Universe

23 pages, 356 KiB

Open AccessReview

Analogous Hawking Effect in Dielectric Media and Solitonic Solutions

by Francesco Belgiorno and Sergio L. Cacciatori

Universe 2020, 6(8), 127; https://doi.org/10.3390/universe6080127 - 17 Aug 2020

Cited by 5 | Viewed by 1993

We review some aspects of our longstanding research concerning the analogous Hawking effect in dispersive dielectric media. We introduce nonlinear contributions in the polarization field in the relativistically covariant version of the Hopfield model and then, in order to provide a simplified description [...] Read more.

We review some aspects of our longstanding research concerning the analogous Hawking effect in dispersive dielectric media. We introduce nonlinear contributions in the polarization field in the relativistically covariant version of the Hopfield model and then, in order to provide a simplified description aimed at avoiding some subtleties in the quantization of the original model, we discuss the so-called

ϕ ψ

-model. We show that the nonlinearity allows for introducing in a self-consistent way the otherwise phenomenological dependence of the susceptibility and of the resonance frequency

ω_{0}

on the spacetime variables, and this is a consequence of the linearization of the model around solitonic solutions representing propagating perturbations of the refractive index, to be then associated with the Hawking effect. Full article

(This article belongs to the Section Field Theory)

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2 pages, 186 KiB

Open AccessEditorial

Seyfert Galaxies Astrophysics

by Luigi Foschini

Universe 2020, 6(8), 126; https://doi.org/10.3390/universe6080126 - 17 Aug 2020

Cited by 1 | Viewed by 1871

Abstract

Looking back in time, it is somehow surprising to see the enormous advancement made by astrophysics in just about one century [...] Full article

(This article belongs to the Special Issue Seyfert Galaxies Astrophysics)

27 pages, 352 KiB

Open AccessReview

Black-Hole Models in Loop Quantum Gravity

by Martin Bojowald

Universe 2020, 6(8), 125; https://doi.org/10.3390/universe6080125 - 14 Aug 2020

Cited by 45 | Viewed by 3919

Abstract

Dynamical black-hole scenarios have been developed in loop quantum gravity in various ways, combining results from mini and midisuperspace models. In the past, the underlying geometry of space-time has often been expressed in terms of line elements with metric components that differ from [...] Read more.

Dynamical black-hole scenarios have been developed in loop quantum gravity in various ways, combining results from mini and midisuperspace models. In the past, the underlying geometry of space-time has often been expressed in terms of line elements with metric components that differ from the classical solutions of general relativity, motivated by modified equations of motion and constraints. However, recent results have shown by explicit calculations that most of these constructions violate general covariance and slicing independence. The proposed line elements and black-hole models are therefore ruled out. The only known possibility to escape this sentence is to derive not only modified metric components but also a new space-time structure which is covariant in a generalized sense. Formally, such a derivation is made available by an analysis of the constraints of canonical gravity, which generate deformations of hypersurfaces in space-time, or generalized versions if the constraints are consistently modified. A generic consequence of consistent modifications in effective theories suggested by loop quantum gravity is signature change at high density. Signature change is an important ingredient in long-term models of black holes that aim to determine what might happen after a black hole has evaporated. Because this effect changes the causal structure of space-time, it has crucial implications for black-hole models that have been missed in several older constructions, for instance in models based on bouncing black-hole interiors. Such models are ruled out by signature change even if their underlying space-times are made consistent using generalized covariance. The causal nature of signature change brings in a new internal consistency condition, given by the requirement of deterministic behavior at low curvature. Even a causally disconnected interior transition, opening back up into the former exterior as some kind of astrophysical white hole, is then ruled out. New versions consistent with both generalized covariance and low-curvature determinism are introduced here, showing a remarkable similarity with models developed in other approaches, such as the final-state proposal or the no-transition principle obtained from the gauge-gravity correspondence. Full article

(This article belongs to the Special Issue Universe: 5th Anniversary)

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15 pages, 1024 KiB

Open AccessArticle

Quark Stars in Massive Brans–Dicke Gravity with Tolman–Kuchowicz Spacetime

by Amal Majid and M. Sharif

Universe 2020, 6(8), 124; https://doi.org/10.3390/universe6080124 - 13 Aug 2020

Cited by 31 | Viewed by 2435

Abstract

In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the [...] Read more.

In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the stellar model to evaluate the unknown constants in terms of mass and radius of the star. The radius of the strange star candidate PSR J1614-2230 is predicted by assuming maximum anisotropy at the surface of the star for different values of the coupling parameter, mass of the scalar field and bag constant. We examine various properties as well as the viability and stability of the anisotropic sphere. We conclude that the astrophysical model agrees with the essential criteria of a physically realistic model for higher values of the coupling parameter as well as mass of the scalar field. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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40 pages, 519 KiB

Open AccessArticle

Dark Side of Weyl Gravity

by Petr Jizba, Lesław Rachwał, Stefano G. Giaccari and Jaroslav Kňap

Universe 2020, 6(8), 123; https://doi.org/10.3390/universe6080123 - 12 Aug 2020

Cited by 7 | Viewed by 3130

Abstract

We address the issue of a dynamical breakdown of scale invariance in quantum Weyl gravity together with related cosmological implications. In the first part, we build on our previous work [Phys. Rev. D2020, 101, 044050], where we found a [...] Read more.

