Universe

24 pages, 4114 KB

Open AccessEditor’s ChoiceArticle

Building a Radio AGN Sample from Cosmic Morning—The Radio High-Redshift Quasar Catalog (RHzQCat): I. Catalog from SDSS Quasars and Radio Surveys at z ≥ 3

by Yingkang Zhang, Ruqiu Lin, Krisztina Perger, Sándor Frey, Tao An, Xiang Ji, Qiqi Wu and Shilong Liao

Universe 2025, 11(12), 392; https://doi.org/10.3390/universe11120392 - 28 Nov 2025

Viewed by 948

Abstract

Radio-loud high-redshift quasars (RHRQs) provide crucial insights into the evolution of relativistic jets and their connection to the growth of supermassive black holes. Beyond the extensively studied population at

z \geq 5

, the cosmic morning epoch (

3 ≲ z ≲ 5

[...] Read more.

Radio-loud high-redshift quasars (RHRQs) provide crucial insights into the evolution of relativistic jets and their connection to the growth of supermassive black holes. Beyond the extensively studied population at

z \geq 5

, the cosmic morning epoch (

3 ≲ z ≲ 5

) marks the peak of active galactic nucleus (AGN) activity and black hole accretion, yet remains relatively unexplored. In this work, we compiled the radio high-redshift quasar catalog (RHzQCat) by cross-matching the SDSS DR16Q catalog with four major radio surveys—FIRST, NVSS, RACS, and GLEAM. Our tier-based cross-matching framework and visual validation ensured reliable source identification across surveys with diverse beam sizes. The catalog included 1629 reliable and 315 candidate RHRQs, with radio luminosities uniformly spanning

10^{25.5}

–

10^{29.3}

W Hz⁻¹. About 95% of the confirmed sources exhibited compact morphologies, consistent with Doppler-boosted or young AGN populations at high redshifts. Our catalog increases the number of known RHRQs at

z \geq 3

by an order of magnitude, representing the largest and most homogeneous catalog of radio quasars at cosmic morning, filling the observational gap between the early (

z > 6

) and local Universe. It provides a robust reference for future statistical studies of jet evolution, AGN feedback, and cosmic magnetism with next-generation facilities such as the Square Kilometer Array (SKA). Full article

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

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24 pages, 372 KB

Open AccessEditor’s ChoiceArticle

An Intrinsic Coordinate Reference Frame Procedure I: Tensorial Canonical Weyl Scalars

by Cooper K. Watson, William Julius, Patrick Brown, Donald Salisbury and Gerald B. Cleaver

Universe 2025, 11(12), 389; https://doi.org/10.3390/universe11120389 - 26 Nov 2025

Viewed by 746

Abstract

The canonical quantization of gravity in general relativity is greatly simplified by the artificial decomposition of space time into a 3 + 1 formalism. Such a simplification appears to come at the cost of general covariance. This quantization procedure requires tangential and perpendicular [...] Read more.

The canonical quantization of gravity in general relativity is greatly simplified by the artificial decomposition of space time into a 3 + 1 formalism. Such a simplification appears to come at the cost of general covariance. This quantization procedure requires tangential and perpendicular infinitesimal diffeomorphisms generated by the symmetry group under the Legendre transformation of the given action. This gauge generator, along with the fact that Weyl curvature scalars may act as “intrinsic coordinates” (or a dynamical reference frame) that depend only on the spatial metric (

g_{a b}

) and the conjugate momenta (

p^{c d}

), allows for an alternative approach to canonical quantization of gravity. In this paper, we present the tensorial solution of the set of Weyl scalars in terms of canonical phase-space variables. Full article

27 pages, 10026 KB

Open AccessEditor’s ChoiceArticle

Dynamical Friction Constraints on the Dark Matter Hypothesis Across Astronomical Scales

by Xavier Hernandez and Pavel Kroupa

Universe 2025, 11(11), 367; https://doi.org/10.3390/universe11110367 - 6 Nov 2025

Cited by 2 | Viewed by 1983

Abstract

Dynamical friction implies a consistency check on any system where dark matter particles are hypothesised to explain orbital dynamics requiring more mass under Newtonian gravity than is directly detectable. Introducing the assumption of a dominant dark matter halo will also imply a decay [...] Read more.

Dynamical friction implies a consistency check on any system where dark matter particles are hypothesised to explain orbital dynamics requiring more mass under Newtonian gravity than is directly detectable. Introducing the assumption of a dominant dark matter halo will also imply a decay timescale for the orbits in question. A self-consistency constraint hence arises, such that the resulting orbital decay timescales must be longer than the lifetimes of the systems in question. While such constraints are often trivially passed, the combined dependencies of dynamical friction timescales on the mass and orbital radius of the orbital tracer and on the density and velocity dispersion of the assumed dark matter particles leads to the existence of a number of astronomical systems where such a consistency test is failed. Here, we review cases from stars in ultrafaint dwarf galaxies, galactic bars, satellite galaxies, and, particularly, the multi-period mutual orbits of the Magellanic Clouds, as recently inferred from the star formation histories of these two galaxies, as well as the nearby M81 group of galaxies, where introducing enough dark matter to explain observed kinematics leads to dynamical friction orbital decay timescales shorter than the lifetimes of the systems in question. Taken together, these observations exclude dark matter halos made of particles as plausible explanations for the observed kinematics of these systems. Full article

(This article belongs to the Section Galaxies and Clusters)

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29 pages, 419 KB

Open AccessEditor’s ChoiceReview

Modified Gravity with Nonminimal Curvature–Matter Couplings: A Framework for Gravitationally Induced Particle Creation

by Francisco S. N. Lobo, Tiberiu Harko and Miguel A. S. Pinto

Universe 2025, 11(11), 356; https://doi.org/10.3390/universe11110356 - 28 Oct 2025

Viewed by 1796

Abstract

Modified gravity theories with a nonminimal coupling between curvature and matter offer a compelling alternative to dark energy and dark matter by introducing an explicit interaction between matter and curvature invariants. Two of the main consequences of such an interaction are the emergence [...] Read more.

Modified gravity theories with a nonminimal coupling between curvature and matter offer a compelling alternative to dark energy and dark matter by introducing an explicit interaction between matter and curvature invariants. Two of the main consequences of such an interaction are the emergence of an additional force and the non-conservation of the energy–momentum tensor, which can be interpreted as an energy exchange between matter and geometry. By adopting this interpretation, one can then take advantage of many different approaches in order to investigate the phenomenon of gravitationally induced particle creation. One of these approaches relies on the so-called irreversible thermodynamics of open systems formalism. By considering the scalar–tensor formulation of one of these theories, we derive the corresponding particle creation rate, creation pressure, and entropy production, demonstrating that irreversible particle creation can drive a late-time de Sitter acceleration through a negative creation pressure, providing a natural alternative to the cosmological constant. Furthermore, we demonstrate that the generalized second law of thermodynamics holds: the total entropy, from both the apparent horizon and enclosed matter, increases monotonically and saturates in the de Sitter phase, imposing constraints on the allowed particle production dynamics. Furthermore, we present brief reviews of other theoretical descriptions of matter creation processes. Specifically, we consider approaches based on the Boltzmann equation and quantum-based aspects and discuss the generalization of the Klein–Gordon equation, as well as the problem of its quantization in time-varying gravitational fields. Hence, gravitational theories with nonminimal curvature–matter couplings present a unified and testable framework, connecting high-energy gravitational physics with cosmological evolution and, possibly, quantum gravity, while remaining consistent with local tests through suitable coupling functions and screening mechanisms. Full article

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

27 pages, 1596 KB

Open AccessEditor’s ChoiceArticle

The Nuclear Astrophysics Program at the CERN n_TOF Facility: Results and Perspectives

by P. M. Milazzo, C. Lederer-Woods and A. Mengoni

Universe 2025, 11(10), 329; https://doi.org/10.3390/universe11100329 - 30 Sep 2025

Viewed by 1151

Abstract

The CERN n_TOF facility is a research infrastructure specifically designed for studying neutron-induced nuclear reactions. Pulsed white neutron beams are delivered toward three experimental areas, two of them at different baselines to apply the time-of-flight technique, and another one very close to the [...] Read more.

The CERN n_TOF facility is a research infrastructure specifically designed for studying neutron-induced nuclear reactions. Pulsed white neutron beams are delivered toward three experimental areas, two of them at different baselines to apply the time-of-flight technique, and another one very close to the neutron source for activation studies. High intensity and high neutron energy resolution make n_TOF a unique facility. A major component of the physics program at n_TOF is dedicated to the measurement of key neutron induced reactions for nuclear astrophysics, relevant to nucleosynthesis in stars, the Big Bang primordial nucleosynthesis as well as Cosmochronology. A review of the relevant results obtained at the n_TOF facility is reported, together with details of challenging new measurements in preparation. Full article

(This article belongs to the Special Issue Advances in Nuclear Astrophysics)

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32 pages, 1122 KB

Open AccessEditor’s ChoiceArticle

Distribution of Heavy-Element Abundances Generated by Decay from a Quasi-Equilibrium State

by Gerd Röpke, David Blaschke and Friedrich K. Röpke

Universe 2025, 11(10), 323; https://doi.org/10.3390/universe11100323 - 23 Sep 2025

Cited by 1 | Viewed by 1330

Abstract

We present a freeze-out approach for describing the formation of heavy elements in expanding nuclear matter. Applying concepts used in modeling heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and [...] Read more.

We present a freeze-out approach for describing the formation of heavy elements in expanding nuclear matter. Applying concepts used in modeling heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and the Mott effect, which describes the dissolution of nuclei at high densities of nuclear matter. With this approach, we search for a universal initial distribution in a quasi-equilibrium state from which the coarse-grained pattern of the solar abundances of heavy elements freezes out and evolves by radioactive decay of the excited states. The universal initial state is characterized by the Lagrange parameters, which are related to temperature and chemical potentials of neutrons and protons. We show that such a state exists and determine a temperature of 5.266 MeV, a neutron chemical potential of 940.317 MeV and a proton chemical potential of 845.069 MeV, with a baryon number density of 0.013 fm⁻³ and a proton fraction of

0.13

. Heavy neutron-rich nuclei such as the hypothetical double-magic nucleus ³⁵⁸Sn appear in the initial distribution and contribute to the observed abundances after fission. We discuss astrophysical scenarios for the realization of this universal initial distribution for heavy-element nucleosynthesis, including supernova explosions, neutron star mergers and the inhomogeneous Big Bang. The latter scenario may be of interest in the light of early massive objects observed with the James Webb Space Telescope and opens new perspectives on the universality of the observed r-process patterns and the lack of observations of population III stars. Full article

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

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12 pages, 366 KB

Open AccessEditor’s ChoiceArticle

Stability Analysis of Magnetized Quark Matter in Tsallis Statistics

by Jia Zhang and Xin-Jian Wen

Universe 2025, 11(9), 312; https://doi.org/10.3390/universe11090312 - 12 Sep 2025

Viewed by 807

Abstract

In this work, we employ the nonextensive Nambu–Jona-Lasinio model to analyze the thermodynamic properties of magnetized quark matter. The nonequilibrium state is described in Tsallis distribution by a dimensionless parameter q. We find that within a reasonable temperature range, the system undergoes [...] Read more.

In this work, we employ the nonextensive Nambu–Jona-Lasinio model to analyze the thermodynamic properties of magnetized quark matter. The nonequilibrium state is described in Tsallis distribution by a dimensionless parameter q. We find that within a reasonable temperature range, the system undergoes a crossover transition at the critical chemical potential, which is decreased by the increase of both the temperature and q value. In contrast to the enhanced stability by magnetic field in Boltzmann statistics, it is found that the stability of chiral restored matter in Tsallis statistics would be reduced by an increase of the magnetic field. Conversely, the increase of the q would enhance the stability of quark matter. Finally, we display the different magnetic effects on the stability in the chiral broken and restored regions. Full article

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

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24 pages, 10348 KB

Open AccessEditor’s ChoiceReview

The Variable Sky Through the OGLE Eye

by Patryk Iwanek

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

Viewed by 2092

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)

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12 pages, 244 KB

Open AccessEditor’s ChoiceArticle

Towards Relational Foundations for Spacetime Quantum Physics

by Pietro Dall’Olio and José A. Zapata

Universe 2025, 11(8), 250; https://doi.org/10.3390/universe11080250 - 29 Jul 2025

Viewed by 1273

Abstract

Rovelli’s relational interpretation of quantum mechanics tells us that the description of a system in the formalism of quantum mechanics is not an absolute but is relative to the observer itself. The interpretation goes further and proposes a set of axioms. In standard [...] Read more.

Rovelli’s relational interpretation of quantum mechanics tells us that the description of a system in the formalism of quantum mechanics is not an absolute but is relative to the observer itself. The interpretation goes further and proposes a set of axioms. In standard non-relational language, one of them states that an observer can only retrieve a finite amount information from a system by means of measurement. Our contribution starts with the observation that quantum mechanics, i.e., quantum field theory (QFT) in dimension 1, radically differs from QFT in higher dimensions. In higher dimensions, boundary data (or initial data) cannot be characterized by finitely many measurements. This calls for a notion of measuring scale, which we provide. At a given measuring scale, the observer has partial information about the system. Our notion of measuring scale generalizes the one implicitly used in Wilsonian QFT. At each measuring scale, there are effective theories, which may be corrected, and if the theory turns out to be renormalizable, the mentioned corrections converge to determine a completely corrected (or renormalized) theory at the given measuring scale. The notion of a measuring scale is the cornerstone of Wilsonian QFT; this notion tells us that we are not describing a system from an absolute perspective. An effective theory at that scale describes the system with respect to the observer, which may retrieve information from the system by means of measurement in a specific way determined by our notion of measuring scale. We claim that a relational interpretation of quantum physics for spacetimes of dimensions greater than 1 is Wilsonian. Full article

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

12 pages, 549 KB

Open AccessEditor’s ChoiceArticle

Disruption of Planetary System Architectures by Stellar Flybys

by Robert Przyłuski, Hans Rickman, Paweł Wajer, Tomasz Wiśniowski, Diego Turrini, Danae Polychroni, Camilla Danielski, J. M. Diederik Kruijssen, Steven Longmore and Mélanie Chevance

Universe 2025, 11(8), 240; https://doi.org/10.3390/universe11080240 - 22 Jul 2025

Cited by 1 | Viewed by 1341

Abstract

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and [...] Read more.

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and is systematically scaled in both planetary mass and orbital compactness to explore the parameter space of observed exoplanetary architectures. Simulations explore a range of stellar encounter scenarios drawn from four distinct cluster environments. Our results show that system survival depends critically on the interplay between planetary mass and orbital scale: compact configurations are more resistant to external perturbations, while increased planetary mass improves resilience only up to a threshold, beyond which internal instabilities dominate. No system whose planets are twice as massive as the ones in the solar system survives stellar encounters. Systems that are at least an order of magnitude more compact than the solar system remain stable under typical encounter conditions. These findings place strong constraints on the initial architectures of planetary systems that can endure stellar-dense birth environments. Full article

(This article belongs to the Section Planetary Sciences)

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16 pages, 4520 KB

Open AccessEditor’s ChoiceReview

SN 2023ixf: The Closest Supernova of the Decade

by Wynn Jacobson-Galán

Universe 2025, 11(7), 231; https://doi.org/10.3390/universe11070231 - 15 Jul 2025

Cited by 4 | Viewed by 1679

Abstract

Supernova 2023ixf occurred on 18 May 2023 in the nearby galaxy Messier 101 (

D \approx 6.85

Mpc), making it the closest supernova in the last decade. Following its discovery, astronomers around the world rushed to observe the explosion across the electromagnetic spectrum [...] Read more.

Supernova 2023ixf occurred on 18 May 2023 in the nearby galaxy Messier 101 (

D \approx 6.85

Mpc), making it the closest supernova in the last decade. Following its discovery, astronomers around the world rushed to observe the explosion across the electromagnetic spectrum in order to uncover its early-time properties. Based on multi-wavelength analysis during its first year after explosion, Supernova 2023ixf is a type II supernova that interacted with dense, confined circumstellar material in its local environment—this material being lost from its red supergiant progenitor in the final years before explosion. In this article, we will review the findings of >80 studies already published on this incredible event and explore how the synthesis of SN 2023ixf observations across the electromagnetic spectrum can be used to constrain type II supernova explosion physics in addition to the uncertain mass loss histories of red supergiant stars in their final years. Full article

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

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12 pages, 1540 KB

Open AccessEditor’s ChoiceReview

Gravitational Waves: Echoes of the Biggest Bangs Since the Big Bang and/or BSM Physics?

by John Ellis

Universe 2025, 11(7), 213; https://doi.org/10.3390/universe11070213 - 26 Jun 2025

Cited by 2 | Viewed by 1480

Abstract

“If one could ever prove the existence of gravitational waves, the processes responsible for their generation would probably be much more curious and interesting than even the waves themselves.” (Gustav Mie, 1868–1957). The discovery of gravitational waves has opened new windows on [...] Read more.

“If one could ever prove the existence of gravitational waves, the processes responsible for their generation would probably be much more curious and interesting than even the waves themselves.” (Gustav Mie, 1868–1957). The discovery of gravitational waves has opened new windows on astrophysics, cosmology and physics beyond the Standard Model (BSM). Measurements by the LIGO, Virgo and KAGRA Collaborations of stellar–mass binaries and neutron star mergers have shown that gravitational waves travel at close to the velocity of light and constrain BSM possibilities, such as a graviton mass and Lorentz violation in gravitational wave propagation. Follow-up measurements of neutron star mergers have provided evidence for the production of heavy elements, possibly including some essential for human life. The gravitational waves in the nanoHz range observed by Pulsar Timing Arrays (PTAs) may have been emitted by supermassive black hole binaries, but might also have originated from BSM cosmological scenarios such as cosmic strings, or phase transitions in the early Universe. The answer to the question in the title may be provided by gravitational-wave detectors at higher frequencies, such as LISA and atom interferometers. KCL-PH-TH/2024-05. Full article

(This article belongs to the Special Issue Particle Physics and Cosmology: A Themed Issue in Honor of Professor Dimitri Nanopoulos)

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31 pages, 926 KB

Open AccessEditor’s ChoiceReview

A Comprehensive Guide to Interpretable AI-Powered Discoveries in Astronomy

by Maggie Lieu

Universe 2025, 11(6), 187; https://doi.org/10.3390/universe11060187 - 11 Jun 2025

Cited by 3 | Viewed by 4799

Abstract

The exponential growth of astronomical data necessitates the adoption of artificial intelligence (AI) and machine learning for timely and efficient scientific discovery. While AI techniques have achieved significant successes across diverse astronomical domains, their inherent complexity often obscures the reasoning behind their predictions, [...] Read more.

The exponential growth of astronomical data necessitates the adoption of artificial intelligence (AI) and machine learning for timely and efficient scientific discovery. While AI techniques have achieved significant successes across diverse astronomical domains, their inherent complexity often obscures the reasoning behind their predictions, hindering scientific trust and verification. This review addresses the crucial need for interpretability in AI-powered astronomy. We survey key applications where AI is making significant impacts and review the foundational concepts of transparency, interpretability, and explainability. A comprehensive overview of various interpretable machine learning methods is presented, detailing their mechanisms, applications in astronomy, and associated challenges. Given that no single method offers a complete understanding, we emphasize the importance of employing a suite of techniques to build robust interpretations. We argue that prioritizing interpretability is essential for validating results, guarding against biases, understanding model limitations, and ultimately enhancing the scientific value of AI in astronomy. Building trustworthy AI through explainable methods is fundamental to advancing our understanding of the universe. Full article

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

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12 pages, 2754 KB

Open AccessEditor’s ChoiceArticle

μPPET: Investigating the Muon Puzzle with J-PET Detectors

by Alessio Porcelli, Kavya Valsan Eliyan, Gabriel Moskal, Nousaba Nasrin Protiti, Diana Laura Sirghi, Ermias Yitayew Beyene, Neha Chug, Catalina Curceanu, Eryk Czerwiński, Manish Das, Marek Gorgol, Jakub Hajduga, Sharareh Jalali, Bożena Jasińska, Krzysztof Kacprzak, Tevfik Kaplanoglu, Łukasz Kapłon, Kamila Kasperska, Aleksander Khreptak, Grzegorz Korcyl, Tomasz Kozik, Deepak Kumar, Karol Kubat, Edward Lisowski, Filip Lisowski, Justyna Mędrala-Sowa, Wiktor Mryka, Simbarashe Moyo, Szymon Niedźwiecki, Szymon Parzych, Piyush Pandey, Elena Perez del Rio, Bartłomiej Rachwał, Martin Rädler, Sushil Sharma, Magdalena Skurzok, Ewa Łucja Stȩpień, Tomasz Szumlak, Pooja Tanty, Keyvan Tayefi Ardebili, Satyam Tiwari and Paweł Moskal Show full author list Hide full author list

Universe 2025, 11(6), 180; https://doi.org/10.3390/universe11060180 - 2 Jun 2025

Cited by 2 | Viewed by 1586

Abstract

The

μ

PPET [mu(

μ

)on Probe with J-PET] project aims to investigate the “Muon Puzzle” seen in cosmic ray air showers. This puzzle arises from the observation of a significantly larger number of muons on Earth’s surface than that predicted by the [...] Read more.

