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Universe, Volume 10, Issue 11 (November 2024) – 30 articles

Cover Story (view full-size image): In the framework of the analogous Hawking effect, the main contribution of our paper consists in finding, in a two-dimensional model, and in the limit of weak dispersive effects, a complete set of relations between ingoing modes P, N, V entering the horizon, and outgoing modes H, \({\overset{-}{H}}\), \({V^{′}}\) reaching the asymptotic region. This is achieved by deducing, on purely mathematical grounds, the connection formulas without introducing heuristical boundary conditions ad hoc. A general picture for calculating all the interesting amplitudes involved in the scattering matrix is provided. The present framework encompasses many interesting systems involved in experiments (BEC, water, dielectric media). We also provide some insights about the four dimensional case and about exact solutions of the equation involving non-dispersive modes. View this paper
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31 pages, 1408 KiB  
Article
Black Hole Solutions in Non-Minimally Coupled Weyl Connection Gravity
by Maria Margarida Lima and Cláudio Gomes
Universe 2024, 10(11), 433; https://doi.org/10.3390/universe10110433 - 20 Nov 2024
Viewed by 358
Abstract
Schwarzschild and Reissner–Nordstrøm black hole solutions are found in the context of a non-minimal matter–curvature coupling with Weyl connection both in vacuum and in the presence of a cosmological constant-like matter content. This model has the advantage of an extra force term which [...] Read more.
Schwarzschild and Reissner–Nordstrøm black hole solutions are found in the context of a non-minimal matter–curvature coupling with Weyl connection both in vacuum and in the presence of a cosmological constant-like matter content. This model has the advantage of an extra force term which can mimic dark matter and dark energy, and simultaneously following Weyl’s idea of unifying gravity and electromagnetism. In fact, vacuum Schwarzschild solutions differ from the ones in a constant curvature scenario in f(R) theories, with the appearance of a coefficient in the term that is linear in r and a corrected “cosmological constant”. Non-vacuum Schwarzschild solutions formally have the same solutions as in the previous case, with the exception being the physical interpretation of a cosmological constant as the source of the matter Lagrangian and not a simple reparameterization of the f(R) description. Reissner–Nordstrøm solutions cannot be found in a vacuum, only in the presence of matter fields, with the result that the solutions also differ from the constant curvature scenario in f(R) theories by the term being linear in r, the corrected/dressed charge, and the cosmological constant. These results have bearings on future numerical simulations for black holes and gravitational waves in next-generation wavelet templates. Full article
(This article belongs to the Section Gravitation)
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17 pages, 584 KiB  
Article
The Properties of an Edge-On Low Surface Brightness Galaxies Sample
by Tian-Wen Cao, Zi-Jian Li, Pei-Bin Chen, Venu M. Kalari, Cheng Cheng, Gaspar Galaz, Hong Wu and Junfeng Wang
Universe 2024, 10(11), 432; https://doi.org/10.3390/universe10110432 - 20 Nov 2024
Viewed by 359
Abstract
We analyzed the properties of a sample of edge-on low-surface brightness galaxies, which are referred to as Cao23 ELSBGs. Cao23 ELSBGs exhibit a wide range of luminosities (−22 < Mr < −13) with a mean scale length of 3.19 ± 1.48 kpc. [...] Read more.
We analyzed the properties of a sample of edge-on low-surface brightness galaxies, which are referred to as Cao23 ELSBGs. Cao23 ELSBGs exhibit a wide range of luminosities (−22 < Mr < −13) with a mean scale length of 3.19 ± 1.48 kpc. Compared to HI-rich dwarf ELSBGs, Cao23 ELSBGs display more extended disk structures and redder (g-r) colors. They are also, on average, more massive than HI-rich dwarf ELSBGs. Star formation rates (SFRs) were calculated using WISE 12 μm luminosity conversions and spectral energy distribution (SED) fitting methods, respectively. Cao23 ELSBGs fall below the main sequence with specific star formation rates (sSFRs) primarily in the range of 0.01–0.1 Gyr−1. More massive Cao23 LSBGs tend to have lower sSFRs. Additionally, we derived the non-parametric star formation histories (SFHs) of Cao23 ELSBGs by SED fitting, dividing the SFHs into seven look back time bins with constant SFRs assumed for each bin. Our analysis indicates that high-mass (M > 109.0M) Cao23 ELSBGs assembled their mass earlier than their lower-mass counterparts, supporting a downsizing trend for LSBGs. Full article
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22 pages, 3167 KiB  
Article
The Composite Spectral Energy Distribution of Quasars Is Surprisingly Universal Since Cosmic Noon
by Zhenyi Cai
Universe 2024, 10(11), 431; https://doi.org/10.3390/universe10110431 - 19 Nov 2024
Viewed by 483
Abstract
Leveraging the photometric data of the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX), we construct mean/median spectral energy distributions (SEDs) for unique bright quasars in redshift bins of 0.2 and up to z3, after taking the GALEX [...] Read more.
Leveraging the photometric data of the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX), we construct mean/median spectral energy distributions (SEDs) for unique bright quasars in redshift bins of 0.2 and up to z3, after taking the GALEX non-detection into account. Further correcting for the absorption of the intergalactic medium, these mean/median quasar SEDs constitute a surprisingly redshift-independent mean/median composite SED from the rest-frame optical down to ≃500 A˚ for quasars with bolometric luminosity brighter than 1045.5ergs1. Moreover, the mean/median composite quasar SED is plausibly also independent of black hole mass and Eddington ratio, and suggests similar properties of dust and gas in the quasar host galaxies since cosmic noon. Both the mean and median composite SEDs are nicely consistent with previous mean composite quasar spectra at wavelengths beyond ≃1000 A˚, but at shorter wavelengths, are redder, indicating, on average, less ionizing radiation than previously expected. Through comparing the model-predicted to the observed composite quasar SEDs, we favor a simply truncated disk model, rather than a standard thin disk model, for the quasar central engine, though we request more sophisticated disk models. Future deep ultraviolet facilities, such as the China Space Station Telescope and the Ultraviolet Explorer, would prompt revolutions in many aspects, including the quasar central engine, production of the broad emission lines in quasars, and cosmic reionization. Full article
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19 pages, 1057 KiB  
Article
Black Holes with a Cloud of Strings and Quintessence in a Non-Linear Electrodynamics Scenario
by Francinaldo Florencio do Nascimento, Valdir Barbosa Bezerra and Jefferson de Morais Toledo
Universe 2024, 10(11), 430; https://doi.org/10.3390/universe10110430 - 19 Nov 2024
Viewed by 515
Abstract
We obtain exact black hole solutions to Einstein gravity coupled with a nonlinear electrodynamics field, in the presence of a cloud of strings and quintessence, as sources. The solutions have four parameters, namely m, k, a, and α, corresponding [...] Read more.
