Article
Astronomy & Astrophysics
M. Mousavi, K. Atazadeh
Summary: The study examines future cosmological singularities in the context of massive gravity and minimal massive bigravity theory. It identifies sudden and big brake singularities in different cosmological models, noting that the sudden singularity can be eliminated when model parameters approach zero.
PHYSICS OF THE DARK UNIVERSE
(2022)
Article
Astronomy & Astrophysics
Joao Luis Rosa
Summary: This study investigates the junction conditions for matching between two spacetimes at a separation hypersurface in the perfect-fluid version of f(R, T) gravity, considering both geometrical and scalar-tensor representations. General junction conditions are derived for a thin shell separating the spacetimes, with a focus on smooth matching when the stress-energy tensor of the thin shell vanishes. The set of junction conditions includes constraints on the continuity of the trace of the stress-energy tensor and its partial derivatives, leading to implications for spherical symmetric thin shells and specific forms of the function f(R, T).
Article
Multidisciplinary Sciences
Bofeng Wu
Summary: A signature invariant geometric algebra framework for spacetime physics is formulated in this paper, including techniques related to relative vector and spacetime split, as well as rotor techniques for Lorentz transformations and relativistic dynamics of a massive particle in curved spacetime. The paper also presents a three-dimensional analogue of Newton's second law for a particle in curved spacetime, and provides a geometric algebra approach to gyroscopic precession in Lense-Thirring spacetime for a gyroscope moving.
SCIENTIFIC REPORTS
(2022)
Article
Physics, Particles & Fields
R. V. Lobato, G. A. Carvalho, N. G. Kelkar, M. Nowakowski
Summary: In this work, the equilibrium configurations of massive white dwarfs in the context of modified gravity are investigated. The theory is applied to white dwarfs for the first time, specifically studying massive white dwarfs. The results predict maximum masses above the Chandrasekhar mass limit and significantly increase the mass for stars with radius < 2000 km.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Multidisciplinary Sciences
Fabrizia Ronco, Michael Matschiner, Astrid Boehne, Anna Boila, Heinz H. Buescher, Athimed El Taher, Adrian Indermaur, Milan Malinsky, Virginie Ricci, Ansgar Kahmen, