [...] Read more.

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. Full article

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

34 pages, 7508 KB

Open AccessArticle

Instabilities in Cylindrical Geometry Using the Minimalist Approach: Formalism and Rotational Instabilities

by Nektarios Vlahakis

Universe 2026, 12(9), 269; https://doi.org/10.3390/universe12090269 - 4 Sep 2026

Viewed by 217

Abstract

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 [...] Read more.

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. Full article

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32 pages, 1048 KB

Open AccessArticle

A Comparative Study of Machine-Learning Methods for Early Classification from Sparse Astronomical Light Curves

by Xueli Lin, Zihan Qian, Cunshi Wang and Yuyang Li

Universe 2026, 12(9), 268; https://doi.org/10.3390/universe12090268 - 3 Sep 2026

Viewed by 348

Abstract

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 [...] Read more.

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. Full article

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

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36 pages, 2722 KB

Open AccessArticle

Gravitational Baryogenesis in Energy-Momentum Squared Gravity

by David S. Pereira, Francisco S. N. Lobo and José Pedro Mimoso

Universe 2026, 12(9), 267; https://doi.org/10.3390/universe12090267 - 1 Sep 2026

Viewed by 267

Abstract

We demonstrate that the matter sector itself can drive baryogenesis in energy-momentum squared gravity,

f (R, T^{2})

with

T^{2} \equiv T_{μ ν} T^{μ ν}

. High-density matter corrections provide new time-dependent sources for the baryon asymmetry [...] Read more.

We demonstrate that the matter sector itself can drive baryogenesis in energy-momentum squared gravity,

f (R, T^{2})

with

T^{2} \equiv T_{μ ν} T^{μ ν}

. High-density matter corrections provide new time-dependent sources for the baryon asymmetry through derivative couplings to

T^{2}

and to the full combination

f (R, T^{2})

. 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. Full article

(This article belongs to the Special Issue Exploring and Constraining Alternative Theories of Gravity)

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9 pages, 428 KB

Open AccessCommunication

Determination of Neutron Star Radius from Pulse Profile Modeling Using Profile Likelihood

by Vyaas Ramakrishnan and Shantanu Desai

Universe 2026, 12(9), 266; https://doi.org/10.3390/universe12090266 - 1 Sep 2026

Viewed by 235

Abstract

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 [...] Read more.

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 <

1 σ

. 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. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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43 pages, 559 KB

Open AccessArticle

Conformal Symmetry, SM and Gravity

by Loriano Bonora

Universe 2026, 12(9), 265; https://doi.org/10.3390/universe12090265 - 31 Aug 2026

Viewed by 193

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Celebrating the 110th Anniversary of General Relativity: Advances, Challenges and Perspectives)

13 pages, 438 KB

Open AccessArticle

Spheroidal Resampling Analysis of High-Redshift Gamma-Ray Burst Spatial Densities

by Istvan Horvath, Zsolt Bagoly, Lajos G. Balazs, Jon Hakkila, Janos Horvath, Sandor Pinter, Istvan I. Racz, Peter Veres and Bendegúz Koncz

Universe 2026, 12(9), 264; https://doi.org/10.3390/universe12090264 - 30 Aug 2026

Viewed by 229

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Section Cosmology)

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32 pages, 19259 KB

Open AccessArticle

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

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

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

Viewed by 255

Abstract

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

ρ

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

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

ρ

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

λ

6678, He i

λ

5875, and Si iii

λ

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

P = 16.46

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

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

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12 pages, 7793 KB

Open AccessArticle

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

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

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

Viewed by 273

Abstract

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

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

m = 1

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

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

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

10^{10} M_{⊙}

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

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

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22 pages, 4010 KB

Open AccessArticle

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

\sqrt{s_{n n}}

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

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

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

Viewed by 288

Abstract

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

p_{T}

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

\sqrt{s_{n n}}

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

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

p_{T}

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

\sqrt{s_{n n}}

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

\sqrt{s_{n n}}

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

p_{T}

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

⟨ N_{p a r t} ⟩

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

⟨ N_{p a r t} ⟩

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

p_{T}

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

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

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34 pages, 861 KB

Open AccessReview

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

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

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

Viewed by 333

Abstract

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

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

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

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60 pages, 8195 KB

Open AccessReview

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

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

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

Viewed by 416

Abstract

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

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

(This article belongs to the Section Astroinformatics and Astrostatistics)

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16 pages, 2368 KB

Open AccessArticle

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

by Elke Pilat-Lohinger, Maximilian Zimmermann and Birgit Loibnegger

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

Viewed by 443

Abstract

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

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

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

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