Universe

30 pages, 3392 KiB

Open AccessReview

Dark Coincidences: Small-Scale Solutions with Refracted Gravity and MOND

by Valentina Cesare

Universe 2023, 9(1), 56; https://doi.org/10.3390/universe9010056 - 16 Jan 2023

Cited by 2 | Viewed by 1622

General relativity and its Newtonian weak field limit are not sufficient to explain the observed phenomenology in the Universe, from the formation of large-scale structures to the dynamics of galaxies, with the only presence of baryonic matter. The most investigated cosmological model, the [...] Read more.

General relativity and its Newtonian weak field limit are not sufficient to explain the observed phenomenology in the Universe, from the formation of large-scale structures to the dynamics of galaxies, with the only presence of baryonic matter. The most investigated cosmological model, the

Λ

CDM, accounts for the majority of observations by introducing two dark components, dark energy and dark matter, which represent ∼95% of the mass-energy budget of the Universe. Nevertheless, the

Λ

CDM model faces important challenges on the scale of galaxies. For example, some very tight relations between the properties of dark and baryonic matters in disk galaxies, such as the baryonic Tully–Fisher relation (BTFR), the mass discrepancy–acceleration relation (MDAR), and the radial acceleration relation (RAR), which see the emergence of the acceleration scale

a_{0} ≃ 1.2 \times 10^{- 10}

m s

^{- 2}

, cannot be intuitively explained by the CDM paradigm, where cosmic structures form through a stochastic merging process. An even more outstanding coincidence is due to the fact that the acceleration scale

a_{0}

, emerging from galaxy dynamics, also seems to be related to the cosmological constant

Λ

. Another challenge is provided by dwarf galaxies, which are darker than what is expected in their innermost regions. These pieces of evidence can be more naturally explained, or sometimes even predicted, by modified theories of gravity, that do not introduce any dark fluid. I illustrate possible solutions to these problems with the modified theory of gravity MOND, which departs from Newtonian gravity for accelerations smaller than

a_{0}

, and with Refracted Gravity, a novel classical theory of gravity introduced in 2016, where the modification of the law of gravity is instead regulated by a density scale. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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Graphical abstract

11 pages, 6791 KiB

Open AccessCommunication

Novel Exact Solution for the Bidirectional Sixth-Order Sawada–Kotera Equation

by Hongcai Ma, Xiaoyu Chen and Aiping Deng

Universe 2023, 9(1), 55; https://doi.org/10.3390/universe9010055 - 15 Jan 2023

Cited by 1 | Viewed by 961

Abstract

In this paper, we take the bidirectional sixth-order Sawada–Kotera equation as an instance and use a new limit approach to generate a multiple-pole solution and the degenerate of the breather wave from the N-order soliton solution. We show not only the substitution [...] Read more.

In this paper, we take the bidirectional sixth-order Sawada–Kotera equation as an instance and use a new limit approach to generate a multiple-pole solution and the degenerate of the breather wave from the N-order soliton solution. We show not only the substitution method, but also the specific mathematical expression of the double-pole, triple-pole, and the degenerate breather solution after the substitution. Meanwhile, we give the dynamic images and trajectories of the different multiple-pole solution. Moreover, we also acquire the interaction between two double-pole solutions and different nonlinear superposition solutions. Full article

(This article belongs to the Section Mathematical Physics)

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35 pages, 2385 KiB

Open AccessArticle

Mu2e Run I Sensitivity Projections for the Neutrinoless μ⁻ → e⁻ Conversion Search in Aluminum

by Mu2e Collaboration

Universe 2023, 9(1), 54; https://doi.org/10.3390/universe9010054 - 13 Jan 2023

Cited by 13 | Viewed by 3095

Abstract

The Mu2e experiment at Fermilab will search for the neutrinoless

μ^{-} \to e^{-}

conversion in the field of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I and Run II, separated by an approximately two-year-long shutdown. This [...] Read more.

The Mu2e experiment at Fermilab will search for the neutrinoless

μ^{-} \to e^{-}

conversion in the field of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I and Run II, separated by an approximately two-year-long shutdown. This paper presents an estimate of the expected Mu2e Run I search sensitivity and includes a detailed discussion of the background sources, uncertainties of their prediction, analysis procedures, and the optimization of the experimental sensitivity. The expected Run I

5 σ

discovery sensitivity is

R_{μ e} = 1.2 \times 10^{- 15}

, with a total expected background of

0.11 \pm 0.03

events. In the absence of a signal, the expected upper limit is

R_{μ e} < 6.2 \times 10^{- 16}

at 90% CL. This represents a three order of magnitude improvement over the current experimental limit of

R_{μ e} < 7 \times 10^{- 13}

at 90% CL set by the SINDRUM II experiment. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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2 pages, 179 KiB

Open AccessEditorial

Editorial to the Special Issue “Solar Wind Structures and Phenomena: Origins, Properties, Geoeffectiveness, and Prediction”

by Yuri I. Yermolaev, Vladimir A. Slemzin and Volker Bothmer

Universe 2023, 9(1), 53; https://doi.org/10.3390/universe9010053 - 12 Jan 2023

Viewed by 818

Abstract

The heliosphere is filled with solar wind, which is formed due to the expansion of the plasma of hot solar corona [...] Full article

(This article belongs to the Special Issue Solar Wind Structures and Phenomena: Origins, Properties, Geoeffectiveness, and Prediction)

