Universe

31 pages, 4153 KiB

Open AccessReview

Optical Polarimetry for Fundamental Physics

by Guido Zavattini and Federico Della Valle

Universe 2021, 7(7), 252; https://doi.org/10.3390/universe7070252 - 20 Jul 2021

Cited by 4 | Viewed by 3043

Sensitive magneto-optical polarimetry was proposed by E. Iacopini and E. Zavattini in 1979 to detect vacuum electrodynamic non-linearity, in particular Vacuum Magnetic Birefringence (VMB). This process is predicted in QED via the fluctuation of electron–positron virtual pairs but can also be due to [...] Read more.

Sensitive magneto-optical polarimetry was proposed by E. Iacopini and E. Zavattini in 1979 to detect vacuum electrodynamic non-linearity, in particular Vacuum Magnetic Birefringence (VMB). This process is predicted in QED via the fluctuation of electron–positron virtual pairs but can also be due to hypothetical Axion-Like Particles (ALPs) and/or MilliCharged Particles (MCP). Today ALPs are considered a strong candidate for Dark Matter. Starting in 1992 the PVLAS collaboration, financed by INFN, Italy, attempted to measure VMB conceptually following the original 1979 scheme based on an optical cavity permeated by a time-dependent magnetic field and heterodyne detection. Two setups followed differing basically in the magnet: the first using a rotating superconducting 5.5 T dipole magnet at the Laboratori Nazionali di Legnaro, Legnaro, Italy and the second using two rotating permanent 2.5 T dipole magnets at the INFN section of Ferrara. At present PVLAS is the experiment which has set the best limit in VMB reaching a noise floor within a factor 7 of the predicted QED signal:

Δ n^{(QED)} = 2.5 \times 10^{- 23}

@ 2.5 T. It was also shown that the noise floor was due to the optical cavity and a larger magnet is the only solution to increase the signal to noise ratio. The PVLAS experiment ended at the end of 2018. A new effort, VMB@CERN, which plans to use a spare LHC dipole magnet at CERN with a new modified optical scheme, is now being proposed. In this review, a detailed description of the PVLAS effort and the comprehension of its limits leading to a new proposal will be given. Full article

(This article belongs to the Special Issue Italian Research Facilities for Fundamental Physics)

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21 pages, 349 KiB

Open AccessReview

Space–Time Physics in Background-Independent Theories of Quantum Gravity

by Martin Bojowald

Universe 2021, 7(7), 251; https://doi.org/10.3390/universe7070251 - 20 Jul 2021

Cited by 7 | Viewed by 2116

Abstract

Background independence is often emphasized as an important property of a quantum theory of gravity that takes seriously the geometrical nature of general relativity. In a background-independent formulation, quantum gravity should determine not only the dynamics of space–time but also its geometry, which [...] Read more.

Background independence is often emphasized as an important property of a quantum theory of gravity that takes seriously the geometrical nature of general relativity. In a background-independent formulation, quantum gravity should determine not only the dynamics of space–time but also its geometry, which may have equally important implications for claims of potential physical observations. One of the leading candidates for background-independent quantum gravity is loop quantum gravity. By combining and interpreting several recent results, it is shown here how the canonical nature of this theory makes it possible to perform a complete space–time analysis in various models that have been proposed in this setting. In spite of the background-independent starting point, all these models turned out to be non-geometrical and even inconsistent to varying degrees, unless strong modifications of Riemannian geometry are taken into account. This outcome leads to several implications for potential observations as well as lessons for other background-independent approaches. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology II)

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20 pages, 809 KiB

Open AccessReview

New Advancements in AdS/CFT in Lower Dimensions

by Yolanda Lozano and Anayeli Ramirez

Universe 2021, 7(7), 250; https://doi.org/10.3390/universe7070250 - 19 Jul 2021

Cited by 6 | Viewed by 1992

Abstract

We review recent developments in the study of the AdS/CFT correspondence in lower dimensions. We start by summarising the classification of AdS

_{3} \times S^{2}

solutions in massive type IIA supergravity with (0, 4) supersymmetries and the construction of their 2D dual [...] Read more.

We review recent developments in the study of the AdS/CFT correspondence in lower dimensions. We start by summarising the classification of AdS

_{3} \times S^{2}

solutions in massive type IIA supergravity with (0, 4) supersymmetries and the construction of their 2D dual quiver CFTs. These theories are the seed for further developments that we review next. First, we construct a new class of AdS

_{3}

solutions in M-theory that describe M-strings in M5-brane intersections. Second, we generate a new class of AdS

_{2} \times S^{3}

solutions in massive IIA with four supercharges that we interpret as describing backreacted baryon vertices within the 5D

N = 1

QFT living in D4-D8 branes. Third, we construct two classes of AdS

_{2}

solutions in Type IIB. The first are dual to discrete light-cone quantised quantum mechanics living in null cylinders. The second class is interpreted as dual to backreacted baryon vertices within 4D

N = 2

QFT living in D3-D7 branes. Explicit dual quiver field theories are given for all classes of solutions. These are used to compute the central charges of the CFTs that are shown to agree with the holographic expressions. Full article

(This article belongs to the Collection Women Physicists in Astrophysics, Cosmology and Particle Physics)

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12 pages, 452 KiB

Open AccessArticle

New Model of 4D Einstein–Gauss–Bonnet Gravity Coupled with Nonlinear Electrodynamics

by Sergey Il’ich Kruglov

Universe 2021, 7(7), 249; https://doi.org/10.3390/universe7070249 - 19 Jul 2021

Cited by 10 | Viewed by 2227

Abstract

New spherically symmetric solution in 4D Einstein–Gauss–Bonnet gravity coupled with nonlinear electrodynamics is obtained. At infinity, this solution has the Reissner–Nordström behavior of the charged black hole. The black hole thermodynamics, entropy, shadow, energy emission rate, and quasinormal modes of black holes are [...] Read more.

New spherically symmetric solution in 4D Einstein–Gauss–Bonnet gravity coupled with nonlinear electrodynamics is obtained. At infinity, this solution has the Reissner–Nordström behavior of the charged black hole. The black hole thermodynamics, entropy, shadow, energy emission rate, and quasinormal modes of black holes are investigated. Full article

(This article belongs to the Special Issue Modified Theories of Gravity and Cosmological Applications)

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14 pages, 1027 KiB

Open AccessArticle

Identification of Fractal Properties in Geomagnetic Data of Southeast Asian Region during Various Solar Activity Levels

by Farhan Naufal Rifqi, Nurul Shazana Abdul Hamid, A. Babatunde Rabiu and Akimasa Yoshikawa

Universe 2021, 7(7), 248; https://doi.org/10.3390/universe7070248 - 16 Jul 2021

Cited by 9 | Viewed by 2326

Abstract

The fractal properties of geomagnetic northward component data (H-component) in the equatorial region during various phases of solar activity over Southeast Asia were investigated and then quantified using the parameter of the Hurst exponent (H). This study began with the identification [...] Read more.

The fractal properties of geomagnetic northward component data (H-component) in the equatorial region during various phases of solar activity over Southeast Asia were investigated and then quantified using the parameter of the Hurst exponent (H). This study began with the identification of existence of spectral peaks and scaling properties in international quiet day H-component data which were measured during three levels of solar activity: low, intermediate, and high. Then, various cases of quiet and disturbed days during different solar activity levels were analyzed using the method that performed the best in the preceding part. In all the years analyzed, multifractal scaling and spectral peaks exist, signifying that the data have fractal properties and that there are external factors driving the fluctuations of geomagnetic activity other than solar activity. The analysis of various cases of quiet and disturbed days generally showed that quiet days had anti-persistence tendencies (H < 0.5) while disturbed days had persistence tendencies (H > 0.5)—generally a higher level of Hurst exponent compared to quiet days. As for long-term quiet day H-component data, it had a Hurst exponent value that was near H ≃ 0.50, while the long-term disturbed day H-component data showed higher values than that of the quiet day. Full article

(This article belongs to the Special Issue Space Weather)

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24 pages, 415 KiB

Open AccessReview

Two Applications of the Analytic Conformal Bootstrap: A Quick Tour Guide

by Agnese Bissi, Parijat Dey and Giulia Fardelli

Universe 2021, 7(7), 247; https://doi.org/10.3390/universe7070247 - 15 Jul 2021

Cited by 4 | Viewed by 2054

Abstract

We reviewed the recent developments in the study of conformal field theories in generic space time dimensions using the methods of the conformal bootstrap, in its analytic aspect. These techniques are solely based on symmetries, particularly on the analytic structure and in the [...] Read more.

We reviewed the recent developments in the study of conformal field theories in generic space time dimensions using the methods of the conformal bootstrap, in its analytic aspect. These techniques are solely based on symmetries, particularly on the analytic structure and in the associativity of the operator product expansion. We focused on two applications of the analytic conformal bootstrap: the study of the

ϵ

expansion of the Wilson–Fisher model via the introduction of a dispersion relation and the large N expansion of the maximally supersymmetric Super Yang–Mills theory in four dimensions. Full article

(This article belongs to the Collection Women Physicists in Astrophysics, Cosmology and Particle Physics)

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61 pages, 3665 KiB

Open AccessArticle

Reactor Antineutrino Anomaly Reanalysis in Context of Inverse-Square Law Violation

by Vadim A. Naumov and Dmitry S. Shkirmanov

Universe 2021, 7(7), 246; https://doi.org/10.3390/universe7070246 - 15 Jul 2021

Cited by 4 | Viewed by 2789

Abstract

We discuss a possibility that the so-called reactor antineutrino anomaly (RAA), which is a deficit of the

{\bar{ν}}_{e}

rates in the reactor experiments in comparison to the theoretical expectations, can at least in part be explained by applying a quantum field-theoretical [...] Read more.

We discuss a possibility that the so-called reactor antineutrino anomaly (RAA), which is a deficit of the

{\bar{ν}}_{e}

rates in the reactor experiments in comparison to the theoretical expectations, can at least in part be explained by applying a quantum field-theoretical approach to neutrino oscillations, which in particular predicts a small deviation from the classical inverse-square law at short (but still macroscopic) distances between the neutrino source and detector. An extensive statistical analysis of the current reactor data on the integrated

{\bar{ν}}_{e}

event rates vs. baseline is performed to examine this speculation. The obtained results are applied to study another long-standing puzzle—gallium neutrino anomaly (GNA), which is a missing

ν_{e}

flux from

^{37}

Ar and

^{51}

Cr electron-capture decays as measured by the gallium–germanium solar neutrino detectors GALLEX and SAGE. Full article

(This article belongs to the Special Issue Innovative Detection Strategies for New Physics Searches)

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16 pages, 347 KiB

Open AccessArticle

A Precessing Jet Scenario for the Multi-Wavelength Long-Term Modulation of LS I +61°303

by Frédéric Jaron

Universe 2021, 7(7), 245; https://doi.org/10.3390/universe7070245 - 14 Jul 2021

Cited by 7 | Viewed by 1837

Abstract

The high-mass X-ray binary LS I +61°303 is detected across the electromagnetic spectrum from radio until the very high energy

γ

-ray regime. The emission is not only highly variable on many time scales, but is also periodic at all observed wavelengths. Periodic [...] Read more.