We obtain exact black hole solutions to Einstein gravity coupled with a nonlinear electrodynamics field, in the presence of a cloud of strings and quintessence, as sources. The solutions have four parameters, namely m, k, a, and α, corresponding to the physical mass of the black hole, the nonlinear charge of a self-gravitating magnetic field, the cloud of strings, and the intensity of the quintessential fluid. The consequences of these sources on the regularity or singularity of the solutions, on their horizons, as well as on the energy conditions, are discussed. We study some aspects concerning the thermodynamics of the black hole, by taking into account the mass, Hawking temperature, and heat capacity and show how these quantities depend on the presence of the cloud of strings and quintessence, in the scenario considered. Full article
(This article belongs to the Collection Open Questions in Black Hole Physics)
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28 pages, 2411 KiB  
Review
Cosmological Models in Lovelock Gravity: An Overview of Recent Progress
by Sergey Pavluchenko
Universe 2024, 10(11), 429; https://doi.org/10.3390/universe10110429 - 18 Nov 2024
Viewed by 421
Abstract
In the current review, we provide a summary of the recent progress made in the cosmological aspect of extra-dimensional Lovelock gravity. Our review covers a wide variety of particular model/matter source combinations: Einstein–Gauss–Bonnet as well as cubic Lovelock gravities with vacuum, cosmological constant, [...] Read more.
In the current review, we provide a summary of the recent progress made in the cosmological aspect of extra-dimensional Lovelock gravity. Our review covers a wide variety of particular model/matter source combinations: Einstein–Gauss–Bonnet as well as cubic Lovelock gravities with vacuum, cosmological constant, perfect fluid, spatial curvature, and some of their combinations. Our analysis suggests that it is possible to set constraints on the parameters of the above-mentioned models from the simple requirement of the existence of a smooth transition from the initial singularity to a realistic low-energy regime. Initially, anisotropic space naturally evolves into a configuration with two isotropic subspaces, and if one of these subspaces is three-dimensional and is expanding while another is contracting, we call it realistic compactification. Of course, the process is not devoid of obstacles, and in our paper, we review the results of the compactification occurrence investigation for the above-mentioned models. In particular, for vacuum and Λ-term EGB models, compactification is not suppressed (but is not the only possible outcome either) if the number of extra dimensions is D2; for vacuum cubic Lovelock gravities it is always present (however, cubic Lovelock gravity is defined only for D3 number of extra dimensions); for the EGB model with perfect fluid it is present for D=2 (we have not considered this model in higher dimensions yet), and in the presence of spatial curvature, the realistic stabilization of extra dimensions is always present (however, such a model is well-defined only in D4 number of extra dimensions). Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
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20 pages, 419 KiB  
Article
Current Density Induced by a Cosmic String in de Sitter Spacetime in the Presence of Two Flat Boundaries
by Wagner Oliveira dos Santos, Herondy F. Santana Mota and Eugênio R. Bezerra de Mello
Universe 2024, 10(11), 428; https://doi.org/10.3390/universe10110428 - 17 Nov 2024
Viewed by 385
Abstract
In this paper, we investigate the vacuum bosonic current density induced by a carrying-magnetic-flux cosmic string in a (D+1)-de Sitter spacetime considering the presence of two flat boundaries perpendicular to it. In this setup, the Robin boundary conditions [...] Read more.
In this paper, we investigate the vacuum bosonic current density induced by a carrying-magnetic-flux cosmic string in a (D+1)-de Sitter spacetime considering the presence of two flat boundaries perpendicular to it. In this setup, the Robin boundary conditions are imposed on the scalar charged quantum field on the boundaries. The particular cases of Dirichlet and Neumann boundary conditions are studied separately. Due to the coupling of the quantum scalar field with the classical gauge field, corresponding to a magnetic flux running along the string’s core, a nonzero vacuum expectation value for the current density operator along the azimuthal direction is induced. The two boundaries divide the space in three regions with different properties of the vacuum states. In this way, our main objective is to calculate the induced currents in these three regions. In order to develop this analysis we calculate, for both regions, the positive frequency Wightman functions. Because the vacuum bosonic current in dS space has been investigated before, in this paper we consider only the contributions induced by the boundaries. We show that for each region the azimuthal current densities are odd functions of the magnetic flux along the string. To probe the correctness of our results, we take the particular cases and analyze some asymptotic limits of the parameters of the model. Also some graphs are presented exhibiting the behavior of the current with relevant physical parameter of the system. Full article
(This article belongs to the Section Field Theory)
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16 pages, 823 KiB  
Article
Post-Newtonian Effects in Compact Binaries with a Dark Matter Spike: A Lagrangian Approach
by Diego Montalvo, Adam Smith-Orlik, Saeed Rastgoo, Laura Sagunski, Niklas Becker and Hazkeel Khan
Universe 2024, 10(11), 427; https://doi.org/10.3390/universe10110427 - 17 Nov 2024
Viewed by 366
Abstract
We apply the Lagrangian method to study the post-Newtonian evolution of a compact binary system with environmental effects, including a dark matter spike, and obtain the resulting gravitational wave emission. This formalism allows one to incorporate post-Newtonian effects up to any desired known [...] Read more.
