Universe

17 pages, 7025 KB

Open AccessReview

by Vindhyawasini Prasad

Universe 2026, 12(1), 20; https://doi.org/10.3390/universe12010020 - 12 Jan 2026

The Standard Model (SM) of particle physics is one of the most successful frameworks in modern physics, yet it leaves several fundamental questions unanswered, including the nature of dark matter (DM). Precise knowledge of DM is crucial for testing astrophysical and cosmological observations [...] Read more.

The Standard Model (SM) of particle physics is one of the most successful frameworks in modern physics, yet it leaves several fundamental questions unanswered, including the nature of dark matter (DM). Precise knowledge of DM is crucial for testing astrophysical and cosmological observations and for determining the matter density of our Universe. Many hidden dark sector models beyond the SM open the possibility of coupling between DM and SM particles via various portals. The corresponding new physics particles include light Higgs bosons, dark photons, axion-like particle, and spin-1/2 fermions. Furthermore, the introduction of a dark baryon could simultaneously explain the origin of DM and the observed matter–antimatter asymmetry in the Universe. If these hypothetical particles have masses of a few GeV, they can be explored at high-intensity

e^{+} e^{-}

colliders, such as in the BaBar, Belle/Belle II, and BESIII experiments. This report reviews the current status of DM searches at

e^{+} e^{-}

colliders, with a focus on portal-based scenarios. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Energy Theories)

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12 pages, 1137 KB

Open AccessArticle

Spectral and Photometric Studies of NGC 4151 in the Optical Range: Current Results

by Nazim Huseynov, Saule Shomshekova, Alexander Serebryanskiy, Luydmila Kondratyeva, Samira Rahimli, Gabit Nazymbekov, Inna Reva and Gaukhar Aimanova

Universe 2026, 12(1), 19; https://doi.org/10.3390/universe12010019 - 9 Jan 2026

Abstract

We present the results of long-term photometric and spectroscopic monitoring of the Seyfert galaxy NGC 4151 based on new observational data complemented by archival material spanning several decades. NGC 4151 is one of the most extensively studied active galactic nuclei, exhibiting pronounced variability [...] Read more.

We present the results of long-term photometric and spectroscopic monitoring of the Seyfert galaxy NGC 4151 based on new observational data complemented by archival material spanning several decades. NGC 4151 is one of the most extensively studied active galactic nuclei, exhibiting pronounced variability in both optical continuum and emission-line fluxes, which makes it a key object for investigating physical processes in the central engine and the broad-line region. Our study covers the optical and near-infrared wavelength ranges, including the Ic band and the standard BVRc photometric filters. Using multi-band optical photometry and optical spectroscopy, we construct light curves of the continuum and emission lines and perform a comparative analysis of their temporal behavior during different activity states of the galaxy. The analysis focuses on variability amplitudes, long-term trends, and correlations between photometric and spectral characteristics, allowing us to examine the relationship between continuum variations and the line-emitting regions. Full article

(This article belongs to the Section Galaxies and Clusters)

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30 pages, 4811 KB

Open AccessArticle

On the Cooling of Compact Stars in Light of the HESS J1731-347 Remnant

by Dimitrios G. Nanopoulos, Pavlos Laskos-Patkos and Charalampos C. Moustakidis

Universe 2026, 12(1), 18; https://doi.org/10.3390/universe12010018 - 8 Jan 2026

Abstract

Recent analyses on the central compact object in the HESS J1731-347 supernova remnant reported not only surprising structural properties (mass M and radius R), but also an interesting thermal evolution. More precisely, it has been estimated that [...] Read more.

Recent analyses on the central compact object in the HESS J1731-347 supernova remnant reported not only surprising structural properties (mass M and radius R), but also an interesting thermal evolution. More precisely, it has been estimated that

M = 0 . 77_{- 0.17}^{+ 0.20} M_{⊙}

and

R = 10 . 4_{- 0.78}^{+ 0.86}

km (at the

1 σ

level), while a redshited surface temperature of

153_{- 2}^{+ 4}

keV at an age of 2–6 kyrs has been reported. In the present work, we conduct an in-depth investigation on the possible nature (hadronic, hybrid, quark) of this compact object by attempting to not only explain its mass and radius but also the corresponding estimations for its temperature and age. In the case of hybrid stars we also examine possible effects of the symmetry energy on the activation of different neutrino emitting process, and hence on the resulting cooling curves. We found that the reported temperature and age may be compatible to hadronic stellar configurations regardless of whether pairing effects are included. In the scenario of hybrid stars, we found that the strange quark matter core has to be in a superconducting state in order to reach an agreement with the observational constraints. In addition, the hadronic phase must be soft enough so that the direct Urca process is not activated. Furthermore, we have shown that the considered cooling constraints can be reconciled within the framework of strange stars. However, quark matter has to be in a superconducting state and the quark direct Urca process needs to be blocked. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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23 pages, 3022 KB

