Universe

24 pages, 1135 KB

Open AccessArticle

Birth of an Isotropic and Homogeneous Universe with a Running Cosmological Constant

by A. Oliveira Castro Júnior, A. Corrêa Diniz, G. Oliveira-Neto and G. A. Monerat

Universe 2025, 11(9), 310; https://doi.org/10.3390/universe11090310 - 11 Sep 2025

The present work discusses the birth of the Universe via quantum tunneling through a potential barrier, based on quantum cosmology, taking a running cosmological constant into account. We consider the Friedmann–Lemaître–Robertson–Walker (FLRW) metric with positively curved spatial sections (

k = 1

) [...] Read more.

The present work discusses the birth of the Universe via quantum tunneling through a potential barrier, based on quantum cosmology, taking a running cosmological constant into account. We consider the Friedmann–Lemaître–Robertson–Walker (FLRW) metric with positively curved spatial sections (

k = 1

) and the matter’s content is a dust perfect fluid. The model was quantized by the Dirac formalism, leading to a Wheeler–DeWitt equation. We solve that equation both numerically and using a WKB approximation. We study the behavior of tunneling probabilities

T P_{W K B}

and

T P_{i n t}

by varying the energy E of the dust perfect fluid, the phenomenological parameter

ν

, the present value of the Hubble function

H_{0}

, and the constant energy density

ρ_{Λ}^{0}

, with the last three parameters all being associated with the running cosmological constant. We observe that both tunneling probabilities,

T P_{W K B}

and

T P_{i n t}

, decrease as one increases

ν

. We also note that

T P_{W K B}

and

T P_{i n t}

grow as E increases, indicating that the Universe is more likely to be born with higher dust energy E values. The same is observed for the parameter

ρ_{Λ}^{0}

, that is,

T P_{W K B}

and

T P_{i n t}

are larger for higher values of

ρ_{Λ}^{0}

. Finally, the tunneling probabilities decrease as one increases the value of

H_{0}

. Therefore, the best conditions for the Universe to be born, in the present model, would be to have the highest possible values for E and

Λ

and the lowest possible values for

ν

and

H_{0}

. Full article

(This article belongs to the Section Cosmology)

► Show Figures

Figure 1

22 pages, 501 KB

Open AccessArticle

Initial Conditions for Tidal Synchronisation of a Planet by Its Moon

by Valeri V. Makarov and Michael Efroimsky

Universe 2025, 11(9), 309; https://doi.org/10.3390/universe11090309 - 10 Sep 2025

Abstract

Moons tidally interact with their host planets and stars. A close moon is quickly synchronised by the planet or becomes captured in a higher spin–orbit resonance. However, the planet requires much more time to significantly alter its rotation rate under the influence of [...] Read more.

Moons tidally interact with their host planets and stars. A close moon is quickly synchronised by the planet or becomes captured in a higher spin–orbit resonance. However, the planet requires much more time to significantly alter its rotation rate under the influence of moon-generated tides. The situation becomes more complex for close-in planets, as star-generated tides come into play and compete with moon-generated tides. The synchronisation of the planet by its moon changes the tidal dynamics of the entire star–planet–moon system and can lead to long-term stable configurations. In this paper, we demonstrate that a certain initial condition must be met for this to occur. Based on the angular momentum conservation, the derived condition is universal and bears no dependence upon the planet’s internal structure or tidal dissipation model. It is applicable to dwindling systems as well as to tidally expanding orbits and cases of initially retrograde motion. We present calculations for specific planet–moon systems (Earth and the Moon; Neptune and Triton; Venus and its hypothetical presently extinct moon Neith; Mars, Phobos, and Deimos; and Pluto and Charon) to constrain dynamically plausible formation and evolution scenarios. Among other things, our analysis prompts the question of whether Pluto and Charon evolved into their current state from an initially more compact configuration (as is commonly assumed) or from a wider orbit—a topic that will be discussed at length elsewhere. Our results are equally applicable to exoplanets. For example, if asynchronous close-in exoplanets are detected, the possibility of tidal synchronisation by an exomoon should be considered. Full article

► Show Figures

Figure 1

15 pages, 290 KB

Open AccessArticle

Non-Linear Equation of Motion for Page–Wootters Mechanism with Interaction and Quasi-Ideal Clocks

by Leandro R. S. Mendes, Frederico Brito and Diogo O. Soares-Pinto

Universe 2025, 11(9), 308; https://doi.org/10.3390/universe11090308 - 10 Sep 2025

Abstract

We explore a timeless approach to quantum theory, in the form of the Page–Wootters mechanism, in which a gravitational interaction is introduced between the system and a finite-dimensional clock. The clock model used is the recently proposed quasi-ideal clock, a construction that can [...] Read more.