We apply the Lagrangian method to study the post-Newtonian evolution of a compact binary system with environmental effects, including a dark matter spike, and obtain the resulting gravitational wave emission. This formalism allows one to incorporate post-Newtonian effects up to any desired known order, as well as any other environmental effect around the binary, as long as their dissipation power or force formulae are known. In particular, in this work, we employ this method to study a black hole–black hole binary system of mass ratio 105 by including post-Newtonian effects of order 1PN and 2.5PN, as well as the effect of relativistic dynamical friction. We obtain the modified orbits and the corresponding modified gravitational waveform. Finally, we contrast these modifications against the LISA sensitivity curve in frequency space and show that this observatory can detect the associated signals. Full article
(This article belongs to the Section Compact Objects)
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11 pages, 255 KiB  
Article
Probing the Dark Universe with Gravitational Waves
by Antonio Enea Romano
Universe 2024, 10(11), 426; https://doi.org/10.3390/universe10110426 - 15 Nov 2024
Viewed by 384
Abstract
Gravitational waves (GWs) are expected to interact with dark energy and dark matter, affecting their propagation on cosmological scales. To model this interaction, we derive a gauge-invariant effective equation and action valid for all GW polarizations. This is achieved by encoding the effects [...] Read more.
Gravitational waves (GWs) are expected to interact with dark energy and dark matter, affecting their propagation on cosmological scales. To model this interaction, we derive a gauge-invariant effective equation and action valid for all GW polarizations. This is achieved by encoding the effects of GW interactions at different orders of perturbation into a polarization-, frequency-, and time-dependent effective speed. The invariance of perturbations under time-dependent conformal transformations and the gauge invariance of GWs allow us to derive the unitary gauge effective action in any conformally related frame, thereby clarifying the relationship between the Einstein and Jordan frames. Tests of the polarization and frequency dependencies in the propagation time and luminosity distance of different GW polarizations allow us to probe the dark Universe, which acts as an effective medium, modeled by the GW effective speed. Full article
3 pages, 145 KiB  
Editorial
Editorial to the Special Issue “Space Missions to Small Bodies: Results and Future Activities”
by Andrea Longobardo
Universe 2024, 10(11), 425; https://doi.org/10.3390/universe10110425 - 14 Nov 2024
Viewed by 456
Abstract
Small bodies (asteroids, comets, and satellites) are the most primitive bodies of our solar system and, for this reason, represent the key to understanding its origin and early evolution [...] Full article
(This article belongs to the Special Issue Space Missions to Small Bodies: Results and Future Activities)
30 pages, 11511 KiB  
Article
Sources and Radiations of the Fermi Bubbles
by Vladimir A. Dogiel and Chung-Ming Ko
Universe 2024, 10(11), 424; https://doi.org/10.3390/universe10110424 - 12 Nov 2024
Viewed by 618
Abstract
Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local [...] Read more.
Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local ambient medium of the galactic halo. A decade or so ago, a phenomenological model of the FBs was suggested as a result of routine star disruptions by the supermassive black hole in the GC which might provide enough energy for large-scale structures, like the FBs. In 2020, analytical and numerical models of the FBs as a process of routine tidal disruption of stars near the GC were developed; these disruption events can provide enough cumulative energy to form and maintain large-scale structures like the FBs. The disruption events are expected to be 104105yr1, providing an average power of energy release from the GC into the halo of E˙3×1041 erg s1, which is needed to support the FBs. Analysis of the evolution of superbubbles in exponentially stratified disks concluded that the FB envelope would be destroyed by the Rayleigh–Taylor (RT) instabilities at late stages. The shell is composed of swept-up gas of the bubble, whose thickness is much thinner in comparison to the size of the envelope. We assume that hydrodynamic turbulence is excited in the FB envelope by the RT instability. In this case, the universal energy spectrum of turbulence may be developed in the inertial range of wavenumbers of fluctuations (the Kolmogorov–Obukhov spectrum). From our model we suppose the power of the FBs is transformed partly into the energy of hydrodynamic turbulence in the envelope. If so, hydrodynamic turbulence may generate MHD fluctuations, which accelerate cosmic rays there and generate gamma-ray and radio emission from the FBs. We hope that this model may interpret the observed nonthermal emission from the bubbles. Full article
(This article belongs to the Special Issue Studying Astrophysics with High-Energy Cosmic Particles)
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16 pages, 1992 KiB  
Article
Exploring γ-Ray Flares from High-Redshift Blazar B3 1343+451 at GeV Energies
by Xiongfei Geng, Yang Liu, Gang Cao, Jing Fan, Xiongbang Yang, Nan Ding, Minghu Gao, Yehui Yang and Zhijie Zhang
Universe 2024, 10(11), 423; https://doi.org/10.3390/universe10110423 - 11 Nov 2024
Viewed by 594
Abstract
We study the temporal and spectral variability properties of the high-redshift blazar B3 1343+451 utilizing Fermi-LAT data from 2008 to 2022 in the energy range of 0.1–300 GeV. We identify six major flares with many substructures and analyze their temporal and spectral properties [...] Read more.