We address the issue of a dynamical breakdown of scale invariance in quantum Weyl gravity together with related cosmological implications. In the first part, we build on our previous work [Phys. Rev. D2020, 101, 044050], where we found a non-trivial renormalization group fixed point in the infrared sector of quantum Weyl gravity. Here, we prove that the ensuing non-Gaussian IR fixed point is renormalization scheme independent. This confirms the feasibility of the analog of asymptotic safety scenario for quantum Weyl gravity in the IR. Some features, including non-analyticity and a lack of autonomy, of the system of

β

-functions near a turning point of the renormalization group at intermediate energies are also described. We further discuss an extension of the renormalization group analysis to the two-loop level. In particular, we show universal properties of the system of

β

-functions related to three couplings associated with

C^{2}

(Weyl square),

G

(Gauss–Bonnet), and

R^{2}

(Ricci curvature square) terms. Finally, we discuss various technical and conceptual issues associated with the conformal (trace) anomaly and propose possible remedies. In the second part, we analyze physics in the broken phase. In particular, we show that, in the low-energy sector of the broken phase, the theory looks like Starobinsky

f (R)

gravity with a gravi-cosmological constant that has a negative sign in comparison to the usual matter-induced cosmological constant. We discuss implications for cosmic inflation and highlight a non-trivial relation between Starobinsky’s parameter and the gravi-cosmological constant. Salient issues, including possible UV completions of quantum Weyl gravity and the role of the trace anomaly matching, are also discussed. Full article

(This article belongs to the Special Issue Shedding Light to the Dark Sides of the Universe: Cosmology from Strong Interactions)

8 pages, 1993 KiB

Open AccessCommunication

Ionospheric Behavior of foF2 over Chinese EIA Region and Its Comparison with IRI-2016

by Peng Zhu, Cong Xie, Chunhua Jiang, Guobin Yang, Jing Liu, Zhengqiang Li and Zhengyu Zhao

Universe 2020, 6(8), 122; https://doi.org/10.3390/universe6080122 - 11 Aug 2020

Cited by 3 | Viewed by 2407

Abstract

The ionograms, which were recorded by the ionosonde located at Pu’er station (PUR, 22.7° N, 101.05° E, Dip Latitude 12.9° N) in the Southwest of China in the year of 2016, were used to study the ionospheric behavior of the ordinary critical frequency [...] Read more.

The ionograms, which were recorded by the ionosonde located at Pu’er station (PUR, 22.7° N, 101.05° E, Dip Latitude 12.9° N) in the Southwest of China in the year of 2016, were used to study the ionospheric behavior of the ordinary critical frequency of the F2 layer (foF2) in the region of the northern equatorial ionization anomaly. To verify the performance of the International Reference Ionosphere (IRI) over the Southwest of China, a comparative study of the observed foF2 and the latest version of the International Reference Ionosphere (IRI-2016) was carried out. We found that the foF2 in equinox months is greater than summer and winter. Moreover, a higher frequency of the observed bite-out of foF2 in January and April than other months and the IRI-2016 cannot represent the bite-out of foF2 in diurnal variations. Compared to the observations at Pu’er Station, the IRI-2016 underestimated foF2 for most time of the year. The IRI with the International Radio Consultative Committee (CCIR) option overestimated foF2 is higher than that with the International Union of Radio Science (URSI) option. Furthermore, the normalized root mean square error of foF2 from the IRI-2016 with the CCIR option is less than that with the URSI. Full article

(This article belongs to the Special Issue Space Weather)

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15 pages, 271 KiB

Open AccessArticle

CPTM Discrete Symmetry, Quantum Wormholes and Cosmological Constant Problem

by Sergey Bondarenko

Universe 2020, 6(8), 121; https://doi.org/10.3390/universe6080121 - 11 Aug 2020

Cited by 9 | Viewed by 2288

Abstract

We discuss the consequences of the charge, parity, time, and mass (CPTM) extended reversal symmetry for the problems of the vacuum energy density and value of the cosmological constant. The results obtained are based on the framework with the separation of extended space-time [...] Read more.

We discuss the consequences of the charge, parity, time, and mass (CPTM) extended reversal symmetry for the problems of the vacuum energy density and value of the cosmological constant. The results obtained are based on the framework with the separation of extended space-time of the interest on the different regions connected by this symmetry with the action of the theory valid for the full space-time and symmetrical with respect to the extended CPTM transformations. The cosmological constant is arising in the model due the gravitational interactions between the different parts of the space-time trough the quantum non-local vertices. It is proposed that the constant’s value depends on the form and geometry of the vertices that glue the separated parts of the extended solution of Einstein equations determining, in turn, its classical geometry. The similarity of the proposed model to the bimetric theories of gravitation is also discussed. Full article

(This article belongs to the Special Issue Recent Advances in Wormhole Physics)

6 pages, 270 KiB

Open AccessCommunication

Possible Wormhole Candidates in Active Galactic Nuclei

by Mikhail Piotrovich, Serguei Krasnikov, Stanislava Buliga and Tinatin Natsvlishvili

Universe 2020, 6(8), 120; https://doi.org/10.3390/universe6080120 - 11 Aug 2020

Cited by 3 | Viewed by 1918

Abstract

The hypothesis is considered that the active galactic nuclei (AGNs) are wormhole (WH) mouths rather than supermassive black holes (SMBHs). We study the difference in the physical properties of such objects from those of AGNs with SMBH, as well as the the corresponding [...] Read more.