21 pages, 908 KiB

Open AccessArticle

Polarimetric Reverberation Mapping in Medium-Band Filters

by Elena Shablovinskaya, Luka Č. Popović, Roman Uklein, Eugene Malygin, Dragana Ilić, Stefano Ciroi, Dmitry Oparin, Luca Crepaldi, Lyuba Slavcheva-Mihova, Boyko Mihov and Yanko Nikolov

Universe 2023, 9(1), 52; https://doi.org/10.3390/universe9010052 - 12 Jan 2023

Cited by 2 | Viewed by 1228

Abstract

Earlier, we suggested the “reload” concept of the polarimetric reverberation mapping of active galactic nuclei (AGN), proposed for the first time more than 10 years ago. We have successfully tested this approach of reverberation mapping of the broad emission line on the galaxy [...] Read more.

Earlier, we suggested the “reload” concept of the polarimetric reverberation mapping of active galactic nuclei (AGN), proposed for the first time more than 10 years ago. We have successfully tested this approach of reverberation mapping of the broad emission line on the galaxy Mrk 6. It was shown that such an idea allows one to look at the AGN central parsec structure literally in a new light. However, the method originally assumed the use of spectropolarimetric observations, expensive in terms of telescope time, and implemented on rare large telescopes. Currently, we propose an adaptation of the polarimetric reverberation mapping of broad lines in medium-band filters following the idea of the photometric reverberation mapping, when filters are selected so that their bandwidth is oriented to the broad line and the surrounding continuum near. In this paper, we present the progress status of such monitoring conducted jointly at the Special astrophysical observatory and Asiago Cima Ekar observatory (OAPd/INAF) with support from Rozhen National Astronomical Observatory (NAO), some first results for the most frequently observed AGNs Mrk 335, Mrk 509, and Mrk 817, and the discussion of the future perspectives of the campaign. Full article

(This article belongs to the Special Issue A Multimessenger View of Supermassive Black Holes and the Quasar Main Sequence)

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13 pages, 277 KiB

Open AccessArticle

Highly Dispersive Optical Solitons with Four Forms of Self-Phase Modulation

by Ahmed M. Elsherbeny, Ahmed H. Arnous, Anjan Biswas, Oswaldo González-Gaxiola, Luminita Moraru, Simona Moldovanu, Catalina Iticescu and Hashim M. Alshehri

Universe 2023, 9(1), 51; https://doi.org/10.3390/universe9010051 - 12 Jan 2023

Cited by 9 | Viewed by 1003

Abstract

This paper implements the enhanced Kudryashov approach to retrieve highly dispersive optical solitons and study it with four nonlinear forms. These are the power law, generalized quadratic-cubic law, triple-power law, and the generalized non-local law. This approach reveals bright and singular optical solitons [...] Read more.

This paper implements the enhanced Kudryashov approach to retrieve highly dispersive optical solitons and study it with four nonlinear forms. These are the power law, generalized quadratic-cubic law, triple-power law, and the generalized non-local law. This approach reveals bright and singular optical solitons along with the respective parameter constraints. Full article

(This article belongs to the Special Issue Research on Optical Soliton Perturbation)

10 pages, 496 KiB

Open AccessArticle

Single Pulse Studies of PSR B0950+08 with FAST

by Heng Yang, Shijun Dang, Qijun Zhi, Lunhua Shang, Xin Xu, Dandan Zhang, Shuo Xiao, Rushuang Zhao, Aijun Dong, Hui Liu, Ziyi You, Qingying Li, Yuanyi Qin, Yanqing Cai and Wei Li

Universe 2023, 9(1), 50; https://doi.org/10.3390/universe9010050 - 12 Jan 2023

Cited by 1 | Viewed by 1495

Abstract

We report detailed polarization and single-pulse studies of PSR B0950+08 with the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) at 1250 MHz. Significant bridge emission was observed between the inter-pulse and the main pulse and the height of the bridge decreased with increase in [...] Read more.

We report detailed polarization and single-pulse studies of PSR B0950+08 with the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) at 1250 MHz. Significant bridge emission was observed between the inter-pulse and the main pulse and the height of the bridge decreased with increase in frequency. Our results support the interpretation that both the inter-pulse and the main pulse of this pulsar are from the same magnetic pole. From the relative peak flux density and the relative energy distribution, we conclude that no giant pulse was detected in PSR B0950+08. Our results provide opportunities to study the origin of the emission from PSR B0950+08 and offer new insights into the origins of pulsar emission and bridge emission. Full article

(This article belongs to the Special Issue Stellar Multi-Band Observational Studies in the Era of Big Data)

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17 pages, 475 KiB

Open AccessArticle

Dynamics of a Cosmological Model in f(R,T) Gravity: II. In Three-Dimensional Space

by Jianwen Liu, Ruifang Wang and Fabao Gao

Universe 2023, 9(1), 49; https://doi.org/10.3390/universe9010049 - 11 Jan 2023

Cited by 2 | Viewed by 1065

Abstract

Based on the results discussed on the invariant planes in the known literature [Universe 2022, 8, 365] for the flat FLRW space-time universe model with ideal fluid under the gravity of [...] Read more.

