Universe

24 pages, 13152 KiB

Open AccessArticle

Radio Observations as a Probe of Cosmic Web Magnetism

by Ettore Carretti and Franco Vazza

Universe 2025, 11(5), 164; https://doi.org/10.3390/universe11050164 - 21 May 2025

The Universe’s magnetogenesis can be investigated with radio observations of cosmic filaments, where the information on the initial magnetic field seeds is expected to be preserved in time. In this work, we update the comparison between recent observational results in filaments with the [...] Read more.

The Universe’s magnetogenesis can be investigated with radio observations of cosmic filaments, where the information on the initial magnetic field seeds is expected to be preserved in time. In this work, we update the comparison between recent observational results in filaments with the predictions from recent cosmological simulations to check whether one of them is favored. The radio probes we use are the rotation measure (RM) of filaments as a function of the redshift (z), stacking of synchrotron emission from filaments, and the RM radial profile away from galaxy groups. The first two probes favor the presence of a dominant primordial magnetic field component and disfavor a sole astrophysical scenario, while the third probe does not yet give an unambiguous outcome. We also estimate the average field strength in filaments. Independently of the scenario and the shape of the astrophysical component RM, it is in the range 10–60 nG at

z = 0

, while, when restricted to the model that gives the best match to the simulations, it gives

43 \pm 7

nG, with an astrophysical component RM rapidly decreasing with the redshift. Full article

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

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9 pages, 17914 KiB

Open AccessArticle

Measurement of Ion Mobilities for the Ion-TPC of NvDEx Experiment

by Tianyu Liang, Meiqiang Zhan, Hulin Wang, Xianglun Wei, Dongliang Zhang, Jun Liu, Chengui Lu, Qiang Hu, Yichen Yang, Chaosong Gao, Le Xiao, Xiangming Sun, Feng Liu, Chengxin Zhao, Hao Qiu and Kai Chen

Universe 2025, 11(5), 163; https://doi.org/10.3390/universe11050163 - 16 May 2025

Abstract

In the N

ν

DEx collaboration, a high-pressure gas TPC is being developed to search for the neutrinoless double beta decay. The use of electronegative ⁸²SeF₆ gas mandates an ion-TPC. The reconstruction of the z coordinate is to be realized by [...] Read more.

In the N

ν

DEx collaboration, a high-pressure gas TPC is being developed to search for the neutrinoless double beta decay. The use of electronegative ⁸²SeF₆ gas mandates an ion-TPC. The reconstruction of the z coordinate is to be realized by exploiting the feature of multiple species of charge carriers. As the initial stage of the development, we studied the properties of the

{SF}_{6}

gas, which is non-toxic and has a similar molecular structure to

{SeF}_{6}

. In the paper, we present the measurement of drift velocities and mobilities of the majority and minority negative charge carriers found in

{SF}_{6}

at a pressure of 750 Torr, slightly higher than the local atmospheric pressure. The reduced fields range between 3.0 and 5.5 Td. This was performed using a laser beam to ionize the gas inside a small TPC, with a drift length of 3.7 cm. A customized charge-sensitive amplifier was developed to read out the anode signals induced by the slowly drifting ions. The closure test of the reconstruction of the z coordinate using the difference in the velocities of the two carriers was also demonstrated. Full article

(This article belongs to the Special Issue Exploring Double Beta Decay: Probing Fundamental Properties of Neutrinos and Beyond)

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68 pages, 5470 KiB

Open AccessFeature PaperArticle

Does Quantum Mechanics Breed Larger, More Intricate Quantum Theories? The Case for Experience-Centric Quantum Theory and the Interactome of Quantum Theories

by Alireza Tavanfar, Sahar Alipour and Ali T. Rezakhani

Universe 2025, 11(5), 162; https://doi.org/10.3390/universe11050162 - 16 May 2025

Abstract

We pose and address the radical question of whether quantum mechanics, known for its firm internal structure and enormous empirical success, carries in itself the genomes of larger quantum theories that have higher internal intricacy and phenomenological versatility. In other words, we consider, [...] Read more.

