Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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22 pages, 617 KiB  
Review
Neutrino Flavor Model Building and the Origins of Flavor and CP 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 12 | Viewed by 1479
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 CP violation from group theory due to the intimate connection between physical CP 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  
Review
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 4 | Viewed by 4904
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  
Article
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 1354
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
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15 pages, 804 KiB  
Article
Closed Timelike Curves Induced by a Buchdahl-Inspired Vacuum Spacetime in R2 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 3 | Viewed by 1531
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 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 k˜ of higher-derivative characteristic, and reduces to the Schwarzschild metric, for k˜=0. For the case k˜(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  
Article
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 1105
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  
Review
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 1242
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
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22 pages, 595 KiB  
Review
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 9 | Viewed by 1084
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  
Opinion
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 81 | Viewed by 8289
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 ΛCDM, 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 ΛCDM, (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 z0) new physics, and I conclude by providing reflections with regards to potentially interesting models which may also help with the S8 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  
Article
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 5 | Viewed by 1100
Abstract
Herein, we investigate scalar–tensor and bi-scalar–tensor modified theories of gravity that can alleviate the H0 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 H0 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 H0 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 H0 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  
Article
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 65 | Viewed by 1745
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  
Article
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 3 | Viewed by 840
Abstract
There are two free coupling parameters c13 and c14 in the Einstein–Æther metric describing a non-rotating black hole. This metric is the Reissner–Nordström black hole solution when 02c13<c14<2, but it is [...] Read more.
There are two free coupling parameters c13 and c14 in the Einstein–Æther metric describing a non-rotating black hole. This metric is the Reissner–Nordström black hole solution when 02c13<c14<2, but it is not for 0c14<2c13<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 c13 and c14 in the two cases of 02c13<c14<2 and 0c14<2c13<2. The distributions of order and chaos in the binary parameter space (c13,c14) rely on different combinations of the other parameters and the initial conditions. Full article
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14 pages, 2098 KiB  
Article
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 5 | Viewed by 832
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 ≃0.2arcseconds level. Major competing effects due to the mismodeling in the zonal multipoles J,=2,3,4, of the Jovian non-spherically symmetric gravity field and in the Jupiter’s spin axis 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  
Review
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 6 | Viewed by 1341
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  
Review
The Warm Inflation Story
by Arjun Berera
Universe 2023, 9(6), 272; https://doi.org/10.3390/universe9060272 - 6 Jun 2023
Cited by 13 | Viewed by 1282
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  
Article
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 24 | Viewed by 1591
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 δρvac(H)νeffmPl2OH2, where H is the Hubble rate and νeff 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 8πG(H)ρvac(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 ΛCDM. 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 ΛCDM model, with particular emphasis on its impact on the σ8 and H0 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 (νeff=0). On fundamental grounds, νeff receives contributions from all the quantized matter fields in FLRW spacetime. We show that with a tiny amount of vacuum dynamics (νeff1) the global fit can improve significantly with respect to the ΛCDM 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  
Article
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 2 | Viewed by 1585
Abstract
The prospects of extracting new physics signals in coherent elastic neutrino–nucleus scattering (CEνNS) 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 (CEνNS) 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 CEνNS cross sections on 12C, 16O, 40Ar, 56Fe and 208Pb 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 208Pb and 40Ar charge form factor predictions with available elastic electron scattering data. Since CEνNS 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 40Ar form factor and CEνNS cross sections by comparing relative differences between recent microscopic nuclear theory and widely-used phenomenological form factor predictions. Future precision measurements of CEνNS 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  