Based on the results discussed on the invariant planes in the known literature [Universe 2022, 8, 365] for the flat FLRW space-time universe model with ideal fluid under the gravity of

f (R, T) = ξ R^{α} + ζ \sqrt{- T}

, this paper continues to describe the global dynamics of this model in the three-dimensional space containing infinity through dynamic system analysis. Moreover, the cosmological solutions of all the physical significance regions restricted by three invariant planes are also fully discussed. Full article

(This article belongs to the Section Cosmology)

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18 pages, 364 KiB

Open AccessArticle

Perusing Buchbinder–Lyakhovich Canonical Formalism for Higher-Order Theories of Gravity

by Dalia Saha and Abhik Kumar Sanyal

Universe 2023, 9(1), 48; https://doi.org/10.3390/universe9010048 - 11 Jan 2023

Viewed by 775

Abstract

Ostrogradsky’s, Dirac’s, and Horowitz’s techniques in terms of higher-order theories of gravity produce identical phase-space structures. The problem with these techniques is manifested in the case of Gauss–Bonnet–dilatonic coupled action in the presence of higher-order term, in which case, classical correspondence cannot be [...] Read more.

Ostrogradsky’s, Dirac’s, and Horowitz’s techniques in terms of higher-order theories of gravity produce identical phase-space structures. The problem with these techniques is manifested in the case of Gauss–Bonnet–dilatonic coupled action in the presence of higher-order term, in which case, classical correspondence cannot be established. Here, we explore another technique developed by Buchbinder and their collaborators (BL) and show that it also suffers from the same disease. However, when expressing the action in terms of the three-space curvature, and removing ‘the total derivative terms’, if Horowitz’s formalism or even Dirac’s constraint analysis is pursued, all pathologies disappear. Here, we show that the same is true for BL formalism, which appears to be the simplest of all the techniques to handle. Full article

(This article belongs to the Section Cosmology)

17 pages, 336 KiB

Open AccessArticle

Isotropic Perfect Fluids in Modified Gravity

by Shavani Naicker, Sunil D. Maharaj and Byron P. Brassel

Universe 2023, 9(1), 47; https://doi.org/10.3390/universe9010047 - 11 Jan 2023

Cited by 5 | Viewed by 1281

Abstract

We generate the Einstein–Gauss–Bonnet field equations in higher dimensions for a spherically symmetric static spacetime. The matter distribution is a neutral fluid with isotropic pressure. The condition of isotropic pressure, an Abel differential equation of the second kind, is transformed to a first [...] Read more.

We generate the Einstein–Gauss–Bonnet field equations in higher dimensions for a spherically symmetric static spacetime. The matter distribution is a neutral fluid with isotropic pressure. The condition of isotropic pressure, an Abel differential equation of the second kind, is transformed to a first order nonlinear canonical differential equation. This provides a mechanism to generate exact solutions systematically in higher dimensions. Our solution generating algorithm is a different approach from those considered earlier. We show that a specific choice of one potential leads to a new solution for the second potential for all spacetime dimensions. Several other families of exact solutions to the condition of pressure isotropy are found for all spacetime dimensions. Earlier results are regained from our treatments. The difference with general relativity is highlighted in our study. Full article

(This article belongs to the Special Issue Spacetime Symmetry)

19 pages, 2554 KiB

Open AccessEssay

Nearly Forgotten Cosmological Concept of E. B. Gliner

by Dmitry Yakovlev and Alexander Kaminker

Universe 2023, 9(1), 46; https://doi.org/10.3390/universe9010046 - 11 Jan 2023

Cited by 1 | Viewed by 2140

Abstract

E. B. Gliner started his scientific career in 1963 at the age of 40. In 1965, when the existence of the cosmological constant

λ

seemed unnecessary to most cosmologists, he renewed interest in the problem by emphasizing a material interpretation of de Sitter [...] Read more.

E. B. Gliner started his scientific career in 1963 at the age of 40. In 1965, when the existence of the cosmological constant

λ

seemed unnecessary to most cosmologists, he renewed interest in the problem by emphasizing a material interpretation of de Sitter space (i.e., the space curved in the presence of

λ

). According to that interpretation, the curvature is produced by a cosmological vacuum (now identified as dark energy of the universe). In 1970, Gliner proposed a description of exponential expansion (or contraction) of the universe at the early (or late) evolution stage dominated by cosmological vacuum. In 1975, Gliner (with I. G. Dyminikova) suggested a model of the early universe free of Big Bang singularity, and developed a scenario of nonsingular Friedmann cosmology. Many of these findings were used in the modern inflation scenarios of the universe, first proposed by A. A. Starobinsky (1979) and A. Guth (1981) and greatly multiplied later. However, these inflation scenarios differ from the scenario of Gliner and Dymnikova, and Gliner’s contribution to cosmology is nearly forgotten. The history and the essence of this contribution are outlined, as well the difference from the inflation theories. Full article

(This article belongs to the Special Issue Elementary Particles in Astrophysics and Cosmology)

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25 pages, 1292 KiB

Open AccessArticle

Dynamics and Stability of the Two-Body Problem with Yukawa Correction to Newton’s Gravity, Revisited and Applied Numerically to the Solar System

by Nawras Abo Hasan, Nabil Joudieh and Nidal Chamoun

Universe 2023, 9(1), 45; https://doi.org/10.3390/universe9010045 - 10 Jan 2023

Viewed by 906

Abstract

In this manuscript, we review the motion of a two-body celestial system (planet–sun) for a Yukawa-type correction on Newton’s gravitational potential using Hamilton’s formulation. We reexamine the stability using the corresponding linearization Jacobian matrix, and verify that the conditions of Bertrand’s theorem are [...] Read more.

