Universe

13 pages, 419 KiB

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 5 | Viewed by 757

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 KiB

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 6 | Viewed by 831

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 KiB

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 760

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 KiB

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 2 | Viewed by 782

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 KiB

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 12 | Viewed by 663

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 KiB

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 2 | Viewed by 852

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 KiB

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

Viewed by 1062

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 KiB

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 3 | Viewed by 1991

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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Graphical abstract

17 pages, 505 KiB

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 3 | Viewed by 1060

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 KiB

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 2 | Viewed by 1145

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 KiB

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 7 | Viewed by 1089

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 KiB

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 3 | Viewed by 1274

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 KiB

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 5 | Viewed by 2156

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 KiB

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 4 | Viewed by 1001

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 KiB

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 3 | Viewed by 1281

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 KiB

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 5 | Viewed by 1554

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 KiB

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 5 | Viewed by 1842

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 KiB

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 21 | Viewed by 2726

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 KiB

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 6 | Viewed by 1736

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 KiB

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 1481

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 KiB

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 2010

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 KiB

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 3 | Viewed by 1565

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 KiB

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 5 | Viewed by 2349

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 KiB

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 13 | Viewed by 1587

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 KiB

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 15 | Viewed by 2089

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 KiB

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 11 | Viewed by 6056

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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26 pages, 1637 KiB

Open AccessEditor’s ChoiceArticle

Uniform Asymptotic Approximation Method with Pöschl–Teller Potential

by Rui Pan, John Joseph Marchetta, Jamal Saeed, Gerald Cleaver, Bao-Fei Li, Anzhong Wang and Tao Zhu

Universe 2023, 9(11), 471; https://doi.org/10.3390/universe9110471 - 31 Oct 2023

Cited by 1 | Viewed by 1897

Abstract

In this paper, we study analytical approximate solutions for second-order homogeneous differential equations with the existence of only two turning points (but without poles) by using the uniform asymptotic approximation (UAA) method. To be more concrete, we consider the Pöschl–Teller (PT) potential, for [...] Read more.

In this paper, we study analytical approximate solutions for second-order homogeneous differential equations with the existence of only two turning points (but without poles) by using the uniform asymptotic approximation (UAA) method. To be more concrete, we consider the Pöschl–Teller (PT) potential, for which analytical solutions are known. Depending on the values of the parameters involved in the PT potential, we find that the upper bounds of the errors of the approximate solutions in general are ≲0.15∼10% for the first-order approximation of the UAA method. The approximations can be easily extended to high orders, for which the errors are expected to be much smaller. Such obtained analytical solutions can be used to study cosmological perturbations in the framework of quantum cosmology as well as quasi-normal modes of black holes. Full article

(This article belongs to the Special Issue Recent Advances in Gravity: A Themed Issue in Honor of Prof. Jorge Pullin on His 60th Anniversary)

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15 pages, 804 KiB

Open AccessEditor’s ChoiceArticle

Closed Timelike Curves Induced by a Buchdahl-Inspired Vacuum Spacetime in

R^{2}

Gravity

by Hoang Ky Nguyen and Francisco S. N. Lobo

Universe 2023, 9(11), 467; https://doi.org/10.3390/universe9110467 - 30 Oct 2023

Cited by 6 | Viewed by 2139

Abstract

The recently obtained special Buchdahl-inspired metric Phys. Rev. D 107, 104008 (2023) describes asymptotically flat spacetimes in pure Ricci-squared gravity. The metric depends on a new (Buchdahl) parameter

\tilde{k}

of higher-derivative characteristic, and reduces to the Schwarzschild metric, for [...] Read more.

The recently obtained special Buchdahl-inspired metric Phys. Rev. D 107, 104008 (2023) describes asymptotically flat spacetimes in pure Ricci-squared gravity. The metric depends on a new (Buchdahl) parameter

\tilde{k}

of higher-derivative characteristic, and reduces to the Schwarzschild metric, for

\tilde{k} = 0

. For the case

\tilde{k} \in (- 1, 0)

, it was shown that it describes a traversable Morris–Thorne–Buchdahl (MTB) wormhole Eur. Phys. J. C 83, 626 (2023), where the weak energy condition is formally violated. In this paper, we briefly review the special Buchdahl-inspired metric, with focuses on the construction of the Kruskal–Szekeres (KS) diagram and the situation for a wormhole to emerge. Interestingly, the MTB wormhole structure appears to permit the formation of closed timelike curves (CTCs). More specifically, a CTC straddles the throat, comprising of two segments positioned in opposite quadrants of the KS diagram. The closed timelike loop thus passes through the wormhole throat twice, causing two reversals in the time direction experienced by the (timelike) traveller on the CTC. The key to constructing a CTC lies in identifying any given pair of antipodal points

(T, X)

and

(- T, - X)

on the wormhole throat in the KS diagram as corresponding to the same spacetime event. It is interesting to note that the Campanelli–Lousto metric in Brans–Dicke gravity is known to support two-way traversable wormholes, and the formation of the CTCs presented herein is equally applicable to the Campanelli–Lousto solution. Full article

(This article belongs to the Special Issue The Physics of Time Travel)

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12 pages, 290 KiB

Open AccessEditor’s ChoiceArticle

A New Approach to String Theory

by Albert Schwarz

Universe 2023, 9(10), 451; https://doi.org/10.3390/universe9100451 - 16 Oct 2023

Cited by 2 | Viewed by 1581

Abstract

In the present paper, we consider quantum theories obtained through the quantization of classical theories with first-class constraints assuming that these constraints form a Lie algebra. We show that in this case, one can construct physical quantities of a new type. We apply [...] Read more.