We pose and address the radical question of whether quantum mechanics, known for its firm internal structure and enormous empirical success, carries in itself the genomes of larger quantum theories that have higher internal intricacy and phenomenological versatility. In other words, we consider, at the basic level of closed quantum systems and regardless of interpretational aspects, whether standard quantum theory (SQT) harbors quantum theories with context-based deformed principles or structures, having definite predictive power within much broader scopes. We answer this question in the affirmative following complementary evidence and reasoning arising from quantum-computation-based quantum simulation and fundamental, general, and abstract rationales within the frameworks of information theory, fundamental or functional emergence, and participatory agency. In this light, as we show, one is led to the recently proposed experience-centric quantum theory (ECQT), which is a larger and richer theory of quantum behaviors with drastically generalized quantum dynamics. ECQT allows the quantum information of the closed quantum system’s developed state history to continually contribute to defining and updating the many-body interactions, the Hamiltonians, and even the internal elements and “particles” of the total system. Hence, the unitary evolutions are continually impacted and become guidable by the agent system’s experience. The intrinsic interplay of unitarity and non-Markovianity in ECQT brings about a host of diverse behavioral phases, which concurrently infuse closed and open quantum system characteristics, and it even surpasses the theory of open systems in SQT. From a broader perspective, a focus of our investigation is the existence of the quantum interactome—the interactive landscape of all coexisting, independent, context-based quantum theories that emerge from inferential participatory agencies—and its predictive phenomenological utility. Full article

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

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32 pages, 1368 KiB

Open AccessReview

On the X-Ray Emission from Supernovae, and Implications for the Mass-Loss Rates of Their Progenitor Stars

by Vikram V. Dwarkadas

Universe 2025, 11(5), 161; https://doi.org/10.3390/universe11050161 - 15 May 2025

Abstract

We summarize the X-ray emission from young SNe. Having accumulated data on most observed X-ray SNe, we display the X-ray lightcurves of young SNe. We also explore the X-ray spectra of various SN types. The X-ray emission from Type Ib/c SNe is non-thermal. [...] Read more.

We summarize the X-ray emission from young SNe. Having accumulated data on most observed X-ray SNe, we display the X-ray lightcurves of young SNe. We also explore the X-ray spectra of various SN types. The X-ray emission from Type Ib/c SNe is non-thermal. It is also likely that the emission from Type IIP SNe with low mass-loss rates (around 10⁻⁷ M_⊙ yr⁻¹) is non-thermal. As the mass-loss rate increases, thermal emission begins to dominate. Type IIn SNe have the highest X-ray luminosities, and are clearly thermal. We do not find evidence of non-thermal emission from Type IIb SNe. The aggregated data are used to obtain approximate mass-loss rates of the progenitor stars of these SNe. Type IIP have progenitors with mass-loss rates

< 10^{- 5} M_{⊙}

yr⁻¹, while Type IIn progenitors generally have mass-loss rates

> 10^{- 3} M_{⊙}

yr⁻¹. However, we emphasize that the density of the ambient medium is the important parameter, and if it is due to a non-steady outflow solution, it cannot be translated into a mass-loss rate. Full article

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

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8 pages, 245 KiB

Open AccessCommunication

Kerr Black Hole as a Pevatron at the Galaxy Center

by Orlando Panella, Simone Pacetti, Giorgio Immirzi and Yogendra Srivastava

Universe 2025, 11(5), 160; https://doi.org/10.3390/universe11050160 - 13 May 2025

Abstract

Conventional general relativity supplies the notion of a vacuum tension and thus a maximum force

F_{\max} = c^{4} / (4 G) ≃ 3 \cdot 10^{43}

N that is realized for a black hole. In conjunction with the Wilson area rule, [...] Read more.

Conventional general relativity supplies the notion of a vacuum tension and thus a maximum force

F_{\max} = c^{4} / (4 G) ≃ 3 \cdot 10^{43}

N that is realized for a black hole. In conjunction with the Wilson area rule, we are thus led to the surface confinement of the mass of a black hole analogous to the surface confinement of quarks. The central result of our paper is that PeV scale protons can be generated on the surface of a Kerr black hole. This result is in concert with the presence at the galactic center of that Pevatron accelerating mechanism first suggested by the H.E.S.S. Collaboration and further confirmed by the HAWC Observatory. Full article

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

14 pages, 865 KiB

Open AccessArticle

Neutron Decay Anomaly and Its Effects on Neutron Star Properties

by H. C. Das and G. F. Burgio

Universe 2025, 11(5), 159; https://doi.org/10.3390/universe11050159 - 12 May 2025

Abstract

We investigate the effects of dark matter (DM) on neutron star (NS) properties using the neutron decay anomaly model within the relativistic mean-field (RMF) framework. Three nucleonic models (HCD0–HCD2) are developed, satisfying astrophysical constraints such as the maximum NS mass (≥ [...] Read more.