The

μ

PPET [mu(

μ

)on Probe with J-PET] project aims to investigate the “Muon Puzzle” seen in cosmic ray air showers. This puzzle arises from the observation of a significantly larger number of muons on Earth’s surface than that predicted by the current theoretical models. The investigated hypothesis is based on recently observed asymmetries in the parameters for the strong interaction cross-section and trajectory of an outgoing particle due to projectile–target polarization. The measurements require detailed information about muons at the ground level, including their track and charge distributions. To achieve this, the two PET scanners developed at the Jagiellonian University in Krakow (Poland), the J-PET detectors, will be employed, taking advantage of their well-known resolution and convenient location for detecting muons that reach long depths in the atmosphere. One station will be used as a muon tracker, while the second will reconstruct the core of the air shower. In parallel, the existing hadronic interaction models will be modified and fine-tuned based on the experimental results. In this work, we present the conceptualization and preliminary designs of

μ

PPET. Full article

(This article belongs to the Special Issue Ultra-High-Energy Cosmic Rays)

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24 pages, 13152 KB

Open AccessEditor’s ChoiceArticle

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

Cited by 1 | Viewed by 1284

Abstract

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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18 pages, 934 KB

Open AccessEditor’s ChoiceArticle

Evidence of Gapless Superfluidity in MXB 1659-29 With and Without Late Time Cooling

by Valentin Allard and Nicolas Chamel

Universe 2025, 11(5), 140; https://doi.org/10.3390/universe11050140 - 27 Apr 2025

Cited by 1 | Viewed by 697

Abstract

The interpretation of the thermal relaxation of some transiently accreting neutron stars in quasipersistent soft X-ray transients, especially MXB 1659-29, has been found to be challenging within the traditional deep crustal heating paradigm. Due to the pinning of quantized vortices, the neutron superfluid [...] Read more.

The interpretation of the thermal relaxation of some transiently accreting neutron stars in quasipersistent soft X-ray transients, especially MXB 1659-29, has been found to be challenging within the traditional deep crustal heating paradigm. Due to the pinning of quantized vortices, the neutron superfluid is not expected to remain at rest in the crust, as was generally assumed. We have recently shown that for sufficiently large relative superflows, the neutron superfluid could become gapless. This dynamical phase could naturally explain the late-time cooling of MXB 1659-29. However, the interpretation of the last observation of MXB 1659-29 in 2013 before its second accretion phase in 2015 remains debated, with some spectral fits being consistent with no further temperature decline. Here, we revisit the cooling of this neutron star considering the different fits. New simulations of the crust cooling are performed, accounting for neutron diffusion and allowing for gapless superfluidity. In all cases, gapless superfluidity is found to provide the best fit to observations. Full article

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

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14 pages, 565 KB

Open AccessEditor’s ChoiceArticle

Scanning the Universe for Large-Scale Structures Using Gamma-Ray Bursts

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

Universe 2025, 11(4), 121; https://doi.org/10.3390/universe11040121 - 6 Apr 2025

Cited by 2 | Viewed by 1552

Abstract

In the past few decades, large universal structures have been found that challenge the homogeneity and isotropy expected in standard cosmological models. The largest of these, identified as the Hercules–Corona Borealis Great Wall, was found in 2014 in the northern galactic hemisphere in [...] Read more.

In the past few decades, large universal structures have been found that challenge the homogeneity and isotropy expected in standard cosmological models. The largest of these, identified as the Hercules–Corona Borealis Great Wall, was found in 2014 in the northern galactic hemisphere in the redshift range of

1.6 \leq z \leq 2.1

. Subsequent studies used an increasing gamma-ray burst database to show that the cluster was unlikely to have been caused by statistical sampling uncertainties. This study re-examines burst clustering in the northern galactic hemisphere using a recently developed methodology. Evidence is provided that the Hercules–Corona Borealis Great Wall cluster is larger than previously thought, with members potentially spanning the redshift range of

0.33 \leq z \leq 2.43

. The extension of this cluster’s size does not appear to have been due to statistical variations or sampling biases. Full article

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16 pages, 1077 KB

Open AccessEditor’s ChoiceArticle

Non-Singular “Gauss” Black Hole from Non-Locality

by Jens Boos

Universe 2025, 11(4), 112; https://doi.org/10.3390/universe11040112 - 29 Mar 2025

Cited by 9 | Viewed by 735

Abstract

Cutting out an infinite tube around

r = 0

formally removes the Schwarzschild singularity, but without a physical mechanism, this procedure seems ad hoc and artificial. In this paper, we provide justification for such a mechanism by means of non-locality. Motivated by the [...] Read more.

Cutting out an infinite tube around

r = 0

formally removes the Schwarzschild singularity, but without a physical mechanism, this procedure seems ad hoc and artificial. In this paper, we provide justification for such a mechanism by means of non-locality. Motivated by the Gauss law, we define a suitable radius variable as the inverse of a regular non-local potential, and use this variable to model a non-singular black hole. The resulting geometry has a de Sitter core, and for generic values of the regulator, there is no inner horizon, saving this model from potential issues via mass inflation. An outer horizon only exists for masses above a critical threshold, thereby reproducing the conjectured “mass gap” for black holes in non-local theories. The geometry’s density and pressure terms decrease exponentially, thereby rendering it an almost-exact vacuum solution of the Einstein equations outside of astrophysical black holes. Its thermodynamic properties resemble those of the Hayward black hole, with the notable exception that for critical mass, the horizon radius is zero. Full article

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20 pages, 909 KB

Open AccessEditor’s ChoiceArticle

Where to Search for Supermassive Binary Black Holes

by Paola Marziani, Edi Bon, Natasa Bon and Mauro D’Onofrio

Universe 2025, 11(3), 76; https://doi.org/10.3390/universe11030076 - 21 Feb 2025

Cited by 2 | Viewed by 1215

Abstract

Supermassive binary black holes (SMBBHs) are the anticipated byproducts of galaxy mergers and play a pivotal role in shaping galaxy evolution, gravitational wave emissions, and accretion physics. Despite their theoretical prevalence, direct observational evidence for SMBBHs remains elusive, with only a handful of [...] Read more.

Supermassive binary black holes (SMBBHs) are the anticipated byproducts of galaxy mergers and play a pivotal role in shaping galaxy evolution, gravitational wave emissions, and accretion physics. Despite their theoretical prevalence, direct observational evidence for SMBBHs remains elusive, with only a handful of candidates identified to date. This paper explores optimal strategies and key environments for locating SMBBHs, focusing on observational signatures in the broad Balmer lines. We present a preliminary analysis on a flux-limited sample of sources belonging to an evolved spectral type along the quasar main sequence, and we discuss the spectroscopic clues indicative of binary activity and highlight the critical role of time-domain spectroscopic surveys in uncovering periodic variability linked to binary systems. Full article

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

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41 pages, 5616 KB

Open AccessEditor’s ChoiceReview

Searching for New Physics in an Ultradense Environment: A Review on Dark Matter Admixed Neutron Stars

by Francesco Grippa, Gaetano Lambiase and Tanmay Kumar Poddar

Universe 2025, 11(3), 74; https://doi.org/10.3390/universe11030074 - 21 Feb 2025

Cited by 22 | Viewed by 2594

Abstract

Neutron stars (NSs), among the densest objects in the universe, are exceptional laboratories for investigating the properties of dark matter (DM). Recent theoretical and observational developments have heightened interest in exploring the impact of DM on NS structure, giving rise to the concept [...] Read more.

Neutron stars (NSs), among the densest objects in the universe, are exceptional laboratories for investigating the properties of dark matter (DM). Recent theoretical and observational developments have heightened interest in exploring the impact of DM on NS structure, giving rise to the concept of dark matter admixed neutron stars (DANSs). This review examines how NSs can accumulate DM over time, potentially altering their fundamental properties. We explore the leading models describing DM behavior within NSs, focusing on the effects of both bosonic and fermionic candidates on key features such as mass, radius, and tidal deformability. Additionally, we review how DM can modify the cooling and heating processes, trigger the formation of a black hole, and impact gravitational wave (GW) emissions from binary systems. By synthesizing recent research, this work highlights how DANSs might produce observable signatures, offering new opportunities to probe DM’s properties through astrophysical phenomena. Full article

(This article belongs to the Special Issue Search for New Physics Through Combined Approaches)

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34 pages, 435 KB

Open AccessEditor’s ChoiceReview

Black Hole Thermodynamics and Generalised Non-Extensive Entropy

by Emilio Elizalde, Shin’ichi Nojiri and Sergei D. Odintsov

Universe 2025, 11(2), 60; https://doi.org/10.3390/universe11020060 - 11 Feb 2025

Cited by 20 | Viewed by 2263

Abstract

The first part of this work provides a review of recent research on generalised entropies and their origin, as well as its application to black hole thermodynamics. To start, it is shown that the Hawking temperature and the Bekenstein–Hawking entropy are, respectively, the [...] Read more.