We study the temporal and spectral variability properties of the high-redshift blazar B3 1343+451 utilizing Fermi-LAT data from 2008 to 2022 in the energy range of 0.1–300 GeV. We identify six major flares with many substructures and analyze their temporal and spectral properties in detail. The fastest rise and decay timescales are found to be 4.8 ± 0.48 h and 5.28 ± 0.72 h, respectively. The size of the emission region is constrained to be R ∼ 5.18 × 1015–1.56 × 1016 cm with the typical Doppler factors of δ ∼ 10–30. Most of the peaks from the flares exhibit a symmetric temporal profile within the error bars, implying that the rise and decay timescales are dominated by the disturbances caused by dense plasma blobs passing through the standing shock front in the jet region. We also find that four flares are better fitted with a log-parabolic distribution, while two flares are better fitted with a power-law distribution. Our results indicate that the emission regions vary from one flare to another, which is consistent with earlier results. Full article
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4 pages, 166 KiB  
Editorial
Solar Radio Emissions
by Baolin Tan and Jing Huang
Universe 2024, 10(11), 422; https://doi.org/10.3390/universe10110422 - 9 Nov 2024
Viewed by 412
Abstract
The radio emission from the Sun covers a very wide frequency band ranging from several hundreds of GHz (sub-millimeter wavelength) down to sub-MHz (kilometer wavelength) [...] Full article
(This article belongs to the Special Issue Solar Radio Emissions)
14 pages, 616 KiB  
Article
Gravitational Lensing Effects from Models of Loop Quantum Gravity with Rigorous Quantum Parameters
by Haida Li and Xiangdong Zhang
Universe 2024, 10(11), 421; https://doi.org/10.3390/universe10110421 - 8 Nov 2024
Viewed by 478
Abstract
Many previous works have studied gravitational lensing effects from Loop Quantum Gravity. So far, gravitational lensing effects from Loop Quantum Gravity have only been studied by choosing large quantum parameters much larger than the Planck scale. However, by construction, the quantum parameters of [...] Read more.
Many previous works have studied gravitational lensing effects from Loop Quantum Gravity. So far, gravitational lensing effects from Loop Quantum Gravity have only been studied by choosing large quantum parameters much larger than the Planck scale. However, by construction, the quantum parameters of the effective models of Loop Quantum Gravity are usually related to the Planck length and, thus, are extremely small. In this work, by strictly imposing the quantum parameters as initially constructed, we study the true quantum corrections of gravitational lensing effects by five effective black hole models of Loop Quantum Gravity. Our study reveals several interesting results, including the different scales of quantum corrections displayed by each model and the connection between the quantum correction of deflection angles and the quantum correction of the metric. Observables related to the gravitational lensing effect are also obtained for all models in the case of SgrA* and M87*. Full article
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41 pages, 549 KiB  
Article
Comparing a Gauge-Invariant Formulation and a “Conventional Complete Gauge-Fixing Approach” for l=0,1-Mode Perturbations on the Schwarzschild Background Spacetime
by Kouji Nakamura
Universe 2024, 10(11), 420; https://doi.org/10.3390/universe10110420 - 7 Nov 2024
Viewed by 495
Abstract
This article provides a comparison of the gauge-invariant formulation for l=0,1-mode perturbations on the Schwarzschild background spacetime, proposed by the same author in 2021, and a “conventional complete gauge-fixing approach” where the spherical harmonic functions Ylm [...] Read more.
This article provides a comparison of the gauge-invariant formulation for l=0,1-mode perturbations on the Schwarzschild background spacetime, proposed by the same author in 2021, and a “conventional complete gauge-fixing approach” where the spherical harmonic functions Ylm as the scalar harmonics are used from the starting point. Although it is often stated that “gauge-invariant formulations in general-relativistic perturbations are equivalent to complete gauge-fixing approaches”, we conclude that, as a result of this comparison, the derived solutions through the proposed gauge-invariant formulation and those through a “conventional complete gauge-fixing approach” are different. It is pointed out that there is a case where the boundary conditions and initial conditions are restricted in a conventional complete gauge-fixing approach. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)
41 pages, 1918 KiB  
Review
Semi-Symmetric Metric Gravity: A Brief Overview
by Himanshu Chaudhary, Lehel Csillag and Tiberiu Harko
Universe 2024, 10(11), 419; https://doi.org/10.3390/universe10110419 - 7 Nov 2024
Viewed by 607
Abstract
We present a review of the Semi-Symmetric Metric Gravity (SSMG) theory, representing a geometric extension of standard general relativity, based on a connection introduced by Friedmann and Schouten in 1924. The semi-symmetric connection is a connection that generalizes the Levi-Civita one by allowing [...] Read more.
We present a review of the Semi-Symmetric Metric Gravity (SSMG) theory, representing a geometric extension of standard general relativity, based on a connection introduced by Friedmann and Schouten in 1924. The semi-symmetric connection is a connection that generalizes the Levi-Civita one by allowing for the presence of a simple form of the torsion, described in terms of a torsion vector. The Einstein field equations are postulated to have the same form as in standard general relativity, thus relating the Einstein tensor constructed with the help of the semi-symmetric connection, with the energy–momentum tensor. The inclusion of the torsion contributions in the field equations has intriguing cosmological implications, particularly during the late-time evolution of the Universe. Presumably, these effects also dominate under high-energy conditions, and thus SSMG could potentially address unresolved issues in general relativity and cosmology, such as the initial singularity, inflation, or the 7Li problem of the Big-Bang Nucleosynthesis. The explicit presence of torsion in the field equations leads to the non-conservation of the energy–momentum tensor, which can be interpreted within the irreversible thermodynamics of open systems as describing particle creation processes. We also review in detail the cosmological applications of the theory, and investigate the statistical tests for several models, by constraining the model parameters via comparison with several observational datasets. Full article
(This article belongs to the Special Issue Dark Energy and Dark Matter)
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26 pages, 416 KiB  
Perspective
Foundational Issues in Dynamical Casimir Effect and Analogue Features in Cosmological Particle Creation
by Jen-Tsung Hsiang and Bei-Lok Hu
Universe 2024, 10(11), 418; https://doi.org/10.3390/universe10110418 - 7 Nov 2024
Viewed by 518
Abstract
Moving mirrors as analogue sources of Hawking radiation from black holes have been explored extensively but less so with cosmological particle creation (CPC), even though the analogy between the dynamical Casimir effect (DCE) and CPC based on the mechanism of the parametric amplification [...] Read more.