The hypothesis is considered that the active galactic nuclei (AGNs) are wormhole (WH) mouths rather than supermassive black holes (SMBHs). We study the difference in the physical properties of such objects from those of AGNs with SMBH, as well as the the corresponding difference in observational data. Firstly, the radiative efficiency for some types of WHs (both static and rotating) can be significantly larger than the theoretical maximal value for the Kerr SMBHs. A number of AGNs is presented, for which the observational data can be interpreted as the result of the presence of WHs in them. Secondly, a sufficiently strong gamma radiation with a characteristic spectrum noticeably differing from that of AGNs jets, can be emitted from a static WH as a result of a collision of accreting flows of matter inside the WH. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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21 pages, 513 KiB

Open AccessReview

The Equation of State of Nuclear Matter: From Finite Nuclei to Neutron Stars

by G. Fiorella Burgio and Isaac Vidaña

Universe 2020, 6(8), 119; https://doi.org/10.3390/universe6080119 - 10 Aug 2020

Cited by 32 | Viewed by 4006

Abstract

Background. We investigate possible correlations between neutron star observables and properties of atomic nuclei. In particular, we explore how the tidal deformability of a 1.4 solar mass neutron star,

M_{1.4}

, and the neutron-skin thickness of

^{48}

Ca and

^{208}

Pb are [...] Read more.

Background. We investigate possible correlations between neutron star observables and properties of atomic nuclei. In particular, we explore how the tidal deformability of a 1.4 solar mass neutron star,

M_{1.4}

, and the neutron-skin thickness of

^{48}

Ca and

^{208}

Pb are related to the stellar radius and the stiffness of the symmetry energy. Methods. We examine a large set of nuclear equations of state based on phenomenological models (Skyrme, NLWM, DDM) and ab initio theoretical methods (BBG, Dirac–Brueckner, Variational, Quantum Monte Carlo). Results: We find strong correlations between tidal deformability and NS radius, whereas a weaker correlation does exist with the stiffness of the symmetry energy. Regarding the neutron-skin thickness, weak correlations appear both with the stiffness of the symmetry energy, and the radius of a

M_{1.4}

. Our results show that whereas the considered EoS are compatible with the largest masses observed up to now, only five microscopic models and four Skyrme forces are simultaneously compatible with the present constraints on L and the PREX experimental data on the

^{208}

Pb neutron-skin thickness. We find that all the NLWM and DDM models and the majority of the Skyrme forces are excluded by these two experimental constraints, and that the analysis of the data collected by the NICER mission excludes most of the NLWM considered. Conclusion. The tidal deformability of a

M_{1.4}

and the neutron-skin thickness of atomic nuclei show some degree of correlation with nuclear and astrophysical observables, which however depends on the ensemble of adopted EoS. Full article

(This article belongs to the Special Issue Shedding Light to the Dark Sides of the Universe: Cosmology from Strong Interactions)

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16 pages, 4765 KiB

Open AccessArticle

Paradigms and Scenarios for the Dark Matter Phenomenon

by Paolo Salucci, Nicola Turini and Chiara di Paolo

Universe 2020, 6(8), 118; https://doi.org/10.3390/universe6080118 - 10 Aug 2020

Cited by 28 | Viewed by 2715

Abstract

Well known scaling laws among the structural properties of the dark and the luminous matter in disc systems are too complex to be arisen by two inert components that just share the same gravitational field. This brings us to critically focus on the [...] Read more.

Well known scaling laws among the structural properties of the dark and the luminous matter in disc systems are too complex to be arisen by two inert components that just share the same gravitational field. This brings us to critically focus on the 30-year-old paradigm, that, resting on a priori knowledge of the nature of Dark Matter (DM), has led us to a restricted number of scenarios, especially favouring the collisionless

Λ

Cold Dark Matter one. Motivated by such observational evidence, we propose to resolve the dark matter mystery by following a new Paradigm: the nature of DM must be guessed/derived by deeply analyzing the properties of the dark and luminous mass distribution at galactic scales. The immediate application of this paradigm leads us to propose the existence of a direct interaction between Dark and Standard Model particles, which has finely shaped the inner regions of galaxies. Full article

(This article belongs to the Special Issue Relevant Tensions in the Standard Cosmological Model and Small Scale Problem of Cold Dark Matter Paradigm)

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9 pages, 295 KiB

Open AccessArticle

Intensity and Polarization of Radiation Emerging from Rotating Accretion Disc with Increasing Height

by Nikolai Silant’ev, Galina Alekseeva, Yulia Ananjevskaja and Viktor Novikov

Universe 2020, 6(8), 117; https://doi.org/10.3390/universe6080117 - 10 Aug 2020

Cited by 2 | Viewed by 1895

Abstract

We consider the radiation emission in continuum and spectral lines from rotating accretion disc with the progressive increasing height. It is known that for the plane accretion disc with homogeneous atmosphere the wave electric field

E

is perpendicular to the plane between line [...] Read more.

We consider the radiation emission in continuum and spectral lines from rotating accretion disc with the progressive increasing height. It is known that for the plane accretion disc with homogeneous atmosphere the wave electric field

E

is perpendicular to the plane between line of sight

n

and the normal to the disc

N

. For the expanding accretion disc the wave electric field from every inclined cone-like part with given azimuthal angle

φ

has direction perpendicular to the plane between

n

and local normal

N^{'}

to the inclined surface. This behaviour is the consequence of homogeneity of inclined atmosphere and has purely geometrical origin. The geometrical consideration shows that the position angles of polarized radiation in the right and left parts relative to plane (nN) of the inclined accretion disc have opposite values. Therefore, for inclined accretion disc the integral continuum radiation has the usual polarization angle perpendicular to the plane

(nN)

, but smaller degree of polarization and less alongated along the normal

N

than that for the plane accretion disc. For spectral line, due to the Doppler effect, the polarization (position) angles have opposite signs for the red and blue wings. Such behaviour is frequently observed in H

α

-radiation. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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14 pages, 1764 KiB

Open AccessArticle

Statistical Analysis of Field-Aligned Alfvénic Turbulence and Intermittency in Fast Solar Wind

by Francesco Carbone, Daniele Telloni, Luca Sorriso-Valvo, Gary Zank, Lingling Zhao, Laxman Adhikari and Roberto Bruno

Universe 2020, 6(8), 116; https://doi.org/10.3390/universe6080116 - 9 Aug 2020

Cited by 5 | Viewed by 2582

Abstract

The statistical properties of fast Alfvénic solar wind turbulence have been analyzed by means of empirical mode decomposition and the associated Hilbert spectral analysis. The stringent criteria employed for the data selection in the Wind spacecraft database, has made possible to sample multiple [...] Read more.

The statistical properties of fast Alfvénic solar wind turbulence have been analyzed by means of empirical mode decomposition and the associated Hilbert spectral analysis. The stringent criteria employed for the data selection in the Wind spacecraft database, has made possible to sample multiple

k_{‖}

field-aligned intervals of the three magnetic field components. The results suggest that the spectral anisotropy predicted by the critical balance theory is not observed in the selected database, whereas a Kolmogorov-like scaling (

E (k_{‖}) \sim k^{- 5 / 3}

) and a weak or absent level of intermittency are robust characteristics of the Alfvénic slab component of solar wind turbulence. Full article

(This article belongs to the Special Issue Solar Wind Turbulence: New Advances from the Parker Solar Probe and Solar Orbiter Space Missions)

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16 pages, 812 KiB

Open AccessArticle

Nuclear Pairing Gaps and Neutron Star Cooling

by Jin-Biao Wei, Fiorella Burgio and Hans-Josef Schulze

Universe 2020, 6(8), 115; https://doi.org/10.3390/universe6080115 - 8 Aug 2020

Cited by 6 | Viewed by 3004

Abstract

We study the cooling of isolated neutron stars with particular regard to the importance of nuclear pairing gaps. A microscopic nuclear equation of state derived in the Brueckner-Hartree-Fock approach is used together with compatible neutron and proton pairing gaps. We then study the [...] Read more.

We study the cooling of isolated neutron stars with particular regard to the importance of nuclear pairing gaps. A microscopic nuclear equation of state derived in the Brueckner-Hartree-Fock approach is used together with compatible neutron and proton pairing gaps. We then study the effect of modifying the gaps on the final deduced neutron star mass distributions. We find that a consistent description of all current cooling data can be achieved and a reasonable neutron star mass distribution can be predicted employing the (slightly reduced by about 40%) proton 1S0 Bardeen-Cooper-Schrieffer (BCS) gaps and no neutron 3P2 pairing. Full article

(This article belongs to the Special Issue Superfluidity and Superconductivity in Neutron Stars)

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17 pages, 704 KiB

Open AccessArticle

A Chi-Squared Analysis of the Measurements of Two Cosmological Parameters over Time

by Timothy Faerber and Martín López-Corredoira

Universe 2020, 6(8), 114; https://doi.org/10.3390/universe6080114 - 7 Aug 2020

Cited by 5 | Viewed by 3228

Abstract

The aim of this analysis was to determine whether or not the given error bars truly represented the dispersion of values in a historical compilation of two cosmological parameters: the amplitude of mass fluctuations (

σ_{8}

) and Hubble’s constant ( [...] Read more.

The aim of this analysis was to determine whether or not the given error bars truly represented the dispersion of values in a historical compilation of two cosmological parameters: the amplitude of mass fluctuations (

σ_{8}

) and Hubble’s constant (

H_{0}

) parameters in the standard cosmological model. For this analysis, a chi-squared test was executed on a compiled list of past measurements. It was found through analysis of the chi-squared (

χ^{2}

) values of the data that for

σ_{8}

(60 data points measured between 1993 and 2019 and

χ^{2}

between 182.4 and 189.0) the associated probability Q is extremely low, with

Q = 1.6 \times 10^{- 15}

for the weighted average and

Q = 8.8 \times 10^{- 15}

for the best linear fit of the data. This was also the case for the

χ^{2}

values of

H_{0}

(163 data points measured between 1976 and 2019 and

χ^{2}

between 480.1 and 575.7), where

Q = 1.8 \times 10^{- 33}

for the linear fit of the data and

Q = 1.0 \times 10^{- 47}

for the weighted average of the data. The general conclusion was that the statistical error bars associated with the observed parameter measurements have been underestimated or the systematic errors were not properly taken into account in at least 20% of the measurements. The fact that the underestimation of error bars for

H_{0}

is so common might explain the apparent 4.4

σ

discrepancy formally known today as the Hubble tension. Full article

(This article belongs to the Special Issue Universe: 5th Anniversary)

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19 pages, 894 KiB

Open AccessArticle

Guessing the Riddle of a Black Hole

by Boris E. Meierovich

Universe 2020, 6(8), 113; https://doi.org/10.3390/universe6080113 - 7 Aug 2020

Cited by 6 | Viewed by 2570

Abstract

A static structure of matter, extremely compressed to the state of a Bose–Einstein condensate by its own gravitational field, is considered. Instead of the widely spread restriction

det g_{i k} < 0

, I used a weaker condition of regularity: all invariants [...] Read more.

A static structure of matter, extremely compressed to the state of a Bose–Einstein condensate by its own gravitational field, is considered. Instead of the widely spread restriction

det g_{i k} < 0

, I used a weaker condition of regularity: all invariants of

g_{i k}

are finite. This makes it possible to find regular static solutions to Einstein equations for a spherically symmetric distribution of matter with no restriction on total mass. In these regular static solutions, the metric component

g^{r r}

changes its sign twice:

g^{r r} (r) = 0

at

r = r_{g}

and at

r = r_{h} > r_{g}

. The signature of the metric tensor is changed to (+,+,−,−) within the spherical layer

r_{g} < r < r_{h}

. Though the gravitation dominates at extremely high density, I assume that it does not violate the exchange interaction of elementary particles of the Standard Model. The found regular static solution to Einstein equations, having no limitation on mass, pretends to describe the state of a black hole to which the gravitational collapse leads. The features of a collapsed black hole, its internal composition depending on total mass and the relation with surrounding dark matter, are considered. An astrophysical application: The pressure balance at the interface between a black hole and dark matter determines the plateau velocity of a galaxy rotation curve as a function of the black hole mass. The plateau velocity is inversely proportional to the black hole mass. The speed of rotation of a star at the periphery of a galaxy is proportional to the square root of the black hole mass (direct attraction to the center) and inversely proportional to the mass of the same black hole (as the influence of dark matter). For a condensate of massive bosons in the Standard Model, the direct attraction to the black hole and the influence of dark matter are equal if the black hole mass is about

\tilde{M}

∼

4.24 \times 10^{37}

g. In galaxies with black hole masses

M ≳ M_{⊙} = 1.989 \times 10^{33}

g (like UMa: NGC 3726 and UMa: NGC 3769 of the Ursa Major cluster), the motion of stars is driven by dark matter. Their rotation curves should have a well-defined plateau. On the contrary, in galaxies with black hole masses

M > > \tilde{M}

(like in our Milky Way with the black hole mass

M = 8.6 \times 10^{39}

g), the motion of stars is regulated by the black hole in the center. Dark matter does not play a significant role in our Milky Way Galaxy. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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19 pages, 321 KiB

Open AccessArticle

On the Role of Einstein–Cartan Gravity in Fundamental Particle Physics

by Carl F. Diether III and Joy Christian

Universe 2020, 6(8), 112; https://doi.org/10.3390/universe6080112 - 5 Aug 2020

Cited by 8 | Viewed by 4840

Abstract

Two of the major open questions in particle physics are: (1) Why do the elementary fermionic particles that are so far observed have such low mass-energy compared to the Planck energy scale? (2) What mechanical energy may be counterbalancing the divergent electrostatic and [...] Read more.

Two of the major open questions in particle physics are: (1) Why do the elementary fermionic particles that are so far observed have such low mass-energy compared to the Planck energy scale? (2) What mechanical energy may be counterbalancing the divergent electrostatic and strong force energies of point-like charged fermions in the vicinity of the Planck scale? In this paper, using a hitherto unrecognised mechanism derived from the non-linear amelioration of the Dirac equation known as the Hehl–Datta equation within the Einstein–Cartan–Sciama–Kibble (ECSK) extension of general relativity, we present detailed numerical estimates suggesting that the mechanical energy arising from the gravitationally coupled self-interaction in the ECSK theory can address both of these questions in tandem. Full article

(This article belongs to the Special Issue Torsion-Gravity and Spinors in Fundamental Theoretical Physics)

34 pages, 518 KiB

Open AccessReview

Dirac, Majorana, Weyl in 4D

by Loriano Bonora, Roberto Soldati and Stav Zalel

Universe 2020, 6(8), 111; https://doi.org/10.3390/universe6080111 - 4 Aug 2020

Cited by 8 | Viewed by 2875

Abstract

This is a review of some elementary properties of Dirac, Weyl and Majorana spinors in 4D. We focus in particular on the differences between massless Dirac and Majorana fermions, on one side, and Weyl fermions, on the other. We review in detail the [...] Read more.

This is a review of some elementary properties of Dirac, Weyl and Majorana spinors in 4D. We focus in particular on the differences between massless Dirac and Majorana fermions, on one side, and Weyl fermions, on the other. We review in detail the definition of their effective actions, when coupled to (vector and axial) gauge fields, and revisit the corresponding anomalies using the Feynman diagram method with different regularisations. Among various well known results we stress in particular the regularisation independence in perturbative approaches, while not all the regularisations fit the non-perturbative ones. As for anomalies, we highlight in particular one perhaps not so well known feature: the rigid relation between chiral and trace anomalies. Full article

(This article belongs to the Special Issue 80 Years of Professor Wigner's Seminal Work "On Unitary Representations of the Inhomogeneous Lorentz Group")

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25 pages, 24668 KiB

Open AccessArticle

Approximate Analytical Periodic Solutions to the Restricted Three-Body Problem with Perturbation, Oblateness, Radiation and Varying Mass

by Fabao Gao and Yongqing Wang

Universe 2020, 6(8), 110; https://doi.org/10.3390/universe6080110 - 4 Aug 2020

Cited by 14 | Viewed by 2731

Abstract

Against the background of a restricted three-body problem consisting of a supergiant eclipsing binary system, the two primaries are composed of a pair of bright oblate stars whose mass changes with time. The zero-velocity surface and curve of the problem are numerically studied [...] Read more.

Against the background of a restricted three-body problem consisting of a supergiant eclipsing binary system, the two primaries are composed of a pair of bright oblate stars whose mass changes with time. The zero-velocity surface and curve of the problem are numerically studied to describe the third body’s motion area, and the corresponding five libration points are obtained. Moreover, the effect of small perturbations, Coriolis and centrifugal forces, radiative pressure, and the oblateness and mass parameters of the two primaries on the third body’s dynamic behavior is discussed through the bifurcation diagram. Furthermore, the second- and third-order approximate analytical periodic solutions around the collinear solution point

L_{3}

in two-dimensional plane and three-dimensional spaces are presented by using the Lindstedt-Poincaré perturbation method. Full article

(This article belongs to the Section Planetary Sciences)

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12 pages, 1396 KiB

Open AccessArticle

A Bayesian Approach to the Simulation Argument

by David Kipping

Universe 2020, 6(8), 109; https://doi.org/10.3390/universe6080109 - 3 Aug 2020

Cited by 3 | Viewed by 49574

Abstract

The Simulation Argument posed by Bostrom suggests that we may be living inside a sophisticated computer simulation. If posthuman civilizations eventually have both the capability and desire to generate such Bostrom-like simulations, then the number of simulated realities would greatly exceed the one [...] Read more.

The Simulation Argument posed by Bostrom suggests that we may be living inside a sophisticated computer simulation. If posthuman civilizations eventually have both the capability and desire to generate such Bostrom-like simulations, then the number of simulated realities would greatly exceed the one base reality, ostensibly indicating a high probability that we do not live in said base reality. In this work, it is argued that since the hypothesis that such simulations are technically possible remains unproven, statistical calculations need to consider not just the number of state spaces, but the intrinsic model uncertainty. This is achievable through a Bayesian treatment of the problem, which is presented here. Using Bayesian model averaging, it is shown that the probability that we are sims is in fact less than 50%, tending towards that value in the limit of an infinite number of simulations. This result is broadly indifferent as to whether one conditions upon the fact that humanity has not yet birthed such simulations, or ignore it. As argued elsewhere, it is found that if humanity does start producing such simulations, then this would radically shift the odds and make it very probably we are in fact simulated. Full article

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15 pages, 325 KiB

Open AccessEditor’s ChoiceArticle

Revisiting the Cosmological Constant Problem within Quantum Cosmology

by Vesselin G. Gueorguiev and Andre Maeder

Universe 2020, 6(8), 108; https://doi.org/10.3390/universe6080108 - 2 Aug 2020

Cited by 9 | Viewed by 4113

Abstract

A new perspective on the Cosmological Constant Problem (CCP) is proposed and discussed within the multiverse approach of Quantum Cosmology. It is assumed that each member of the ensemble of universes has a characteristic scale a that can be used as integration variable [...] Read more.