The first part of this work provides a review of recent research on generalised entropies and their origin, as well as its application to black hole thermodynamics. To start, it is shown that the Hawking temperature and the Bekenstein–Hawking entropy are, respectively, the only possible thermodynamical temperature and entropy of the Schwarzschild black hole. Moreover, it is investigated if the other known generalised entropies, which include Rényi’s entropy, Tsallis entropy, and the four- and five-parameter generalised entropies, could correctly yield the Hawking temperature and the ADM mass. The possibility that generalised entropies could describe hairy black hole thermodynamics is also considered, both for the Reissner–Nordström black hole and for Einstein’s gravity coupled with two scalar fields. Two possibilities are investigated, namely, the case when the ADM mass does not yield the Bekenstein–Hawking entropy, and the case in which the effective mass expressing the energy inside the horizon does not yield the Hawking temperature. For the model with two scalar fields, the radii of the photon sphere and of the black hole shadow are calculated, which gives constraints on the BH parameters. These constraints are seen to be consistent, provided that the black hole is of the Schwarzschild type. Subsequently, the origin of the generalised entropies is investigated, by using their microscopic particle descriptions in the frameworks of a microcanonical ensemble and canonical ensemble, respectively. Finally, the McLaughlin expansion for the generalised entropies is used to derive, in each case, the microscopic interpretation of the generalised entropies, via the canonical and the grand canonical ensembles. Full article

(This article belongs to the Section Gravitation)

39 pages, 23123 KB

Open AccessEditor’s ChoiceArticle

Core to Cosmic Edge: SIMBA-C’s New Take on Abundance Profiles in the Intragroup Medium at z = 0

by Aviv Padawer-Blatt, Zhiwei Shao, Renier T. Hough, Douglas Rennehan, Ruxin Barré, Vida Saeedzadeh, Arif Babul, Romeel Davé, Chiaki Kobayashi, Weiguang Cui, François Mernier and Ghassem Gozaliasl

Universe 2025, 11(2), 47; https://doi.org/10.3390/universe11020047 - 1 Feb 2025

Cited by 2 | Viewed by 2166

Abstract

We employ the simba-c cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo [...] Read more.

We employ the simba-c cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo mass (

10^{13} \leq M_{500} / M_{⊙} \leq 10^{15}

). Typically, simba-c generates lower-amplitude abundance profiles than simba with flatter cores, in better agreement with observations. For low-mass groups, both simulations over-enrich the IGrM with Si, S, Ca, and Fe compared to observations, a trend likely related to inadequate modeling of metal dispersal and mixing. We analyze the 3D mass-weighted abundance profiles, concluding that the lower simba-c IGrM abundances are primarily a consequence of fewer metals in the IGrM, driven by reduced metal yields in Chem5, and the removal of the instantaneous recycling of metals approximation employed by simba. Additionally, an increased IGrM mass in low-mass simba-c groups is likely triggered by changes to the AGN and stellar feedback models. Our study suggests that a more realistic chemical enrichment model broadly improves agreement with observations, but physically motivated sub-grid models for other key processes, like AGN and stellar feedback and turbulent diffusion, are required to realistically reproduce observed group environments. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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60 pages, 821 KB

Open AccessEditor’s ChoiceReview

Introduction to Thermal Field Theory: From First Principles to Applications

by Alberto Salvio

Universe 2025, 11(1), 16; https://doi.org/10.3390/universe11010016 - 11 Jan 2025

Cited by 11 | Viewed by 1999

Abstract

This review article provides the basics and discusses some important applications of thermal field theory, namely, the combination of statistical mechanics and relativistic quantum field theory. In the first part, the fundamentals are covered: the density matrix, the corresponding averages, and the treatment [...] Read more.

This review article provides the basics and discusses some important applications of thermal field theory, namely, the combination of statistical mechanics and relativistic quantum field theory. In the first part, the fundamentals are covered: the density matrix, the corresponding averages, and the treatment of fields of various spin in a medium. The second part is dedicated to the computation of thermal Green’s function for scalars, vectors, and fermions with path-integral methods. These functions play a crucial role in thermal field theory as explained here. A more applicative part of the review is dedicated to the production of particles in a medium and to phase transitions in field theory, including the process of vacuum decay in a general theory featuring a first-order phase transition. To understand this review, the reader should have good knowledge of non-statistical quantum field theory. Full article

(This article belongs to the Section Field Theory)

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54 pages, 671 KB

Open AccessEditor’s ChoiceArticle

Quantum-Ordering Ambiguities in Weak Chern—Simons 4D Gravity and Metastability of the Condensate-Induced Inflation

by Panagiotis Dorlis, Nick E. Mavromatos and Sotirios-Neilos Vlachos

Universe 2025, 11(1), 15; https://doi.org/10.3390/universe11010015 - 11 Jan 2025

Cited by 11 | Viewed by 4452

Abstract

In this work, we elaborate further on a (3+1)-dimensional cosmological Running-Vacuum-type-Model (RVM) of inflation based on string-inspired Chern-Simons(CS) gravity, involving axions coupled to gravitational-CS(gCS) anomalous terms. Inflation in such models is caused by primordial-gravitational-waves(GW)-induced condensation of the gCS terms, which leads to a [...] Read more.

In this work, we elaborate further on a (3+1)-dimensional cosmological Running-Vacuum-type-Model (RVM) of inflation based on string-inspired Chern-Simons(CS) gravity, involving axions coupled to gravitational-CS(gCS) anomalous terms. Inflation in such models is caused by primordial-gravitational-waves(GW)-induced condensation of the gCS terms, which leads to a linear-axion potential. We demonstrate that this inflationary phase may be metastable, due to the existence of imaginary parts of the gCS condensate. These are quantum effects, proportional to commutators of GW perturbations, hence vanishing in the classical theory. Their existence is quantum-ordering-scheme dependent. We argue in favor of a physical importance of such imaginary parts, which we compute to second order in the GW (tensor) perturbations in the framework of a gauge-fixed effective Lagrangian, within a (mean field) weak-quantum-gravity-path-integral approach. We thus provide estimates of the inflation lifetime. On matching our results with the inflationary phenomenology, we fix the quantum-ordering ambiguities, and obtain an order-of-magnitude constraint on the String-Mass-Scale-to-Planck-Mass ratio, consistent with previous estimates by the authors in the framework of a dynamical-system approach to linear-axion RVM inflation. Finally, we examine the role of periodic modulations in the axion potential induced by non-perturbative effects on the slow-roll inflationary parameters, and find compatibility with the cosmological data. Full article

(This article belongs to the Special Issue Particle Physics and Cosmology: A Themed Issue in Honor of Professor Dimitri Nanopoulos)

13 pages, 419 KB

Open AccessEditor’s ChoiceArticle

The Two Alternative Explosion Mechanisms of Core-Collapse Supernovae: 2024 Status Report

by Noam Soker

Universe 2024, 10(12), 458; https://doi.org/10.3390/universe10120458 - 16 Dec 2024

Cited by 19 | Viewed by 3486

Abstract

In comparing the two alternative explosion mechanisms of core-collapse supernovae (CCSNe), I examine recent three-dimensional (3D) hydrodynamical simulations of CCSNe in the frame of the delayed neutrino explosion mechanism (neutrino mechanism) and argue that these valuable simulations show that neutrino heating can supply [...] Read more.

In comparing the two alternative explosion mechanisms of core-collapse supernovae (CCSNe), I examine recent three-dimensional (3D) hydrodynamical simulations of CCSNe in the frame of the delayed neutrino explosion mechanism (neutrino mechanism) and argue that these valuable simulations show that neutrino heating can supply a non-negligible fraction of the explosion energy but not the observed energies, and hence cannot be the primary explosion mechanism. In addition to the energy crisis, the neutrino mechanism predicts many failed supernovae that are not observed. The most challenging issue of the neutrino mechanism is that it cannot account for point-symmetric morphologies of CCSN remnants, many of which were identified in 2024. These contradictions with observations imply that the neutrino mechanism cannot be the primary explosion mechanism of CCSNe. The alternative jittering jets explosion mechanism (JJEM) seems to be the primary explosion mechanism of CCSNe; neutrino heating boosts the energy of the jittering jets. Even if some simulations show explosions of stellar models (but usually with energies below that observed), it does not mean that the neutrino mechanism is the explosion mechanism. Jittering jets, which simulations do not include, can explode the core before the neutrino heating process does. Morphological signatures of jets in many CCSN remnants suggest that jittering jets are the primary driving mechanism, as expected by the JJEM. Full article

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

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12 pages, 535 KB

Open AccessEditor’s ChoiceArticle

Reanalysis of the MACHO Constraints on PBH in the Light of Gaia DR3 Data

by Juan García-Bellido and Michael Hawkins

Universe 2024, 10(12), 449; https://doi.org/10.3390/universe10120449 - 6 Dec 2024

Cited by 17 | Viewed by 1785

Abstract

The recent astrometric data of hundreds of millions of stars from Gaia DR3 has allowed for a precise determination of the Milky Way rotation curve up to 28 kpc. The data suggest a rapid decline in the density of dark matter beyond 19 [...] Read more.

The recent astrometric data of hundreds of millions of stars from Gaia DR3 has allowed for a precise determination of the Milky Way rotation curve up to 28 kpc. The data suggest a rapid decline in the density of dark matter beyond 19 kpc. We fit the whole rotation curve with four components (gas, disk, bulge, and halo), and compute the microlensing optical depth to the Large Magellanic Cloud. With this model of the galaxy we reanalyse the microlensing events of the MACHO and EROS-2 Collaborations. Using the published MACHO efficiency function for the duration of their survey, together with the rate of expected events according to the new density profile, we find that the Dark Matter halo could be composed of up to 20% of massive compact halo objects for any mass between

0.001

to

1 M_{⊙}

. For the EROS-2 survey, using a modified efficiency curve for consistency with the MACHO analysis, we also find compatibility with a MACHO halo, but with a tighter constraint around

0.005 M_{⊙}

where the halo fraction cannot be larger than ∼10%. This result assumes that all the lenses have the same mass. If these were distributed in an extended mass function like that of the Thermal History Model, the constraints are weakened, allowing 100% of all DM in the form of Primordial Black Holes. Full article

(This article belongs to the Section Cosmology)

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13 pages, 632 KB

Open AccessEditor’s ChoiceArticle

Using the Difference of the Inclinations of a Pair of Counter-Orbiting Satellites to Measure the Lense–Thirring Effect

by Lorenzo Iorio

Universe 2024, 10(12), 447; https://doi.org/10.3390/universe10120447 - 5 Dec 2024

Viewed by 1498

Abstract

Let two test particles A and B, revolving about a spinning primary along ideally identical orbits in opposite directions, be considered. From the general expressions of the precessions of the orbital inclination induced by the post-Newtonian gravitomagnetic and Newtonian quadrupolar fields of the [...] Read more.