In this manuscript, we review the motion of a two-body celestial system (planet–sun) for a Yukawa-type correction on Newton’s gravitational potential using Hamilton’s formulation. We reexamine the stability using the corresponding linearization Jacobian matrix, and verify that the conditions of Bertrand’s theorem are met for radii

≪ 10^{15}

m, meaning that bound closed orbits are expected. Applied to the solar system, we present the equation of motion of the planet, then solve it both analytically and numerically. Making use of the analytical expression of the orbit, we estimate the Yukawa strength

α

and find it to be larger than the nominal value (

10^{- 8}

) adopted in previous studies, in that it is of order (

α = 10^{- 4} - 10^{- 5}

) for the terrestrial planets (Mercury, Venus, earth, Mars, and Pluto) and even larger (

α = 10^{- 3}

) for the giant planets (Jupiter, Saturn, Uranus, and Neptune). Taking the inputs (

r_{m i n}, v_{m a s}, e

) observed by NASA, we analyse the orbits analytically and numerically for both the estimated and nominal values of

α

and determine the corresponding trajectories. For each obtained orbit, we recalculate the characterizing parameters (

r_{m i n}, r_{m a x}, a, b, e

) and compare their values according to the potential (Newton with/without Yukawa correction) and method (analytical and/or numerical) used. When compared to the observational data, we conclude that the path correction due to Yukawa correction is on the order of up to 80 million km (20 million km) as the maximum deviation occurring for Neptune (Pluto) for a nominal (estimated) value of

α

. Full article

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11 pages, 478 KiB

Open AccessArticle

Dependence of Freeze-Out Parameters on Collision Energies and Cross-Sections

by Muhammad Waqas, Atef AbdelKader, Muhammad Ajaz, Abdel Nasser Tawfik, Zafar Wazir, Abd Al Karim Haj Ismail, Shi Jun Luo and Hafsa Zar Khan

Universe 2023, 9(1), 44; https://doi.org/10.3390/universe9010044 - 10 Jan 2023

Cited by 2 | Viewed by 1187

Abstract

We analyzed the transverse momentum spectra (

p_{T}

) reported by the NA61/SHINE and NA49 experiments in inelastic proton–proton (

p p

) and central Lead–Lead (

P b - P b

), Argon–Scandium (

A r - S c

), and [...] Read more.

We analyzed the transverse momentum spectra (

p_{T}

) reported by the NA61/SHINE and NA49 experiments in inelastic proton–proton (

p p

) and central Lead–Lead (

P b - P b

), Argon–Scandium (

A r - S c

), and Beryllium–Beryllium (

B e - B e

) collisions with the Blast-wave model with Boltzmann–Gibbs (BWBG) statistics. The BGBW model was in good agreement with the experimental data. We were able to extract the transverse flow velocity (

β_{T}

), the kinetic freeze-out temperature (

T_{0}

), and the kinetic freeze-out volume (V) from the

p_{T}

spectra using the BGBW model. Furthermore, we also obtained the initial temperature (

T_{i}

) and the mean transverse momentum (<

p_{T}

>) by the alternative method. We observed that

T_{0}

increases with increasing collision energy and collision cross-section, representing the colliding system’s size. The transverse flow velocity was observed to remain invariant with increasing collision energy, while it showed a random change with different collision cross-sections. In the same way, the kinetic freeze-out volume and mean transverse momentum increased with an increase in collision energy or collision cross-section. The same behavior was also seen in the freeze-out temperature, which increased with increasing collision cross-sections. At chemical freeze-out, we also determined both the chemical potential and temperature and compared these with the hadron resonance gas model (HRG) and different experimental data. We report that there is an excellent agreement with the HRG model and various experiments, which reveals the ability of the fit function to manifest features of the chemical freeze-out. Full article

(This article belongs to the Special Issue Collectivity in High-Energy Proton-Proton and Heavy-Ion Collisions)

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21 pages, 690 KiB

Open AccessArticle

The Higgs Trilinear Coupling and the Scale of New Physics for the SM-Axion-Seesaw-Higgs Portal Inflation (SMASH) Model

by C.R. Das, Katri Huitu and Timo J. Kärkkäinen

Universe 2023, 9(1), 43; https://doi.org/10.3390/universe9010043 - 09 Jan 2023

Viewed by 1023

Abstract

In the extended scalar sector of the SMASH (Standard Model - Axion-Seesaw-Higgs portal inflation) framework, we conduct a phenomenological investigation of the observable effects. In a suitable region of the SMASH scalar parameter spaces, we solve the vacuum metastability problem and discuss the [...] Read more.