Moving mirrors as analogue sources of Hawking radiation from black holes have been explored extensively but less so with cosmological particle creation (CPC), even though the analogy between the dynamical Casimir effect (DCE) and CPC based on the mechanism of the parametric amplification of quantum field fluctuations has also been known for a long time. This ‘perspective’ essay intends to convey some of the rigor and thoroughness of quantum field theory in curved spacetime, which serves as the theoretical foundation of CPC, to DCE, which enjoys a variety of active experimental explorations. We have selected seven issues of relevance to address, starting from the naively simple ones, e.g., why one should be bothered with ‘curved’ spacetime when performing a laboratory experiment in ostensibly flat space, to foundational theoretical ones, such as the frequent appearance of nonlocal dissipation in the system dynamics induced by colored noises in its field environment, the existence of quantum Lenz law and fluctuation–dissipation relations in the backreaction effects of DCE emission on the moving atom/mirror or the source, and the construction of a microphysics model to account for the dynamical responses of a mirror or medium. The strengthening of the theoretical ground for DCE is not only useful for improving conceptual clarity but needed for the development of the proof-of-concept type of future experimental designs for DCE. The results from the DCE experiments in turn will enrich our understanding of quantum field effects in the early universe because they are, in the spirit of analogue gravity, our best hopes for the verification of these fundamental processes. Full article
(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)
24 pages, 6959 KiB  
Article
Linking Turbulent Interplanetary Magnetic Field Fluctuations and Current Sheets
by Maria O. Riazantseva, Timofey V. Treves, Olga Khabarova, Liudmila S. Rakhmanova, Yuri I. Yermolaev and Alexander A. Khokhlachev
Universe 2024, 10(11), 417; https://doi.org/10.3390/universe10110417 - 7 Nov 2024
Viewed by 545
Abstract
The study aims to understand the role of solar wind current sheets (CSs) in shaping the spectrum of turbulent fluctuations and driving dissipation processes in space plasma. Local non-adiabatic heating and acceleration of charged particles in the solar wind is one of the [...] Read more.
The study aims to understand the role of solar wind current sheets (CSs) in shaping the spectrum of turbulent fluctuations and driving dissipation processes in space plasma. Local non-adiabatic heating and acceleration of charged particles in the solar wind is one of the most intriguing challenges in space physics. Leading theories attribute these effects to turbulent heating, often associated with magnetic reconnection at small-scale coherent structures in the solar wind, such as CSs and flux ropes. We identify CSs observed at 1 AU in different types of the solar wind around and within an interplanetary coronal mass ejection (ICME) and analyze the corresponding characteristics of the turbulent cascade. It is found that the spectra of fluctuations of the interplanetary magnetic field may be reshaped due to the CS impact potentially leading to local disruptions in energy transfer along the cascade of turbulent fluctuations. Case studies of the spectra behavior at the peak of the CS number show their steepening at MHD scales, flattening at kinetic scales, and merging of the spectra into a single form, with the break almost disappearing. In the broader vicinity of the CS number peak, the behavior of spectral parameters changes sharply, but not always following the same pattern. The statistical analysis shows a clear correlation between the break frequency and the CS number. These results are consistent with the picture of turbulent reconnection at CSs. The CS occurrence is found to be statistically linked with the increased temperature. In the ICME sheath, there are two CS populations observed in the hottest and coldest plasma. Full article
(This article belongs to the Section Space Science)
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14 pages, 592 KiB  
Article
Profile Variation in PSR B0355+54 over a Narrow Frequency Range
by Shibo Jiang, Lin Li, Rai Yuen, Jianping Yuan, Jumei Yao, Xun Shi, Yonghua Xu, Jianling Chen and Zhigang Wen
Universe 2024, 10(11), 416; https://doi.org/10.3390/universe10110416 - 6 Nov 2024
Viewed by 520
Abstract
We investigate changes in the shape of the averaged pulse profile in PSR B0355+54 (PSR J0358+5413) based on data obtained at the center frequency of 1250 MHz using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our dataset consists of 12 non-consecutive observations, each [...] Read more.
We investigate changes in the shape of the averaged pulse profile in PSR B0355+54 (PSR J0358+5413) based on data obtained at the center frequency of 1250 MHz using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our dataset consists of 12 non-consecutive observations, each lasting between 1 and 2 h. Considerable variation is observed in the averaged profiles across the observations even though each is folded from thousands of single pulses. Changes in the profile are measured through the ratio (R) between the peak intensities of the leading and trailing components. We find that the averaged pulse profile exhibits significant variation across observations, but distinctive from typical profile mode-changing. By dividing the frequency bandwidth into eight sub-bands, we demonstrate that the shape of the averaged profile undergoes significant evolution with frequency. In general, the changes in R across the sub-bands are different in different observations, but its value is uniform at low frequencies implying a more consistent emission. We demonstrate that the profile stabilization timescale for this pulsar is much longer than commonly suggested for ordinary pulsars, which is likely due to non-uniform and varying arrangement of the emission sources in the emission region. Full article
(This article belongs to the Section Compact Objects)
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14 pages, 449 KiB  
Article
Neutron Beta Decay and Exact Conservation of Charged Weak Hadronic Vector Current in the Standard Model
by Derar Altarawneh, Roman Höllwieser and Markus Wellenzohn
Universe 2024, 10(11), 415; https://doi.org/10.3390/universe10110415 - 6 Nov 2024
Viewed by 447
Abstract
We investigate the reliability of the hypothesis of exact conservation of the charged weak hadronic vector current in neutron β-decay with a polarized neutron and an unpolarized proton and electron. We calculate the contributions of the phenomenological term responsible for Exact [...] Read more.