Review
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 73 | Viewed by 3190
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  
Review
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 1685
Abstract
The setting of the renormalization scale (μr) 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 (μr) 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 μr 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  
Article
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 2 | Viewed by 1757
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 loga(t), where a(t) is the scale factor, while the particle contributions dilute as 1/a3(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 103 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 (1.5×103) to (8×103) 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  
Review
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 15 | Viewed by 1544
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  
Article
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 937
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  
Review
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 69 | Viewed by 3978
Abstract
The ΛCDM 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 ΛCDM 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 4σ to 6σ deviation between the Hubble constant (H0) estimations measured by the local distance ladder and the cosmic microwave background (CMB) measurement. In this review, we revisit the H0 tension based on the latest research and sort out evidence from solutions to this tension that might imply new physics beyond the ΛCDM 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  
Article
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 56 | Viewed by 3042
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 , 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 kBMc/, instead of the Bekenstein–Hawking entropy, SBH=4πkBGM2/c. Unlike BHs, a collapsed star of this kind is consistent with quantum theory, thermodynamically stable, and suffers from no information paradox. Full article
20 pages, 535 KiB  
Article
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 8 | Viewed by 1669
Abstract
Alternative cosmological models have been under deep scrutiny in recent years, aiming to address the main shortcomings of the ΛCDM 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 ΛCDM 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 ΛCDM. 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 H0 and σ8 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  
Review
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 3 | Viewed by 1540
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  
Review
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 1313
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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18 pages, 6617 KiB  
Article
Inhomogeneous Jets from Neutron Star Mergers: One Jet to Rule Them All
by Gavin P. Lamb, Lorenzo Nativi, Stephan Rosswog, D. Alexander Kann, Andrew Levan, Christoffer Lundman and Nial Tanvir
Universe 2022, 8(12), 612; https://doi.org/10.3390/universe8120612 - 23 Nov 2022
Cited by 13 | Viewed by 1272
Abstract
Using the resultant profiles from 3D hydrodynamic simulations of relativistic jets interacting with neutron star merger wind ejecta, we show how the inhomogeneity of energy and velocity across the jet surface profile can alter the observed afterglow lightcurve. We find that the peak [...] Read more.
Using the resultant profiles from 3D hydrodynamic simulations of relativistic jets interacting with neutron star merger wind ejecta, we show how the inhomogeneity of energy and velocity across the jet surface profile can alter the observed afterglow lightcurve. We find that the peak afterglow flux depends sensitively on the observer’s line-of-sight, not only via the jet inclination but also through the jet rotation: for an observer viewing the afterglow within the GRB-bright jet core, we find a peak flux variability on the order <0.5 dex through rotational orientation and <1.3 dex for the polar inclination. An observed afterglow’s peak flux can be used to infer the jet kinetic energy, and where a top-hat jet is assumed, we find the range of inferred jet kinetic energies for our various model afterglow lightcurves (with fixed model parameters), covers ∼1/3 of the observed short GRB population. Additionally, we present an analytic jet structure function that includes physically motivated parameter uncertainties due to variability through the rotation of the source. An approximation for the change in collimation due to the merger ejecta mass is included and we show that by considering the observed range of merger ejecta masses from short GRB kilonova candidates, a population of merger jets with a fixed intrinsic jet energy is capable of explaining the observed broad diversity seen in short GRB afterglows. Full article
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222 pages, 2537 KiB  
Article
A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei
by Luigi Foschini, Matthew L. Lister, Heinz Andernach, Stefano Ciroi, Paola Marziani, Sonia Antón, Marco Berton, Elena Dalla Bontà, Emilia Järvelä, Maria J. M. Marchã, Patrizia Romano, Merja Tornikoski, Stefano Vercellone and Amelia Vietri
Universe 2022, 8(11), 587; https://doi.org/10.3390/universe8110587 - 4 Nov 2022
Cited by 18 | Viewed by 3192
Abstract
We considered the fourth catalog of gamma-ray point sources produced by the Fermi Large Area Telescope (LAT) and selected only jetted active galactic nuclei (AGN) or sources with no specific classification, but with a low-frequency counterpart. Our final list is composed of 2980 [...] Read more.
We considered the fourth catalog of gamma-ray point sources produced by the Fermi Large Area Telescope (LAT) and selected only jetted active galactic nuclei (AGN) or sources with no specific classification, but with a low-frequency counterpart. Our final list is composed of 2980 gamma-ray point sources. We then searched for optical spectra in all the available literature and publicly available databases, to measure redshifts and to confirm or change the original LAT classification. Our final list of gamma-ray emitting jetted AGN is composed of BL Lac Objects (40%), flat-spectrum radio quasars (23%), misaligned AGN (2.8%), narrow-line Seyfert 1, Seyfert, and low-ionization nuclear emission-line region galaxies (1.9%). We also found a significant number of objects changing from one type to another, and vice versa (changing-look AGN, 1.1%). About 30% of gamma-ray sources still have an ambiguous classification or lack one altogether. Full article
(This article belongs to the Special Issue Black Holes and Relativistic Jets)
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20 pages, 2415 KiB  
Review
Searches for Ultra-High-Energy Photons at the Pierre Auger Observatory
by The Pierre Auger Collaboration
Universe 2022, 8(11), 579; https://doi.org/10.3390/universe8110579 - 2 Nov 2022
Cited by 8 | Viewed by 2563
Abstract
The Pierre Auger Observatory, which is the largest air-shower experiment in the world, offers unprecedented exposure to neutral particles at the highest energies. Since the start of data collection more than 18 years ago, various searches for ultra-high-energy (UHE, [...] Read more.
The Pierre Auger Observatory, which is the largest air-shower experiment in the world, offers unprecedented exposure to neutral particles at the highest energies. Since the start of data collection more than 18 years ago, various searches for ultra-high-energy (UHE, E1017eV) photons have been performed, either for a diffuse flux of UHE photons, for point sources of UHE photons or for UHE photons associated with transient events such as gravitational wave events. In the present paper, we summarize these searches and review the current results obtained using the wealth of data collected by the Pierre Auger Observatory. Full article
(This article belongs to the Special Issue Ultra High Energy Photons)
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12 pages, 271 KiB  
Article
Quantum Clones inside Black Holes
by Gerard ’t Hooft
Universe 2022, 8(10), 537; https://doi.org/10.3390/universe8100537 - 18 Oct 2022
Cited by 15 | Viewed by 24485
Abstract
A systematic procedure is proposed for better understanding the evolution laws of black holes in terms of pure quantum states. We start with the two opposed regions I and II in the Penrose diagram, and study the evolution of matter in these [...] Read more.
A systematic procedure is proposed for better understanding the evolution laws of black holes in terms of pure quantum states. We start with the two opposed regions I and II in the Penrose diagram, and study the evolution of matter in these regions, using the algebra derived earlier from the Shapiro effect in quantum particles. Since this spacetime has two distinct asymptotic regions, one must assume that there is a mechanism that reduces the number of states. In earlier work we proposed that region II describes the angular antipodes of region I, the ‘antipodal identification’, but this eventually leads to contradictions. Our much simpler proposal is now that all states defined in region II are exact quantum clones of those in region I. This indicates more precisely how to restore unitarity by making all quantum states observable, and in addition suggests that generalisations towards other black hole structures will be possible. An apparent complication is that the wave function must evolve with a purely antisymmetric, imaginary-valued Hamiltonian, but this complication can be well-understood in a realistic interpretation of quantum mechanics. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
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13 pages, 1519 KiB  
Article
Exploring Anisotropic Lorentz Invariance Violation from the Spectral-Lag Transitions of Gamma-Ray Bursts
by Jin-Nan Wei, Zi-Ke Liu, Jun-Jie Wei, Bin-Bin Zhang and Xue-Feng Wu
Universe 2022, 8(10), 519; https://doi.org/10.3390/universe8100519 - 6 Oct 2022
Cited by 1 | Viewed by 1365
Abstract
The observed spectral lags of gamma-ray bursts (GRBs) have been widely used to explore possible violations of Lorentz invariance. However, these studies were generally performed by concentrating on the rough time lag of a single highest-energy photon and ignoring the intrinsic time lag [...] Read more.
The observed spectral lags of gamma-ray bursts (GRBs) have been widely used to explore possible violations of Lorentz invariance. However, these studies were generally performed by concentrating on the rough time lag of a single highest-energy photon and ignoring the intrinsic time lag at the source. A new way to test nonbirefringent Lorentz-violating effects has been proposed by analyzing the multi-photon spectral-lag behavior of a GRB that displays a positive-to-negative transition. This method gives both a plausible description of the intrinsic energy-dependent time lag and comparatively robust constraints on Lorentz-violating effects. In this work, we conduct a systematic search for Lorentz-violating photon dispersion from the spectral-lag transition features of 32 GRBs. By fitting the spectral-lag data of these 32 GRBs, we place constraints on a variety of isotropic and anisotropic Lorentz-violating coefficients with mass dimension d=6 and 8. While our dispersion constraints are not competitive with existing bounds, they have the promise to complement the full coefficient space. Full article
(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)
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20 pages, 382 KiB  
Article
Nonlocal Gravity: Modification of Newtonian Gravitational Force in the Solar System
by Mahmood Roshan and Bahram Mashhoon
Universe 2022, 8(9), 470; https://doi.org/10.3390/universe8090470 - 8 Sep 2022
Cited by 5 | Viewed by 1716
Abstract
Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation developed in close analogy with the nonlocal electrodynamics of media. It appears that the nonlocal aspect of the universal gravitational interaction could simulate dark matter. Within the Newtonian regime of [...] Read more.
Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation developed in close analogy with the nonlocal electrodynamics of media. It appears that the nonlocal aspect of the universal gravitational interaction could simulate dark matter. Within the Newtonian regime of NLG, we investigate the deviation of the gravitational force from the Newtonian inverse square law as a consequence of the existence of the effective dark matter. In particular, we work out the magnitude of this deviation in the solar system out to 100 astronomical units. Moreover, we give an improved lower limit for the short-range parameter of the reciprocal kernel of NLG. Full article
(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)
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13 pages, 337 KiB  
Article
Effect of Some Modified Models of Gravity on the Radial Velocity of Binary Systems
by Lorenzo Iorio and Matteo Luca Ruggiero
Universe 2022, 8(9), 443; https://doi.org/10.3390/universe8090443 - 25 Aug 2022
Viewed by 1333
Abstract
For many classes of astronomical and astrophysical binary systems, long observational records of their radial velocity V, which is their directly observable quantity, are available. For exoplanets close to their parent stars, they cover several full orbital revolutions, while for wide binaries [...] Read more.
For many classes of astronomical and astrophysical binary systems, long observational records of their radial velocity V, which is their directly observable quantity, are available. For exoplanets close to their parent stars, they cover several full orbital revolutions, while for wide binaries such as, e.g., the Proxima/α Centauri AB system, only relatively short orbital arcs are sampled by existing radial velocity measurements. Here, the changes ΔV induced on a binary’s radial velocity by some long-range modified models of gravity are analytically calculated. In particular, extra-potentials proportional to rN,N=2,3 and r2 are considered; the Cosmological Constant Λ belongs to the latter group. Both the net shift per orbit and the instantaneous one are explicitly calculated for each model. The Cosmological Constant induces a shift in the radial velocity of the Proxima/α Centauri AB binary as little as ΔV107ms1, while the present-day accuracy in measuring its radial velocity is σV30ms1. The calculational scheme presented here is quite general, and can be straightforwardly extended to any other modified gravity. Full article
(This article belongs to the Section Gravitation)
9 pages, 399 KiB  
Article
The Galactic Interstellar Medium Has a Preferred Handedness of Magnetic Misalignment
by Zhiqi Huang
Universe 2022, 8(8), 423; https://doi.org/10.3390/universe8080423 - 17 Aug 2022
Cited by 2 | Viewed by 1615
Abstract
The Planck mission detected a positive correlation between the intensity (T) and B-mode polarization of the Galactic thermal dust emission. The TB correlation is a parity-odd signal, whose statistical mean vanishes in models with mirror symmetry. Recent work has [...] Read more.
The Planck mission detected a positive correlation between the intensity (T) and B-mode polarization of the Galactic thermal dust emission. The TB correlation is a parity-odd signal, whose statistical mean vanishes in models with mirror symmetry. Recent work has shown, with strong evidence, that local handedness of the misalignment between the dust filaments and the sky-projected magnetic field produces TB signals. However, it remains unclear whether the observed global TB signal is caused by statistical fluctuations of magnetic misalignment angles or whether some parity-violating physics in the interstellar medium sets a preferred misalignment handedness. The present work aims to make a quantitative statement about how confidently the statistical fluctuation interpretation is ruled out by filament-based simulations of polarized dust emission. We use the publicly available DUSTFILAMENTS code to simulate the dust emission from filaments whose magnetic misalignment angles are symmetrically randomized and construct the probability density function of ξp, a weighted sum of the TB power spectrum. We find that the Planck data have a ≳10σ tension with the simulated ξp distribution. Our results strongly support the idea that the Galactic filament misalignment has a preferred handedness, whose physical origin is yet to be identified. Full article
(This article belongs to the Special Issue Cosmic Microwave Background)
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17 pages, 349 KiB  
Article
Galaxy Phase-Space Density Data Preclude That Bose–Einstein Condensate Be the Total Dark Matter
by Héctor J. de Vega and Norma G. Sanchez
Universe 2022, 8(8), 419; https://doi.org/10.3390/universe8080419 - 11 Aug 2022
Cited by 13 | Viewed by 1854
Abstract
Ultralight scalars with a typical mass of the order m1022 eV and light scalars forming a Bose–Einstein condensate (BEC) exhibit a Jeans length in the kpc scale and were therefore proposed as dark matter (DM) candidates. Our treatment here [...] Read more.
Ultralight scalars with a typical mass of the order m1022 eV and light scalars forming a Bose–Einstein condensate (BEC) exhibit a Jeans length in the kpc scale and were therefore proposed as dark matter (DM) candidates. Our treatment here is generic, independent of the particle physics model and applies to all DM BEC, in both in or out of equilibrium situations. Two observed quantities crucially constrain DM in an inescapable way: the average DM density ρDM and the phase-space density Q. The observed values of ρDM and Q in galaxies today constrain both the possibility to form a BEC, and the DM mass m. These two constraints robustly exclude the axion DM that decouples after inflation. Moreover, the value m1022 eV can only be obtained with a number of ultrarelativistic degrees of freedom at decoupling in the trillions, which is impossible for decoupling in the radiation dominated era. In addition, we find for the axion vacuum misalignment scenario that axions are produced strongly out of thermal equilibrium and that the axion mass in such a scenario turns to be 17 orders of magnitude too large to reproduce the observed galactic structures. Moreover, we also consider inhomogenous gravitationally bounded BEC’s supported by the bosonic quantum pressure independently of any particular particle physics scenario. For a typical size R kpc and compact object masses M107M they remarkably lead to the same particle mass m1022 eV as the BEC free-streaming length. However, the phase-space density for the gravitationally bounded BEC’s turns out to be more than sixty orders of magnitude smaller than the galaxy-observed values. We conclude that the BEC cannot be the total DM. The axion can be candidates to be only part of the DM of the universe. Besides, an axion in the mili-eV scale may be a relevant source of dark energy through the zero point cosmological quantum fluctuations. Full article
(This article belongs to the Collection Women Physicists in Astrophysics, Cosmology and Particle Physics)
16 pages, 4000 KiB  
Article
Trajectory Analysis and Optimization of Hesperides Mission
by Giovanni Mengali and Alessandro A. Quarta
Universe 2022, 8(7), 364; https://doi.org/10.3390/universe8070364 - 1 Jul 2022
Cited by 4 | Viewed by 1985
Abstract
A challenging problem from a technological viewpoint is to send a spacecraft at a distance of about 600 au from the Sun, comparable with that of the Sun’s gravitational focus (that is, the general relativistic focusing of light rays, whose minimum solar distance [...] Read more.