A new perspective on the Cosmological Constant Problem (CCP) is proposed and discussed within the multiverse approach of Quantum Cosmology. It is assumed that each member of the ensemble of universes has a characteristic scale a that can be used as integration variable in the partition function. An averaged characteristic scale of the ensemble is estimated by using only members that satisfy the Einstein field equations. The averaged characteristic scale is compatible with the Planck length when considering an ensemble of solutions to the Einstein field equations with an effective cosmological constant. The multiverse ensemble is split in Planck-seed universes with vacuum energy density of order one; thus,

\tilde{Λ} \approx 8 π

in Planck units and a-derivable universes. For a-derivable universe with a characteristic scale of the order of the observed Universe

a \approx 8 \times 10^{60}

, the cosmological constant

Λ = \tilde{Λ} / a^{2}

is in the range

10^{- 121}

–

10^{- 122}

, which is close in magnitude to the observed value

10^{- 123}

. We point out that the smallness of

Λ

can be viewed to be natural if its value is associated with the entropy of the Universe. This approach to the CCP reconciles the Planck-scale huge vacuum energy–density predicted by QFT considerations, as valid for Planck-seed universes, with the observed small value of the cosmological constant as relevant to an a-derivable universe as observed. Full article

(This article belongs to the Special Issue Quantum Models for Cosmology)

86 pages, 23085 KiB

Open AccessReview

Dark Matters on the Scale of Galaxies

by Ivan de Martino, Sankha S. Chakrabarty, Valentina Cesare, Arianna Gallo, Luisa Ostorero and Antonaldo Diaferio

Universe 2020, 6(8), 107; https://doi.org/10.3390/universe6080107 - 1 Aug 2020

Cited by 74 | Viewed by 7399

Abstract

The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales [...] Read more.

The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some of the solutions proposed within the cold dark-matter model, and solutions when including warm dark matter, self-interacting dark matter, axion-like particles, or fuzzy dark matter. We also illustrate some modifications of the theory of gravity: Modified Newtonian Dynamics (MOND), MOdified Gravity (MOG), and

f (R)

gravity. Full article

(This article belongs to the Special Issue Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models)

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23 pages, 2452 KiB

Open AccessArticle

Caustics in Gravitational Lensing by Mixed Binary Systems

by Valerio Bozza, Silvia Pietroni and Chiara Melchiorre

Universe 2020, 6(8), 106; https://doi.org/10.3390/universe6080106 - 31 Jul 2020

Cited by 9 | Viewed by 3352

Abstract

We investigated binary lenses with

1 / r^{n}

potentials in the asymmetric case with two lenses with different indexes n and m. These kinds of potentials have been widely used in several contexts, ranging from galaxies with halos described by different [...] Read more.

We investigated binary lenses with

1 / r^{n}

potentials in the asymmetric case with two lenses with different indexes n and m. These kinds of potentials have been widely used in several contexts, ranging from galaxies with halos described by different power laws to lensing by wormholes or exotic matter. In this paper, we present a complete atlas of critical curves and caustics for mixed binaries, starting from the equal-strength case, and then exploring unequal-strength systems. We also calculate the transitions between all different topology regimes. Finally we find some useful analytic approximations for the wide binary case and for the extreme unequal-strength case. Full article

(This article belongs to the Special Issue Gravitational Lensing and Optical Geometry: A Centennial Perspective)

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18 pages, 854 KiB

Open AccessArticle

Formation Dynamics of Black- and White-Hole Horizons in an Analogue Gravity Model

by Manuele Tettamanti and Alberto Parola

Universe 2020, 6(8), 105; https://doi.org/10.3390/universe6080105 - 31 Jul 2020

Cited by 3 | Viewed by 2441

Abstract

We investigate the formation dynamics of sonic horizons in a Bose gas confined in a (quasi) one-dimensional trap. This system is one of the most promising realizations of the analogue gravity paradigm and has already been successfully studied experimentally. Taking advantage of the [...] Read more.

We investigate the formation dynamics of sonic horizons in a Bose gas confined in a (quasi) one-dimensional trap. This system is one of the most promising realizations of the analogue gravity paradigm and has already been successfully studied experimentally. Taking advantage of the exact solution of the one-dimensional, hard-core, Bose model (Tonks–Girardeau gas), we show that by switching on a step potential, either a sonic, black-hole-like horizon or a black/white hole pair may form, according to the initial velocity of the fluid. Our simulations never suggest the formation of an isolated white-hole horizon, although a stable stationary solution of the dynamical equations with those properties is analytically found. Moreover, we show that the semiclassical dynamics, based on the Gross–Pitaevskii equation, conforms to the exact solution only in the case of fully subsonic flows while a stationary solution exhibiting a supersonic transition is never reached dynamically. Full article

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18 pages, 327 KiB

Open AccessArticle

Critical Tidal Currents in General Relativity

by Bahram Mashhoon

Universe 2020, 6(8), 104; https://doi.org/10.3390/universe6080104 - 30 Jul 2020

Cited by 4 | Viewed by 2115 | Correction

Abstract

Relativistic tidal equations are formulated with respect to the rest frame of a central gravitational source and their solutions are studied. The existence of certain relativistic critical tidal currents are thereby elucidated. Specifically, observers that are spatially at rest in the exterior Kerr [...] Read more.