We investigate the reliability of the hypothesis of exact conservation of the charged weak hadronic vector current in neutron β-decay with a polarized neutron and an unpolarized proton and electron. We calculate the contributions of the phenomenological term responsible for Exact Conservation of the charged weak hadronic Vector Current (or the ECVC effect) in neutron β-decay, even for different masses of the neutron and proton, to the correlation coefficients, together with a complete set of contributions of scalar and tensor interactions beyond the Standard Model (SM). We argue that if the total contributions of scalar and tensor interactions beyond the SM fail to reconcile the experimental data on the correlation coefficients with the contributions of the ECVC effect, one may conclude that the charged weak hadronic vector current is not conserved in the hadronic transitions of weak processes with different masses of incoming and outgoing hadrons. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
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9 pages, 225 KiB  
Article
Estimation of the Chances to Find New Phenomena at the LHC in a Model-Agnostic Combinatorial Analysis
by Sergei V. Chekanov
Universe 2024, 10(11), 414; https://doi.org/10.3390/universe10110414 - 5 Nov 2024
Cited by 1 | Viewed by 794
Abstract
In this paper, we estimate the number of event topologies that have the potential to be produced in pp collisions at the Large Hadron Collider (LHC) without violating kinematic and other constraints. We use numerical calculations and combinatorics, guided by large-scale Monte [...] Read more.
In this paper, we estimate the number of event topologies that have the potential to be produced in pp collisions at the Large Hadron Collider (LHC) without violating kinematic and other constraints. We use numerical calculations and combinatorics, guided by large-scale Monte Carlo simulations of Standard Model (SM) processes. Then, we set the upper limit on the probability that new physics may escape detection, assuming a model-agnostic approach. The calculated probability is unexpectedly large, and the fact that LHC has not found new physics until now is not entirely surprising. Theoretical limitations and experimental challenges in observing new physics within the studied exclusive event classes are examined. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
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17 pages, 528 KiB  
Article
Studying the Properties of Spacetime with an Improved Dynamical Model of the Inner Solar System
by Dmitry Pavlov and Ivan Dolgakov
Universe 2024, 10(11), 413; https://doi.org/10.3390/universe10110413 - 3 Nov 2024
Viewed by 507
Abstract
Physical properties of the Sun (orientation of rotation axis, oblateness coefficient J2, and change rate of the gravitational parameter μ˙) are determined using a dynamical model describing the motion of the Sun, planets, the Moon, asteroids, and [...] Read more.
Physical properties of the Sun (orientation of rotation axis, oblateness coefficient J2, and change rate of the gravitational parameter μ˙) are determined using a dynamical model describing the motion of the Sun, planets, the Moon, asteroids, and Trans-Neptunian objects (TNOs). Among the many kinds of observations used to determine the orbits and physical properties of the bodies, the most important for our study are precise interplanetary ranging data: Earth–Mercury ranges from MESSENGER spacecraft and Earth–Mars ranges from Odyssey and MRO. The findings allow us to improve the model of the Sun in modern planetary ephemerides. First, the dynamically determined direction of the Sun’s pole is ≈2° off the visible axis of rotation of the Sun’s surface, which is corroborated by present knowledge of the Sun’s interior. Second, the change rate of the Sun’s gravitational parameter is found to be smaller (in absolute value) than the nominal value derived from the estimate of mass loss through radiation and solar wind. Possible interpretations are discussed. Full article
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29 pages, 440 KiB  
Article
Analogous Hawking Radiation in Dispersive Media
by Francesco Belgiorno, Sergio L. Cacciatori and Simone Trevisan
Universe 2024, 10(11), 412; https://doi.org/10.3390/universe10110412 - 2 Nov 2024
Viewed by 489
Abstract
In the framework of the analogous Hawking effect, we significantly improve our previous analysis of the master equation that encompasses very relevant physical systems, like Bose–Einstein condensates (BECs), dielectric media, and water. In particular, we are able to provide two significant improvements to [...] Read more.
In the framework of the analogous Hawking effect, we significantly improve our previous analysis of the master equation that encompasses very relevant physical systems, like Bose–Einstein condensates (BECs), dielectric media, and water. In particular, we are able to provide two significant improvements to the analysis. As our main result, we provide a complete set of connection formulas for both the subluminal and superluminal cases without resorting to suitable boundary conditions, first introduced by Corley, but simply on the grounds of a rigorous mathematical setting. Moreover, we provide an extension to the four-dimensional case, showing explicitly that, apart from obvious changes, adding transverse dimensions does not substantially modify the Hawking temperature in the dispersive case. Furthermore, an important class of exact solutions of the so-called reduced equation that governs the behavior of non-dispersive modes is also provided. Full article
(This article belongs to the Section Field Theory)
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16 pages, 4105 KiB  
Article
Structure Formation Through Magnetohydrodynamical Instabilities in Primordial Disks
by Koichi Noguchi, Toshiki Tajima and Wendell Horton
Universe 2024, 10(11), 411; https://doi.org/10.3390/universe10110411 - 31 Oct 2024
Viewed by 508
Abstract
The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational [...] Read more.
The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational and kinematic dynamo instabilities by comparing the properties of these instabilities. We calculate the mass spectra of coagulated density structures by the above mechanism in the radial direction for an axisymmetric magnetohydrodynamic (MHD) torus equilibrium and power density profile models. Our local three-dimensional MHD simulation indicates that the coupling of the Parker and magnetorotational instability creates spiral arms and gas blobs in an accretion disk, reinforcing the theory and model. Such a mechanism for the early structure formation may be tested in a laboratory. The recent progress in experiments involving shear flows in rotating tokamak, field reversed configuration (FRC) and laser plasmas may become a key element to advance in nonlinear studies. Full article
(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)
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23 pages, 3287 KiB  
Article
Relational Lorentzian Asymptotically Safe Quantum Gravity: Showcase Model
by Renata Ferrero and Thomas Thiemann
Universe 2024, 10(11), 410; https://doi.org/10.3390/universe10110410 - 31 Oct 2024
Viewed by 525
Abstract
In a recent contribution, we identified possible points of contact between the asymptotically safe and canonical approaches to quantum gravity. The idea is to start from the reduced phase space (often called relational) formulation of canonical quantum gravity, which provides a reduced (or [...] Read more.