In the extended scalar sector of the SMASH (Standard Model - Axion-Seesaw-Higgs portal inflation) framework, we conduct a phenomenological investigation of the observable effects. In a suitable region of the SMASH scalar parameter spaces, we solve the vacuum metastability problem and discuss the one-loop correction to the triple Higgs coupling,

λ_{H H H}

. The

λ_{H H H}

and SM Higgs quartic coupling

λ_{H}

corrections are found to be proportional to the threshold correction. A large

λ_{H H H}

correction (≳5%) implies vacuum instability in the model and thus limits the general class of theories that use threshold correction. We performed a full two-loop renormalization group analysis of the SMASH model. The SMASH framework has also been used to estimate the evolution of lepton asymmetry in the universe. Full article

(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)

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26 pages, 696 KiB

Open AccessArticle

Multiverse Predictions for Habitability: Origin of Life Scenarios

by McCullen Sandora, Vladimir Airapetian, Luke Barnes, Geraint F. Lewis and Ileana Pérez-Rodríguez

Universe 2023, 9(1), 42; https://doi.org/10.3390/universe9010042 - 09 Jan 2023

Cited by 1 | Viewed by 2557

Abstract

If the origin of life is rare and sensitive to the local conditions at the site of its emergence, then, using the principle of mediocrity within a multiverse framework, we may expect to find ourselves in a universe that is better than usual [...] Read more.

If the origin of life is rare and sensitive to the local conditions at the site of its emergence, then, using the principle of mediocrity within a multiverse framework, we may expect to find ourselves in a universe that is better than usual at creating these necessary conditions. We use this reasoning to investigate several origin of life scenarios to determine whether they are compatible with the multiverse, including the prebiotic soup scenario, hydrothermal vents, delivery of prebiotic material from impacts, and panspermia. We find that most of these scenarios induce a preference toward weaker-gravity universes, and that panspermia and scenarios involving solar radiation or large impacts as a disequilibrium source are disfavored. Additionally, we show that several hypothesized habitability criteria which are disfavored when the origin of life is not taken into account become compatible with the multiverse, and that the emergence of life and emergence of intelligence cannot both be sensitive to disequilibrium production conditions. Full article

(This article belongs to the Section Planetary Sciences)

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16 pages, 304 KiB

Open AccessArticle

Avoidance of Singularity during the Gravitational Collapse with String T-Duality Effects

by Kimet Jusufi

Universe 2023, 9(1), 41; https://doi.org/10.3390/universe9010041 - 09 Jan 2023

Cited by 5 | Viewed by 1303

Abstract

In this paper, we explore the gravitational collapse of matter (dust) under the effect of zero-point length

l_{0}

. During the gravitational collapse, we neglect the backreaction effect of pre-Hawking radiation (in the sense that it is a small effect and cannot [...] Read more.

In this paper, we explore the gravitational collapse of matter (dust) under the effect of zero-point length

l_{0}

. During the gravitational collapse, we neglect the backreaction effect of pre-Hawking radiation (in the sense that it is a small effect and cannot prevent the formation of an apparent horizon), then we recast the internal metric of a collapsing star as a closed FRW universe for any spherically symmetric case and, finally, we obtain the minimal value for the scale factor, meaning that the particles never hit the singularity. We argue that the object emerging at the end of the gravitational collapse can be interpreted as Planck stars (black hole core) hidden inside the event horizon of the black hole, with a radius proportional to

{(G M l_{0}^{2} / c^{2})}^{1 / 3}

. Quite interestingly, we found the same result for the radius of the Planck star using a free-falling observer point of view. In addition, we point out a correspondence between the modified Friedmann’s equations in loop quantum gravity and the modified Friedmann’s equation in string T-duality. In the end, we discuss two possibilities regarding the final stage of the black hole. The first possibility is that we end up with Planck-size black hole remnants. The second possibility is that the inner core can be unstable and, due to the quantum tunneling effect, the spacetime can undergo a black-hole-to-white-hole transition (a bouncing Planck star). Full article

(This article belongs to the Section Gravitation)

14 pages, 2722 KiB

Open AccessArticle

Hot Spots in Sgr A* Accretion Disk: Hydrodynamic Insights

by Elizabeth P. Tito, Victor P. Goncharov and Vadim I. Pavlov

Universe 2023, 9(1), 40; https://doi.org/10.3390/universe9010040 - 08 Jan 2023

Viewed by 1208

Abstract

The recent image of our galaxy’s supermassive black hole Sgr A* derived from the 7 April 2017 data of the Event Horizon Telescope Collaboration shows multiple hot spots in its accretion disk. Using the analytical framework, we demonstrate that the observed hot spots [...] Read more.

The recent image of our galaxy’s supermassive black hole Sgr A* derived from the 7 April 2017 data of the Event Horizon Telescope Collaboration shows multiple hot spots in its accretion disk. Using the analytical framework, we demonstrate that the observed hot spots may not be disjoint elements but causally linked components (“petals”) of one rotating quasi-stationary macro-structure formed in the thermo-vorticial field within the accretion disk. Full article

(This article belongs to the Section Compact Objects)

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31 pages, 3206 KiB

Open AccessArticle

Recent Cross-Section Measurements of Top-Quark Pair Production in Association with Gauge Bosons

by Joshuha Thomas-Wilsker

Universe 2023, 9(1), 39; https://doi.org/10.3390/universe9010039 - 06 Jan 2023

Viewed by 1243

Abstract

This article reviews recent cross-section measurements of

t \bar{t}

production in association with a photon, W or Z boson at the Large Hadron Collider (LHC). All measurements reviewed use proton–proton (pp) datasets collected by the ATLAS and CMS experiments between 2016 and [...] Read more.