Relativistic tidal equations are formulated with respect to the rest frame of a central gravitational source and their solutions are studied. The existence of certain relativistic critical tidal currents are thereby elucidated. Specifically, observers that are spatially at rest in the exterior Kerr spacetime are considered in detail; in effect, these fiducial observers define the rest frame of the Kerr source. The general tidal equations for the free motion of test particles are worked out with respect to the Kerr background. The analytic solutions of these equations are investigated and the existence of a tidal acceleration mechanism is emphasized. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

14 pages, 1192 KiB

Open AccessCommunication

Spinning Test Particle in Four-Dimensional Einstein–Gauss–Bonnet Black Holes

by Yu-Peng Zhang, Shao-Wen Wei and Yu-Xiao Liu

Universe 2020, 6(8), 103; https://doi.org/10.3390/universe6080103 - 28 Jul 2020

Cited by 107 | Viewed by 4018

Abstract

In this paper, we investigate the motion of a classical spinning test particle in a background of a spherically symmetric black hole based on the novel four-dimensional Einstein–Gauss–Bonnet gravity [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)]. We find that [...] Read more.

In this paper, we investigate the motion of a classical spinning test particle in a background of a spherically symmetric black hole based on the novel four-dimensional Einstein–Gauss–Bonnet gravity [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)]. We find that the effective potential of a spinning test particle in this background could have two minima when the Gauss–Bonnet coupling parameter

α

is nearly in a special range

- 8 < α / M^{2} < - 2

(M is the mass of the black hole), which means a particle can be in two separate orbits with the same spin-angular momentum and orbital angular momentum, and the accretion disc could have discrete structures. We also investigate the innermost stable circular orbits of the spinning test particle and find that the corresponding radius could be smaller than the cases in general relativity. Full article

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

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59 pages, 1816 KiB

Open AccessArticle

Cosmic-Ray Database Update: Ultra-High Energy, Ultra-Heavy, and Antinuclei Cosmic-Ray Data (CRDB v4.0)

by David Maurin, Hans Peter Dembinski, Javier Gonzalez, Ioana Codrina Mariş and Frédéric Melot

Universe 2020, 6(8), 102; https://doi.org/10.3390/universe6080102 - 24 Jul 2020

Cited by 34 | Viewed by 5160

Abstract

We present an update on CRDB, the cosmic-ray database for charged species. CRDB is based on MySQL, queried and sorted by jquery and table-sorter libraries, and displayed via PHP web pages through the AJAX protocol. We review the modifications made on the structure [...] Read more.

We present an update on CRDB, the cosmic-ray database for charged species. CRDB is based on MySQL, queried and sorted by jquery and table-sorter libraries, and displayed via PHP web pages through the AJAX protocol. We review the modifications made on the structure and outputs of the database since the first release (Maurin et al., 2014). For this update, the most important feature is the inclusion of ultra-heavy nuclei (

Z > 30

), ultra-high energy nuclei (from

10^{15}

to

10^{20}

eV), and limits on antinuclei fluxes (

Z \leq - 1

for

A > 1

); more than 100 experiments, 350 publications, and 40,000 data points are now available in CRDB. We also revisited and simplified how users can retrieve data and submit new ones. For questions and requests, please contact [email protected]. Full article

(This article belongs to the Special Issue High Energy Cosmic Ray Direct Detection)

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23 pages, 799 KiB

Open AccessReview

The Fundamental Roles of the de Sitter Vacuum

by Irina Dymnikova

Universe 2020, 6(8), 101; https://doi.org/10.3390/universe6080101 - 24 Jul 2020

Cited by 12 | Viewed by 3172

Abstract

We overview the fundamental roles of the de Sitter vacuum in cosmology where it is responsible for powering the early inflationary stage(s) and the present accelerated expansion, in black hole physics where it provides the existence of a wide class of regular black [...] Read more.

We overview the fundamental roles of the de Sitter vacuum in cosmology where it is responsible for powering the early inflationary stage(s) and the present accelerated expansion, in black hole physics where it provides the existence of a wide class of regular black holes and self-gravitating solitons replacing naked singularities, and in particle physics where it ensures the intrinsic relation of the Higgs mechanism with gravity and spacetime symmetry breaking. Full article

(This article belongs to the Special Issue Universe: 5th Anniversary)

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16 pages, 1096 KiB

Open AccessArticle

Holographic Interpretation of Relative State Complexity

by Alexander Yosifov, Aditya Iyer and Lachezar Filipov

Universe 2020, 6(8), 100; https://doi.org/10.3390/universe6080100 - 22 Jul 2020

Cited by 1 | Viewed by 2189

Abstract

We investigate a large-N conformal field theory (CFT) in a high-energy pure state coupled to a small auxiliary system of M weakly-interacting degrees of freedom, and argue the relative state complexity of the auxiliary system is holographically dual to an effective low-energy [...] Read more.

We investigate a large-N conformal field theory (CFT) in a high-energy pure state coupled to a small auxiliary system of M weakly-interacting degrees of freedom, and argue the relative state complexity of the auxiliary system is holographically dual to an effective low-energy notion of computational cost in the bulk, i.e., to the minimal depth of the quantum circuit required to decode its gravitational dual. In light of this, using Nielsen’s approach, a new measure of quantum chaos in terms of the evolution of circuit complexity is proposed. It suggests that studying the evolution of circuit complexity of the auxiliary system can teach us about the chaotic properties of the large-N CFT. This new diagnostic for quantum chaos has important implications for the interior dynamics of evaporating black holes as it implies the radiated Hawking cloud is pseudorandom. Full article

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

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