We explore a timeless approach to quantum theory, in the form of the Page–Wootters mechanism, in which a gravitational interaction is introduced between the system and a finite-dimensional clock. The clock model used is the recently proposed quasi-ideal clock, a construction that can approximate the time–energy canonical commutation relation. We derive equations of motion for the case in which the system is in a pure and mixed state, obtaining a Schrödinger-like equation that leads to a non-linear equation exhibiting decoherence due to the non-ideal nature of the clock and gravitational coupling. A distinctive feature of this equation is that it exhibits dependence on the system’s initial conditions. Full article

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

17 pages, 749 KB

Open AccessArticle

Probing the Cosmic Distance Duality Relation via Non-Parametric Reconstruction for High Redshifts

by Felipe Avila, Fernanda Oliveira, Camila Franco, Maria Lopes, Rodrigo Holanda, Rafael C. Nunes and Armando Bernui

Universe 2025, 11(9), 307; https://doi.org/10.3390/universe11090307 - 9 Sep 2025

Abstract

We test the validity of the cosmic distance duality relation (CDDR) by combining angular diameter distance and luminosity distance measurements from recent cosmological observations. For the angular diameter distance, we use data from transverse baryon acoustic oscillations and galaxy clusters. On the other [...] Read more.

We test the validity of the cosmic distance duality relation (CDDR) by combining angular diameter distance and luminosity distance measurements from recent cosmological observations. For the angular diameter distance, we use data from transverse baryon acoustic oscillations and galaxy clusters. On the other hand, the luminosity distance is obtained from Type Ia supernovae in the Pantheon+ sample and from quasar catalogs. To reduce the large dispersion in quasar luminosity distances, we apply a selection criterion based on their deviation from the

Λ

CDM model and implement a binning procedure to suppress statistical noise. We reconstruct the CDDR using Gaussian Processes, a non-parametric supervised machine learning method. Our results show no significant deviation from the CDDR within the

2 σ

confidence level across the redshift range explored, supporting its validity even at high redshifts. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

► Show Figures

Figure 1

19 pages, 1517 KB

Open AccessArticle

Probing the Topology of the Early Universe Using CMB Temperature and Polarization Anisotropies

by Miguel-Angel Sanchis-Lozano

Universe 2025, 11(9), 306; https://doi.org/10.3390/universe11090306 - 9 Sep 2025

Abstract

The temperature and polarization anisotropies of the cosmic microwave background (CMB) as measured today can offer key insights into the topology of the early universe prior to inflation, for example by discriminating between flat and warped geometries. In this paper, we focus on [...] Read more.

The temperature and polarization anisotropies of the cosmic microwave background (CMB) as measured today can offer key insights into the topology of the early universe prior to inflation, for example by discriminating between flat and warped geometries. In this paper, we focus on a Kaluza–Klein model with an extra spatial dimension that compactifies at the Grand Unified Theory (GUT) epoch, subject to mixed Neumann/Dirichlet boundary conditions at fixed points. As a consequence, a set of Infrared (IR) cutoffs emerges in both the scalar and tensor spectra, leading to observable consequences in the CMB. We examine the possible signatures of such a topology in detail, particularly in relation to the even–odd parity imbalance already reported by the COBE, WMAP and Planck missions in the temperature angular correlations. Furthermore, we extend our analysis to the existing Planck E-mode polarization data and to the high-precision B-mode polarization measurements expected from the forthcoming LiteBIRD mission. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

► Show Figures

Figure 1

17 pages, 531 KB

Open AccessArticle

Black Hole Solution Free of Ghosts in f(R) Gravity Coupled with Two Scalar Fields

by G. G. L. Nashed and A. Eid

Universe 2025, 11(9), 305; https://doi.org/10.3390/universe11090305 - 9 Sep 2025

Abstract

One extension of general relativity, known as

f (R)

gravity, where

R

denotes the Ricci scalar, is regarded as a promising candidate for addressing the anomalies observed in conventional general relativity. In this work, we apply the field equations of [...] Read more.