We investigate the effects of dark matter (DM) on neutron star (NS) properties using the neutron decay anomaly model within the relativistic mean-field (RMF) framework. Three nucleonic models (HCD0–HCD2) are developed, satisfying astrophysical constraints such as the maximum NS mass (≥

2 M_{⊙}

), the NICER mass–radius limits, and the tidal deformability constraint from the GW170817 event. The equation of states of the NS admixed with DM (DMANS) is calculated by incorporating the self-interactions between them. The macroscopic properties, such as mass, radius, and tidal deformability of the NSs, are obtained for HCD models along with five others by varying self-interaction strength. By combining NS observations with scattering cross-section constraints from galaxy clusters, we explore model-dependent trends in the DM self-interaction parameter space. While the quantitative bounds may vary with hadronic model choice, our analysis offers insights into the interplay between DM interactions and NS observables within the RMF framework. Full article

(This article belongs to the Section Compact Objects)

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23 pages, 655 KiB

Open AccessArticle

Screening Mechanisms on White Dwarfs: Symmetron and Dilaton

by Joan Bachs-Esteban, Ilídio Lopes and Javier Rubio

Universe 2025, 11(5), 158; https://doi.org/10.3390/universe11050158 - 12 May 2025

Abstract

This work provides the first comparison of the symmetron and dilaton fields in white dwarfs. We show how these screening mechanisms behave inside such stars and their impact on stellar properties. Employing a custom-developed shooting method, we solve the scalar–tensor equilibrium equations in [...] Read more.

This work provides the first comparison of the symmetron and dilaton fields in white dwarfs. We show how these screening mechanisms behave inside such stars and their impact on stellar properties. Employing a custom-developed shooting method, we solve the scalar–tensor equilibrium equations in the Newtonian approximation. We consider a Chandrasekhar equation of state and examine a range of potential mass scales and coupling strengths for both fields. Both fields enhance the pressure drop in low-density white dwarfs, leading to smaller stellar masses, radii, and luminosities. Unlike chameleon models, their effects are suppressed in more massive stars, with symmetron fields fully decoupling and dilaton fields weakening but not vanishing. Consequently, no mass–radius curve for screened white dwarfs exceeds the Newtonian prediction in any of these three mechanisms. The mass–radius deviations are generally more pronounced at lower densities, depending on model parameters. Due to their common runaway potential, we confirm that dilaton and chameleon fields display similar field and gradient profiles. In contrast, due to their environment-dependent coupling, the dilaton and symmetron mechanisms exhibit stronger density-dependent screening effects. These findings highlight both phenomenological differences and theoretical similarities among these mechanisms, motivating asteroseismology studies to constrain the symmetron and dilaton parameter spaces. Full article

(This article belongs to the Special Issue Exotic Scenarios for Compact Astrophysical Objects)

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16 pages, 10616 KiB

Open AccessArticle

Superluminal Motion and Jet Parameters in the High-Redshift Blazar J1429+5406

by Dávid Koller and Sándor Frey

Universe 2025, 11(5), 157; https://doi.org/10.3390/universe11050157 - 11 May 2025

Abstract

We investigate the relativistic jet of the powerful radio-emitting blazar J1429+5406 at redshift

z = 3.015

. Our understanding of jet kinematics in

z \geq 3

quasars is still rather limited, based on a sample of less than about 50 objects. The blazar [...] Read more.