Open AccessArticle

Single-Point Search for eV-Scale Axion-like Particles with Variable-Angle Three-Beam-Stimulated Resonant Photon Collider

by Takumi Hasada, Kensuke Homma, Airi Kodama, Haruhiko Nishizaki, Yuri Kirita, Shin-ichiro Masuno, Shigeki Tokita, Masaki Hashida and ᵗSAPPHIRES Collaboration

Universe 2026, 12(1), 17; https://doi.org/10.3390/universe12010017 - 5 Jan 2026

Abstract

We report a laboratory search for axion-like particles (ALPs) in the eV mass range using a variable-angle three-beam-stimulated resonant photon collider. The scheme independently focuses and collides three laser beams, providing a cosmology- and astrophysics-independent test. By varying the angles of incidence, the [...] Read more.

We report a laboratory search for axion-like particles (ALPs) in the eV mass range using a variable-angle three-beam-stimulated resonant photon collider. The scheme independently focuses and collides three laser beams, providing a cosmology- and astrophysics-independent test. By varying the angles of incidence, the center-of-mass energy can be scanned continuously across the eV range. In this work, we operated the collider in a vacuum chamber at a large-angle configuration, verified the spacetime overlap of the three short pulses, and performed a first search centered at

m_{a} ≃ 2.27 eV

. No excess was observed. Thus, we set a

95 %

C.L. upper limit on the pseudoscalar two-photon coupling, with a minimum sensitivity of

g / M ≃ 4.2 \times 10^{- 10} {GeV}^{- 1}

at

m_{a} = 2.27 eV

. This provides the first model-independent upper limit on the coupling that reaches the KSVZ benchmark in the eV regime and demonstrates the feasibility of eV-scale mass scans in the near future. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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16 pages, 488 KB

Open AccessArticle

Analysis of 14 Years of X-Ray Emission from SN 2011DH

by Elisa J. Gao and Vikram V. Dwarkadas

Universe 2026, 12(1), 16; https://doi.org/10.3390/universe12010016 - 3 Jan 2026

Abstract

Ejecta from core-collapse supernovae interact with the circumstellar medium shed by the progenitor star, producing X-ray emission. Previous studies analyzed the X-ray spectrum of the Type IIb supernova SN 2011dh up to ∼500 days after explosion. Long-term monitoring of X-ray emission provides valuable [...] Read more.

Ejecta from core-collapse supernovae interact with the circumstellar medium shed by the progenitor star, producing X-ray emission. Previous studies analyzed the X-ray spectrum of the Type IIb supernova SN 2011dh up to ∼500 days after explosion. Long-term monitoring of X-ray emission provides valuable constraints on supernova evolution and progenitor systems, yet such studies remain rare for Type IIb events due to limited data. Here we present the most comprehensive X-ray light curve of SN 2011dh to date, combining all available Chandra and XMM-Newton data with previously published and newly released Swift observations, extending coverage to ∼5100 days. We measure a luminosity decline consistent with

L_{X} \propto t^{- 0.74 \pm 0.04}

and infer a mass-loss rate of

(1.0

–

2.2) \times 10^{- 6} M_{⊙} {yr}^{- 1}

for

v_{w} = 10 km s^{- 1}

, or

(2.0

–

4.4) \times 10^{- 6} M_{⊙} {yr}^{- 1}

for

v_{w} = 20 km s^{- 1}

. These estimates agree with earlier results, supporting the interpretation that the X-ray emission has been dominated by an adiabatic reverse shock. The consistency of our late-time results with previous studies demonstrates that SN 2011dh has evolved steadily for nearly 14 years. Full article

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

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16 pages, 310 KB

Open AccessArticle

Momentum Transport in Ferromagnetic–Plasmon Heterostructures Within the Keldysh Formalism

by Feiyi Liu, Min Guo, Mingyang Liu and Yang Wang

Universe 2026, 12(1), 15; https://doi.org/10.3390/universe12010015 - 3 Jan 2026

Abstract

We investigate momentum transport in ferromagnetic–plasmon heterostructures using Keldysh field theory and energy–momentum tensor formalism. A three-layer model reveals that plasmon frequency shifts generate a non-zero expectation value for the

x z

-component of the energy–momentum tensor

⟨ T_{x z} ⟩

through [...] Read more.