One extension of general relativity, known as

f (R)

gravity, where

R

denotes the Ricci scalar, is regarded as a promising candidate for addressing the anomalies observed in conventional general relativity. In this work, we apply the field equations of

f (R)

gravity to a spacetime with spherical symmetry with distinct metric potentials, i.e.,

g_{t t} \neq g_{r r}

. By solving the resulting nonlinear differential equations, we derive a novel black hole solution without imposing constraints on the Ricci scalar or on the specific form of

f (R)

gravity. This solution does not reduce to the Schwarzschild solution of Einstein’s general relativity. This solution is notable because it includes a gravitational mass and extra terms that make the singularities in the curvature stronger than those in black holes from Einstein’s general relativity. We analyze these black holes within the framework of thermodynamics and demonstrate their consistency with standard thermodynamic quantities. Furthermore, we investigate the stability by examining odd-type perturbation modes and show that the resulting black hole is stable. Finally, we derive the coefficients of the two scalar fields and demonstrate that the black hole obtained in this study is free from ghosts. Full article

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

► Show Figures

Figure 1

24 pages, 10348 KB

Open AccessReview

The Variable Sky Through the OGLE Eye

by Patryk Iwanek

Universe 2025, 11(9), 304; https://doi.org/10.3390/universe11090304 - 8 Sep 2025

Abstract

The Optical Gravitational Lensing Experiment (OGLE) is one of the most productive and influential photometric sky surveys in the history of observational astronomy. Originally designed to detect dark matter through gravitational microlensing events, OGLE has evolved into a cornerstone of time-domain astrophysics, delivering [...] Read more.

The Optical Gravitational Lensing Experiment (OGLE) is one of the most productive and influential photometric sky surveys in the history of observational astronomy. Originally designed to detect dark matter through gravitational microlensing events, OGLE has evolved into a cornerstone of time-domain astrophysics, delivering three decades of two-band, high-cadence observations of approximately two billion stars across the Galactic bulge, disk, and Magellanic System. This review summarizes OGLE’s key contributions to variable star research, including the discovery, classification and characterization of pulsating stars, eclipsing, ellipsoidal, and rotating variables, or irregular and eruptive stars. Particular emphasis is placed on the OGLE Collection of Variable Stars (OCVS), a publicly available and systematically expanded dataset that has become a fundamental resource for studies of stellar variability and evolution, Milky Way and other galaxies structure, microlensing, compact objects, exoplanets and more. The synergy between OGLE and other major sky surveys, including ASAS, ASAS-SN, ATLAS, Gaia, KMTNet, MACHO, MOA, TESS, PLATO, or ZTF further amplifies its scientific reach. Full article

(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)

► Show Figures

Figure 1

19 pages, 1748 KB

Open AccessArticle

On the True Significance of the Hubble Tension: A Bayesian Error Decomposition Accounting for Information Loss

by Nathalia M. N. da Rocha, Andre L. B. Ribeiro and Francisco B. S. Oliveira

Universe 2025, 11(9), 303; https://doi.org/10.3390/universe11090303 - 6 Sep 2025

Abstract

The Hubble tension, a persistent discrepancy between early and late Universe measurements of

H_{0}

, poses a significant challenge to the standard cosmological model. In this work, we present a new Bayesian hierarchical framework designed to meticulously decompose this observed tension into [...] Read more.

The Hubble tension, a persistent discrepancy between early and late Universe measurements of

H_{0}

, poses a significant challenge to the standard cosmological model. In this work, we present a new Bayesian hierarchical framework designed to meticulously decompose this observed tension into its constituent parts: standard measurement errors, information loss arising from parameter-space projection, and genuine physical tension. Our approach, employing Fisher matrix analysis with MCMC-estimated loss coefficients and explicitly modeling information loss via variance inflation factors (

λ

), is particularly important in high-precision analysis where even seemingly small information losses can impact conclusions. We find that the real tension component (

T_{r e a l}

) has a mean value of 5.94 km/s/Mpc (95% CI: [3.32, 8.64] km/s/Mpc). Quantitatively, approximately 78% of the observed tension variance is attributed to real tension, 13% to measurement error, and 9% to information loss. Despite this, our decomposition indicates that the observed ∼