Let two test particles A and B, revolving about a spinning primary along ideally identical orbits in opposite directions, be considered. From the general expressions of the precessions of the orbital inclination induced by the post-Newtonian gravitomagnetic and Newtonian quadrupolar fields of the central object, it turns out that the Lense–Thirring inclination rates of A and B are equal and opposite, while the Newtonian ones oblateness are identical, due to the primary’s oblateness. Thus, the differences in the inclination shifts of the two orbiters would allow, in principle, for the classical effects to be cancelled out by enhancing the general relativistic ones. The conditions affecting the orbital configurations that must be satisfied for this to occur and possible observable consequences regarding the Earth are investigated. In particular, a scenario involving two spacecraft in polar orbits, branded POLAr RElativity Satellites (POLARES) and reminiscent of an earlier proposal by Van Patten and Everitt in the mid-1970s, is considered. A comparison with the ongoing experiment with the LAser GEOdynamics Satellite (LAGEOS) and LAser RElativity Satellite (LARES) 2 is made. Full article

(This article belongs to the Section Gravitation)

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26 pages, 996 KB

Open AccessEditor’s ChoiceReview

Primordial Black Holes: Formation, Spin and Type II

by Tomohiro Harada

Universe 2024, 10(12), 444; https://doi.org/10.3390/universe10120444 - 30 Nov 2024

Cited by 10 | Viewed by 2135

Abstract

Primordial black holes (PBHs) may have formed through the gravitational collapse of cosmological perturbations that were generated and stretched during the inflationary era, later entering the cosmological horizon during the decelerating phase, if their amplitudes were sufficiently large. In this review paper, we [...] Read more.

Primordial black holes (PBHs) may have formed through the gravitational collapse of cosmological perturbations that were generated and stretched during the inflationary era, later entering the cosmological horizon during the decelerating phase, if their amplitudes were sufficiently large. In this review paper, we will briefly introduce the basic concept of PBHs and review the formation dynamics through this mechanism, the estimation of the initial spins of PBHs and the time evolution of type II fluctuations, with a focus on the radiation-dominated and (early) matter-dominated phases. Full article

(This article belongs to the Special Issue Primordial Black Holes from Inflation)

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11 pages, 904 KB

Open AccessEditor’s ChoiceArticle

An Analysis of Variance of the Pantheon+ Dataset: Systematics in the Covariance Matrix?

by Ryan E. Keeley, Arman Shafieloo and Benjamin L’Huillier

Universe 2024, 10(12), 439; https://doi.org/10.3390/universe10120439 - 28 Nov 2024

Cited by 15 | Viewed by 6608

Abstract

We investigate the statistics of the available Pantheon+ dataset. Noticing that the

χ^{2}

value for the best-fit

Λ

CDM model to the real data is small, we quantify how significant its smallness is by calculating the distribution of

χ^{2}

values for [...] Read more.

We investigate the statistics of the available Pantheon+ dataset. Noticing that the

χ^{2}

value for the best-fit

Λ

CDM model to the real data is small, we quantify how significant its smallness is by calculating the distribution of

χ^{2}

values for the best-fit

Λ

CDM model fit to mock Pantheon+-like datasets, using the provided covariance matrix. We further investigate the distribution of the residuals of the Pantheon+ dataset with respect to the best-fit

Λ

CDM model, and notice that they scatter less than would be expected from the covariance matrix but find no significant kurtosis. These results point to the conclusion that the Pantheon+ covariance matrix is over-estimated. One simple interpretation of these results is a ∼7% overestimation of errors on SN distance moduli in Pantheon+ data. When the covariance matrix is reduced by subtracting an intrinsic scatter term from the diagonal terms of the covariance matrix, the best-fit

χ^{2}

for the

Λ

CDM model achieves a normal value of 1580 and no deviation from

Λ

CDM is detected. We further quantify how consistent the

Λ

CDM model is with respect to the modified data with the subtracted covariance matrix using model-independent reconstruction techniques such as the iterative smoothing method. We find that the standard model is consistent with the data. There are a number of potential explanations for this smallness of the

χ^{2}

, such as a Malmquist bias at high redshift, or accounting for systematic uncertainties by adding them to the covariance matrix, thus approximating systematic uncertainties as statistical ones. Full article

(This article belongs to the Section Cosmology)

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26 pages, 416 KB

Open AccessEditor’s ChoicePerspective

Foundational Issues in Dynamical Casimir Effect and Analogue Features in Cosmological Particle Creation

by Jen-Tsung Hsiang and Bei-Lok Hu

Universe 2024, 10(11), 418; https://doi.org/10.3390/universe10110418 - 7 Nov 2024

Cited by 4 | Viewed by 2247

Abstract

Moving mirrors as analogue sources of Hawking radiation from black holes have been explored extensively but less so with cosmological particle creation (CPC), even though the analogy between the dynamical Casimir effect (DCE) and CPC based on the mechanism of the parametric amplification [...] Read more.

Moving mirrors as analogue sources of Hawking radiation from black holes have been explored extensively but less so with cosmological particle creation (CPC), even though the analogy between the dynamical Casimir effect (DCE) and CPC based on the mechanism of the parametric amplification of quantum field fluctuations has also been known for a long time. This ‘perspective’ essay intends to convey some of the rigor and thoroughness of quantum field theory in curved spacetime, which serves as the theoretical foundation of CPC, to DCE, which enjoys a variety of active experimental explorations. We have selected seven issues of relevance to address, starting from the naively simple ones, e.g., why one should be bothered with ‘curved’ spacetime when performing a laboratory experiment in ostensibly flat space, to foundational theoretical ones, such as the frequent appearance of nonlocal dissipation in the system dynamics induced by colored noises in its field environment, the existence of quantum Lenz law and fluctuation–dissipation relations in the backreaction effects of DCE emission on the moving atom/mirror or the source, and the construction of a microphysics model to account for the dynamical responses of a mirror or medium. The strengthening of the theoretical ground for DCE is not only useful for improving conceptual clarity but needed for the development of the proof-of-concept type of future experimental designs for DCE. The results from the DCE experiments in turn will enrich our understanding of quantum field effects in the early universe because they are, in the spirit of analogue gravity, our best hopes for the verification of these fundamental processes. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

14 pages, 334 KB

Open AccessEditor’s ChoiceArticle

An Update of the Hypothetical X17 Particle

by Attila J. Krasznahorkay, Attila Krasznahorkay, Margit Csatlós, János Timár, Marcell Begala, Attila Krakó, István Rajta, István Vajda and Nándor J. Sas

Universe 2024, 10(11), 409; https://doi.org/10.3390/universe10110409 - 31 Oct 2024

Cited by 9 | Viewed by 2288

Abstract

Recently, when examining the differential internal pair creation coefficients of ⁸Be, ⁴He and ¹²C nuclei, we observed peak-like anomalies in the angular correlation of the e⁺e⁻ pairs. This was interpreted as the creation and immediate decay of [...] Read more.

Recently, when examining the differential internal pair creation coefficients of ⁸Be, ⁴He and ¹²C nuclei, we observed peak-like anomalies in the angular correlation of the e⁺e⁻ pairs. This was interpreted as the creation and immediate decay of an intermediate bosonic particle with a mass of

m_{X} c^{2} \approx

17 MeV, receiving the name X17 in subsequent publications. In this paper, our latest results obtained for the X17 particle are presented by investigating the e⁺e⁻ pair correlations in the decay of the Giant Dipole Resonance (GDR) of ⁸Be. Our results initiated a significant number of new experiments all over the world to detect the X17 particle and determine its properties. In this paper, we will also conduct a mini-review of the experiments whose results are already published, as well as the ones closest to being published. Full article

(This article belongs to the Special Issue Multiparticle Dynamics)

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18 pages, 14984 KB

Open AccessEditor’s ChoiceArticle

The Mother’s Day Solar Storm of 11 May 2024 and Its Effect on Earth’s Radiation Belts

by Viviane Pierrard, Alexandre Winant, Edith Botek and Maximilien Péters de Bonhome

Universe 2024, 10(10), 391; https://doi.org/10.3390/universe10100391 - 10 Oct 2024

Cited by 22 | Viewed by 5515

Abstract

The month of May 2024 was characterized by solar energetic particles events directed towards the Earth, especially the big event causing a strong terrestrial geomagnetic storm during the night from 10 to 11 May 2024, with auroras observed everywhere in Europe. This was [...] Read more.

The month of May 2024 was characterized by solar energetic particles events directed towards the Earth, especially the big event causing a strong terrestrial geomagnetic storm during the night from 10 to 11 May 2024, with auroras observed everywhere in Europe. This was the strongest storm for the last 20 years with a Disturbed Storm Time index Dst < −400 nT. In the present work, we show with observations of GOES, PROBA-V/EPT and MetOP/MEPED that this exceptional event was associated with the injection of energetic protons in the proton radiation belt, with important consequences for the South part of the South Atlantic Anomaly (SAA). In addition, the geomagnetic storm caused by the solar eruption has had tremendous impacts on the electron radiation belts. Indeed, we show that for 0.3 to 1 MeV electrons, the storm led to a long lasting four belts configuration which was not observed before with EPT launched in 2013, until a smaller geomagnetic storm took place at the end of June 2024. Moreover, for the first time since its launch, observations of the EPT show that ultra-relativistic electrons with

E > 2

MeV have been injected into the inner belt down to McIlwain parameter L = 2.4, violating the impenetrable barrier previously estimated to be located at L = 2.8. Full article

(This article belongs to the Section Space Science)

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17 pages, 505 KB

Open AccessEditor’s ChoiceArticle

Prigogine’s Second Law and Determination of the EUP and GUP Parameters in Small Black Hole Thermodynamics

by Giorgio Sonnino

Universe 2024, 10(10), 390; https://doi.org/10.3390/universe10100390 - 7 Oct 2024

Cited by 4 | Viewed by 1866

Abstract

In 1974, Stephen Hawking made the groundbreaking discovery that black holes emit thermal radiation, characterized by a specific temperature now known as the Hawking temperature. While his original derivation is intricate, retrieving the exact expressions for black hole temperature and entropy in a [...] Read more.