We investigate momentum transport in ferromagnetic–plasmon heterostructures using Keldysh field theory and energy–momentum tensor formalism. A three-layer model reveals that plasmon frequency shifts generate a non-zero expectation value for the

x z

-component of the energy–momentum tensor

⟨ T_{x z} ⟩

through magnon–plasmon coupling. The momentum transport exhibits linear velocity dependence, with temperature behavior transitioning from exponential suppression at low temperatures to linear growth at high temperatures, governed by the magnon energy gap. Spatial oscillations follow

sin (2 n π z / h)

patterns within the ferromagnetic layer. This framework provides fundamental insights into quantum momentum transport mechanisms in magnetic systems. Full article

(This article belongs to the Section Field Theory)

19 pages, 486 KB

Open AccessArticle

Late-Time Constraints on Future Singularity Dark Energy Models from Geometry and Growth

by Tomasz Denkiewicz

Universe 2026, 12(1), 14; https://doi.org/10.3390/universe12010014 - 3 Jan 2026

Abstract

We confront two future-singularity dark-energy templates—sudden future singularities (SFSs) and finite scale factor singularities (FSFSs)—with late-time geometric probes and redshift-space distortion growth data. We compute the observable growth

f σ_{8} (z)

by solving the full linear perturbation system (including the [...] Read more.

We confront two future-singularity dark-energy templates—sudden future singularities (SFSs) and finite scale factor singularities (FSFSs)—with late-time geometric probes and redshift-space distortion growth data. We compute the observable growth

f σ_{8} (z)

by solving the full linear perturbation system (including the standard fiducial cosmology rescaling of RSD measurements) and build a joint

χ^{2}

from Pantheon+SH0ES SNe Ia,

H (z)

, DESI AP-only BAO, and

f σ_{8}

. Parameter constraints are obtained via grid-based profiling over nuisance parameters and the singularity time location parameter. We compare the viability and goodness of fit of the singularity scenarios to the

Λ

CDM reference. Full article

(This article belongs to the Section Cosmology)

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51 pages, 20151 KB

Open AccessReview

Tetraquark-Jet Systems at the High-Luminosity LHC

by Francesco Giovanni Celiberto

Universe 2026, 12(1), 13; https://doi.org/10.3390/universe12010013 - 2 Jan 2026

Abstract

We investigate the high-energy production of tetraquark-jet systems at the LHC and its forthcoming high-luminosity upgrade. In this review, we examine the leading–power fragmentation of fully heavy tetraquarks (

T_{4 Q}

) in hadronic collisions, highlighting their relevance as novel probes of [...] Read more.

We investigate the high-energy production of tetraquark-jet systems at the LHC and its forthcoming high-luminosity upgrade. In this review, we examine the leading–power fragmentation of fully heavy tetraquarks (

T_{4 Q}

) in hadronic collisions, highlighting their relevance as novel probes of multiquark dynamics in QCD. Our analysis relies on the hadron–structure–oriented TQ4Q1.1 fragmentation functions, built within a nonrelativistic QCD framework that incorporates both gluon- and heavy-quark-initiated channels. Threshold-consistent DGLAP evolution is performed through the HF-NRevo scheme, enabling a unified treatment of mass thresholds and scale variations. We also provide a systematic discussion of uncertainties arising from color-composite long-distance matrix elements (LDMEs) and from perturbative hard- and fragmentation-scale inputs (H- and F-MHOUs). Phenomenological predictions are obtained using the (sym)Jethad framework at NLL/NLO⁺ accuracy for semi-inclusive tetraquark-jet production at the LHC and beyond. This review connects the emerging spectroscopy of fully heavy exotics with modern fragmentation-based approaches to hadron structure and high-energy QCD. Full article

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

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41 pages, 1152 KB

Open AccessArticle

Incoherent Processes in Dilepton Production in Proton–Nucleus Scattering at High Energies

by Sergei P. Maydanyuk and Gyorgy Wolf

Universe 2026, 12(1), 12; https://doi.org/10.3390/universe12010012 - 1 Jan 2026

Abstract

(1) Purpose: Incoherent processes in production of lepton pairs (dileptons) are studied for the scattering of protons on nuclei. Methods: New quantum mechanical model is constructed on the basis (1) generalization of the nuclear model of emission of photons in the proton-nucleus reactions [...] Read more.