In the present paper, we consider quantum theories obtained through the quantization of classical theories with first-class constraints assuming that these constraints form a Lie algebra. We show that in this case, one can construct physical quantities of a new type. We apply this construction to string theory. We find that scattering amplitudes in critical bosonic closed string theory can be expressed in terms of physical quantities of the new type. Our techniques can also be applied to superstrings and heterotic strings. Full article

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

15 pages, 1618 KiB

Open AccessEditor’s ChoiceReview

Main Results from the ISSI International Team “Characterization of 67P Cometary Activity”

by Andrea Longobardo, Minjae Kim, Boris Pestoni, Mauro Ciarniello, Giovanna Rinaldi, Stavro Ivanovski, Fabrizio Dirri, Marco Fulle, Vincenzo Della Corte, Alessandra Rotundi and Martin Rubin

Universe 2023, 9(10), 446; https://doi.org/10.3390/universe9100446 - 11 Oct 2023

Cited by 1 | Viewed by 1693

Abstract

The ESA/Rosetta mission accompanied the Jupiter Family Comet 67P/Churyumov-Gerasimenko and provided a huge amount of data which are providing important results about cometary activity mechanisms. We summarize the results obtained within the ISSI International Team Characterization of 67P cometary activity, which studied [...] Read more.

The ESA/Rosetta mission accompanied the Jupiter Family Comet 67P/Churyumov-Gerasimenko and provided a huge amount of data which are providing important results about cometary activity mechanisms. We summarize the results obtained within the ISSI International Team Characterization of 67P cometary activity, which studied dust and gas ejection in different stages of the comet’s orbit, by means of a data fusion between instruments onboard the Rosetta orbiter, i.e., the OSIRIS camera, the VIRTIS imaging spectrometer, the GIADA dust detector, the MIDAS atomic force microscope, the COSIMA dust mass spectrometer, and the ROSINA gas mass spectrometer, supported by numerical models and experimental work. The team reconstructed the motion of the dust particles ejected from the comet surface, finding a correlation between dust ejection and solar illumination as well as larger occurrence of fluffy (pristine) particles in less processed and more pebble-rich terrains. Dust activity is larger in ice-rich terrains, indicating that water sublimation is the dominant activity process during the perihelion phase. The comparison of dust fluxes of different particle size suggests a link between dust morphology and ejection speed, generation of micrometric dust from fragmentation of millimetric dust, and homogeneity of physical properties of compact dust particles across the 67P surface. The comparison of fluxes of refractory and ice particles suggests the occurrence of a small amount of ice in fluffy particles, which is released when they are fragmented. A new model of cometary activity has been finally developed, according to which the comet nucleus includes Water-Ice-Enriched Blocks (WEBs), that, when exposed by CO₂ activity, are the main sources of water sublimation and dust ejection. Full article

(This article belongs to the Special Issue Comets: Tracers of Solar System Formation and Evolution—Celebrating the 20th Anniversary of Rosetta Mission Launch)

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22 pages, 595 KiB

Open AccessEditor’s ChoiceReview

The Statistics of Primordial Black Holes in a Radiation-Dominated Universe: Recent and New Results

by Cristiano Germani and Ravi K. Sheth

Universe 2023, 9(9), 421; https://doi.org/10.3390/universe9090421 - 16 Sep 2023

Cited by 16 | Viewed by 1604

Abstract

We review the nonlinear statistics of Primordial Black Holes that form from the collapse of over-densities in a radiation-dominated Universe. We focus on the scenario in which large over-densities are generated by rare and Gaussian curvature perturbations during inflation. As new results, we [...] Read more.

We review the nonlinear statistics of Primordial Black Holes that form from the collapse of over-densities in a radiation-dominated Universe. We focus on the scenario in which large over-densities are generated by rare and Gaussian curvature perturbations during inflation. As new results, we show that the mass spectrum follows a power law determined by the critical exponent of the self-similar collapse up to a power spectrum dependent cutoff, and that the abundance related to very narrow power spectra is exponentially suppressed. Related to this, we discuss and explicitly show that both the Press–Schechter approximation and the statistics of mean profiles lead to wrong conclusions for the abundance and mass spectrum. Finally, we clarify that the transfer function in the statistics of initial conditions for Primordial Black Holes formation (the abundance) does not play a significant role. Full article

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

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56 pages, 3425 KiB

Open AccessFeature PaperEditor’s ChoiceOpinion

Seven Hints That Early-Time New Physics Alone Is Not Sufficient to Solve the Hubble Tension

by Sunny Vagnozzi

Universe 2023, 9(9), 393; https://doi.org/10.3390/universe9090393 - 30 Aug 2023

Cited by 169 | Viewed by 10360

Abstract

The Hubble tension has now grown to a level of significance which can no longer be ignored and calls for a solution which, despite a huge number of attempts, has so far eluded us. Significant efforts in the literature have focused on early-time [...] Read more.

The Hubble tension has now grown to a level of significance which can no longer be ignored and calls for a solution which, despite a huge number of attempts, has so far eluded us. Significant efforts in the literature have focused on early-time modifications of

?

CDM, introducing new physics operating prior to recombination and reducing the sound horizon. In this opinion paper I argue that early-time new physics alone will always fall short of fully solving the Hubble tension. I base my arguments on seven independent hints, related to (1) the ages of the oldest astrophysical objects, (2) considerations on the sound horizon-Hubble constant degeneracy directions in cosmological data, (3) the important role of cosmic chronometers, (4) a number of “descending trends” observed in a wide variety of low-redshift datasets, (5) the early integrated Sachs-Wolfe effect as an early-time consistency test of

?

CDM, (6) early-Universe physics insensitive and uncalibrated cosmic standard constraints on the matter density, and finally (7) equality wavenumber-based constraints on the Hubble constant from galaxy power spectrum measurements. I argue that a promising way forward should ultimately involve a combination of early- and late-time (but non-local—in a cosmological sense, i.e., at high redshift) new physics, as well as local (i.e., at

z ? 0

) new physics, and I conclude by providing reflections with regards to potentially interesting models which may also help with the

S_{8}

tension. Full article

(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)

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13 pages, 396 KiB

Open AccessEditor’s ChoiceArticle

Alleviating the H₀ Tension in Scalar–Tensor and Bi-Scalar–Tensor Theories

by Maria Petronikolou and Emmanuel N. Saridakis

Universe 2023, 9(9), 397; https://doi.org/10.3390/universe9090397 - 30 Aug 2023

Cited by 13 | Viewed by 1621

Abstract

Herein, we investigate scalar–tensor and bi-scalar–tensor modified theories of gravity that can alleviate the

H_{0}

tension. In the first class of theories, we show that by choosing particular models with a shift-symmetric friction term we are able to alleviate the tension by [...] Read more.

Herein, we investigate scalar–tensor and bi-scalar–tensor modified theories of gravity that can alleviate the

H_{0}

tension. In the first class of theories, we show that by choosing particular models with a shift-symmetric friction term we are able to alleviate the tension by obtaining a smaller effective Newton’s constant at intermediate times, a feature that cannot be easily obtained in modified gravity. In the second class of theories, which involve two extra propagating degrees of freedom, we show that the

H_{0}

tension can be alleviated, and the mechanism behind this is the phantom behavior of the effective dark-energy equation-of-state parameter. Hence, scalar–tensor and bi-scalar–tensor theories have the ability to alleviate the

H_{0}

tension with both known sufficient late-time mechanisms. Full article

(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)

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20 pages, 559 KiB

Open AccessEditor’s ChoiceArticle

Black Holes Hint towards De Sitter Matrix Theory

by Leonard Susskind

Universe 2023, 9(8), 368; https://doi.org/10.3390/universe9080368 - 11 Aug 2023

Cited by 72 | Viewed by 2449

Abstract

De Sitter black holes and other non-perturbative configurations can be used to probe the holographic degrees of freedom of de Sitter space. For small black holes, evidence was first provided in the seminal work of Banks, Fiol, and Morrise and follow-ups by Banks [...] Read more.

De Sitter black holes and other non-perturbative configurations can be used to probe the holographic degrees of freedom of de Sitter space. For small black holes, evidence was first provided in the seminal work of Banks, Fiol, and Morrise and follow-ups by Banks and Fischler, showing that dS is described by a form of matrix theory. For large black holes, the evidence provided here is new: Gravitational calculations and matrix theory calculations of the rates of exponentially rare fluctuations match one another in surprising detail. The occurrences of Nariai geometry and the “inside-out” transition are particularly interesting examples, which I explain in this paper. Full article

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12 pages, 8396 KiB

Open AccessEditor’s ChoiceArticle

Effects of Coupling Constants on Chaos of Charged Particles in the Einstein–Æther Theory

by Caiyu Liu and Xin Wu

Universe 2023, 9(8), 365; https://doi.org/10.3390/universe9080365 - 7 Aug 2023

Cited by 5 | Viewed by 1282

Abstract

There are two free coupling parameters

c_{13}

and

c_{14}

in the Einstein–Æther metric describing a non-rotating black hole. This metric is the Reissner–Nordström black hole solution when

0 \leq 2 c_{13} < c_{14} < 2

, but it is [...] Read more.

There are two free coupling parameters

c_{13}

and

c_{14}

in the Einstein–Æther metric describing a non-rotating black hole. This metric is the Reissner–Nordström black hole solution when

0 \leq 2 c_{13} < c_{14} < 2

, but it is not for

0 \leq c_{14} < 2 c_{13} < 2

. When the black hole is immersed in an external asymptotically uniform magnetic field, the Hamiltonian system describing the motion of charged particles around the black hole is not integrable; however, the Hamiltonian allows for the construction of explicit symplectic integrators. The proposed fourth-order explicit symplectic scheme is used to investigate the dynamics of charged particles because it exhibits excellent long-term performance in conserving the Hamiltonian. No universal rule can be given to the dependence of regular and chaotic dynamics on varying one or two parameters

c_{13}

and

c_{14}

in the two cases of

0 \leq 2 c_{13} < c_{14} < 2

and

0 \leq c_{14} < 2 c_{13} < 2

. The distributions of order and chaos in the binary parameter space

(c_{13}, c_{14})

rely on different combinations of the other parameters and the initial conditions. Full article

(This article belongs to the Special Issue Testing General Relativity from the Solar System, Black Holes to Cosmology)

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14 pages, 2098 KiB

Open AccessEditor’s ChoiceArticle

The Lense–Thirring Effect on the Galilean Moons of Jupiter

by Lorenzo Iorio

Universe 2023, 9(7), 304; https://doi.org/10.3390/universe9070304 - 23 Jun 2023

Cited by 6 | Viewed by 1396

Abstract

The perspectives of detecting the general relativistic gravitomagnetic Lense–Thirring effect on the orbits of the Galilean moons of Jupiter induced by the angular momentum

S

of the latter are preliminarily investigated. Numerical integrations over one century show that the expected gravitomagnetic signatures of [...] Read more.

The perspectives of detecting the general relativistic gravitomagnetic Lense–Thirring effect on the orbits of the Galilean moons of Jupiter induced by the angular momentum

S

of the latter are preliminarily investigated. Numerical integrations over one century show that the expected gravitomagnetic signatures of the directly observable right ascension

α

and declination

δ

of the satellites are as large as tens of arcseconds for Io, while for Callisto they drop to the ≃

0.2 arcseconds

level. Major competing effects due to the mismodeling in the zonal multipoles

J_{ℓ}, ℓ = 2, 3, 4, \dots

of the Jovian non-spherically symmetric gravity field and in the Jupiter’s spin axis

\hat{k}

should have a limited impact, especially in view of the future improvements in determining such parameters expected after the completion of the ongoing Juno mission in the next few years. On the other hand, the masses of the satellites, responsible of their mutual

N

-body perturbations, should be known better than now. Such a task should be accomplished with the future JUICE and Clipper missions to the Jovian system. Present-day accuracy in knowing the orbits of the Jovian Galilean satellites is of the order of 10 milliarcseconds, to be likely further improved thanks to the ongoing re-reduction of old photographic plates. This suggests that, in the next future, the Lense–Thirring effect in the main Jovian system of moons might be detectable with dedicated data reductions in which the gravitomagnetic field is explicitly modeled and solved-for. Full article

(This article belongs to the Section Gravitation)

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69 pages, 5960 KiB

Open AccessEditor’s ChoiceReview

Cosmological Probes of Structure Growth and Tests of Gravity

by Jiamin Hou, Julian Bautista, Maria Berti, Carolina Cuesta-Lazaro, César Hernández-Aguayo, Tilman Tröster and Jinglan Zheng

Universe 2023, 9(7), 302; https://doi.org/10.3390/universe9070302 - 22 Jun 2023

Cited by 10 | Viewed by 2230

Abstract

The current standard cosmological model is constructed within the framework of general relativity with a cosmological constant

Λ

, which is often associated with dark energy, and phenomenologically explains the accelerated cosmic expansion. Understanding the nature of dark energy is one of the [...] Read more.

The current standard cosmological model is constructed within the framework of general relativity with a cosmological constant

Λ

, which is often associated with dark energy, and phenomenologically explains the accelerated cosmic expansion. Understanding the nature of dark energy is one of the most appealing questions in achieving a self-consistent physical model at cosmological scales. Modification of general relativity could potentially provide a more natural and physical solution to the accelerated expansion. The growth of the cosmic structure is sensitive in constraining gravity models. In this paper, we aim to provide a concise introductory review of modified gravity models from an observational point of view. We will discuss various mainstream cosmological observables, and their potential advantages and limitations as probes of gravity models. Full article

(This article belongs to the Special Issue Cosmological Constant)

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41 pages, 519 KiB

Open AccessEditor’s ChoiceReview

The Warm Inflation Story

by Arjun Berera

Universe 2023, 9(6), 272; https://doi.org/10.3390/universe9060272 - 6 Jun 2023

Cited by 25 | Viewed by 2011

Abstract

Warm inflation has normalized two ideas in cosmology, that in the early universe the initial primordial density perturbations generally could be of classical rather than quantum origin and that during inflation, particle production from interactions amongst quantum field, and its backreaction effects, can [...] Read more.

Warm inflation has normalized two ideas in cosmology, that in the early universe the initial primordial density perturbations generally could be of classical rather than quantum origin and that during inflation, particle production from interactions amongst quantum field, and its backreaction effects, can occur concurrent with inflationary expansion. When we first introduced these ideas, both were met with resistance, but today they are widely accepted as possibilities with many models and applications based on them, which is an indication of the widespread influence of warm inflation. Open quantum field theory, which has been utilized in studies of warm inflation, is by now a relevant subject in cosmology, in part due to this early work. In this review I first discuss the basic warm inflation dynamics. I then outline how to compute warm inflation dynamics from first-principles quantum field theory (QFT) and in particular how a dissipative term arises. Warm inflation models can have an inflaton mass bigger than the Hubble scale and the inflaton field excursion can remain sub-Planckian, thus overcoming the most prohibitive problems of inflation model building. I discuss the early period of my work in developing warm inflation that helped me arrive at these important features of its dynamics. Inflationary cosmology today is immersed in hypothetical models, which by now are acting as a diversion from reaching any endgame in this field. I discuss better ways to approach model selection and give necessary requirements for a well constrained and predictive inflation model. A few warm inflation models are pointed out that could be developed to this extent. I discuss how, at this stage, more progress would be made in this subject by taking a broader view on the possible early universe solutions that include not just inflation but the diverse range of options. Full article

(This article belongs to the Special Issue Warm Inflation)

40 pages, 942 KiB

Open AccessEditor’s ChoiceArticle

Running Vacuum in the Universe: Phenomenological Status in Light of the Latest Observations, and Its Impact on the σ₈ and H₀ Tensions

by Joan Solà Peracaula, Adrià Gómez-Valent, Javier de Cruz Pérez and Cristian Moreno-Pulido

Universe 2023, 9(6), 262; https://doi.org/10.3390/universe9060262 - 30 May 2023

Cited by 43 | Viewed by 2403

Abstract

A substantial body of phenomenological and theoretical work over the last few years strengthens the possibility that the vacuum energy density (VED) of the universe is dynamical, and in particular that it adopts the ‘running vacuum model’ (RVM) form, in which the VED [...] Read more.

A substantial body of phenomenological and theoretical work over the last few years strengthens the possibility that the vacuum energy density (VED) of the universe is dynamical, and in particular that it adopts the ‘running vacuum model’ (RVM) form, in which the VED evolves mildly as

δ ρ_{vac} (H) \sim ν_{eff} m_{Pl}^{2} O (H^{2})

, where H is the Hubble rate and

ν_{eff}

is a (small) free parameter. This dynamical scenario is grounded on recent studies of quantum field theory (QFT) in curved spacetime and also on string theory. It turns out that what we call the ‘cosmological constant’,

Λ

, is no longer a rigid parameter but the nearly sustained value of

8 π G (H) ρ_{vac} (H)

around any given epoch

H (t)

, where

G (H)

is the gravitational coupling, which can also be very mildly running (logarithmically). Of particular interest is the possibility suggested in past works that such a running may help to cure the cosmological tensions afflicting the

Λ

CDM. In the current study, we reanalyze the RVM in full and we find it becomes further buttressed. Using modern cosmological data, namely a compilation of the latest SNIa+BAO+

H (z)

+LSS+CMB observations, we probe to what extent the RVM provides a quality fit better than the concordance

Λ

CDM model, with particular emphasis on its impact on the

σ_{8}

and

H_{0}

tensions. We utilize the Einstein–Boltzmann system solver CLASS and the Monte Carlo sampler MontePython for the statistical analysis, as well as the statistical DIC criterion to compare the running vacuum against the rigid vacuum (

ν_{eff} = 0

). On fundamental grounds,

ν_{eff}

receives contributions from all the quantized matter fields in FLRW spacetime. We show that with a tiny amount of vacuum dynamics (

ν_{eff} ≪ 1

) the global fit can improve significantly with respect to the

Λ

CDM and the mentioned tensions may subside to inconspicuous levels. Full article

(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)

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20 pages, 2481 KiB

Open AccessEditor’s ChoiceArticle

Cross Sections for Coherent Elastic and Inelastic Neutrino-Nucleus Scattering

by Nils Van Dessel, Vishvas Pandey, Heather Ray and Natalie Jachowicz

Universe 2023, 9(5), 207; https://doi.org/10.3390/universe9050207 - 25 Apr 2023

Cited by 25 | Viewed by 2387

Abstract

The prospects of extracting new physics signals in coherent elastic neutrino–nucleus scattering (CE

ν

NS) processes are limited by the precision with which the underlying nuclear structure physics, embedded in the weak nuclear form factor, is known. We present calculations of charge and [...] Read more.

The prospects of extracting new physics signals in coherent elastic neutrino–nucleus scattering (CE

ν

NS) processes are limited by the precision with which the underlying nuclear structure physics, embedded in the weak nuclear form factor, is known. We present calculations of charge and weak nuclear form factors and CE

ν

NS cross sections on

^{12}

C,

^{16}

O,

^{40}

Ar,

^{56}

Fe and

^{208}

Pb nuclei. We obtain the proton and neutron densities, and charge and weak form factors by solving Hartree–Fock (HF) equations with a Skyrme (SkE2) nuclear potential. We validate our approach by comparing

^{208}

Pb and

^{40}

Ar charge form factor predictions with available elastic electron scattering data. Since CE

ν

NS experiments at stopped-pion sources are also well suited to measure inelastic charged–current and neutral–current neutrino–nucleus cross sections, we also present calculations for these processes, incorporating a continuum Random Phase Approximation (CRPA) description on top of the HF–SkE2 picture of the nucleus. Providing both coherent as well as inelastic cross sections in a consistent framework, we aim at obtaining a reliable and detailed comparison of the strength of these processes in the energy region below 100 MeV. Furthermore, we attempt to gauge the level of theoretical uncertainty pertaining to the description of the

^{40}

Ar form factor and CE

ν

NS cross sections by comparing relative differences between recent microscopic nuclear theory and widely-used phenomenological form factor predictions. Future precision measurements of CE

ν

NS will potentially help in constraining these nuclear structure details that will in turn improve prospects of extracting new physics. Full article

(This article belongs to the Special Issue Many Body Theory)

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78 pages, 2968 KiB

Open AccessEditor’s ChoiceReview

Inflation and Primordial Black Holes

by Ogan Özsoy and Gianmassimo Tasinato

Universe 2023, 9(5), 203; https://doi.org/10.3390/universe9050203 - 24 Apr 2023

Cited by 119 | Viewed by 4705

Abstract

We review conceptual aspects of inflationary scenarios able to produce primordial black holes by amplifying the size of curvature fluctuations to the level required to trigger black hole formation. We identify general mechanisms to do so, both for single- and multiple-field inflation. In [...] Read more.

We review conceptual aspects of inflationary scenarios able to produce primordial black holes by amplifying the size of curvature fluctuations to the level required to trigger black hole formation. We identify general mechanisms to do so, both for single- and multiple-field inflation. In single-field inflation, the spectrum of curvature fluctuations is enhanced by pronounced gradients of background quantities controlling the cosmological dynamics, which can induce brief phases of non-slow-roll inflationary evolution. In multiple-field inflation, the amplification occurs through appropriate couplings with additional sectors characterized by tachyonic instabilities that enhance the size of their fluctuations. As representative examples, we consider axion inflation and two-field models of inflation with rapid turns in field space. We develop our discussion in a pedagogical manner by including some of the most relevant calculations and by guiding the reader through the existing theoretical literature, emphasizing general themes common to several models. Full article

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

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19 pages, 561 KiB

Open AccessEditor’s ChoiceReview

Elimination of QCD Renormalization Scale and Scheme Ambiguities

by Sheng-Quan Wang, Stanley J. Brodsky, Xing-Gang Wu, Jian-Ming Shen and Leonardo Di Giustino

Universe 2023, 9(4), 193; https://doi.org/10.3390/universe9040193 - 17 Apr 2023

Cited by 4 | Viewed by 2186

Abstract

The setting of the renormalization scale (

μ_{r}

) in the perturbative QCD (pQCD) is one of the crucial problems for achieving precise fixed-order pQCD predictions. The conventional prescription is to take its value as the typical momentum transfer Q in a [...] Read more.

The setting of the renormalization scale (

μ_{r}

) in the perturbative QCD (pQCD) is one of the crucial problems for achieving precise fixed-order pQCD predictions. The conventional prescription is to take its value as the typical momentum transfer Q in a given process, and theoretical uncertainties are then evaluated by varying it over an arbitrary range. The conventional scale-setting procedure introduces arbitrary scheme-and-scale ambiguities in fixed-order pQCD predictions. The principle of maximum conformality (PMC) provides a systematic way to eliminate the renormalization scheme-and-scale ambiguities. The PMC method has rigorous theoretical foundations; it satisfies the renormalization group invariance (RGI) and all of the self-consistency conditions derived from the renormalization group. The PMC has now been successfully applied to many physical processes. In this paper, we summarize recent PMC applications, including event shape observables and heavy quark pair production near the threshold region in

e^{+} e^{-}

annihilation and top-quark decay at hadronic colliders. In addition, estimating the contributions related to the uncalculated higher-order terms is also summarized. These results show that the major theoretical uncertainties caused by different choices of

μ_{r}

are eliminated, and the improved pQCD predictions are thus obtained, demonstrating the generality and applicability of the PMC. Full article

(This article belongs to the Special Issue The Quantum Chromodynamics: 50th Anniversary of the Discovery)

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30 pages, 493 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Dark Energy Is the Cosmological Quantum Vacuum Energy of Light Particles—The Axion and the Lightest Neutrino

by Héctor J. de Vega and Norma G. Sanchez

Universe 2023, 9(4), 167; https://doi.org/10.3390/universe9040167 - 30 Mar 2023

Cited by 3 | Viewed by 2412

Abstract

We uncover the general mechanism and the nature of today’s dark energy (DE). This is only based on well-known quantum physics and cosmology. We show that the observed DE today originates from the cosmological quantum vacuum of light particles, which provides a continuous [...] Read more.

We uncover the general mechanism and the nature of today’s dark energy (DE). This is only based on well-known quantum physics and cosmology. We show that the observed DE today originates from the cosmological quantum vacuum of light particles, which provides a continuous energy distribution able to reproduce the data. Bosons give positive contributions to the DE, while fermions yield negative contributions. As usual in field theory, ultraviolet divergences are subtracted from the physical quantities. The subtractions respect the symmetries of the theory, and we normalize the physical quantities to be zero for the Minkowski vacuum. The resulting finite contributions to the energy density and the pressure from the quantum vacuum grow as

log a (t)

, where

a (t)

is the scale factor, while the particle contributions dilute as

1 / a^{3} (t)

, as it must be for massive particles. We find the explicit dark energy equation of state of today to be

P = w (z) H

: it turns to be slightly

w (z) < - 1

with

w (z)

asymptotically reaching the value

- 1

from below. A scalar particle can produce the observed dark energy through its quantum cosmological vacuum provided that (i) its mass is of the order of

10^{- 3}

eV = 1 meV, (ii) it is very weakly coupled, and (iii) it is stable on the time scale of the age of the universe. The axion vacuum thus appears as a natural candidate. The neutrino vacuum (especially the lightest mass eigenstate) can give negative contributions to the dark energy. We find that

w (z = 0)

is slightly below

- 1

by an amount ranging from

(- 1.5 \times 10^{- 3})

to

(- 8 \times 10^{- 3})

and we predict the axion mass to be in the range between 4 and 5

meV

. We find that the universe will expand in the future faster than the de Sitter universe as an exponential in the square of the cosmic time. Dark energy today arises from the quantum vacuum of light particles in FRW cosmological space-time in an analogous way to the Casimir vacuum effect of quantum fields in Minkowski space-time with non-trivial boundary conditions. Full article

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

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65 pages, 3005 KiB

Open AccessEditor’s ChoiceReview

Quiescent and Active Galactic Nuclei as Factories of Merging Compact Objects in the Era of Gravitational Wave Astronomy

by Manuel Arca Sedda, Smadar Naoz and Bence Kocsis

Universe 2023, 9(3), 138; https://doi.org/10.3390/universe9030138 - 6 Mar 2023

Cited by 35 | Viewed by 2643

Abstract

Galactic nuclei harbouring a central supermassive black hole (SMBH), possibly surrounded by a dense nuclear cluster (NC), represent extreme environments that house a complex interplay of many physical processes that uniquely affect stellar formation, evolution, and dynamics. The discovery of gravitational waves (GWs) [...] Read more.

Galactic nuclei harbouring a central supermassive black hole (SMBH), possibly surrounded by a dense nuclear cluster (NC), represent extreme environments that house a complex interplay of many physical processes that uniquely affect stellar formation, evolution, and dynamics. The discovery of gravitational waves (GWs) emitted by merging black holes (BHs) and neutron stars (NSs), funnelled a huge amount of work focused on understanding how compact object binaries (COBs) can pair up and merge together. Here, we review from a theoretical standpoint how different mechanisms concur with the formation, evolution, and merger of COBs around quiescent SMBHs and active galactic nuclei (AGNs), summarising the main predictions for current and future (GW) detections and outlining the possible features that can clearly mark a galactic nuclei origin. Full article

(This article belongs to the Special Issue Binary Evolution in Galactic Nuclei)

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20 pages, 390 KiB

Open AccessEditor’s ChoiceArticle

An Analytical Approach to Bayesian Evidence Computation

by Juan García-Bellido

Universe 2023, 9(3), 118; https://doi.org/10.3390/universe9030118 - 24 Feb 2023

Cited by 1 | Viewed by 1355

Abstract

Bayesian evidence is a key tool in model selection, allowing a comparison of models with different numbers of parameters. Its use in the analysis of cosmological models has been limited by difficulties in calculating it, with current numerical algorithms requiring supercomputers. In this [...] Read more.

Bayesian evidence is a key tool in model selection, allowing a comparison of models with different numbers of parameters. Its use in the analysis of cosmological models has been limited by difficulties in calculating it, with current numerical algorithms requiring supercomputers. In this paper we give exact formulae for the Bayesian evidence in the case of Gaussian likelihoods with arbitrary correlations and top-hat priors, and approximate formulae for the case of likelihood distributions with leading non-Gaussianities (skewness and kurtosis). We apply these formulae to cosmological models with and without isocurvature components, and compare with results we previously obtained using numerical thermodynamic integration. We find that the results are of lower precision than the thermodynamic integration, while still being good enough to be useful. Full article

(This article belongs to the Section Cosmology)

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35 pages, 1365 KiB

Open AccessEditor’s ChoiceReview

Hubble Tension: The Evidence of New Physics

by Jian-Ping Hu and Fa-Yin Wang

Universe 2023, 9(2), 94; https://doi.org/10.3390/universe9020094 - 10 Feb 2023

Cited by 136 | Viewed by 6243

Abstract

The

Λ

CDM model provides a good fit to most astronomical observations but harbors large areas of phenomenology and ignorance. With the improvements in the precision and number of observations, discrepancies between key cosmological parameters of this model have emerged. Among them, the [...] Read more.

The

Λ

CDM model provides a good fit to most astronomical observations but harbors large areas of phenomenology and ignorance. With the improvements in the precision and number of observations, discrepancies between key cosmological parameters of this model have emerged. Among them, the most notable tension is the 4

σ

to 6

σ

deviation between the Hubble constant (

H_{0}

) estimations measured by the local distance ladder and the cosmic microwave background (CMB) measurement. In this review, we revisit the

H_{0}

tension based on the latest research and sort out evidence from solutions to this tension that might imply new physics beyond the

Λ

CDM model. The evidence leans more towards modifying the late-time universe. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

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15 pages, 361 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Gravitational Condensate Stars: An Alternative to Black Holes

by Pawel O. Mazur and Emil Mottola

Universe 2023, 9(2), 88; https://doi.org/10.3390/universe9020088 - 7 Feb 2023

Cited by 539 | Viewed by 5899

Abstract

A new final endpoint of complete gravitational collapse is proposed. By extending the concept of Bose–Einstein condensation to gravitational systems, a static, spherically symmetric solution to Einstein’s equations is obtained, characterized by an interior de Sitter region of

p = - ρ

gravitational [...] Read more.