The high-mass X-ray binary LS I +61°303 is detected across the electromagnetic spectrum from radio until the very high energy

γ

-ray regime. The emission is not only highly variable on many time scales, but is also periodic at all observed wavelengths. Periodic modulation was observed on different time-scales, ranging from hours, over months to several years. The subject of this article is a super-orbital, long-term modulation of ∼4.6 years. We review the observation of this periodic modulation at multiple wavelengths and investigate systematic relationships between them. IN particular, radio observations reveal that the long-term modulation is a very stable feature of the source. Observations at other wavelengths result in a phase-shift of the modulationpattern that is a systematic function of energy. The stability of this period favors a scenario in which the long-term modulation is the result of a precessing jet giving rise to periodic changes in the Doppler factor, beating with the orbital modulation of the accretion rate. We explain the phase-shifts across energy bands in a scenario with shorter wavelengths originating closer to the base of the presessing jet. A significant deviation of the TeV emission from this trend possibly requires a different explanation related to magnetic reconnection events. Full article

(This article belongs to the Special Issue Multi-Wavelength Properties of Gamma-Ray Binaries)

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12 pages, 608 KiB

Open AccessArticle

Dynamics in Interacting Scalar-Torsion Cosmology

by Andronikos Paliathanasis

Universe 2021, 7(7), 244; https://doi.org/10.3390/universe7070244 - 14 Jul 2021

Cited by 14 | Viewed by 1949

Abstract

In a spatially flat Friedmann–Lemaître–Robertson–Walker background space, we consider a scalar-torsion gravitational model which has similar properties to the dilaton theory. This teleparallel model is invariant under a discrete transformation similar to the Gasperini–Veneziano duality transformation. Moreover, in the gravitational action integral, we [...] Read more.

In a spatially flat Friedmann–Lemaître–Robertson–Walker background space, we consider a scalar-torsion gravitational model which has similar properties to the dilaton theory. This teleparallel model is invariant under a discrete transformation similar to the Gasperini–Veneziano duality transformation. Moreover, in the gravitational action integral, we introduce the Lagrangian function of a pressureless fluid source which is coupled to the teleparallel dilaton field. This specific gravitational theory with interaction in the dark sector of the universe was investigated by using methods of the dynamical system analysis. We calculate that the theory provides various areas of special interest for the evolution of the cosmological history. Inflationary scaling solutions and the de Sitter universe are recovered. Furthermore, we calculate that there exist an attractor which provides a stable solution where the two fluid components, the scalar field and the pressureless matter, contribute in the cosmological fluid. This solution is of special interest because it can describe the present epoch. Finally, the qualitative evolution of the cosmographic parameters is discussed. Full article

(This article belongs to the Special Issue Probing the Dark Universe with Theory and Observations)

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11 pages, 3396 KiB

Open AccessArticle

The Ionization Energies of Dust-Forming Metal Oxide Clusters

by David Gobrecht, Jan Philip Sindel, Helena Lecoq-Molinos and Leen Decin

Universe 2021, 7(7), 243; https://doi.org/10.3390/universe7070243 - 14 Jul 2021

Cited by 5 | Viewed by 1926

Abstract

Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, [...] Read more.

Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigated the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They included clusters of magnesia (MgO)_n, silicon monoxide (SiO)_n, alumina (Al₂O₃)_n, and titania (TiO₂)_n with stoichiometric sizes of n = 1–8. The magnesia, alumina, and titania clusters showed relatively little variation in their ionization energies with respect to the cluster size n: 7.1–8.2 eV for (MgO)_n, from 8.9–10.0 eV for (Al₂O₃)_n, and 9.3–10.5 eV for (TiO₂)_n. In contrast, the (SiO)_n ionization energies decrease with size n, starting from 11.5 eV for n = 1, and decreasing to 6.6 eV for n = 8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral–neutral reactions. Full article

(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)

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11 pages, 600 KiB

Open AccessCommunication

Multi-Wavelength Properties of the 2021 Periastron Passage of PSR B1259-63

by Maria Chernyakova, Denys Malyshev, Brian van Soelen, Shane O’Sullivan, Charlotte Sobey, Sergey Tsygankov, Samuel Mc Keague, Jacob Green, Matthew Kirwan, Andrea Santangelo, Gerd Pühlhofer and Itumeleng M. Monageng

Universe 2021, 7(7), 242; https://doi.org/10.3390/universe7070242 - 13 Jul 2021

Cited by 16 | Viewed by 2477

Abstract

PSR B1259-63 is a gamma-ray binary system hosting a radio pulsar orbiting around a O9.5Ve star, LS 2883, with a period of ∼3.4 years. The interaction of the pulsar wind with the LS 2883 outflow leads to unpulsed broadband emission in the radio, [...] Read more.