(1) Purpose: Incoherent processes in production of lepton pairs (dileptons) are studied for the scattering of protons on nuclei. Methods: New quantum mechanical model is constructed on the basis (1) generalization of the nuclear model of emission of photons in the proton-nucleus reactions from low to intermediate energies, (2) formalism of dilepton production. Results: (1) The coherent cross sections of dilepton production in

p + Be

at proton beam energy

E_{p}

of 2.1 GeV calculated by model are in good agreement with experimental data of DLS Collaboration. (2) Dilepton production for ⁹Be, ¹²C, ¹⁶O, ²⁴Mg, ⁴⁴Ca, ¹⁹⁷Au at

E_{p} = 2.1

GeV are studied. Coherent cross sections of dilepton production are monotonously decreased with increasing mass of nuclei. (3) At larger

E_{p}

dileptons are produced more intensively. (4) Incoherent processes in production of dileptons are studied for p + ⁹Be at E_p = 2.1 GeV. Agreement between experimental data and calculated cross sections is better, in to include incoherent processes to the model. A new phenomenon of suppression of production of dileptons at low energies due to incoherent processes is observed. This is explained by dominant coherent contribution at very low energies. (5) Longitudinal amplitude of virtual photon suppresses the cross section of dilepton production a little (effect is observed for p + ⁹Be at E_p = 2.1 GeV). (6) The contribution from incoherent processes plays a leading role in the dilepton production ((the ratio between the incoherent and coherent terms is 10–100). Also our model provides the tendencies of the full spectrum for p + ⁹³Nb at E_p = 3.5 GeV in good agreement with experimental data obtained by HADES collaboration, and shows large role of incoherent processes. Conclusions: Incoherent processes are much more important than coherent ones in study of dilepton production in this reaction. 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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19 pages, 13896 KB

Open AccessArticle

Day-Time Seeing Changes at the Huairou Solar Observing Station Site

by Artem Y. Shikhovtsev

Universe 2026, 12(1), 11; https://doi.org/10.3390/universe12010011 - 1 Jan 2026

Abstract

In this paper, a simple method of estimating reference optical turbulence profiles at the Huairou Solar Observing Station (HSOS) from a large meteorological dataset is used. These reference profiles can be used in simulations of atmospheric variability above the station and the impact [...] Read more.

In this paper, a simple method of estimating reference optical turbulence profiles at the Huairou Solar Observing Station (HSOS) from a large meteorological dataset is used. These reference profiles can be used in simulations of atmospheric variability above the station and the impact of climate change on image quality. By analyzing the statistics of measured optical turbulence and using the ERA-5 reanalysis data, vertical distributions of optical turbulence above HSOS were obtained for different time periods (1940–1969, 1970–1999, 1989–2010, 2000–2025). It has been shown that the intensity of optical turbulence in the surface layer has been decreasing in recent decades, while the intensity in the upper troposphere has a tendency to increase. Trends are also assessed in total cloud cover and atmospheric boundary layer height at the HSOS site. Observed changes are associated with global warming. Full article

(This article belongs to the Special Issue Measurements, Observations and Theoretical Studies on the Solar Magnetic Field—Celebrating the 40th Anniversary of the Huairou Solar Observing Station)

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29 pages, 1605 KB

Open AccessArticle

Conditional Cosmological Recurrence in Finite Hilbert Spaces and Holographic Bounds Within Causal Patches

by Nikolaos Chronis and Nikolaos Sifakis

Universe 2026, 12(1), 10; https://doi.org/10.3390/universe12010010 - 30 Dec 2025

Abstract

A conditional framework of Conditional Cosmological Recurrence (CCR) is introduced, as follows: if a causal patch admits a finite operational Hilbert space dimension D (as motivated by holographic and entropy bounds), then unitary quantum dynamics guarantee almost-periodic evolution, leading to recurrences. The central [...] Read more.