This article reviews recent cross-section measurements of

t \bar{t}

production in association with a photon, W or Z boson at the Large Hadron Collider (LHC). All measurements reviewed use proton–proton (pp) datasets collected by the ATLAS and CMS experiments between 2016 and 2018 from collisions at a centre-of-mass energy of 13 TeV during the LHC Run 2. Differential and inclusive cross-section measurements are discussed along with the constraints on the effective field theory operators accessible through each process. Finally, we discuss the potential for measurements of these processes at future colliders. Full article

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

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13 pages, 350 KiB

Open AccessArticle

Search for Manifestations of Spin–Torsion Coupling

by Mariya Iv. Trukhanova, Pavel Andreev and Yuri N. Obukhov

Universe 2023, 9(1), 38; https://doi.org/10.3390/universe9010038 - 06 Jan 2023

Cited by 2 | Viewed by 1287

Abstract

We investigate the axial vector spin–torsion coupling effects in the framework of the Poincaré gauge theory of gravity with the general Yang–Mills type Lagrangian. The dynamical equations for the “electric” and “magnetic” components of the torsion field variable are obtained in the general [...] Read more.

We investigate the axial vector spin–torsion coupling effects in the framework of the Poincaré gauge theory of gravity with the general Yang–Mills type Lagrangian. The dynamical equations for the “electric” and “magnetic” components of the torsion field variable are obtained in the general form and it is shown that the helicity density and the spin density of the electromagnetic field appear as the physical sources. The modified Maxwell’s equations for the electromagnetic field are derived, and the electromagnetic wave propagation under the action of the uniform homogeneous torsion field is considered. We demonstrate the Faraday effect of rotation of the polarization for such a wave and establish the strong bound on the possible cosmic axial torsion field from the astrophysical data. Full article

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

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7 pages, 314 KiB

Open AccessCommunication

Might the 2PN Perihelion Precession of Mercury Become Measurable in the Next Future?

by Lorenzo Iorio

Universe 2023, 9(1), 37; https://doi.org/10.3390/universe9010037 - 04 Jan 2023

Viewed by 1098

Abstract

The Hermean average perihelion rate

{\dot{ω}}^{2} PN

, calculated to the second post-Newtonian (2PN) order with the Gauss perturbing equations and the osculating Keplerian orbital elements, ranges from

- 18

to

- 4

microarcseconds per century

(μ as {cty}^{- 1})

[...] Read more.

The Hermean average perihelion rate

{\dot{ω}}^{2} PN

, calculated to the second post-Newtonian (2PN) order with the Gauss perturbing equations and the osculating Keplerian orbital elements, ranges from

- 18

to

- 4

microarcseconds per century

(μ as {cty}^{- 1})

, depending on the true anomaly at epoch

f_{0}

. It is the sum of four contributions: one of them is the direct consequence of the 2PN acceleration entering the equations of motion, while the other three are indirect effects of the 1PN component of the Sun’s gravitational field. An evaluation of the merely formal uncertainty of the experimental Mercury’s perihelion rate

{\dot{ω}}_{\exp}

recently published by the present author, based on 51 years of radiotechnical data processed with the EPM2017 planetary ephemerides by the astronomers E.V. Pitjeva and N.P. Pitjev, is

σ_{{\dot{ω}}_{\exp}} ≃ 8 μ as {cty}^{- 1}

, corresponding to a relative accuracy of

2 \times 10^{- 7}

for the combination

(2 + 2 γ - β) / 3

of the PPN parameters

β

and

γ

scaling the well known 1PN perihelion precession. In fact, the realistic uncertainty may be up to ≃10–50 times larger, despite reprocessing the now available raw data of the former MESSENGER mission with a recently improved solar corona model should ameliorate our knowledge of the Hermean orbit. The BepiColombo spacecraft, currently en route to Mercury, might reach a

≃ 10^{- 7}

accuracy level in constraining

β

and

γ

in an extended mission, despite

≃ 10^{- 6}

seems more likely according to most of the simulations currently available in the literature. Thus, it might be that in the not-too-distant future, it will be necessary to include the 2PN acceleration in the Solar System’s dynamics as well. Full article

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

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14 pages, 494 KiB

Open AccessArticle

Revisiting Angular Momentum Conservation in Transport Simulations of Intermediate-Energy Heavy-Ion Collisions

by Rong-Jun Liu and Jun Xu

Universe 2023, 9(1), 36; https://doi.org/10.3390/universe9010036 - 03 Jan 2023

Cited by 3 | Viewed by 956

Abstract

Based on the well-calibrated IBUU transport model, we have studied the dynamical effect of incorporating rigorous angular momentum conservation in each collision of particles with homework setups. The constraint of the rigorous angular momentum conservation requires in-plane collisions and side jumps of particles [...] Read more.

Based on the well-calibrated IBUU transport model, we have studied the dynamical effect of incorporating rigorous angular momentum conservation in each collision of particles with homework setups. The constraint of the rigorous angular momentum conservation requires in-plane collisions and side jumps of particles after their collision. Since the option is not unique, we have compared two typical prescriptions with the original one. While the results depend quantitatively on the choice of the prescription, we found that the angular momentum conservation generally reduces local density fluctuations and thus the collision rate and may have some influence on the density evolution, the collective flow, and even the pion production in transport simulations of intermediate-energy heavy-ion collisions. Full article

(This article belongs to the Special Issue Intermediate-Energy Heavy-Ion Collisions and Studies with Transport Models)

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27 pages, 857 KiB

Open AccessArticle

Asymptotic Forms of Solutions to System of Nonlinear Partial Differential Equations

by Alexander D. Bruno and Alexander B. Batkhin

Universe 2023, 9(1), 35; https://doi.org/10.3390/universe9010035 - 03 Jan 2023

Cited by 1 | Viewed by 1039

Abstract

Here, we considerably develop the methods of power geometry for a system of partial differential equations and apply them to two different fluid dynamics problems: computing the boundary layer on a needle in the first approximation and computing the asymptotic forms of solutions [...] Read more.