In a recent contribution, we identified possible points of contact between the asymptotically safe and canonical approaches to quantum gravity. The idea is to start from the reduced phase space (often called relational) formulation of canonical quantum gravity, which provides a reduced (or physical) Hamiltonian for the true (observable) degrees of freedom. The resulting reduced phase space is then canonically quantized, and one can construct the generating functional of time-ordered Wightman (i.e., Feynman) or Schwinger distributions, respectively, from the corresponding time-translation unitary group or contraction semigroup, respectively, as a path integral. For the unitary choice, that path integral can be rewritten in terms of the Lorentzian Einstein–Hilbert action plus observable matter action and a ghost action. The ghost action depends on the Hilbert space representation chosen for the canonical quantization and a reduction term that encodes the reduction of the full phase space to the phase space of observables. This path integral can then be treated with the methods of asymptotically safe quantum gravity in its Lorentzian version. We also exemplified the procedure using a concrete, minimalistic example, namely Einstein–Klein–Gordon theory, with as many neutral and massless scalar fields as there are spacetime dimensions. However, no explicit calculations were performed. In this paper, we fill in the missing steps. Particular care is needed due to the necessary switch to Lorentzian signature, which has a strong impact on the convergence of “heat” kernel time integrals in the heat kernel expansion of the trace involved in the Wetterich equation and which requires different cut-off functions than in the Euclidian version. As usual we truncate at relatively low order and derive and solve the resulting flow equations in that approximation. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
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14 pages, 334 KiB  
Article
An Update of the Hypothetical X17 Particle
by Attila J. Krasznahorkay, Attila Krasznahorkay, Margit Csatlós, János Timár, Marcell Begala, Attila Krakó, István Rajta, István Vajda and Nándor J. Sas
Universe 2024, 10(11), 409; https://doi.org/10.3390/universe10110409 - 31 Oct 2024
Viewed by 521
Abstract
Recently, when examining the differential internal pair creation coefficients of 8Be, 4He and 12C nuclei, we observed peak-like anomalies in the angular correlation of the e+e pairs. This was interpreted as the creation and immediate decay of [...] Read more.
Recently, when examining the differential internal pair creation coefficients of 8Be, 4He and 12C nuclei, we observed peak-like anomalies in the angular correlation of the e+e pairs. This was interpreted as the creation and immediate decay of an intermediate bosonic particle with a mass of mXc2 17 MeV, receiving the name X17 in subsequent publications. In this paper, our latest results obtained for the X17 particle are presented by investigating the e+e pair correlations in the decay of the Giant Dipole Resonance (GDR) of 8Be. Our results initiated a significant number of new experiments all over the world to detect the X17 particle and determine its properties. In this paper, we will also conduct a mini-review of the experiments whose results are already published, as well as the ones closest to being published. Full article
(This article belongs to the Special Issue Multiparticle Dynamics)
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22 pages, 347 KiB  
Article
Generalized Schwarzschild Spacetimes with a Linear Term and a Cosmological Constant
by Orchidea Maria Lecian
Universe 2024, 10(11), 408; https://doi.org/10.3390/universe10110408 - 30 Oct 2024
Viewed by 643
Abstract
Particular Kottler spacetimes are analytically investigated. The investigated spacetimes are spherically symmetric nonrotating spacetimes endowed with a Schwarzschild solid-angle element. SchwarzschildNairiai spacetimes, Schwarzschild spacetimes with a linear term, and Schwarzschild spacetimes with a linear term and a cosmological constant are studied. The infinite-redshift [...] Read more.
Particular Kottler spacetimes are analytically investigated. The investigated spacetimes are spherically symmetric nonrotating spacetimes endowed with a Schwarzschild solid-angle element. SchwarzschildNairiai spacetimes, Schwarzschild spacetimes with a linear term, and Schwarzschild spacetimes with a linear term and a cosmological constant are studied. The infinite-redshift surfaces are analytically written. To this aim, the parameter spaces of the models are analytically investigated, and the conditions for which the analytical radii are reconducted to the physical horizons are used to set and to constrain the parameter spaces. The coordinate-singularity-avoiding coordinate extensions are newly written. Schwarzschild spacetimes with a linear term and a cosmological constant termare analytically studied, and the new singularity-avoiding coordinate extensions are detailed. The new roles of the linear term and of the cosmological constant term in characterizing the Schwarzschild radius are traced. The generalized Schwarzschild–deSitter case and generalized Schwarzschild–anti-deSitter case are characterized in a different manner. The weak field limit is newly recalled. The embeddings are newly provided. The quantum implementation is newly envisaged. The geometrical objects are newly calculated. As a result, for the Einstein field equations, the presence of quintessence is newly excluded. The Birkhoff theorem is newly proven to be obeyed. Full article
(This article belongs to the Section Gravitation)
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34 pages, 5374 KiB  
Review
Ultra-Low Frequency Waves of Foreshock Origin Upstream and Inside of the Magnetospheres of Earth, Mercury, and Saturn Related to Solar Wind–Magnetosphere Coupling
by Zsofia Bebesi, Navin Kumar Dwivedi, Arpad Kis, Antal Juhász and Balazs Heilig
Universe 2024, 10(11), 407; https://doi.org/10.3390/universe10110407 - 30 Oct 2024
Viewed by 721
Abstract
This review examines ultra-low frequency (ULF) waves across different planetary environments, focusing on Earth, Mercury, and Saturn. Data from spacecraft missions (CHAMP, Swarm, and Oersted for Earth; MESSENGER for Mercury; and Cassini for Saturn) provide insights into ULF wave dynamics. At Earth, compressional [...] Read more.