A conditional framework of Conditional Cosmological Recurrence (CCR) is introduced, as follows: if a causal patch admits a finite operational Hilbert space dimension D (as motivated by holographic and entropy bounds), then unitary quantum dynamics guarantee almost-periodic evolution, leading to recurrences. The central contribution is the explicit formulation of a micro-to-macro bridge, as follows: (i) finite regions discretize field modes; (ii) gravitational bounds cap entropy and energy; and (iii) the number of accessible states is finite, yielding CCR. The analysis differentiates global microstate recurrences (with double-exponential timescales in

S_{max}

) from operationally relevant coarse-grained returns (exponential in subsystem entropy), with conservative timescale estimates. For predictivity in eternally inflating settings, a causal-diamond measure with xerographic typicality and a single no-Boltzmann-brain constraint is employed, thereby avoiding volume-weighting pathologies. The scope is explicitly conditional: if future quantum gravity demonstrates

D = \infty

for causal patches, CCR is falsified. Full article

(This article belongs to the Section Cosmology)

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20 pages, 32141 KB

Open AccessArticle

Photometric Variability of B-Type Stars in the Young Open Cluster NGC 663

by Xiangfang Xiao, Kun Wang and Licai Deng

Universe 2026, 12(1), 9; https://doi.org/10.3390/universe12010009 - 30 Dec 2025

Abstract

We present a comprehensive study of variable stars in the young open cluster NGC 663, combining ground-based 50BiN photometry, space-based TESS time-series observations, and astrometric measurements from Gaia DR3. A total of 60 variable candidates were identified, and 46 of them appear consistent [...] Read more.

We present a comprehensive study of variable stars in the young open cluster NGC 663, combining ground-based 50BiN photometry, space-based TESS time-series observations, and astrometric measurements from Gaia DR3. A total of 60 variable candidates were identified, and 46 of them appear consistent with B-type variables according to their effective temperatures and spectral classifications. Cross-matching with the VSX catalog shows that variability of 31 objects has been reported previously, while 29 have no prior entries. Using Gaia astrometry, we estimated membership probabilities and found that 40 of the B-type variables are likely associated with the cluster. Light-curve morphology, frequency analysis, and spectral information suggest a mixture of variability types, including seven candidate

α

Cygni stars, three

β

Cephei variables, ten SPB candidates, one possible BCEP/SPB hybrid, twenty Be stars, and five additional variables. These results indicate that NGC 663 provides a valuable environment for studying variability phenomena in massive stars across a range of evolutionary stages. Full article

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

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19 pages, 3629 KB

Open AccessArticle

Thin Accretion Disk Around Bardeen Black Hole Surrounded by Perfect Fluid Dark Matter

by Dan-Dan Cui and Haiyuan Feng

Universe 2026, 12(1), 8; https://doi.org/10.3390/universe12010008 - 29 Dec 2025

Abstract

We investigate the thin accretion disk around Bardeen black hole (BH) surrounded by perfect fluid dark matter (PFDM), focusing on how the magnetic charge g and dark matter (DM) parameter b affect its radiative properties. The results show that increasing g slightly enhances [...] Read more.

We investigate the thin accretion disk around Bardeen black hole (BH) surrounded by perfect fluid dark matter (PFDM), focusing on how the magnetic charge g and dark matter (DM) parameter b affect its radiative properties. The results show that increasing g slightly enhances the energy flux, radiation temperature, luminosity, and efficiency, while shifting the innermost stable circular orbit (

r_{isco}

) inward. Additionally, the influence of b is found to be dominant, making it a key parameter in distinguishing PFDM-surrounded Bardeen BH from their Schwarzschild counterparts. Full article

(This article belongs to the Special Issue New Progress of Black Hole Accretion Disk)

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36 pages, 3703 KB

Open AccessReview

Millihertz Quasi-Periodic Oscillations in Accreting X-Ray Pulsars

by Wen Yang and Wei Wang

Universe 2026, 12(1), 7; https://doi.org/10.3390/universe12010007 - 27 Dec 2025

Abstract

Accreting neutron stars exhibit pulsed X-rays and complex temporal variability across multi-wavelengths and different timescales. This variability could be driven by various physical processes including instability or inhomogeneous motions within the accretion flow, thermonuclear bursts on the neutron star surface. In this review, [...] Read more.