A challenging problem from a technological viewpoint is to send a spacecraft at a distance of about 600 au from the Sun, comparable with that of the Sun’s gravitational focus (that is, the general relativistic focusing of light rays, whose minimum solar distance is obtained when the light rays are assumed to graze the Sun’s surface), and reach it in a time interval on the order of a human working lifetime. A suitably oriented telescope at that distance would be theoretically able to observe exoplanets tens of light years far away and possibly to discover new life forms. The transfer trajectory of this mission is rather complex and requires a close selection of a suitable propulsion system, which must be able to provide the probe with the necessary energy to cruise at a velocity greater than 10 au/year. An effective outline of the these concepts is given by the Hesperides mission, originally proposed by Matloff in 2014. An interesting aspect of this mission proposal is the combination of a nuclear electric propulsion system and a classical solar sail that are jointly exploited to reach the necessary solar system escape velocity. However, the trajectory analysis reported by Matloff is very simplified and is essentially concentrated on a rough estimate of the time required by the spacecraft to reach a distance of 600au. Starting from the Hesperides baseline mission proposal, including the vehicle mass distribution, the aim of this work is to give a detailed mission analysis in an optimal framework. In particular, the spacecraft minimum time trajectory is calculated with indirect methods and a parametric analysis is made to highlight the impact of the main design parameters on the total flight time. The simulations show a substantial reduction of the mission time when compared with the original study. Full article
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45 pages, 3683 KiB  
Review
Critical Tests of Leading Gamma Ray Burst Theories
by Shlomo Dado, Arnon Dar and Alvaro De Rújula
Universe 2022, 8(7), 350; https://doi.org/10.3390/universe8070350 - 24 Jun 2022
Cited by 10 | Viewed by 2683
Abstract
It has been observationally established that supernovae (SNe) of Type Ic produce long duration gamma-ray bursts (GRBs) and that neutron star mergers generate short hard GRBs. SN-Less GRBs presumably originate in a phase transition of a neutron star in a high mass X-ray [...] Read more.
It has been observationally established that supernovae (SNe) of Type Ic produce long duration gamma-ray bursts (GRBs) and that neutron star mergers generate short hard GRBs. SN-Less GRBs presumably originate in a phase transition of a neutron star in a high mass X-ray binary. How these phenomena actually generate GRBs is debated. The fireball and cannonball models of GRBs and their afterglows have been widely confronted with the huge observational data, with their defenders claiming success. The claims, however, may reflect multiple choices and the use of many adjustable parameters, rather than the validity of the models. Only a confrontation of key falsifiable predictions of the models with solid observational data can test their validity. Such critical tests are reviewed in this report. Full article
(This article belongs to the Special Issue GRBs Phenomenology, Models and Applications: A Beginner Guide)
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23 pages, 588 KiB  
Article
Onset of Electron Captures and Shallow Heating in Magnetars
by Nicolas Chamel and Anthea Francesca Fantina
Universe 2022, 8(6), 328; https://doi.org/10.3390/universe8060328 - 11 Jun 2022
Cited by 1 | Viewed by 1945
Abstract
The loss of magnetic pressure accompanying the decay of the magnetic field in a magnetar may trigger exothermic electron captures by nuclei in the shallow layers of the stellar crust. Very accurate analytical formulas are obtained for the threshold density and pressure, as [...] Read more.