We investigate the relativistic jet of the powerful radio-emitting blazar J1429+5406 at redshift

z = 3.015

. Our understanding of jet kinematics in

z \geq 3

quasars is still rather limited, based on a sample of less than about 50 objects. The blazar J1429+5406 was observed at a high angular resolution using the method of very long baseline interferometry over more than two decades, between 1994 and 2018. These observations were conducted at five radio frequencies, covering a wide range from

1.7

to 15 GHz. The outer jet components at ∼20–40 milliarcsecond (mas) separations from the core do not show discernible apparent motion. On the other hand, three jet components within the central 10 mas region exhibit significant proper motion in the range of (0.045–0.16) mas year⁻¹, including one that is among the fastest-moving jet components at

z \geq 3

known to date. Based on the proper motion of the innermost jet component and the measured brightness temperature of the core, we estimated the Doppler factor, the bulk Lorentz factor, and the inclination angle of the jet with respect to the line of sight. The core brightness temperature is at least

3.6 \times 10^{11}

K, well exceeding the equipartition limit, indicating Doppler-boosted radio emission. The low jet inclination (≲5.4°) firmly places J1429+5406 into the blazar category. Full article

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

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13 pages, 2788 KiB

Open AccessArticle

Possible Multi-Band Afterglows of FRB 20171020A and Their Implications

by Ke Bian and Can-Min Deng

Universe 2025, 11(5), 156; https://doi.org/10.3390/universe11050156 - 9 May 2025

Abstract

Fast Radio Bursts (FRBs) are millisecond-duration radio transients of mysterious origin, with growing evidence linking at least some of them to magnetars. While FRBs are primarily observed in the radio band, their potential multi-wavelength afterglows remain largely unexplored. We investigate the possible afterglow [...] Read more.

Fast Radio Bursts (FRBs) are millisecond-duration radio transients of mysterious origin, with growing evidence linking at least some of them to magnetars. While FRBs are primarily observed in the radio band, their potential multi-wavelength afterglows remain largely unexplored. We investigate the possible afterglow of FRB 20171020A, a rare nearby and bright FRB localized in a galaxy at only 37 Mpc. Assuming that this source produces a future bright burst, we model the expected afterglow emission in the radio, optical, and X-ray bands under both uniform and wind-like ambient media, within the framework of the magnetar model. Our results show that the optical afterglow is the most promising for detection, but it fades rapidly and requires follow-up within a few hundred seconds post-burst. The radio afterglow may be detectable under favorable conditions in a dense stellar wind, whereas the X-ray counterpart is too faint for current telescopes. These findings suggest that rapid optical follow-up offers the best opportunity to detect the afterglow of the next bright burst from FRB 20171020A, providing unique insights into the progenitor and its environment. To assess observational feasibility, we estimate the event rate of nearby FRBs with sufficient energy to power detectable afterglows, finding a rate of ∼0.3 per year for CHIME surveys. Although this rate is low and the optical detection timescale is short, coordinated fast-response strategies using global telescope networks could significantly improve the chance of success. As more nearby FRBs are discovered, multi-wavelength observations will be essential in unveiling the physical nature of these enigmatic events. Full article

(This article belongs to the Section Space Science)

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15 pages, 486 KiB

Open AccessArticle

Dust Universes in Higher Dimensions with Gauss–Bonnet Corrections

by Sumeekha Singh, Byron P. Brassel and Sunil D. Maharaj

Universe 2025, 11(5), 155; https://doi.org/10.3390/universe11050155 - 8 May 2025

Abstract

We study dust models in higher-dimensional general relativity and the Einstein–Gauss–Bonnet theory in a maximally symmetric spacetime. We show that the dynamics of the model is governed by an Abel differential equation of the second kind. This equation is solved in general for [...] Read more.

We study dust models in higher-dimensional general relativity and the Einstein–Gauss–Bonnet theory in a maximally symmetric spacetime. We show that the dynamics of the model is governed by an Abel differential equation of the second kind. This equation is solved in general for all arbitrary dimensions and spatial curvature to yield solutions for the cosmic scale factor. We show that the existence of the first integral determining the dynamics of dust can be demonstrated without additional assumptions. Analysis of the first integral shows that explicit and implicit solutions are possible depending on the presence of the cosmological constant. Comparisons between the higher-order theory and general relativity are discussed in detail. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

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17 pages, 15127 KiB

Open AccessArticle

Toward Automated Coronal Observations: A New Integrated System Based on the Lijiang 10 cm Coronagraph

by Tengfei Song, Yu Liu, Xuefei Zhang, Mingyu Zhao, Xiaobo Li, Qiwang Luo, Feiyang Sha, Qiang Liu, Jacob Oloketuyi and Xinjian Wang

Universe 2025, 11(5), 154; https://doi.org/10.3390/universe11050154 - 7 May 2025

Abstract

About ten years ago, we established the first coronagraph that has been continuously operating on the high plateau of western China. This coronagraph is an internal occulting, 10 cm aperture instrument, installed at Lijiang Station through a collaboration with the Norikura Station of [...] Read more.