PSR B1259-63 is a gamma-ray binary system hosting a radio pulsar orbiting around a O9.5Ve star, LS 2883, with a period of ∼3.4 years. The interaction of the pulsar wind with the LS 2883 outflow leads to unpulsed broadband emission in the radio, X-ray, GeV, and TeV domains. One of the most unusual features of the system is an outburst of GeV energies around the periastron, during which the energy release substantially exceeds the spin down luminosity under the assumption of the isotropic emission. In this paper, we present the first results of a recent multi-wavelength campaign (radio, optical, and X-ray bands) accompanied by the analysis of publicly available GeV Fermi/LAT data. The campaign covered a period of more than 100 days around the 2021 periastron and revealed substantial differences from previously observed passages. We report a major delay of the GeV flare, weaker X-ray flux during the peaks, which are typically attributed to the times when the pulsar crosses the disk, and the appearance of a third X-ray peak never observed before. We argue that these features are consistent with the emission cone model proposed by us previously, in the case of a sparser and clumpier disk of the Be star. Full article

(This article belongs to the Special Issue Multi-Wavelength Properties of Gamma-Ray Binaries)

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31 pages, 527 KiB

Open AccessArticle

Firewall from Effective Field Theory

by Pei-Ming Ho and Yuki Yokokura

Universe 2021, 7(7), 241; https://doi.org/10.3390/universe7070241 - 13 Jul 2021

Cited by 17 | Viewed by 2477

Abstract

For an effective field theory in the background of an evaporating black hole with spherical symmetry, we consider non-renormalizable interactions and their relevance to physical effects. The background geometry is determined by the semi-classical Einstein equation for an uneventful horizon where the vacuum [...] Read more.

For an effective field theory in the background of an evaporating black hole with spherical symmetry, we consider non-renormalizable interactions and their relevance to physical effects. The background geometry is determined by the semi-classical Einstein equation for an uneventful horizon where the vacuum energy–momentum tensor is small for freely falling observers. Surprisingly, after Hawking radiation appears, the transition amplitude from the Unruh vacuum to certain multi-particle states grows exponentially with time for a class of higher-derivative operators after the collapsing matter enters the near-horizon region, despite the absence of large curvature invariants. Within the scrambling time, the uneventful horizon transitions towards a firewall, and eventually the effective field theory breaks down. Full article

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

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22 pages, 15100 KiB

Open AccessArticle

New Sources of Leptonic CP Violation at the DUNE Neutrino Experiment

by Alessio Giarnetti and Davide Meloni

Universe 2021, 7(7), 240; https://doi.org/10.3390/universe7070240 - 12 Jul 2021

Cited by 9 | Viewed by 1878

Abstract

We check the capability of the DUNE neutrino experiment to detect new sources of leptonic CP violation beside the single phase expected in the Standard Model. We illustrate our strategy based on the measurement of CP asymmetries in the case that new physics [...] Read more.

We check the capability of the DUNE neutrino experiment to detect new sources of leptonic CP violation beside the single phase expected in the Standard Model. We illustrate our strategy based on the measurement of CP asymmetries in the case that new physics will show up as nonstandard neutrino interactions and sterile neutrino states and show that the most promising one, once the experimental errors are taken into account in both scenarios, is the one related to the

ν_{μ} \to ν_{e}

transition. Full article

(This article belongs to the Special Issue Investigating the Matter-Antimatter Asymmetry)

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10 pages, 510 KiB

Open AccessArticle

s-Processing in Asymptotic Giant Branch Stars in the Light of Revised Neutron-Capture Cross Sections

by Diego Vescovi and René Reifarth

Universe 2021, 7(7), 239; https://doi.org/10.3390/universe7070239 - 11 Jul 2021

Cited by 3 | Viewed by 2585

Abstract

Current AGB stellar models provide an adequate description of the s-process nucleosynthesis that occurs. Nonetheless, they still suffer from many uncertainties related to the modeling of the

^{13} C

pocket formation and the adopted nuclear reaction rates. For many important s-process isotopes, a [...] Read more.

Current AGB stellar models provide an adequate description of the s-process nucleosynthesis that occurs. Nonetheless, they still suffer from many uncertainties related to the modeling of the

^{13} C

pocket formation and the adopted nuclear reaction rates. For many important s-process isotopes, a best set of neutron-capture cross sections was recently re-evaluated. Using stellar models prescribing that the

^{13} C

pocket is a by-product of magnetic-buoyancy-induced mixing phenomena, s-process calculations were carried out with this database. Significant effects are found for a few s-only and branching point isotopes, pointing out the need for improved neutron-capture cross section measurements at low energy. Full article

(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)

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10 pages, 301 KiB

Open AccessArticle

Ellis–Bronnikov Wormholes in Asymptotically Safe Gravity

by G. Alencar, V. B. Bezerra, C. R. Muniz and H. S. Vieira

Universe 2021, 7(7), 238; https://doi.org/10.3390/universe7070238 - 10 Jul 2021

Cited by 14 | Viewed by 2119

Abstract

In this paper, we investigate the simplest wormhole solution—the Ellis–Bronnikov one—in the context of the asymptotically safe gravity (ASG) at the Planck scale. We work with three models, which employ the Ricci scalar, Kretschmann scalar, and squared Ricci tensor to improve the field [...] Read more.

In this paper, we investigate the simplest wormhole solution—the Ellis–Bronnikov one—in the context of the asymptotically safe gravity (ASG) at the Planck scale. We work with three models, which employ the Ricci scalar, Kretschmann scalar, and squared Ricci tensor to improve the field equations by turning the Newton constant into a running coupling constant. For all the cases, we check the radial energy conditions of the wormhole solution and compare them with those that are valid in general relativity (GR). We verified that asymptotic safety guarantees that the Ellis–Bronnikov wormhole can satisfy the radial energy conditions at the throat radius,

r_{0}

, within an interval of values of the latter, which is quite different from the result found in GR. Following this, we evaluate the effective radial state parameter,

ω (r)

, at

r_{0}

, showing that the quantum gravitational effects modify Einstein’s field equations in such a way that it is necessary to have a very exotic source of matter to generate the wormhole spacetime–phantom or quintessence-like matter. This occurs within some ranges of the throat radii, even though the energy conditions are or are not violated there. Finally, we find that, although at

r_{0}

we have a quintessence-like matter, upon growing r, we inevitably came across phantom-like regions. We speculate whether such a phantom fluid must always be present in wormholes in the ASG context or even in more general quantum gravity scenarios. Full article

(This article belongs to the Special Issue Asymptotic Safety in Quantum Gravity)

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39 pages, 3021 KiB

Open AccessReview

The Casimir Effect in Topological Matter

by Bing-Sui Lu

Universe 2021, 7(7), 237; https://doi.org/10.3390/universe7070237 - 9 Jul 2021

Cited by 21 | Viewed by 3148

Abstract

We give an overview of the work done during the past ten years on the Casimir interaction in electronic topological materials, our focus being solids, which possess surface or bulk electronic band structures with nontrivial topologies, which can be evinced through optical properties [...] Read more.