Here, we considerably develop the methods of power geometry for a system of partial differential equations and apply them to two different fluid dynamics problems: computing the boundary layer on a needle in the first approximation and computing the asymptotic forms of solutions to the problem of evolution of the turbulent flow. For each equation of the system, its Newton polyhedron and its hyperfaces with their normals and truncated equations are calculated. To simplify the truncated systems, power-logarithmic transformations are used and the truncated systems are further extracted. Here, we propose algorithms for computing unimodular matrices of power transformations for differential equations. Results: (1) the boundary layer on the needle is absent in liquid, while in gas it is described in the first approximation; (2) the solutions to the problem of evolution of turbulent flow have eight asymptotic forms, presented explicitly. Full article

(This article belongs to the Section Mathematical Physics)

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13 pages, 870 KiB

Open AccessArticle

How to Strengthen Constraints on Non-Newtonian Gravity from Measuring the Lateral Casimir Force

by Galina L. Klimchitskaya and Vladimir M. Mostepanenko

Universe 2023, 9(1), 34; https://doi.org/10.3390/universe9010034 - 03 Jan 2023

Cited by 1 | Viewed by 1036

Abstract

It has been known that in the nanometer interaction range the available experimental data do not exclude the Yukawa-type corrections to Newton’s gravitational law, which exceed the Newtonian gravitational force by many orders of magnitude. The strongest constraints on the parameters of Yukawa-type [...] Read more.

It has been known that in the nanometer interaction range the available experimental data do not exclude the Yukawa-type corrections to Newton’s gravitational law, which exceed the Newtonian gravitational force by many orders of magnitude. The strongest constraints on the parameters of Yukawa-type interaction in this interaction range follow from the experiments on neutron scattering and from measurements of the lateral and normal Casimir forces between corrugated surfaces. In this work, we demonstrate that by optimizing the experimental configuration at the expense of the higher corrugation amplitudes and smaller periods of corrugations it is possible to considerably strengthen the currently available constraints within the wide interaction range from 4.5 to 37 nm. We show that the maximum strengthening by more than a factor of 40 is reachable for the interaction range of 19 nm. Full article

(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)

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24 pages, 15180 KiB

Open AccessReview

Mathematical Singularities in the Farthest Confines of the Universe—And a Brief Report on Its Evolutionary History

by Emilio Elizalde

Universe 2023, 9(1), 33; https://doi.org/10.3390/universe9010033 - 02 Jan 2023

Viewed by 1528

Abstract

It is advisable to avoid and, even better, demystify such grandiose terms as “infinity” or “singularity” in the description of the cosmos. Its proliferation does not positively contribute to the understanding of key concepts that are essential for an updated account of its [...] Read more.

It is advisable to avoid and, even better, demystify such grandiose terms as “infinity” or “singularity” in the description of the cosmos. Its proliferation does not positively contribute to the understanding of key concepts that are essential for an updated account of its origin and evolutionary history. It will be here argued that, as a matter of fact, there are no infinities in physics, in the real world: all that appears, in any given formulation of nature by means of mathematical equations, actually arises from extrapolations, which are made beyond the bounds of validity of the equations themselves. Such a crucial point is rather well known, but too often forgotten, and is discussed in this paper with several examples; namely, the famous Big Bang singularity and others, which appeared before in classical mechanics and electrodynamics, and notably in the quantization of field theories. A brief description of the Universe’s history and evolution follows. Special emphasis is put on what is presently known, from detailed observations of the cosmos and, complementarily, from advanced experiments of very high-energy physics. To conclude, a future perspective on how this knowledge might soon improve is given. Full article

(This article belongs to the Section Cosmology)

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10 pages, 316 KiB

Open AccessArticle

Surface Density of Disk Galaxies in MOND

by Antonino Del Popolo and Morgan Le Delliou

Universe 2023, 9(1), 32; https://doi.org/10.3390/universe9010032 - 01 Jan 2023

Cited by 2 | Viewed by 1387

Abstract

In this paper, we extend a paper by Milgrom (2009, MNRAS 398, 1023) dealing with the existence of a quasi-universal surface density for object of all mass and structure, if they are in the Newtonian regime, i.e., that their mean acceleration is larger [...] Read more.