About ten years ago, we established the first coronagraph that has been continuously operating on the high plateau of western China. This coronagraph is an internal occulting, 10 cm aperture instrument, installed at Lijiang Station through a collaboration with the Norikura Station of the National Astronomical Observatory of Japan. To ensure high efficiency in current and future coronal observations, developing integrated observation systems is essential for reliable, autonomous, and remote operation of coronagraphs. This paper introduces an advanced integrated observation and control system, based on the Lijiang 10 cm coronagraph. The coronagraph focuses on the observations for the solar inner corona, capturing the coronal green-line emission within a field range from

1.03 R_{⨀}

to

2.5 R_{⨀}

. To enhance the observational precision and efficiency, a comprehensive integrated system has been designed, incorporating various subsystems, including precise pointing and tracking mechanisms, a multi-band filter system, a protective dome system, and a robust data storage infrastructure. This paper details the hardware architecture and software frameworks supporting each subsystem. Results from extended operational testing confirm the stability of the system, its capacity for autonomous and remote observations, and significant improvements in the automation and efficiency of coronal imaging. The automated observation system will be further improved and used for our future coronagraphs to be developed for coronal magnetism diagnosis. 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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20 pages, 12161 KiB

Open AccessReview

THEMIS Vector Magnetograph in Canary Islands

by Brigitte Schmieder, Véronique Bommier and Bernard Gelly

Universe 2025, 11(5), 153; https://doi.org/10.3390/universe11050153 - 7 May 2025

Abstract

The Télescope Héliographique pour l’Etude du Magnétisme et des Instabilités Solaires (THEMIS) has been operating in the Canary Islands since 1998. A total of 187 publications are listed in the THEMIS database. The telescope was upgraded in 2019 with adaptive optics and was [...] Read more.

The Télescope Héliographique pour l’Etude du Magnétisme et des Instabilités Solaires (THEMIS) has been operating in the Canary Islands since 1998. A total of 187 publications are listed in the THEMIS database. The telescope was upgraded in 2019 with adaptive optics and was fully operational in 2024. When operated in polarimetric mode, the telescope is calibration-free and has a high polarimetric sensitivity, which enables important results to be obtained. We will summarize a few of these results, obtained mainly during coordinate campaigns with the multi-spacecraft, outlined as follows: the horizontal magnetic field in prominences, the existence of flux rope in flare regions, and the magnetic field interchange reconnection between jets and filaments. 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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14 pages, 2915 KiB

Open AccessArticle

Black Holes as Gravitational Mirrors

by Luis C. N. Santos, Franciele M. da Silva, Celio R. Muniz and Valdir B. Bezerra

Universe 2025, 11(5), 152; https://doi.org/10.3390/universe11050152 - 7 May 2025

Abstract

Retrolensing is a gravitational lensing effect in which light emitted by a background source is deflected by a black hole and redirected toward the observer after undergoing nearly complete loops around the black hole. In this context, we explore the possibility of seeing [...] Read more.

Retrolensing is a gravitational lensing effect in which light emitted by a background source is deflected by a black hole and redirected toward the observer after undergoing nearly complete loops around the black hole. In this context, we explore the possibility of seeing objects of the solar system in past eras through telescope observations by using black holes as a gravitational mirror. We consider the motion of the light around Reissner–Nordström space–time and discuss the properties of the trajectories of boomerang photons. It was shown that, depending on the angle of emission and the position of the source, the photons could return to the emission point. Afterward, we explore the possibility of considering the returning photons in retrolensing geometry where the observer is between the source and the lens in which two classes of black holes are explored: The supermassive Sgr A* black hole at the galactic center and a nearby stellar black hole. For the first time in the literature, we propose the study of the returning photons of planets instead of stars in retrolensing geometry. Full article

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

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19 pages, 1784 KiB

Open AccessArticle

On the Relationship Between ULF Wave Power and Changes of Relativistic Electron Fluxes in the Outer Radiation Belt

by Christopher Lara, Victor A. Pinto, Javier Silva, Bea Zenteno-Quinteros and Pablo S. Moya

Universe 2025, 11(5), 151; https://doi.org/10.3390/universe11050151 - 6 May 2025

Abstract

We performed a statistical study on the correlation between electromagnetic Ultra Low Frequency (ULF) waves and the evolution of relativistic electron fluxes in the outer radiation belt for

3.1 < L^{*} < 6.0

during 101 geomagnetic storms that occurred between January 2013 [...] Read more.