We give an overview of the work done during the past ten years on the Casimir interaction in electronic topological materials, our focus being solids, which possess surface or bulk electronic band structures with nontrivial topologies, which can be evinced through optical properties that are characterizable in terms of nonzero topological invariants. The examples we review are three-dimensional magnetic topological insulators, two-dimensional Chern insulators, graphene monolayers exhibiting the relativistic quantum Hall effect, and time reversal symmetry-broken Weyl semimetals, which are fascinating systems in the context of Casimir physics. Firstly, this is for the reason that they possess electromagnetic properties characterizable by axial vectors (because of time reversal symmetry breaking), and, depending on the mutual orientation of a pair of such axial vectors, two systems can experience a repulsive Casimir–Lifshitz force, even though they may be dielectrically identical. Secondly, the repulsion thus generated is potentially robust against weak disorder, as such repulsion is associated with the Hall conductivity that is topologically protected in the zero-frequency limit. Finally, the far-field low-temperature behavior of the Casimir force of such systems can provide signatures of topological quantization. Full article

(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)

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15 pages, 5271 KiB

Open AccessArticle

Dark Matter Searches at LNF

by Claudio Gatti, Paola Gianotti, Carlo Ligi, Mauro Raggi and Paolo Valente

Universe 2021, 7(7), 236; https://doi.org/10.3390/universe7070236 - 9 Jul 2021

Cited by 11 | Viewed by 2318

Abstract

In recent years, the absence of experimental evidence for searches dedicated to dark matter has triggered the development of new ideas on the nature of this entity, which manifests at the cosmological level. Some of these can be explored by small experiments with [...] Read more.

In recent years, the absence of experimental evidence for searches dedicated to dark matter has triggered the development of new ideas on the nature of this entity, which manifests at the cosmological level. Some of these can be explored by small experiments with a short timescale and an investment that can be afforded by national laboratories, such as the Frascati one. This is the main reason why a laboratory that, traditionally, was focused in particle physics studies with accelerators has begun intense activity in this field of research. Full article

(This article belongs to the Special Issue Italian Research Facilities for Fundamental Physics)

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23 pages, 515 KiB

Open AccessArticle

Probing Gravitational Waves from Pulsars in Brans–Dicke Theory

by Paritosh Verma

Universe 2021, 7(7), 235; https://doi.org/10.3390/universe7070235 - 9 Jul 2021

Cited by 6 | Viewed by 4852

Abstract

This paper comprises the theoretical background for the data analysis of gravitational waves (GWs) from spinning neutron stars in Brans–Dicke (BD) theory. Einstein’s general theory of relativity (GR) predicts only two tensor polarization states, but a generic metric theory of gravity can also [...] Read more.

This paper comprises the theoretical background for the data analysis of gravitational waves (GWs) from spinning neutron stars in Brans–Dicke (BD) theory. Einstein’s general theory of relativity (GR) predicts only two tensor polarization states, but a generic metric theory of gravity can also possess scalar and vector polarization states. The BD theory attempts to modify the GR by varying gravitational constant G, and it has three polarization states. The first two states are the same as in GR, and the third one is scalar polarization. We derive the response of a laser interferometric detector to the GW signal from a spinning neutron star in BD theory. We obtain a statistic based on the maximum likelihood principle to identify the signal in BD theory in the detector’s noise. This statistic generalizes the well known

F

-statistic used in the case of GR. We perform Monte Carlo simulations in Gaussian noise to test the detectability of the signal and the accuracy of estimation of its parameters. Full article

(This article belongs to the Special Issue Continuous Gravitational Waves)

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43 pages, 1529 KiB

Open AccessArticle

Next Generation Design and Prospects for Cannex

by René I. P. Sedmik and Mario Pitschmann

Universe 2021, 7(7), 234; https://doi.org/10.3390/universe7070234 - 9 Jul 2021

Cited by 17 | Viewed by 3106

Abstract

The Casimir And Non-Newtonian force EXperiment (Cannex) implements the unique geometry of macroscopic plane parallel plates that guarantees an optimum sensitivity with respect to interfacial forces and their gradients. Based on experience from the recently completed proof-of-principle phase, we have started [...] Read more.

The Casimir And Non-Newtonian force EXperiment (Cannex) implements the unique geometry of macroscopic plane parallel plates that guarantees an optimum sensitivity with respect to interfacial forces and their gradients. Based on experience from the recently completed proof-of-principle phase, we have started a re-design of the setup aiming to reduce systematic effects and maximize the achievable sensitivity. Several propositions have been made to measure Casimir forces in and out of thermal equilibrium, hypothetical axion and axion-like dark matter interactions, and forces originating from chameleon or symmetron dark energy interactions. In the present article, we give details on the design for the next implementation stage of Cannex and discuss the experimental opportunities, as well as limitations expected for this new setup. Full article

(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)

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17 pages, 734 KiB

Open AccessArticle

Dust Production around Carbon-Rich Stars: The Role of Metallicity

by Ambra Nanni, Sergio Cristallo, Jacco Th. van Loon and Martin A. T. Groenewegen

Universe 2021, 7(7), 233; https://doi.org/10.3390/universe7070233 - 7 Jul 2021

Cited by 7 | Viewed by 2008

Abstract

Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended [...] Read more.

Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended circumstellar envelopes. Among these stars, carbon-rich TP-AGB stars (C-stars) are particularly relevant for the chemical enrichment of galaxies. We here investigated the role of the metallicity in the dust formation process from a theoretical viewpoint. Methods: We coupled an up-to-date description of dust growth and dust-driven wind, which included the time-averaged effect of shocks, with FRUITY stellar evolutionary tracks. We compared our predictions with observations of C-stars in our Galaxy, in the Magellanic Clouds (LMC and SMC) and in the Galactic Halo, characterised by metallicity between solar and 1/10 of solar. Results: Our models explained the variation of the gas and dust content around C-stars derived from the IRS Spitzer spectra. The wind speed of the C-stars at varying metallicity was well reproduced by our description. We predicted the wind speed at metallicity down to 1/10 of solar in a wide range of mass-loss rates. Full article

(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)

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18 pages, 333 KiB

Open AccessReview

Remarks on Some Results Related to the Thermal Casimir Effect in Einstein and Closed Friedmann Universes with a Cosmic String

by Valdir Barbosa Bezerra, Herondy Francisco Santana Mota, Celio Rodrigues Muniz and Carlos Augusto Romero Filho

Universe 2021, 7(7), 232; https://doi.org/10.3390/universe7070232 - 7 Jul 2021

Cited by 4 | Viewed by 1971

Abstract

In this paper, we present a review of some recent results concerning the thermal corrections to the Casimir energy of massless scalar, electromagnetic, and massless spinor fields in the Einstein and closed Friedmann universes with a cosmic string. In the case of a [...] Read more.

In this paper, we present a review of some recent results concerning the thermal corrections to the Casimir energy of massless scalar, electromagnetic, and massless spinor fields in the Einstein and closed Friedmann universes with a cosmic string. In the case of a massless scalar field, it is shown that the Casimir energy can be written as a simple sum of two terms; the first one corresponds to the Casimir energy for the massless scalar field in the Einstein and Friedmann universes without a cosmic string, whereas the second one is simply the Casimir energy of the electromagnetic field in these backgrounds, multiplied by a parameter

λ = (1 / α) - 1

, where

α

is a constant that codifies the presence of the cosmic string, and is related to its linear mass density,

μ

, by the expression

α = 1 - G μ

. The Casimir free energy and the internal energy at a temperature different from zero, as well as the Casimir entropy, are given by similar sums. In the cases of the electromagnetic and massless spinor fields, the Casimir energy, free energy, internal energy, and Casimir entropy are also given by the sum of two terms, similarly to the previous cases, but now with both terms related to the same field. Using the results obtained concerning the mentioned thermodynamic quantities, their behavior at high and low temperatures limits are studied. All these results are particularized to the scenario in which the cosmic string is absent. Some discussions concerning the validity of the Nernst heat theorem are included as well. Full article

(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)

58 pages, 10202 KiB

Open AccessReview

Borexino Results on Neutrinos from the Sun and Earth

by Sindhujha Kumaran, Livia Ludhova, Ömer Penek and Giulio Settanta

Universe 2021, 7(7), 231; https://doi.org/10.3390/universe7070231 - 6 Jul 2021

Cited by 15 | Viewed by 3869

Abstract

Borexino is a 280-ton liquid scintillator detector located at the Laboratori Nazionali del Gran Sasso in Italy. Since the start of its data-taking in May 2007, it has provided several measurements of low-energy neutrinos from various sources. At the base of its success [...] Read more.

Borexino is a 280-ton liquid scintillator detector located at the Laboratori Nazionali del Gran Sasso in Italy. Since the start of its data-taking in May 2007, it has provided several measurements of low-energy neutrinos from various sources. At the base of its success lie unprecedented levels of radio-purity and extensive thermal stabilization, both resulting from a years-long effort of the collaboration. Solar neutrinos, emitted in the Hydrogen-to-Helium fusion in the solar core, are important for the understanding of our star, as well as neutrino properties. Borexino is the only experiment that has performed a complete spectroscopy of the pp chain solar neutrinos (with the exception of the hep neutrinos contributing to the total flux at

10^{- 5}

level), through the detection of pp,

^{7}

Be, pep, and

^{8}

B solar neutrinos and has experimentally confirmed the existence of the CNO fusion cycle in the Sun. Borexino has also detected geoneutrinos, antineutrinos from the decays of long-lived radioactive elements inside the Earth, that can be exploited as a new and unique tool to study our planet. This paper reviews the most recent Borexino results on solar and geoneutrinos, from highlighting the key elements of the analyses up to the discussion and interpretation of the results for neutrino, solar, and geophysics. Full article

(This article belongs to the Special Issue Italian Research Facilities for Fundamental Physics)

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8 pages, 521 KiB

Open AccessCommunication

The Possibility of a Non-Lagrangian Theory of Gravity

by Celia Escamilla-Rivera and Júlio C. Fabris

Universe 2021, 7(7), 230; https://doi.org/10.3390/universe7070230 - 6 Jul 2021

Cited by 1 | Viewed by 2448

Abstract

General Relativity resembles a very elegant crystal glass: If we touch its principles, that is, its Lagrangian, there is a risk of breaking everything. Or, if we will, it is like a short blanket: Curing some problems creates new problems. This paper is [...] Read more.