In 1974, Stephen Hawking made the groundbreaking discovery that black holes emit thermal radiation, characterized by a specific temperature now known as the Hawking temperature. While his original derivation is intricate, retrieving the exact expressions for black hole temperature and entropy in a simpler, more intuitive way without losing the core physical principles behind Hawking’s assumptions is possible. This is obtained by employing the Heisenberg Uncertainty Principle, which is known to be connected to thenvacuum fluctuation. This exercise allows us to easily perform more complex calculations involving the effects of quantum gravity. This work aims to answer the following question: Is it possible to reconcile Prigogine’s second law of thermodynamics for open systems and the second law of black hole dynamics with Hawking radiation? Due to quantum gravity effects, the Heisenberg Uncertainty Principle has been extended to the Generalized Uncertainty Principle (GUP) and successively to the Extended Uncertainty Principle (EUP). The expression for the EUP parameter is obtained by conjecturing that Prigogine’s second law of thermodynamics and the second law of black holes are not violated by the Hawking thermal radiation mechanism. The modified expression for the entropy of a Schwarzschild black hole is also derived. Full article

(This article belongs to the Section Cosmology)

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19 pages, 349 KB

Open AccessEditor’s ChoiceReview

Spinning Systems in Quantum Mechanics: An Overview and New Trends

by E. Brito, Júlio E. Brandão and Márcio M. Cunha

Universe 2024, 10(10), 389; https://doi.org/10.3390/universe10100389 - 4 Oct 2024

Cited by 6 | Viewed by 2749

Abstract

The study of spinning systems plays a question of interest in several research branches in physics. It allows the understanding of simple classical mechanical systems but also provides us with tools to investigate a wide range of phenomena, from condensed matter physics to [...] Read more.

The study of spinning systems plays a question of interest in several research branches in physics. It allows the understanding of simple classical mechanical systems but also provides us with tools to investigate a wide range of phenomena, from condensed matter physics to gravitation and cosmology. In this contribution, we review some remarkable theoretical aspects involving the description of spinning quantum systems. We explore the nonrelativistic and relativistic domains and their respective applications in fields such as graphene physics and topological defects in gravitation. Full article

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

113 pages, 1053 KB

Open AccessEditor’s ChoiceArticle

Quantum Field Theory of Black Hole Perturbations with Backreaction: I General Framework

by Thomas Thiemann

Universe 2024, 10(9), 372; https://doi.org/10.3390/universe10090372 - 18 Sep 2024

Cited by 8 | Viewed by 2159

Abstract

In a seminal work, Hawking showed that natural states for free quantum matter fields on classical spacetimes that solve the spherically symmetric vacuum Einstein equations are KMS states of non-vanishing temperature. Although Hawking’s calculation does not include the backreaction of matter on geometry, [...] Read more.

In a seminal work, Hawking showed that natural states for free quantum matter fields on classical spacetimes that solve the spherically symmetric vacuum Einstein equations are KMS states of non-vanishing temperature. Although Hawking’s calculation does not include the backreaction of matter on geometry, it is more than plausible that the corresponding Hawking radiation leads to black hole evaporation which is, in principle, observable. Obviously, an improvement of Hawking’s calculation including backreaction is a problem of quantum gravity. Since no commonly accepted quantum field theory of general relativity is available yet, it has been difficult to reliably derive the backreaction effect. An obvious approach is to use the black hole perturbation theory of a Schwarzschild black hole of fixed mass and to quantize those perturbations. However, it is not clear how to reconcile perturbation theory with gauge invariance beyond linear perturbations. In recent work, we proposed a new approach to this problem that applies when the physical situation has an approximate symmetry, such as homogeneity (cosmology), spherical symmetry (Schwarzschild), or axial symmetry (Kerr). The idea, which is surprisingly feasible, is to first construct the non-perturbative physical (reduced) Hamiltonian of the reduced phase space of fully gauge invariant observables and only then apply perturbation theory directly in terms of observables. The task to construct observables is then disentangled from perturbation theory, thus allowing to unambiguously develop perturbation theory to arbitrary orders. In this first paper of the series we outline and showcase this approach for spherical symmetry and second order in the perturbations for Einstein–Klein–Gordon–Maxwell theory. Details and generalizations to other matter and symmetry and higher orders will appear in subsequent companion papers. Full article

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30 pages, 2416 KB

Open AccessEditor’s ChoiceReview

Stellar Flares, Superflares, and Coronal Mass Ejections—Entering the Big Data Era

by Krisztián Vida, Zsolt Kővári, Martin Leitzinger, Petra Odert, Katalin Oláh, Bálint Seli, Levente Kriskovics, Robert Greimel and Anna Mária Görgei

Universe 2024, 10(8), 313; https://doi.org/10.3390/universe10080313 - 31 Jul 2024

Cited by 16 | Viewed by 3372

Abstract

Flares, sometimes accompanied by coronal mass ejections (CMEs), are the result of sudden changes in the magnetic field of stars with high energy release through magnetic reconnection, which can be observed across a wide range of the electromagnetic spectrum from radio waves to [...] Read more.

Flares, sometimes accompanied by coronal mass ejections (CMEs), are the result of sudden changes in the magnetic field of stars with high energy release through magnetic reconnection, which can be observed across a wide range of the electromagnetic spectrum from radio waves to the optical range to X-rays. In our observational review, we attempt to collect some fundamental new results, which can largely be linked to the Big Data era that has arrived due to the expansion of space photometric observations over the last two decades. We list the different types of stars showing flare activity and their observation strategies and discuss how their main stellar properties relate to the characteristics of the flares (or even CMEs) they emit. Our goal is to focus, without claiming to be complete, on those results that may, in one way or another, challenge the “standard” flare model based on the solar paradigm. Full article

(This article belongs to the Special Issue Solar and Stellar Activity: Exploring the Cosmic Nexus)

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43 pages, 639 KB

Open AccessEditor’s ChoiceTutorial

Graviton Physics: A Concise Tutorial on the Quantum Field Theory of Gravitons, Graviton Noise, and Gravitational Decoherence

by Jen-Tsung Hsiang, Hing-Tong Cho and Bei-Lok Hu

Universe 2024, 10(8), 306; https://doi.org/10.3390/universe10080306 - 24 Jul 2024

Cited by 14 | Viewed by 4736

Abstract

The detection of gravitational waves in 2015 ushered in a new era of gravitational wave (GW) astronomy capable of probing the strong field dynamics of black holes and neutron stars. It has opened up an exciting new window for laboratory and space tests [...] Read more.

The detection of gravitational waves in 2015 ushered in a new era of gravitational wave (GW) astronomy capable of probing the strong field dynamics of black holes and neutron stars. It has opened up an exciting new window for laboratory and space tests of Einstein’s theory of classical general relativity (GR). In recent years, two interesting proposals have aimed to reveal the quantum nature of perturbative gravity: (1) theoretical predictions on how graviton noise from the early universe, after the vacuum of the gravitational field was strongly squeezed by inflationary expansion; (2) experimental proposals using the quantum entanglement between two masses, each in a superposition (gravitational cat, or gravcat) state. The first proposal focuses on the stochastic properties of quantum fields (QFs), and the second invokes a key concept of quantum information (QI). An equally basic and interesting idea is to ask whether (and how) gravity might be responsible for a quantum system becoming classical in appearance, known as gravitational decoherence. Decoherence due to gravity is of special interest because gravity is universal, meaning, gravitational interaction is present for all massive objects. This is an important issue in macroscopic quantum phenomena (MQP), underlining many proposals in alternative quantum theories (AQTs). To fully appreciate or conduct research in these exciting developments requires a working knowledge of classical GR, QF theory, and QI, plus some familiarity with stochastic processes (SPs), namely, noise in quantum fields and decohering environments. Traditionally a new researcher may be conversant in one or two of these four subjects: GR, QFT, QI, and SP, depending on his/her background. This tutorial attempts to provide the necessary connective tissues between them, helping an engaged reader from any one of these four subjects to leapfrog to the frontier of these interdisciplinary research topics. In the present version, we shall address the three topics listed in the title, excluding gravitational entanglement, because, despite the high attention some recent experimental proposals have received, its nature and implications in relation to quantum gravity still contain many controversial elements. Full article

(This article belongs to the Special Issue Quantum Field Theory of Open Systems)

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11 pages, 5168 KB

Open AccessEditor’s ChoiceReview

X17: Status and Perspectives

by Carlo Gustavino

Universe 2024, 10(7), 285; https://doi.org/10.3390/universe10070285 - 29 Jun 2024

Cited by 10 | Viewed by 1997

Abstract

Recently, a group directed by A. J. Krasznahorkay observed an anomaly in the emission of electron–positron pairs in three different nuclear reactions, namely, the

^{3}

H(p,e

^{-}

e

^{+}

)

^{4}

He,

^{7}

Li(p,e

^{-}

e [...] Read more.

Recently, a group directed by A. J. Krasznahorkay observed an anomaly in the emission of electron–positron pairs in three different nuclear reactions, namely, the

^{3}

H(p,e

^{-}

e

^{+}

)

^{4}

He,

^{7}

Li(p,e

^{-}

e

^{+}

)

^{8}

Be, and

^{11}

B(p,e

^{-}

e

^{+}

)

^{12}

C processes. Kinematics indicate that this anomaly might be due to the de-excitation of

^{4}

He,

^{8}

Be, and

^{12}

C nuclei with the emission of a boson with a mass of about 17 MeV, rapidly decaying into e

^{-}

e

^{+}

pairs. The result of the experiments performed with the singletron accelerator of ATOMKI is reviewed, and the consequences of the so-called X17 boson in particle physics and in cosmology are discussed. Forthcoming experiments designed to shed light on the possible existence of the X17 boson are also reported. Full article

(This article belongs to the Special Issue Recent Outcomes and Future Challenges in Nuclear Astrophysics)

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7 pages, 1120 KB

Open AccessEditor’s ChoiceCommunication

Detecting Wandering Intermediate-Mass Black Holes with AXIS in the Milky Way and Local Massive Galaxies

by Fabio Pacucci, Bryan Seepaul, Yueying Ni, Nico Cappelluti and Adi Foord

Universe 2024, 10(5), 225; https://doi.org/10.3390/universe10050225 - 17 May 2024

Cited by 4 | Viewed by 1986

Abstract

This white paper explores the detectability of intermediate-mass black holes (IMBHs) wandering in the Milky Way (MW) and massive local galaxies, with a particular emphasis on the role of AXIS. IMBHs, ranging within

10^{3 - 6} M_{⊙}

, are commonly found [...] Read more.