In this paper, we extend a paper by Milgrom (2009, MNRAS 398, 1023) dealing with the existence of a quasi-universal surface density for object of all mass and structure, if they are in the Newtonian regime, i.e., that their mean acceleration is larger than MOND typical acceleration

a_{0}

. This result is in agreement with Donato et al. (2009)’s results, claiming the existence of a quasi-universal surface density in all masses in galaxies. The Milgrom paper also predicts that objects with mean inner acceleration smaller than the values discussed do not show the quasi-universal behavior of the surface density discussed. In the present paper, we extend the result of Milgrom’s paper, based on a point mass model, considering spiral galaxies, modelled with a double exponential disk. Similar to Milgrom’s results, we find the existence of a universal surface density for galaxies with large surface density, and a different behavior for galaxies having small surface density. Full article

(This article belongs to the Section Cosmology)

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10 pages, 337 KiB

Open AccessArticle

Discrete Gravity in Emergent Space Theory: Hidden Conical Defects and Teleparallel Gravity

by Richard Pincak, Alexander Pigazzini, Saeid Jafari and Cenap Özel

Universe 2023, 9(1), 31; https://doi.org/10.3390/universe9010031 - 01 Jan 2023

Cited by 2 | Viewed by 1052

Abstract

In the first part of the paper, we try to identify the presence of gravity, at a microscopic level, by introducing conical defects and maintaining an approach that assumes topological equivalence among the underlying manifolds that form the tissue of the D-brane itself. [...] Read more.

In the first part of the paper, we try to identify the presence of gravity, at a microscopic level, by introducing conical defects and maintaining an approach that assumes topological equivalence among the underlying manifolds that form the tissue of the D-brane itself. In the second part, we will present an alternative to the conical defects, considering the theory of teleparallel gravity, in which the presence of the torsion and the construction of the brane always occurs within the context of our new interpretation of emergent spaces. Full article

(This article belongs to the Section Gravitation)

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17 pages, 324 KiB

Open AccessArticle

The Cosmological Arrow of Time and the Retarded Potentials

by Alla A. Yurova, Artyom Yurov and Valerian A. Yurov

Universe 2023, 9(1), 30; https://doi.org/10.3390/universe9010030 - 31 Dec 2022

Viewed by 1231

Abstract

We demonstrate that the cosmological arrow of time is the cause for the arrow of time associated with the retarded radiation. This implies that the proposed mathematical model serves to confirm the hypothesis of Gold and Wheeler that the stars radiate light instead [...] Read more.

We demonstrate that the cosmological arrow of time is the cause for the arrow of time associated with the retarded radiation. This implies that the proposed mathematical model serves to confirm the hypothesis of Gold and Wheeler that the stars radiate light instead of consuming it only because the universe is expanding—just like the darkness of the night sky is a side-effect of the global cosmological expansion. Full article

(This article belongs to the Section Cosmology)

24 pages, 524 KiB

Open AccessArticle

Gauge-Invariant Perturbations at a Quantum Gravity Bounce

by Steffen Gielen and Lisa Mickel

Universe 2023, 9(1), 29; https://doi.org/10.3390/universe9010029 - 31 Dec 2022

Cited by 3 | Viewed by 1107

Abstract

We study the dynamics of gauge-invariant scalar perturbations in cosmological scenarios with a modified Friedmann equation, such as quantum gravity bouncing cosmologies. We work within a separate universe approximation which captures wavelengths larger than the cosmological horizon; this approximation has been successfully applied [...] Read more.

We study the dynamics of gauge-invariant scalar perturbations in cosmological scenarios with a modified Friedmann equation, such as quantum gravity bouncing cosmologies. We work within a separate universe approximation which captures wavelengths larger than the cosmological horizon; this approximation has been successfully applied to loop quantum cosmology and group field theory. We consider two variables commonly used to characterise scalar perturbations: the curvature perturbation on uniform-density hypersurfaces

ζ

and the comoving curvature perturbation

R

. For standard cosmological models in general relativity as well as in loop quantum cosmology, these quantities are conserved and equal on super-horizon scales for adiabatic perturbations. Here we show that while these statements can be extended to a more general form of modified Friedmann equations similar to that of loop quantum cosmology, in other cases, such as the simplest group field theory bounce scenario,

ζ

is conserved across the bounce whereas

R

is not. We relate our results to approaches based on a second-order equation for a single perturbation variable, such as the Mukhanov–Sasaki equation. Full article

(This article belongs to the Special Issue Probing the Quantum Space-Time)

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26 pages, 512 KiB

Open AccessArticle

Generalized Unitarity Method for Unstable Particles

by Gabriel Menezes

Universe 2023, 9(1), 28; https://doi.org/10.3390/universe9010028 - 30 Dec 2022

Cited by 6 | Viewed by 1057

Abstract

In theories with unstable particles, unitarity is satisfied by the inclusion of only stable states in unitarity sums. Therefore, unitarity cuts are not to be taken through unstable particles. This presents a challenge to the generalized unitarity method, whose aim is to reconstruct [...] Read more.

In theories with unstable particles, unitarity is satisfied by the inclusion of only stable states in unitarity sums. Therefore, unitarity cuts are not to be taken through unstable particles. This presents a challenge to the generalized unitarity method, whose aim is to reconstruct amplitudes by analyzing sets of unitarity cuts. Nevertheless, under some general physical conditions, and perhaps some methodological modifications, we prove that the method is still reliable for one-loop amplitudes containing resonances. We discuss some simple examples which illustrate these features. Full article

(This article belongs to the Section Field Theory)

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20 pages, 535 KiB

Open AccessEditor’s ChoiceArticle

Addressing Cosmological Tensions by Non-Local Gravity

by Filippo Bouché, Salvatore Capozziello and Vincenzo Salzano

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

Cited by 5 | Viewed by 1545

Abstract

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

Λ

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

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

Λ

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

Λ

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

H_{0}

and

σ_{8}

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

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

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