Accreting neutron stars exhibit pulsed X-rays and complex temporal variability across multi-wavelengths and different timescales. This variability could be driven by various physical processes including instability or inhomogeneous motions within the accretion flow, thermonuclear bursts on the neutron star surface. In this review, we present a concise overview of the observational features for millihertz (mHz) quasi-periodic oscillations (QPOs) at a frequency range of ∼1–1000 mHz observed in light curves of X-ray pulsars for both low-mass X-ray binaries and high-mass X-ray binaries, based on recent X-ray missions, e.g., NICER, Insight-HXMT and NuSTAR. We further summarize current theoretical interpretations, discuss remaining challenges and propose potential directions for future studies to advance the understanding of the nature and physical origin of these QPOs. Full article

(This article belongs to the Section Compact Objects)

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34 pages, 549 KB

Open AccessArticle

Primordial Gravitational Wave Birefringence in a de Sitter Background with Chern–Simons Coupling

by Abhishek Rout and Brett Altschul

Universe 2026, 12(1), 6; https://doi.org/10.3390/universe12010006 - 26 Dec 2025

Abstract

In this work, we investigate tensor perturbations in a de Sitter background within the framework of Chern–Simons modified gravity. We introduce transverse-traceless perturbations and analyze how the Chern–Simons Cotton tensor induces parity-violating modifications to gravitational wave propagation, while the Pontryagin density vanishes at [...] Read more.

In this work, we investigate tensor perturbations in a de Sitter background within the framework of Chern–Simons modified gravity. We introduce transverse-traceless perturbations and analyze how the Chern–Simons Cotton tensor induces parity-violating modifications to gravitational wave propagation, while the Pontryagin density vanishes at linear order. Using a mode decomposition of the scalar background field, we derive the sub- and super-horizon limits of the wave equations and uncover chiral corrections in the dispersion relations of tensor modes. The resulting birefringence exhibits both amplitude and velocity components, alternating with the phase of the scalar field. Particular solutions sourced by the scalar background show helicity-dependent amplification and a characteristic scaling of the radiated flux that reduces smoothly to the Minkowski limit. The accumulated phase difference between right- and left-handed modes grows quadratically inside the horizon and becomes frozen outside, leaving a permanent parity-violating imprint in the primordial tensor spectrum. Finally, by promoting the Chern–Simons field to a massive dark matter candidate, we demonstrate how its mass-dependent dynamics connect gravitational birefringence to axion-like dark matter phenomenology. Full article

(This article belongs to the Section Gravitation)

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16 pages, 1002 KB

Open AccessArticle

Spectral Instability in Modified Pöschl–Teller Effective Potential Triggered by Deterministic and Random Perturbations

by Shui-Fa Shen, Guan-Ru Li, Ramin G. Daghigh, Jodin C. Morey, Michael D. Green, Wei-Liang Qian and Rui-Hong Yue

Universe 2026, 12(1), 5; https://doi.org/10.3390/universe12010005 - 24 Dec 2025

Abstract

Owing to its substantial implications for black hole spectroscopy, spectral instability has attracted considerable attention in the literature. While the emergence of such instability is attributed to the non-Hermitian nature of the gravitational system, it remains sensitive to various factors. In this work, [...] Read more.

Owing to its substantial implications for black hole spectroscopy, spectral instability has attracted considerable attention in the literature. While the emergence of such instability is attributed to the non-Hermitian nature of the gravitational system, it remains sensitive to various factors. In this work, we conduct a focused analysis of black hole spectral instability using the Pöschl–Teller potential as a toy model. We investigate the dependence of the resulting spectral instability on the magnitude, spatial scale, and localization of deterministic and random perturbations in the effective potential of the wave equation, and discuss the underlying physical interpretations. It is observed that small perturbations in the potential initially have a limited impact on the less damped black hole quasinormal modes, with deviations typically around their unperturbed values, a phenomenon first derived by Skakala and Visser in a more restrictive context. In the higher-overtone region, the deviation propagates, amplifies, and eventually gives rise to spectral instability and, inclusively, bifurcation in the quasinormal mode spectrum. While deterministic perturbations give rise to a deformed but well-defined quasinormal spectrum, random perturbations lead to uncertainties in the resulting spectrum. Nonetheless, the primary trend of the spectral instability remains consistent, being sensitive to both the strength and location of the perturbation. However, we demonstrate that the observed spectral instability might be suppressed for perturbations that are physically appropriate. Full article

(This article belongs to the Collection Open Questions in Black Hole Physics)

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33 pages, 17584 KB

Open AccessReview

Radio Supernovae

by Esha Kundu

Universe 2026, 12(1), 4; https://doi.org/10.3390/universe12010004 - 24 Dec 2025

Abstract

Supernovae (SNe), the catastrophic end of stars’ lives, are among the most energetic phenomena in the universe. Mapping the aftermath of the explosions to the properties of pre-SN stars is challenging due to the lack of knowledge about the evolution of different types [...] Read more.