6.39 σ

discrepancy is predominantly a real physical phenomenon, with real tension contributing ∼

5.64 σ

. Our findings strongly suggest that the Hubble tension is robust and probably points toward new physics beyond the ΛCDM model. Full article

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

► Show Figures

Figure 1

42 pages, 17899 KB

Open AccessArticle

A Systematic Search for New δ Scuti and γ Doradus Stars Using TESS Data

by Ai-Ying Zhou

Universe 2025, 11(9), 302; https://doi.org/10.3390/universe11090302 - 5 Sep 2025

Abstract

Focusing on the discovery of new

δ

Scuti and

γ

Doradus stars, we analyzed the Transiting Exoplanet Survey Satellite (TESS) light curves for 193,940 A-F stars selected from four legacy catalogs—the Henry Draper Catalogue (HD), the Smithsonian Astrophysical Observatory (SAO) Star [...] Read more.

Focusing on the discovery of new

δ

Scuti and

γ

Doradus stars, we analyzed the Transiting Exoplanet Survey Satellite (TESS) light curves for 193,940 A-F stars selected from four legacy catalogs—the Henry Draper Catalogue (HD), the Smithsonian Astrophysical Observatory (SAO) Star Catalog, the Positions and Proper Motions Catalog (PPM), and the Bonner Durchmusterung (BD, including its extensions). Through visual inspection of light curve morphologies and periodograms, combined with evaluation of stellar parameters, we identified over 51,850 previously unreported variable stars. These include 15,380

δ

Scuti, 18,560

γ

Doradus, 28 RR Lyrae stars, 260 heartbeat candidates, and 2645 eclipsing binaries, along with thousands of other variable types. Notably, over 4145 variables exhibit hybrid

δ

Scuti-

γ

Doradus pulsations, and more than 380 eclipsing binaries feature pulsating primary components. This study reveals a substantial population of bright, previously undetected variables, providing a valuable resource for ensemble asteroseismology, binary evolution studies, and Galactic structure research. Our results also highlight the surprising richness in variability still hidden within well-known stellar catalogs and the continued importance of high-precision, time-domain surveys such as TESS. Full article

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

► Show Figures

Figure 1

75 pages, 17108 KB

Open AccessArticle

A Catalog of 73 B-Type Stars and Their Brightness Variation from K2 Campaign 13–18

by Bergerson V. H. V. da Silva, Jéssica M. Eidam, Alan W. Pereira, M. Cristina Rabello-Soares, Eduardo Janot-Pacheco, Laerte Andrade and Marcelo Emilio

Universe 2025, 11(9), 301; https://doi.org/10.3390/universe11090301 - 3 Sep 2025

Abstract

The variability of B-type stars offers valuable insights into the interiors of stars and the processes that drive pulsation and rotation in massive stars. In this study, we present the classification of the variability of 197 B-type stars observed in various Kepler/K2 [...] Read more.

The variability of B-type stars offers valuable insights into the interiors of stars and the processes that drive pulsation and rotation in massive stars. In this study, we present the classification of the variability of 197 B-type stars observed in various Kepler/K2 campaigns, including 73 newly classified stars from Campaigns 13–18. For these stars, we derived atmospheric and evolutionary parameters using space-based photometry and ground-based spectroscopy. We obtained spectroscopic data for 34 targets with high-resolution instruments at OPD/LNA, which were supplemented by archival LAMOST spectra. After correcting for instrumental systematics, we analyzed the light curves using Fourier transforms and wavelet decomposition to identify both periodic and stochastic signals. The identified variability types included SPB stars,

β

Cephei/SPB hybrids, fast-rotating pulsators, stochastic low-frequency variables, eclipsing binaries, and rotational variables. We also revised classifications of misidentified stars using Gaia astrometry, confirming the main-sequence nature of objects once considered subdwarfs. Our results indicate that hot-star variability exists along a continuum shaped by mass, rotation, and internal mixing rather than distinct instability domains. This study enhances our understanding of B-type star variability and supports future asteroseismic modeling with missions like PLATO. Full article

(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)

► Show Figures

Figure 1

13 pages, 324 KB

Open AccessArticle

Gauge Symmetry and Radiatively Induced Terms in Dimension-5 Non-Minimal Lorentz-Violating QED

by A. P. B. Scarpelli and A. R. Vieira

Universe 2025, 11(9), 300; https://doi.org/10.3390/universe11090300 - 3 Sep 2025

Abstract

In this work, we derive the conditions that ensure gauge invariance of non-minimal dimension-5 Lorentz-violating QED. The two- and three-point functions at one loop are computed. The gauge Ward identities are checked, as well as the conditions, to ensure that the gauge symmetry [...] Read more.