This white paper explores the detectability of intermediate-mass black holes (IMBHs) wandering in the Milky Way (MW) and massive local galaxies, with a particular emphasis on the role of AXIS. IMBHs, ranging within

10^{3 - 6} M_{⊙}

, are commonly found at the centers of dwarf galaxies and may exist, yet undiscovered, in the MW. By using model spectra for advection-dominated accretion flows (ADAFs), we calculated the expected fluxes emitted by a population of wandering IMBHs with masses of

10^{5} M_{⊙}

in various MW environments and extrapolated our results to massive local galaxies. Around

40 %

of the potential population of wandering IMBHs in the MW can be detected in an AXIS deep field. We proposed criteria to aid with selecting IMBH candidates using already available optical surveys. We also showed that IMBHs wandering in >200 galaxies within 10 Mpc can be easily detected with AXIS when passing within dense galactic environments (e.g., molecular clouds and cold neutral medium). In summary, we highlighted the potential X-ray detectability of wandering IMBHs in local galaxies and provided insights for guiding future surveys. Detecting wandering IMBHs is crucial for understanding their demographics and evolution and the merging history of galaxies. This white paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS white papers can be found at the AXIS website. Full article

(This article belongs to the Section Galaxies and Clusters)

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13 pages, 1872 KB

Open AccessEditor’s ChoiceReview

Shanghai Tianma Radio Telescope and Its Role in Pulsar Astronomy

by Zhen Yan, Zhiqiang Shen, Yajun Wu, Rongbing Zhao, Jie Liu, Zhipeng Huang, Rui Wang, Xiaowei Wang, Qinghui Liu, Bin Li, Jinqing Wang, Weiye Zhong, Wu Jiang and Bo Xia

Universe 2024, 10(5), 195; https://doi.org/10.3390/universe10050195 - 26 Apr 2024

Cited by 7 | Viewed by 2769

Abstract

After two phases of on-site construction and testing (2010–2013 and 2013–2017), the Shanghai Tianma Radio Telescope (TMRT) can work well, with efficiencies better than 50% from 1.3 to 50.0 GHz, mainly benefiting from its low-noise cryogenic receivers and active surface system. Pulsars were [...] Read more.

After two phases of on-site construction and testing (2010–2013 and 2013–2017), the Shanghai Tianma Radio Telescope (TMRT) can work well, with efficiencies better than 50% from 1.3 to 50.0 GHz, mainly benefiting from its low-noise cryogenic receivers and active surface system. Pulsars were chosen as important targets of research at the TMRT because of their important scientific and applied values. To meet the demands of pulsar-related observations, TMRT is equipped with some necessary backends, including a digital backend system (DIBAS) supporting normal pulsar observation modes, a real-time fast-radio-burst-monitoring backend, and baseband backends for very-long-baseline interferometry (VLBI) observations. Utilizing its high sensitivity and simultaneous dual-frequency observation capacity, a sequence of pulsar research endeavors has been undertaken, such as long-term pulsar timing, magnetar monitoring, multi-frequency (or high-frequency) observations, interstellar scintillation, pulsar VLBI, etc. In this paper, we give a short introduction about pulsar observation systems at the TMRT and briefly review the results obtained by these pulsar research projects. Full article

(This article belongs to the Special Issue Pulsar Astronomy)

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25 pages, 2208 KB

Open AccessEditor’s ChoiceReview

Measuring the Lense–Thirring Orbital Precession and the Neutron Star Moment of Inertia with Pulsars

by Huanchen Hu and Paulo C. C. Freire

Universe 2024, 10(4), 160; https://doi.org/10.3390/universe10040160 - 28 Mar 2024

Cited by 11 | Viewed by 4821

Abstract

Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on [...] Read more.

Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on the equation of state (EoS) of dense matter, accurately determining the radius or moment of inertia (MoI) of an NS remains a major challenge. This article presents a detailed review on measuring the Lense–Thirring (LT) precession effect in the orbit of binary pulsars, which would give access to the MoI of NSs and offer further constraints on the EoS. We discuss the suitability of certain classes of binary pulsars for measuring the LT precession from the perspective of binary star evolution and highlight five pulsars that exhibit properties promising to realise these goals in the near future. Finally, discoveries of compact binaries with shorter orbital periods hold the potential to greatly enhance measurements of the MoI of NSs. The MoI measurements of binary pulsars are pivotal to advancing our understanding of matter at supranuclear densities, as well as improving the precision of gravity tests, such as the orbital decay due to gravitational wave emission, and of tests of alternative gravity theories. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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30 pages, 1088 KB

Open AccessEditor’s ChoiceReview

The Physics of Core-Collapse Supernovae: Explosion Mechanism and Explosive Nucleosynthesis

by Luca Boccioli and Lorenzo Roberti

Universe 2024, 10(3), 148; https://doi.org/10.3390/universe10030148 - 19 Mar 2024

Cited by 42 | Viewed by 6344

Abstract

Recent developments in multi-dimensional simulations of core-collapse supernovae have considerably improved our understanding of this complex phenomenon. In addition to that, one-dimensional (1D) studies have been employed to study the explosion mechanism and its causal connection to the pre-collapse structure of the star, [...] Read more.

Recent developments in multi-dimensional simulations of core-collapse supernovae have considerably improved our understanding of this complex phenomenon. In addition to that, one-dimensional (1D) studies have been employed to study the explosion mechanism and its causal connection to the pre-collapse structure of the star, as well as to explore the vast parameter space of supernovae. Nonetheless, many uncertainties still affect the late stages of the evolution of massive stars, their collapse, and the subsequent shock propagation. In this review, we will briefly summarize the state-of-the-art of both 1D and 3D simulations and how they can be employed to study the evolution of massive stars, supernova explosions, and shock propagation, focusing on the uncertainties that affect each of these phases. Finally, we will illustrate the typical nucleosynthesis products that emerge from the explosion. Full article

(This article belongs to the Special Issue Recent Outcomes and Future Challenges in Nuclear Astrophysics)

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19 pages, 755 KB

Open AccessEditor’s ChoiceArticle

The Relevance of Dynamical Friction for the MW/LMC/SMC Triple System

by Wolfgang Oehm and Pavel Kroupa

Universe 2024, 10(3), 143; https://doi.org/10.3390/universe10030143 - 14 Mar 2024

Cited by 14 | Viewed by 4109

Abstract

Simulations of structure formation in the standard cold dark matter cosmological model quantify the dark matter halos of galaxies. Taking into account dynamical friction between dark matter halos, we investigate the past orbital dynamical evolution of the Magellanic Clouds in the presence of [...] Read more.

Simulations of structure formation in the standard cold dark matter cosmological model quantify the dark matter halos of galaxies. Taking into account dynamical friction between dark matter halos, we investigate the past orbital dynamical evolution of the Magellanic Clouds in the presence of the Galaxy. Our calculations are based on a three-body model of rigid Navarro–Frenk–White profiles for dark matter halos but were verified in a previous publication by comparison to high-resolution N-body simulations of live self-consistent systems. Under the requirement that the LMC and SMC had an encounter within 20 kpc between 1 and 4 Gyr ago in order to allow the development of the Magellanic Stream, using the latest astrometric data, the dynamical evolution of the MW/LMC/SMC system is calculated backwards in time. With the employment of the genetic algorithm and a Markov-Chain Monte-Carlo method, the present state of this system is unlikely, with a probability of

< 10^{- 9}

(

6 σ

complement), because the solutions found do not fit into the error bars for the observed plane-of-sky velocity components of the Magellanic Clouds. This implies that orbital solutions that assume dark matter halos, according to cosmological structure formation theory, to exist around the Magellanic Clouds and the Milky Way are not possible with a confidence of more than 6 sigma. Full article

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13 pages, 285 KB

Open AccessEditor’s ChoiceArticle

Regular Friedmann Universes and Matter Transformations

by Alexander Kamenshchik and Polina Petriakova

Universe 2024, 10(3), 137; https://doi.org/10.3390/universe10030137 - 13 Mar 2024

Cited by 5 | Viewed by 2048

Abstract

We apply a very simple procedure to construct non-singular cosmological models for flat Friedmann universes filled with minimally coupled scalar fields or by tachyon Born–Infeld-type fields. Remarkably, for the minimally coupled scalar field and the tachyon field, the regularity of the cosmological evolution, [...] Read more.

We apply a very simple procedure to construct non-singular cosmological models for flat Friedmann universes filled with minimally coupled scalar fields or by tachyon Born–Infeld-type fields. Remarkably, for the minimally coupled scalar field and the tachyon field, the regularity of the cosmological evolution, or in other words, the existence of bounce, implies the necessity of the transition between scalar fields with standard kinetic terms to those with phantom ones. In both cases, the potentials in the vicinity of the point of the transition have a non-analyticity of the cusp form that is characterized by the same exponent and is equal to

\frac{2}{3}

. If, in the tachyon model’s evolution, the pressure changes its sign, then another transformation of the Born–Infeld-type field occurs: the tachyon transforms into a pseudotachyon, and vice versa. We also undertake an analysis of the stability of the cosmological evolution in our models; we rely on the study of the speed of sound squared. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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16 pages, 505 KB

Open AccessEditor’s ChoiceArticle

A Simple Direct Empirical Observation of Systematic Bias of the Redshift as a Distance Indicator

by Lior Shamir

Universe 2024, 10(3), 129; https://doi.org/10.3390/universe10030129 - 6 Mar 2024

Cited by 4 | Viewed by 3101

Abstract

Recent puzzling observations, such as the

H_{0}

tension, large-scale anisotropies, and massive disk galaxies at high redshifts, have been challenging the standard cosmological model. While one possible explanation is that the standard model is incomplete, other theories are based on the contention [...] Read more.