Supernovae (SNe), the catastrophic end of stars’ lives, are among the most energetic phenomena in the universe. Mapping the aftermath of the explosions to the properties of pre-SN stars is challenging due to the lack of knowledge about the evolution of different types of stars. The immediate surroundings of pre-SN stars carry the signature of the progenitors, and radio observations are the best way to examine the ambient media. Since radio emission originates from the interaction of supersonic SN ejecta with the relatively stationary circumstellar medium, with a few years of radio study, the mass-loss history of progenitor stars can be probed from just before the explosion of the star to thousands of years before the onset of the SN. Moreover, this can provide crucial details about the explosions, which are poorly understood to date. In this paper, we review the radio properties of different types of core-collapse explosions and thermonuclear runaways to understand their mass-loss evolution—which allows us to unravel the imprints of the progenitors on the surrounding media and thus the nature of the exploded stars. Additionally, we discuss the current state of the art in this field, including existing and the next-generation radio facilities with enhanced capabilities that provide further details about these explosions. Full article

(This article belongs to the Special Issue A Multiwavelength View of Supernovae)

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14 pages, 13792 KB

Open AccessArticle

Probing Lorentz Invariance Violation at High Energies Using LHAASO Observations of GRB221009A via DisCan Algorithm

by Yu-Chen Hua, Xiao-Jun Bi, Yu-Ming Yang and Peng-Fei Yin

Universe 2026, 12(1), 3; https://doi.org/10.3390/universe12010003 - 24 Dec 2025

Abstract

The Lorentz invariance violation (LIV) predicted by some quantum gravity theories would manifest as an energy-dependent speed of light, which may potentially distort the observed temporal profile of photons from astrophysical sources at cosmological distances. The dispersion cancellation (DisCan) algorithm offers a powerful [...] Read more.

The Lorentz invariance violation (LIV) predicted by some quantum gravity theories would manifest as an energy-dependent speed of light, which may potentially distort the observed temporal profile of photons from astrophysical sources at cosmological distances. The dispersion cancellation (DisCan) algorithm offers a powerful methodology for investigating such effects by employing quantities such as Shannon entropy, which reflects the initial temporal characteristics. In this study, we apply the DisCan algorithm to search for LIV effects in the LHAASO observations of GRB 221009A, combining data from both the Water Cherenkov Detector Array (WCDA) and Kilometer Squared Array (KM2A) detectors that collectively span an energy range of ∼0.2–13 TeV. Our analysis accounts for the uncertainties from both energy resolution and temporal binning. We derive

95 %

confidence level lower limits on the LIV energy scale of

E_{QG, 1} / 10^{19} GeV > 14.6

(11.2) for the first-order subluminal (superluminal) scenario, and

E_{QG, 2} / 10^{11} GeV > 13.7

(12.5) for the second-order subluminal (superluminal) scenario. Full article

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22 pages, 2095 KB

Open AccessArticle

The Geometry and Flow of Informational Time

by Florian Neukart

Universe 2026, 12(1), 2; https://doi.org/10.3390/universe12010002 - 20 Dec 2025

Abstract

Time remains one of the most elusive concepts in physics, lying at the intersection of quantum mechanics, relativity, and thermodynamics. This work proposes a reformulation in which time arises as a local informational field rather than as a universal coordinate. Temporal direction is [...] Read more.

Time remains one of the most elusive concepts in physics, lying at the intersection of quantum mechanics, relativity, and thermodynamics. This work proposes a reformulation in which time arises as a local informational field rather than as a universal coordinate. Temporal direction is identified with gradients in stored information, linking geometry and entropy through an informational potential that generates both curvature and the arrow of time. The resulting field

T^{a} (x) = \partial^{a} S_{info} (x)

defines causal order and temporal flow through local information exchange, unifying dynamical evolution and entropic asymmetry within a single framework. The formulation preserves general relativity in the macroscopic limit while extending its validity to microscopic regimes where information dynamics supersede geometric structure. It thus offers a coherent physical basis for temporal asymmetry, a bridge between quantum and gravitational descriptions, and a platform for simulating time as an observable field. Full article

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