We performed a statistical study on the correlation between electromagnetic Ultra Low Frequency (ULF) waves and the evolution of relativistic electron fluxes in the outer radiation belt for

3.1 < L^{*} < 6.0

during 101 geomagnetic storms that occurred between January 2013 and November 2018. We used the Van Allen Probes MagEIS and REPT instruments to study electron fluxes from 0.47 MeV to 5.2 MeV, and we utilized magnetic field data from EMFISIS to calculate magnetic field fluctuations parallel and perpendicular to the background magnetic field direction and obtain the ULF integrated power between 1 mHz and 10 mHz. We analyzed the data during the following three different time intervals: the main phase, the recovery phase, and the entire storm. We computed the Pearson’s correlation coefficient and mutual information score between the ratio of fluxes before and after each given phase and the total integrated ULF power during the same time interval. Our results show a significant correlation between ULF wave power and changes in fluxes of hundreds of keV electrons during the main phase of the storms and for MeV electrons during the recovery phase of the storms. By studying fluxes at independent

L^{*}

, the largest correlations correspond to changes in fluxes before and after the entire storm and ULF fluctuations parallel to the field, especially for

L^{*} < 4.6

. We evaluated the drift resonance frequency for azimuthal wavenumber

1 \leq m \leq 10

and found that for all considered energies and frequencies, the drift resonance with Pc5 ULF waves may occur in our region of study, which is consistent with the statistical results. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2025—Space Science)

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23 pages, 319 KiB

Open AccessArticle

Nonrelativistic Superfluids in Cosmology from a Relativistic Approach: Revisiting Two Formulations of Superfluidity

by Aline Favero and Heliudson Bernardo

Universe 2025, 11(5), 150; https://doi.org/10.3390/universe11050150 - 5 May 2025

Abstract

Two formulations of superfluidity are reviewed: Landau’s phenomenological two-fluid model and a relativistic effective field theory description. We demonstrate how the two-fluid formalism can be recovered from the nonrelativistic limit of the relativistic effective theory at finite temperatures. We show how self-gravitating, nonrelativistic [...] Read more.

Two formulations of superfluidity are reviewed: Landau’s phenomenological two-fluid model and a relativistic effective field theory description. We demonstrate how the two-fluid formalism can be recovered from the nonrelativistic limit of the relativistic effective theory at finite temperatures. We show how self-gravitating, nonrelativistic superfluids are obtained from the Newtonian limit of the relativistic approach on curved spaces. The concepts are presented in an accessible manner for readers who may not be deeply familiar with superfluidity from a condensed matter perspective. Full article

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

19 pages, 322 KiB

Open AccessArticle

Weak Gravity Limit in Newer General Relativity

by Alexey Golovnev, Sofia Klimova, Alla N. Semenova and Vyacheslav P. Vandeev

Universe 2025, 11(5), 149; https://doi.org/10.3390/universe11050149 - 3 May 2025

Abstract

We analyse linearised field equations around the Minkowski metric, with its standard flat parallel transport structure, in models of newer GR, which refers to quadratic actions in terms of a nonmetricity tensor. We show that half of the freedom in choosing the model [...] Read more.

We analyse linearised field equations around the Minkowski metric, with its standard flat parallel transport structure, in models of newer GR, which refers to quadratic actions in terms of a nonmetricity tensor. We show that half of the freedom in choosing the model parameters is immediately fixed by asking for reasonable properties of tensors and vectors, defined with respect to spatial rotations, and we accurately describe the much more complicated sector of scalars. In particular, we show that, from the teleparallel viewpoint, the STEGR model with an additional term of a gradient squared of the metric determinant exhibits one and a half new dynamical modes, and not just one new dynamical mode as it was previously claimed. Full article

(This article belongs to the Special Issue Geometric Theories of Gravity)

8 pages, 3820 KiB

Open AccessCommunication

Ultraviolet Background Radiation from Not-So-Dark Matter in the Galactic Halo

by Richard Conn Henry, Jayant Murthy and James Overduin

Universe 2025, 11(5), 148; https://doi.org/10.3390/universe11050148 - 3 May 2025

Abstract

Murthy et al. (2025) (hereafter Paper I) have recently reported the discovery of unexpectedly bright diffuse extreme-ultraviolet radiation at high latitudes in both the Northern and Southern Galactic Hemispheres. After correction for extinction by the total interstellar dust in the direction of each [...] Read more.