General Relativity resembles a very elegant crystal glass: If we touch its principles, that is, its Lagrangian, there is a risk of breaking everything. Or, if we will, it is like a short blanket: Curing some problems creates new problems. This paper is devoted to bring to light the reasons why we pursue the possibility of a non-Lagrangian theory of gravity under the hypothesis of an extension of the original general relativity with an ansatz inspired in the fundamental principles of classical and quantum physics. Full article

(This article belongs to the Special Issue New Challenges in Modified Gravity at Cosmological and Astrophysical Scales)

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29 pages, 436 KiB

Open AccessReview

Perturbative versus Non-Perturbative Quantum Field Theory: Tao’s Method, the Casimir Effect, and Interacting Wightman Theories

by Walter Felipe Wreszinski

Universe 2021, 7(7), 229; https://doi.org/10.3390/universe7070229 - 5 Jul 2021

Cited by 3 | Viewed by 2822

Abstract

We dwell upon certain points concerning the meaning of quantum field theory: the problems with the perturbative approach, and the question raised by ’t Hooft of the existence of the theory in a well-defined (rigorous) mathematical sense, as well as some of the [...] Read more.

We dwell upon certain points concerning the meaning of quantum field theory: the problems with the perturbative approach, and the question raised by ’t Hooft of the existence of the theory in a well-defined (rigorous) mathematical sense, as well as some of the few existent mathematically precise results on fully quantized field theories. Emphasis is brought on how the mathematical contributions help to elucidate or illuminate certain conceptual aspects of the theory when applied to real physical phenomena, in particular, the singular nature of quantum fields. In a first part, we present a comprehensive review of divergent versus asymptotic series, with qed as background example, as well as a method due to Terence Tao which conveys mathematical sense to divergent series. In a second part, we apply Tao’s method to the Casimir effect in its simplest form, consisting of perfectly conducting parallel plates, arguing that the usual theory, which makes use of the Euler-MacLaurin formula, still contains a residual infinity, which is eliminated in our approach. In the third part, we revisit the general theory of nonperturbative quantum fields, in the form of newly proposed (with Christian Jaekel) Wightman axioms for interacting field theories, with applications to “dressed” electrons in a theory with massless particles (such as qed), as well as unstable particles. Various problems (mostly open) are finally discussed in connection with concrete models. Full article

(This article belongs to the Special Issue Development of Modern Methods of QFT and Their Applications)

21 pages, 4050 KiB

Open AccessArticle

Can Gravitational Waves Halt the Expansion of the Universe?

by Jörg Frauendiener, Jonathan Hakata and Chris Stevens

Universe 2021, 7(7), 228; https://doi.org/10.3390/universe7070228 - 5 Jul 2021

Viewed by 2481

Abstract

We numerically investigate the propagation of plane gravitational waves in the form of an initial boundary value problem with de Sitter initial data. The full non-linear Einstein equations with positive cosmological constant

λ

are written in the Friedrich–Nagy gauge which yields a wellposed [...] Read more.

We numerically investigate the propagation of plane gravitational waves in the form of an initial boundary value problem with de Sitter initial data. The full non-linear Einstein equations with positive cosmological constant

λ

are written in the Friedrich–Nagy gauge which yields a wellposed system. The propagation of a single wave and the collision of two with colinear polarization are studied and contrasted with their Minkowskian analogues. Unlike with

λ = 0

, critical behaviours are found with

λ > 0

and are based on the relationship between the wave profile and

λ

. We find that choosing boundary data close to one of these bifurcations results in a “false” steady state which violates the constraints. Simulations containing (approximate) impulsive wave profiles are run and general features are discussed. Analytic results of Tsamis and Woodard, which describe how gravitational waves could affect an expansion rate at an initial instance of time, are explored and generalized to the entire space–time. Finally we put forward boundary conditions that, at least locally, slow down the expansion considerably for a time. Full article

(This article belongs to the Special Issue Gravitational Waves)

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24 pages, 442 KiB

Open AccessArticle

Gravitationally Induced Particle Production through a Nonminimal Torsion–Matter Coupling

by Tiberiu Harko, Francisco S. N. Lobo and Emmanuel N. Saridakis

Universe 2021, 7(7), 227; https://doi.org/10.3390/universe7070227 - 5 Jul 2021

Cited by 19 | Viewed by 1947

Abstract

We investigate the possibility of gravitationally generated particle production via the mechanism of nonminimal torsion–matter coupling. An intriguing feature of this theory is that the divergence of the matter energy–momentum tensor does not vanish identically. We explore the physical and cosmological implications of [...] Read more.

We investigate the possibility of gravitationally generated particle production via the mechanism of nonminimal torsion–matter coupling. An intriguing feature of this theory is that the divergence of the matter energy–momentum tensor does not vanish identically. We explore the physical and cosmological implications of the nonconservation of the energy–momentum tensor by using the formalism of irreversible thermodynamics of open systems in the presence of matter creation/annihilation. The particle creation rates, pressure, and the expression of the comoving entropy are obtained in a covariant formulation and discussed in detail. Applied together with the gravitational field equations, the thermodynamics of open systems lead to a generalization of the standard

Λ

CDM cosmological paradigm, in which the particle creation rates and pressures are effectively considered as components of the cosmological fluid energy–momentum tensor. We consider specific models, and we show that cosmology with a torsion–matter coupling can almost perfectly reproduce the

Λ

CDM scenario, while it additionally gives rise to particle creation rates, creation pressures, and entropy generation through gravitational matter production in both low and high redshift limits. Full article

(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)

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12 pages, 317 KiB

Open AccessArticle

Anisotropic Cosmological Model in a Modified Theory of Gravitation

by Rishi Kumar Tiwari, Aroonkumar Beesham, Soma Mishra and Vipin Dubey

Universe 2021, 7(7), 226; https://doi.org/10.3390/universe7070226 - 4 Jul 2021

Cited by 10 | Viewed by 2521

Abstract

Current observations indicate that, on a large enough scale, the universe is homogeneous and isotropic. However, this does not preclude the possibility of some anisotropy having occurred during the early stages of the evolution of the universe, which could then have been damped [...] Read more.