Recent puzzling observations, such as the

H_{0}

tension, large-scale anisotropies, and massive disk galaxies at high redshifts, have been challenging the standard cosmological model. While one possible explanation is that the standard model is incomplete, other theories are based on the contention that the redshift model as a distance indicator might be biased. These theories can explain the recent observations, but they are challenged by the absence of a direct empirical reproducible observation that the redshift model can indeed be inconsistent. Here, I describe a simple experiment that shows that the spectra of galaxies depend on their rotational velocity relative to the rotational velocity of the Milky Way. Moreover, it shows that the redshift of galaxies that rotate in the opposite direction relative to the Milky Way is significantly smaller compared with the redshift of galaxies that rotate in the same direction relative to the Milky Way (p < 0.006). Three different datasets were used independently, each one was prepared in a different manner, and all of them showed similar redshift bias. A fourth dataset of galaxies from the Southern Galactic pole was also analyzed and shows similar results. All four datasets are publicly available. While a maximum average z difference of ∼0.012 observed with galaxies of relatively low redshift (z < 0.25) is not extreme, the bias is consistent and canpotentially lead to explanations to puzzling observations such as the

H_{0}

tension. Full article

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27 pages, 389 KB

Open AccessEditor’s ChoiceArticle

Theoretically Motivated Dark Electromagnetism as the Origin of Relativistic Modified Newtonian Dynamics

by Felix Finster, José M. Isidro, Claudio F. Paganini and Tejinder P. Singh

Universe 2024, 10(3), 123; https://doi.org/10.3390/universe10030123 - 4 Mar 2024

Cited by 5 | Viewed by 2524

Abstract

The present paper is a modest attempt to initiate the research program outlined in this abstract. We propose that general relativity and relativistic MOND (RelMOND) are analogues of broken electroweak symmetry. That is, [...] Read more.

The present paper is a modest attempt to initiate the research program outlined in this abstract. We propose that general relativity and relativistic MOND (RelMOND) are analogues of broken electroweak symmetry. That is,

S U {(2)}_{R} \times U {(1)}_{Y D E M} \to U {(1)}_{D E M}

(DEM stands for dark electromagnetism), and GR is assumed to arise from the broken

S U {(2)}_{R}

symmetry and is analogous to the weak force. RelMOND is identified with dark electromagnetism

U {(1)}_{D E M}

, which is the remaining unbroken symmetry after the spontaneous symmetry breaking of the dark electro-grav sector

S U {(2)}_{R} \times U {(1)}_{Y D E M}

. This sector, as well as the electroweak sector, arises from the breaking of an

E_{8} \times E_{8}

symmetry in a recently proposed model of unification of the standard model with pre-gravitation, with the latter based on an

S U {(2)}_{R}

gauge theory. The source charge for the dark electromagnetic force is the square root of mass, motivated by the experimental fact that the ratio of the square roots of the masses of the electron, up-quark, and down-quark is 1:2:3, which is the opposite of the ratio of their electric charges at 3:2:1. The introduction of the dark electromagnetic force helps us understand the peculiar mass ratios of the second and third generations of charged fermions. We also note that in the deep MOND regime, acceleration is proportional to the square root of mass, which motivates us to propose the relativistic

U {(1)}_{D E M}

gauge symmetry as the origin of MOND. We explain why the dark electromagnetic force falls inversely with distance, as in MOND, rather than following the inverse square of distance. We conclude that dark electromagnetism effectively mimics cold dark matter, and the two are essentially indistinguishable in cosmological situations where CDM successfully explains observations, such as CMB anisotropies and gravitational lensing. Full article

(This article belongs to the Special Issue The Large-Scale Structure of the Universe: Theory and Observation)

32 pages, 454 KB

Open AccessEditor’s ChoiceReview

Gamma-ray Bursts: 50 Years and Counting!

by Alessandro Armando Vigliano and Francesco Longo

Universe 2024, 10(2), 57; https://doi.org/10.3390/universe10020057 - 26 Jan 2024

Cited by 9 | Viewed by 5823

Abstract

Gamma-ray bursts were discovered by the Vela satellites in the late 1960s, but they were announced for the first time exactly 50 years ago, in 1973. The history of our understanding of gamma-ray bursts can be subdivided into several eras. We will highlight [...] Read more.

Gamma-ray bursts were discovered by the Vela satellites in the late 1960s, but they were announced for the first time exactly 50 years ago, in 1973. The history of our understanding of gamma-ray bursts can be subdivided into several eras. We will highlight the main discoveries about GRBs, as well as the path toward the future that each GRB era could still indicate. Full article

(This article belongs to the Special Issue Recent Advances in Gamma Ray Astrophysics and Future Perspectives)

12 pages, 441 KB

Open AccessEditor’s ChoiceArticle

Femtoscopy with Lévy Sources from SPS through RHIC to LHC

by Máté Csanád and Dániel Kincses

Universe 2024, 10(2), 54; https://doi.org/10.3390/universe10020054 - 23 Jan 2024

Cited by 19 | Viewed by 3362

Abstract

Femtoscopy is a unique tool to investigate the space-time geometry of the matter created in ultra-relativistic collisions. If the probability density distribution of hadron emission is parametrized, then the dependence of its parameters on particle momentum, collision energy, and collision geometry can be [...] Read more.

Femtoscopy is a unique tool to investigate the space-time geometry of the matter created in ultra-relativistic collisions. If the probability density distribution of hadron emission is parametrized, then the dependence of its parameters on particle momentum, collision energy, and collision geometry can be given. In recent years, several measurements came to light that indicated the adequacy of assuming a Lévy-stable shape for the mentioned distribution. In parallel, several new phenomenological developments appeared, aiding the interpretation of the experimental results or providing tools for the measurements. In this paper, we discuss important aspects of femtoscopy with Lévy sources in light of some of these advances, including phenomenological and experimental ones. Full article

(This article belongs to the Special Issue Multiparticle Dynamics)

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22 pages, 617 KB

Open AccessEditor’s ChoiceReview

Neutrino Flavor Model Building and the Origins of Flavor and

C

P

Violation

by Yahya Almumin, Mu-Chun Chen, Murong Cheng, Víctor Knapp-Pérez, Yulun Li, Adreja Mondol, Saúl Ramos-Sánchez, Michael Ratz and Shreya Shukla

Universe 2023, 9(12), 512; https://doi.org/10.3390/universe9120512 - 12 Dec 2023

Cited by 32 | Viewed by 3938

Abstract

The neutrino sector offers one of the most sensitive probes of new physics beyond the Standard Model of Particle Physics (SM). The mechanism of neutrino mass generation is still unknown. The observed suppression of neutrino masses hints at a large scale, conceivably of [...] Read more.

The neutrino sector offers one of the most sensitive probes of new physics beyond the Standard Model of Particle Physics (SM). The mechanism of neutrino mass generation is still unknown. The observed suppression of neutrino masses hints at a large scale, conceivably of the order of the scale of a rand unified theory (GUT), which is a unique feature of neutrinos that is not shared by the charged fermions. The origin of neutrino masses and mixing is part of the outstanding puzzle of fermion masses and mixings, which is not explained ab initio in the SM. Flavor model building for both quark and lepton sectors is important in order to gain a better understanding of the origin of the structure of mass hierarchy and flavor mixing, which constitute the dominant fraction of the SM parameters. Recent activities in neutrino flavor model building based on non-Abelian discrete flavor symmetries and modular flavor symmetries have been shown to be a promising direction to explore. The emerging models provide a framework that has a significantly reduced number of undetermined parameters in the flavor sector. In addition, such a framework affords a novel origin of

C

P

violation from group theory due to the intimate connection between physical

C

P

transformation and group theoretical properties of non-Abelian discrete groups. Model building based on non-Abelian discrete flavor symmetries and their modular variants enables the particle physics community to interpret the current and anticipated upcoming data from neutrino experiments. Non-Abelian discrete flavor symmetries and their modular variants can result from compactification of a higher-dimensional theory. Pursuit of flavor model building based on such frameworks thus also provides the connection to possible UV completions: in particular, to string theory. We emphasize the importance of constructing models in which the uncertainties of theoretical predictions are smaller than, or at most compatible with, the error bars of measurements in neutrino experiments. While there exist proof-of-principle versions of bottom-up models in which the theoretical uncertainties are under control, it is remarkable that the key ingredients of such constructions were discovered first in top-down model building. We outline how a successful unification of bottom-up and top-down ideas and techniques may guide us towards a new era of precision flavor model building in which future experimental results can give us crucial insights into the UV completion of the SM. Full article

(This article belongs to the Special Issue CP Violation and Flavor Physics)

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53 pages, 5295 KB

Open AccessFeature PaperEditor’s ChoiceReview

The Unsettled Number: Hubble’s Tension

by Jorge L. Cervantes-Cota, Salvador Galindo-Uribarri and George F. Smoot

Universe 2023, 9(12), 501; https://doi.org/10.3390/universe9120501 - 29 Nov 2023

Cited by 25 | Viewed by 10203

Abstract

One of main sources of uncertainty in modern cosmology is the present rate of the universe’s expansion, H₀, called the Hubble constant. Once again, different observational techniques bring about different results, causing new “Hubble tension”. In the present work, we review [...] Read more.

One of main sources of uncertainty in modern cosmology is the present rate of the universe’s expansion, H₀, called the Hubble constant. Once again, different observational techniques bring about different results, causing new “Hubble tension”. In the present work, we review the historical roots of the Hubble constant from the beginning of the twentieth century, when modern cosmology originated, to the present. We develop the arguments that gave rise to the importance of measuring the expansion of the Universe and its discovery, and we describe the different pioneering works attempting to measure it. There has been a long dispute on this matter, even in the present epoch, which is marked by high-tech instrumentation and, therefore, in smaller uncertainties in the relevant parameters. It is, again, currently necessary to conduct a careful and critical revision of the different methods before one invokes new physics to solve the so-called Hubble tension. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)

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