A new final endpoint of complete gravitational collapse is proposed. By extending the concept of Bose–Einstein condensation to gravitational systems, a static, spherically symmetric solution to Einstein’s equations is obtained, characterized by an interior de Sitter region of

p = - ρ

gravitational vacuum condensate and an exterior Schwarzschild geometry of arbitrary total mass M. These are separated by a phase boundary with a small but finite thickness ℓ, replacing both the Schwarzschild and de Sitter classical horizons. The resulting collapsed cold, compact object has no singularities, no event horizons, and a globally defined Killing time. Its entropy is maximized under small fluctuations and is given by the standard hydrodynamic entropy of the thin shell, which is of order

k_{B} ℓ M c / ℏ

, instead of the Bekenstein–Hawking entropy,

S_{B H} = 4 π k_{B} G M^{2} / ℏ c

. Unlike BHs, a collapsed star of this kind is consistent with quantum theory, thermodynamically stable, and suffers from no information paradox. Full article

(This article belongs to the Special Issue Dark Matter and Dark Energy: Particle Physics, Cosmology, and Experimental Searches)

20 pages, 535 KiB

Open AccessEditor’s ChoiceArticle

Addressing Cosmological Tensions by Non-Local Gravity

by Filippo Bouché, Salvatore Capozziello and Vincenzo Salzano

Universe 2023, 9(1), 27; https://doi.org/10.3390/universe9010027 - 30 Dec 2022

Cited by 12 | Viewed by 2266

Abstract

Alternative cosmological models have been under deep scrutiny in recent years, aiming to address the main shortcomings of the

Λ

CDM model. Moreover, as the accuracy of cosmological surveys improved, new tensions have risen between the model-dependent analysis of the Cosmic Microwave Background [...] Read more.

Alternative cosmological models have been under deep scrutiny in recent years, aiming to address the main shortcomings of the

Λ

CDM model. Moreover, as the accuracy of cosmological surveys improved, new tensions have risen between the model-dependent analysis of the Cosmic Microwave Background and lower redshift probes. Within this framework, we review two quantum-inspired non-locally extended theories of gravity, whose main cosmological feature is a geometrically driven accelerated expansion. The models are especially investigated in light of the Hubble and growth tension, and promising features emerge for the Deser–Woodard one. On the one hand, the cosmological analysis of the phenomenological formulation of the model shows a lowered growth of structures but an equivalent background with respect to

Λ

CDM. On the other hand, the study of the lensing features at the galaxy cluster scale of a new formulation of non-local cosmology, based on Noether symmetries, makes room for the possibility of alleviating both the

H_{0}

and

σ_{8}

tension. However, the urgent need for a screening mechanism arises for this non-local theory of gravity. Full article

(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)

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30 pages, 3503 KiB

Open AccessEditor’s ChoiceReview

Dark Matter Searches with Top Quarks

by J. Katharina Behr and Alexander Grohsjean

Universe 2023, 9(1), 16; https://doi.org/10.3390/universe9010016 - 27 Dec 2022

Cited by 4 | Viewed by 2273

Abstract

Collider signatures with top quarks provide sensitive probes of dark matter (DM) production at the Large Hadron Collider (LHC). In this article, we review the results of DM searches in final states with top quarks conducted by the ATLAS and CMS Collaborations at [...] Read more.

Collider signatures with top quarks provide sensitive probes of dark matter (DM) production at the Large Hadron Collider (LHC). In this article, we review the results of DM searches in final states with top quarks conducted by the ATLAS and CMS Collaborations at the LHC, including the most recent results on the full LHC Run 2 dataset. We highlight the complementarity of DM searches in final states with top quarks with searches in other final states in the framework of various simplified models of DM. A reinterpretation of a DM search with top quarks in the context of an effective field theory description of scalar dark energy is also discussed. Finally, we give an outlook on the potential of DM searches with top quarks in LHC Run 3, at the high-luminosity LHC, and possible future colliders. In this context, we highlight new benchmark models that could be probed by existing and future searches as well as those that predict still-uncovered signatures of anomalous top-quark production and decays at the LHC. Full article

(This article belongs to the Special Issue Top Quark at the New Physics Frontier)

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15 pages, 514 KiB

Open AccessEditor’s ChoiceReview

Four-top quark physics at the LHC

by Freya Blekman, Fréderic Déliot, Valentina Dutta and Emanuele Usai

Universe 2022, 8(12), 638; https://doi.org/10.3390/universe8120638 - 30 Nov 2022

Cited by 9 | Viewed by 1929

Abstract

The production of four top quarks presents a rare process in the Standard Model that provides unique opportunities and sensitivity to Standard Model observables including potential enhancement of many popular new physics extensions. This article summarises the latest experimental measurements of the four-top [...] Read more.

The production of four top quarks presents a rare process in the Standard Model that provides unique opportunities and sensitivity to Standard Model observables including potential enhancement of many popular new physics extensions. This article summarises the latest experimental measurements of the four-top quark production cross section at the LHC. An overview is provided detailing interpretations of the experimental results regarding the top quark Yukawa coupling in addition to the limits on physics beyond the Standard Model. Further, prospects for future measurements and opportunities offered by this challenging final state are given herein. Full article

(This article belongs to the Special Issue Top Quark at the New Physics Frontier)

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