In this work, we derive the conditions that ensure gauge invariance of non-minimal dimension-5 Lorentz-violating QED. The two- and three-point functions at one loop are computed. The gauge Ward identities are checked, as well as the conditions, to ensure that the gauge symmetry of this non-minimal framework is found to be the same of the usual QED. Induced terms are also investigated, and it is shown that the non-minimal Lorentz-violating

a_{F}^{(5)}

-term of the fermion sector can radiatively induce a non-minimal Lorentz-violating term in the photon sector. Full article

(This article belongs to the Section Field Theory)

► Show Figures

Figure 1

4 pages, 154 KB

Open AccessEditorial

Editorial for the Special Issue “New Discoveries in Astronomical Data”

by Yanxia Zhang and A-Li Luo

Universe 2025, 11(9), 299; https://doi.org/10.3390/universe11090299 - 3 Sep 2025

Abstract

Over the past decade, astronomy has shifted from a data-starved to a data-drenched science [...] Full article

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

1 pages, 159 KB

Open AccessCorrection

Correction: Kuić et al. Finite Time Path Field Theory and a New Type of Universal Quantum Spin Chain Quench Behavior. Universe 2025, 11, 230

by Domagoj Kuić, Alemka Knapp and Diana Šaponja-Milutinović

Universe 2025, 11(9), 298; https://doi.org/10.3390/universe11090298 - 2 Sep 2025

Abstract

We would like to report an error in the published paper [...] Full article

9 pages, 1664 KB

Open AccessArticle

Quantized Nuclear Recoil in the Search for Sterile Neutrinos in Tritium Beta Decay with PTOLEMY

by Wonyong Chung, Mark Farino, Andi Tan, Christopher G. Tully and Shiran Zhang

Universe 2025, 11(9), 297; https://doi.org/10.3390/universe11090297 - 2 Sep 2025

Abstract

The search for keV-scale sterile neutrinos in tritium beta decay is made possible through the theoretically allowed small admixture of electron flavor in right-handed, singlet, massive neutrino states. A distinctive feature of keV-scale sterile-neutrino–induced threshold distortions in the tritium beta spectrum is the [...] Read more.

The search for keV-scale sterile neutrinos in tritium beta decay is made possible through the theoretically allowed small admixture of electron flavor in right-handed, singlet, massive neutrino states. A distinctive feature of keV-scale sterile-neutrino–induced threshold distortions in the tritium beta spectrum is the presence of quantized nuclear-recoil effects, as predicted for atomic tritium bound to two-dimension materials such as graphene. The sensitivities to the sterile neutrino mass and electron-flavor mixing are considered in the context of the PTOLEMY detector simulation with tritiated graphene substrates. The ability to scan the entire tritium energy spectrum with a narrow energy window, low backgrounds, and high-resolution differential energy measurements provides the opportunity to pinpoint the quantized nuclear-recoil effects. providing an additional tool for identifying the kinematics of the production of sterile neutrinos. Background suppression is achieved by transversely accelerating electrons into a high magnetic field, where semi-relativistic electron tagging can be performed with cyclotron resonance emission RF antennas followed by deceleration through the PTOLEMY filter into a high-resolution differential energy detector operating in a zero-magnetic-field region. The PTOLEMY-based approach to keV-scale searches for sterile neutrinos involves a novel precision apparatus utilizing two-dimensional materials to yield high-resolution, sub-eV mass determination for electron-flavor mixing fractions of

| U_{e 4} |^{2} ≃ 10^{- 5}

and smaller. Full article

(This article belongs to the Special Issue keV Warm Dark Matter (ΛWDM) in Agreement with Observations in Tribute to Héctor J. de Vega (2nd Edition))

► Show Figures

Figure 1

10 pages, 21975 KB

Open AccessArticle

A Comparison Study of Collisions at Relativistic Energies Involving Light Nuclei

by Hai-Cheng Wang, Song-Jie Li, Jun Xu and Zhong-Zhou Ren

Universe 2025, 11(9), 296; https://doi.org/10.3390/universe11090296 - 1 Sep 2025

Abstract

We present extensive comparisons of ¹⁶O+¹⁶O collisions at a center-of-mass energy per nucleon pair

\sqrt{s_{N N}} = 200

GeV and ²⁰⁸Pb+¹⁶O collisions at

\sqrt{s_{N N}} = 68.5

GeV as well as ²⁰Ne+²⁰Ne [...] Read more.