The loss of magnetic pressure accompanying the decay of the magnetic field in a magnetar may trigger exothermic electron captures by nuclei in the shallow layers of the stellar crust. Very accurate analytical formulas are obtained for the threshold density and pressure, as well as for the maximum amount of heat that can be possibly released, taking into account the Landau–Rabi quantization of electron motion. These formulas are valid for arbitrary magnetic field strengths, from the weakly quantizing regime to the most extreme situation in which electrons are all confined to the lowest level. Numerical results are also presented based on experimental nuclear data supplemented with predictions from the Brussels-Montreal model HFB-24. This same nuclear model has been already employed to calculate the equation of state in all regions of magnetars. Full article
(This article belongs to the Special Issue Advances in Magnetars)
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31 pages, 464 KiB  
Review
Lorentz Symmetry Violation of Cosmic Photons
by Ping He and Bo-Qiang Ma
Universe 2022, 8(6), 323; https://doi.org/10.3390/universe8060323 - 9 Jun 2022
Cited by 20 | Viewed by 2564
Abstract
As a basic symmetry of space-time, Lorentz symmetry has played important roles in various fields of physics, and it is a glamorous question whether Lorentz symmetry breaks. Since Einstein proposed special relativity, Lorentz symmetry has withstood very strict tests, but there are still [...] Read more.
As a basic symmetry of space-time, Lorentz symmetry has played important roles in various fields of physics, and it is a glamorous question whether Lorentz symmetry breaks. Since Einstein proposed special relativity, Lorentz symmetry has withstood very strict tests, but there are still motivations for Lorentz symmetry violation (LV) research from both theoretical consideration and experimental feasibility, that attract physicists to work on LV theories, phenomena and experimental tests with enthusiasm. There are many theoretical models including LV effects, and different theoretical models predict different LV phenomena, from which we can verify or constrain LV effects. Here, we introduce three types of LV theories: quantum gravity theory, space-time structure theory and effective field theory with extra-terms. Limited by the energy of particles, the experimental tests of LV are very difficult; however, due to the high energy and long propagation distance, high-energy particles from astronomical sources can be used for LV phenomenological researches. Especially with cosmic photons, various astronomical observations provide rich data from which one can obtain various constraints for LV researches. Here, we review four common astronomical phenomena which are ideal for LV studies, together with current constraints on LV effects of photons. Full article
(This article belongs to the Special Issue Ultra High Energy Photons)
21 pages, 1450 KiB  
Review
Multi-Messenger Constraints on the Hubble Constant through Combination of Gravitational Waves, Gamma-Ray Bursts and Kilonovae from Neutron Star Mergers
by Mattia Bulla, Michael W. Coughlin, Suhail Dhawan and Tim Dietrich
Universe 2022, 8(5), 289; https://doi.org/10.3390/universe8050289 - 21 May 2022
Cited by 21 | Viewed by 2691
Abstract
The simultaneous detection of gravitational waves and light from the binary neutron star merger GW170817 led to independent measurements of distance and redshift, providing a direct estimate of the Hubble constant H0 that does not rely on a cosmic distance ladder, nor [...] Read more.
The simultaneous detection of gravitational waves and light from the binary neutron star merger GW170817 led to independent measurements of distance and redshift, providing a direct estimate of the Hubble constant H0 that does not rely on a cosmic distance ladder, nor assumes a specific cosmological model. By using gravitational waves as “standard sirens”, this approach holds promise to arbitrate the existing tension between the H0 value inferred from the cosmic microwave background and those obtained from local measurements. However, the known degeneracy in the gravitational-wave analysis between distance and inclination of the source led to a H0 value from GW170817 that was not precise enough to resolve the existing tension. In this review, we summarize recent works exploiting the viewing-angle dependence of the electromagnetic signal, namely the associated short gamma-ray burst and kilonova, to constrain the system inclination and improve on H0. We outline the key ingredients of the different methods, summarize the results obtained in the aftermath of GW170817 and discuss the possible systematics introduced by each of these methods. Full article
(This article belongs to the Special Issue GRBs Phenomenology, Models and Applications: A Beginner Guide)
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16 pages, 1258 KiB  
Article
Is the Hubble Crisis Connected with the Extinction of Dinosaurs?
by Leandros Perivolaropoulos
Universe 2022, 8(5), 263; https://doi.org/10.3390/universe8050263 - 26 Apr 2022
Cited by 26 | Viewed by 2779
Abstract
It has recently been suggested that a gravitational transition of the effect