Current observations indicate that, on a large enough scale, the universe is homogeneous and isotropic. However, this does not preclude the possibility of some anisotropy having occurred during the early stages of the evolution of the universe, which could then have been damped out later. This idea has aroused interest in the Bianchi models, which are homogeneous but anisotropic. Secondly, there is much interest in modified gravity these days due to the problems that the usual ΛCDM model faces in general relativity. Hence, in this paper, a study was conducted on the Bianchi type-I cosmological model in f(R,T)-modified gravity. Following some ideas from cosmography, a specific form of the deceleration parameter was assumed, leading to a model that exhibited a transition from early deceleration to late-time acceleration. The derived model approached isotropy at late times. The physical properties of the model were discussed, and expressions for the various parameters of the model were derived. It is also possible to make progress towards solving the cosmological constant problem, since in this model in f(R,T) gravity, a variable cosmological-type parameter arose, which was large early on but decreased to a constant value in later times. Full article

(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))

34 pages, 867 KiB

Open AccessArticle

Unifying Theory for Casimir Forces: Bulk and Surface Formulations

by Giuseppe Bimonte and Thorsten Emig

Universe 2021, 7(7), 225; https://doi.org/10.3390/universe7070225 - 4 Jul 2021

Cited by 8 | Viewed by 2473

Abstract

The principles of the electromagnetic fluctuation-induced phenomena such as Casimir forces are well understood. However, recent experimental advances require universal and efficient methods to compute these forces. While several approaches have been proposed in the literature, their connection is often not entirely clear, [...] Read more.

The principles of the electromagnetic fluctuation-induced phenomena such as Casimir forces are well understood. However, recent experimental advances require universal and efficient methods to compute these forces. While several approaches have been proposed in the literature, their connection is often not entirely clear, and some of them have been introduced as purely numerical techniques. Here we present a unifying approach for the Casimir force and free energy that builds on both the Maxwell stress tensor and path integral quantization. The result is presented in terms of either bulk or surface operators that describe corresponding current fluctuations. Our surface approach yields a novel formula for the Casimir free energy. The path integral is presented both within a Lagrange and Hamiltonian formulation yielding different surface operators and expressions for the free energy that are equivalent. We compare our approaches to previously developed numerical methods and the scattering approach. The practical application of our methods is exemplified by the derivation of the Lifshitz formula. Full article

(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)

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9 pages, 194 KiB

Open AccessReview

Astro-Particle Physics at INFN

by Oliviero Cremonesi

Universe 2021, 7(7), 224; https://doi.org/10.3390/universe7070224 - 4 Jul 2021

Cited by 1 | Viewed by 1571

Abstract

In Italy, INFN coordinates the research in the field of astro-particle physics. The supported experimental activities include the study of the cosmic radiation, the search of gravitational waves, the study of dark universe, general and quantum physics, and the study of the neutrino [...] Read more.

In Italy, INFN coordinates the research in the field of astro-particle physics. The supported experimental activities include the study of the cosmic radiation, the search of gravitational waves, the study of dark universe, general and quantum physics, and the study of the neutrino properties. A rich program of experiments installed on the earth, in the space, and underground or underwater is being supported to provide a possible answer to some of the most relevant open questions of particle physics, astrophysics, and cosmology. A short overview of the ongoing effort is presented. Full article

(This article belongs to the Special Issue Italian Research Facilities for Fundamental Physics)

48 pages, 2527 KiB

Open AccessReview

The Gamma-ray Window to Intergalactic Magnetism

by Rafael Alves Batista and Andrey Saveliev

Universe 2021, 7(7), 223; https://doi.org/10.3390/universe7070223 - 2 Jul 2021

Cited by 59 | Viewed by 3723

Abstract

One of the most promising ways to probe intergalactic magnetic fields (IGMFs) is through gamma rays produced in electromagnetic cascades initiated by high-energy gamma rays or cosmic rays in the intergalactic space. Because the charged component of the cascade is sensitive to magnetic [...] Read more.

One of the most promising ways to probe intergalactic magnetic fields (IGMFs) is through gamma rays produced in electromagnetic cascades initiated by high-energy gamma rays or cosmic rays in the intergalactic space. Because the charged component of the cascade is sensitive to magnetic fields, gamma-ray observations of distant objects such as blazars can be used to constrain IGMF properties. Ground-based and space-borne gamma-ray telescopes deliver spectral, temporal, and angular information of high-energy gamma-ray sources, which carries imprints of the intervening magnetic fields. This provides insights into the nature of the processes that led to the creation of the first magnetic fields and into the phenomena that impacted their evolution. Here we provide a detailed description of how gamma-ray observations can be used to probe cosmic magnetism. We review the current status of this topic and discuss the prospects for measuring IGMFs with the next generation of gamma-ray observatories. Full article

(This article belongs to the Special Issue High-Energy Gamma-Ray Astronomy: Results on Fundamental Questions after 30 Years of Ground-Based Observations)

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