We present extensive comparisons of ¹⁶O+¹⁶O collisions at a center-of-mass energy per nucleon pair

\sqrt{s_{N N}} = 200

GeV and ²⁰⁸Pb+¹⁶O collisions at

\sqrt{s_{N N}} = 68.5

GeV as well as ²⁰Ne+²⁰Ne collisions at

\sqrt{s_{N N}} = 200

GeV and ²⁰⁸Pb+²⁰Ne collisions at

\sqrt{s_{N N}} = 68.5

GeV based on a multiphase transport (AMPT) model. We recommend measuring the ratio of the elliptic flow to the triangular flow, which shows appreciable sensitivity to the structure of light nuclei, as also found in other studies. This is especially so if the observable is measured near the target rapidity in ²⁰⁸Pb+¹⁶O or ²⁰⁸Pb+²⁰Ne collisions, as originally found in the present study. Our study serves as a useful reference for understanding the effect of structure on observables in collisions involving light nuclei under analysis or on the schedule. Full article

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

► Show Figures

Figure 1

20 pages, 16141 KB

Open AccessArticle

Low-Latitude Ionospheric Anomalies During Geomagnetic Storm on 10–12 October 2024

by Plamen Mukhtarov and Rumiana Bojilova

Universe 2025, 11(9), 295; https://doi.org/10.3390/universe11090295 - 1 Sep 2025

Abstract

This research examines in detail the behavior of the Equatorial Ionization Anomaly (EIA) during a severe geomagnetic storm that occurred on 10–11 October 2024. The global data of Total Electron Content (TEC) represented by relative deviation, giving information about the variations compared to [...] Read more.

This research examines in detail the behavior of the Equatorial Ionization Anomaly (EIA) during a severe geomagnetic storm that occurred on 10–11 October 2024. The global data of Total Electron Content (TEC) represented by relative deviation, giving information about the variations compared to quiet conditions, were used. The main attention is paid to the appearance of an additional “fountain effect” under the action of disturbed dynamo currents and the vertical drift of the ionospheric plasma caused by them. The results show that the area in which a positive response (increase) of TEC is observed occurs in an area corresponding to local time around 18–20 h (longitude around 60 °W) at magnetic latitudes ±30° and during the storm shifts westward to around 180 °W. The westward drift of the storm-induced “fountain effect” is moving at a speed much slower than the Earth’s rotation speed. As a result, the area of positive TEC response (vertical upward drift) and the area of negative response (vertical downward drift) are localized in both nighttime and daytime conditions. In this investigation, an example of a very similar geomagnetic storm registered on 25 September 1998 is given for comparison, in which a similar stationing of the storm-induced EIA was observed at longitudes around 180 °E. Full article

(This article belongs to the Section Space Science)

► Show Figures

Figure 1

11 pages, 711 KB

Open AccessCommunication

What Do Radio Emission Constraints Tell Us About Little Red Dots as Tidal Disruption Events?

by Krisztina Perger, Judit Fogasy and Sándor Frey

Universe 2025, 11(9), 294; https://doi.org/10.3390/universe11090294 - 1 Sep 2025

Abstract

The real nature of little red dots (LRDs), a class of very compact galaxies in the early Universe recently discovered by the James Webb Space Telescope, is still poorly understood. The most popular theories competing to interpret the phenomena include active galactic nuclei [...] Read more.