This review examines ultra-low frequency (ULF) waves across different planetary environments, focusing on Earth, Mercury, and Saturn. Data from spacecraft missions (CHAMP, Swarm, and Oersted for Earth; MESSENGER for Mercury; and Cassini for Saturn) provide insights into ULF wave dynamics. At Earth, compressional ULF waves, particularly Pc3 waves, show significant power near the equator and peak around Magnetic Local Time (MLT) = 11. These waves interact complexly with Alfvén waves, impacting ionospheric responses and geomagnetic field line resonances. At Mercury, ULF waves transition from circular to linear polarization, indicating resonant interactions influenced by compressional components. MESSENGER data reveal a lower occurrence rate of ULF waves in Mercury’s foreshock compared to Earth’s, attributed to reduced backstreaming protons and lower solar wind Alfvénic Mach numbers, as ULF wave activity increases with heliocentric distance. Short Large-Amplitude Magnetic Structures (SLAMS) observed at Mercury and Saturn show distinct characteristics compared to those of Earth, including the presence of whistler precursos waves. However, due to the large differences in heliospheric distances, SLAMS (their temporal scale size correlate with the ULF wave frequency) at Mercury are significantly shorter in duration than at Earth or Saturn, since the ULF wave frequency primarily depends on the strength of the interplanetary magnetic field. This review highlights the variability of ULF waves and SLAMS across planetary environments, emphasizing Earth’s well-understood ionospheric interactions and the unique behaviours observed for Mercury and Saturn. These findings enhance our understanding of space plasma dynamics and underline the need for further research regarding planetary magnetospheres. Full article
(This article belongs to the Section Space Science)
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24 pages, 2185 KiB  
Article
2D BAO vs. 3D BAO: Solving the Hubble Tension with Bimetric Cosmology
by Sowmaydeep Dwivedi and Marcus Högås
Universe 2024, 10(11), 406; https://doi.org/10.3390/universe10110406 - 28 Oct 2024
Viewed by 589
Abstract
Ordinary 3D Baryon Acoustic Oscillations (BAO) data are model-dependent, requiring the assumption of a cosmological model to calculate comoving distances during data reduction. Throughout the present-day literature, the assumed model is ΛCDM. However, it has been pointed out in several recent works [...] Read more.
Ordinary 3D Baryon Acoustic Oscillations (BAO) data are model-dependent, requiring the assumption of a cosmological model to calculate comoving distances during data reduction. Throughout the present-day literature, the assumed model is ΛCDM. However, it has been pointed out in several recent works that this assumption can be inadequate when analyzing alternative cosmologies, potentially biasing the Hubble constant (H0) low, thus contributing to the Hubble tension. To address this issue, 3D BAO data can be replaced with 2D BAO data, which are only weakly model-dependent. The impact of using 2D BAO data, in combination with alternative cosmological models beyond ΛCDM, has been explored for several phenomenological models, showing a promising reduction in the Hubble tension. In this work, we accommodate these models in the theoretically robust framework of bimetric gravity. This is a modified theory of gravity that exhibits a transition from a (possibly) negative cosmological constant in the early universe to a positive one in the late universe. By combining 2D BAO data with cosmic microwave background and type Ia supernovae data, we find that the inverse distance ladder in this theory yields a Hubble constant of H0=(71.0±0.9)km/s/Mpc, consistent with the SH0ES local distance ladder measurement of H0=(73.0±1.0)km/s/Mpc. Replacing 2D BAO with 3D BAO results in H0=(68.6±0.5)km/s/Mpc from the inverse distance ladder. We conclude that the choice of BAO data significantly impacts the Hubble tension, with ordinary 3D BAO data exacerbating the tension, while 2D BAO data provide results consistent with the local distance ladder. Full article
(This article belongs to the Special Issue Current Status of the Hubble Tension)
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20 pages, 2995 KiB  
Article
Leveraging the Interplanetary Superhighway for Propellant–Optimal Orbit Insertion into Saturn–Titan System
by Giuseppe Papalia and Davide Conte
Universe 2024, 10(11), 405; https://doi.org/10.3390/universe10110405 - 28 Oct 2024
Viewed by 606
Abstract
This paper presents an innovative approach using Dynamical Systems Theory (DST) for interplanetary orbit insertion into Saturn−Titan three−body orbits. By leveraging DST, this study identifies invariant manifolds guiding a spacecraft into Titan−centered Distant Retrograde Orbits (DROs), strategically selected for their scientific significance. Subsequently, [...] Read more.
This paper presents an innovative approach using Dynamical Systems Theory (DST) for interplanetary orbit insertion into Saturn−Titan three−body orbits. By leveraging DST, this study identifies invariant manifolds guiding a spacecraft into Titan−centered Distant Retrograde Orbits (DROs), strategically selected for their scientific significance. Subsequently, Particle Swarm Optimization (PSO) is employed to fine−tune the insertion parameters, thereby minimizing ΔV. The results demonstrate that the proposed method allows for a reduction in ΔV of over 70% compared to conventional approaches like patched conics−based flybys (2.68 km/s vs. 9.23 km/s), albeit with an extended time of flight, which remains notably faster than weak stability boundary transfers. This paper serves as an interplanetary mission planning methodology to optimize spacecraft trajectories for the exploration of the Saturn−Titan system. Full article
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11 pages, 278 KiB  
Article
The Variation of G and Λ in Cosmology
by Aroonkumar Beesham
Universe 2024, 10(11), 404; https://doi.org/10.3390/universe10110404 - 23 Oct 2024
Viewed by 619
Abstract
The idea of varying constants of nature is very old, and has commanded a lot of attention since first mooted. The variation in the gravitational parameter G and cosmological parameter Λ is still an active area of research. Since the idea of a [...] Read more.
The idea of varying constants of nature is very old, and has commanded a lot of attention since first mooted. The variation in the gravitational parameter G and cosmological parameter Λ is still an active area of research. Since the idea of a varying G was introduced by Dirac almost a century ago, there are even theories that have variable G such as the Brans–Dicke theory and the scale covariant theory. Both these theories also have a varying Λ in their full generalisations. A varying Λ was also introduced around the same time as that of varying G. It is interesting to note that a possible solution to the cosmological constant problem can be realised from a dynamic Λ. In this work, we focus on a varying Λ and G framework. In almost all studies in the simplest framework of variables Λ and G, it is found that one of them has to increase with time. However, observations and theoretical considerations indicate that both Λ and G should decrease with time. In this paper, we propose a solution to this problem, finding theories in which both Λ and G decrease with time. Full article
(This article belongs to the Special Issue Dark Energy and Dark Matter)
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