Murthy et al. (2025) (hereafter Paper I) have recently reported the discovery of unexpectedly bright diffuse extreme-ultraviolet radiation at high latitudes in both the Northern and Southern Galactic Hemispheres. After correction for extinction by the total interstellar dust in the direction of each observation, the spectra are nearly identical, suggesting that the radiation has a unique source and likely originates in the halo of our galaxy. The observed spectrum extends down to 912 Å, the interstellar hydrogen absorption edge. Radiation even slightly short of that edge would, if ubiquitous, be sufficient to explain the high degree of ionization in our galaxy and throughout the universe. We hypothesize that this newly discovered radiation originates in the slow decay of dark matter. The intensity of the radiation implies that the decay cannot be via the weak interaction, suggesting the existence of a new, even weaker fundamental interaction, consistent with the exceedingly long decay lifetime required. Full article

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

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14 pages, 933 KiB

Open AccessArticle

Evolution of the Early Universe in Einstein–Cartan Theory

by Qihong Huang, He Huang, Bing Xu and Kaituo Zhang

Universe 2025, 11(5), 147; https://doi.org/10.3390/universe11050147 - 2 May 2025

Abstract

Einstein–Cartan theory is a generalization of general relativity that introduces spacetime torsion. In this paper, we perform phase space analysis to investigate the evolution of the early universe in Einstein–Cartan theory. By studying the stability of critical points in the dynamical system, we [...] Read more.

Einstein–Cartan theory is a generalization of general relativity that introduces spacetime torsion. In this paper, we perform phase space analysis to investigate the evolution of the early universe in Einstein–Cartan theory. By studying the stability of critical points in the dynamical system, we find that there exist two stable critical points which represent an Einstein static solution and an expanding solution, respectively. After analyzing the phase diagram of the dynamical system, we find that the early universe may exhibit an Einstein static state, an oscillating state, or a bouncing state. By assuming the equation of state

ω

can decrease over time t, the universe can depart from the initial Einstein static state, oscillating state, or bouncing state and then evolve into an inflationary phase. Then, we analyze four different inflationary evolution cases in Einstein–Cartan theory and find that a time-variable equation of state

ω

cannot yield values of

n_{s}

and r consistent with observations, while a time-invariant equation of state

ω

is supported by the Planck 2018 results. Thus, in Einstein–Cartan theory, the universe likely originates from a bouncing state rather than an Einstein static state or an oscillating state. Full article

(This article belongs to the Special Issue Bouncing Cosmology and the Early Universe: From Theory to Observations)

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22 pages, 685 KiB

Open AccessArticle

Radial Oscillations of the HESS J1731-347 Compact Object Imposing the Karmarkar Condition

by Grigoris Panotopoulos

Universe 2025, 11(5), 146; https://doi.org/10.3390/universe11050146 - 1 May 2025

Abstract

We model the light HESS J1731-347 compact object (of known stellar mass and radius) within Einstein’s General Relativity, imposing the Karmarkar condition in gravity for anisotropic stars. The three free parameters of the analytic solution are determined by imposing the matching conditions at [...] Read more.

We model the light HESS J1731-347 compact object (of known stellar mass and radius) within Einstein’s General Relativity, imposing the Karmarkar condition in gravity for anisotropic stars. The three free parameters of the analytic solution are determined by imposing the matching conditions at the surface of the star for objects of known stellar mass and radius. Finally, using well-established criteria, it is shown that the solution is compatible with all requirements for well-behaved and realistic solutions. Furthermore, we study the radial oscillation modes, and we compare them to the ones corresponding to an isotropic star modeled by the Tolman IV exact analytic solution obtained a long time ago. A comparison between the large frequency separations is made as well. Full article

(This article belongs to the Special Issue Challenges and Future Directions in Neutron Star Research)

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