The real nature of little red dots (LRDs), a class of very compact galaxies in the early Universe recently discovered by the James Webb Space Telescope, is still poorly understood. The most popular theories competing to interpret the phenomena include active galactic nuclei and enhanced star formation in dusty galaxies. To date, however, neither model gives a completely satisfactory explanation to the population as a whole; thus, alternative theories have arisen, including tidal disruption events (TDEs). By considering observational constraints on the radio emission of LRDs, we discuss whether TDEs are adequate alternatives solving these high-redshift enigmas. We utilise radio flux density upper limits from LRD stacking analyses, TDE peak radio luminosities, and volumetric density estimates. We find that the characteristic values of flux densities and luminosities allow radio-quiet TDEs as the underlying process of LRDs in any case, while the less common radio-loud TDEs are compatible with the model under special constraints only. Considering other factors, such as volumetric density estimates, delayed and long-term radio flares of TDEs, and cosmological time dilation, TDEs appear to be a plausible explanation for LRDs from the radio point of view. Full article

(This article belongs to the Special Issue Advances in Studies of Galaxies at High Redshift)

► Show Figures

Figure 1

11 pages, 407 KB

Open AccessCommunication

On Restrictions of Current Warp Drive Spacetimes and Immediate Possibilities of Improvement

by Hamed Barzegar and Thomas Buchert

Universe 2025, 11(9), 293; https://doi.org/10.3390/universe11090293 - 1 Sep 2025

Abstract

Looking at current proposals of so-called ‘warp drive spacetimes’, they appear to employ General Relativity only at an elementary level. A number of strong restrictions are imposed such as flow-orthogonality of the spacetime foliation, vanishing spatial Ricci tensor, and dimensionally reduced and coordinate-dependent [...] Read more.

Looking at current proposals of so-called ‘warp drive spacetimes’, they appear to employ General Relativity only at an elementary level. A number of strong restrictions are imposed such as flow-orthogonality of the spacetime foliation, vanishing spatial Ricci tensor, and dimensionally reduced and coordinate-dependent velocity fields, to mention the main restrictions. We here provide a brief summary of our proposal of a general and covariant description of spatial motions within General Relativity, then discuss the restrictions that are employed in the majority of the current literature. That current warp drive models are discussed to be unphysical may not be surprising; they lack important ingredients such as covariantly non-vanishing spatial velocity, acceleration, vorticity together with curved space, and a warp mechanism. Full article

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

► Show Figures

Figure 1

15 pages, 425 KB

Open AccessArticle

Application of the Magnetar Engine to an Intermediate-Luminosity Gamma-Ray Burst Associated with the Supernova GRB 201015A/SN 201015A

by Xingling Li, Defeng Kong, Liangjun Chen, Xianggao Wang and Enwei Liang

Universe 2025, 11(9), 292; https://doi.org/10.3390/universe11090292 - 31 Aug 2025

Abstract

We present optical photometry for the afterglow of GRB 201015A, which can be classified as a medium-luminosity gamma-ray burst (

L_{γ, iso} \approx 2.55 \times 10^{49} erg s^{- 1}

) and the associated underlying supernova SN 201015A. A millisecond [...] Read more.

We present optical photometry for the afterglow of GRB 201015A, which can be classified as a medium-luminosity gamma-ray burst (

L_{γ, iso} \approx 2.55 \times 10^{49} erg s^{- 1}

) and the associated underlying supernova SN 201015A. A millisecond magnetar engine has been widely suggested to exist in gamma-ray burst (GRB) phenomena. In this paper, we study the effects of the magnetar engine on GRB 201015A/SN 201015A by light curve analysis. We use a smooth broken power-law plus magnetar spin-down model to fit the X-ray and optical light curves of GRB 201015A/SN 201015A. The best-fitting results reveal that the magnetar initial spin period and surface magnetic field at the pole are constrained to be

P_{0} = 16 . 80_{- 0.47}^{+ 0.24} ms

and

B_{p} = 0 . 80_{- 0.32}^{+ 0.34} \times 10^{15} G

, respectively, and the SN ejected a total mass of

M_{ej} = 2 . 55_{- 0.37}^{+ 1.12} M_{⊙}

and an ejecta velocity of

v_{ej}

=

{30,000}_{- 2500}^{+ 4800} km s^{- 1}

, inferring a kinetic energy of

E_{SN, K} \approx 1.37 \times 10^{52} erg

. From our analysis, we find that the central engine of GRB 201015A/SN 201015A may well be a magnetar, and the emission from a magnetar central engine can be solely responsible for powering SN 201015A. Full article

► Show Figures

Figure 1

Journal Description

Universe

Latest Articles

Journal Menu

Journal Browser

Highly Accessed Articles

Latest Books

E-Mail Alert

News

Topics

Conferences

Special Issues

Topical Collections

Further Information

Guidelines

MDPI Initiatives

Follow MDPI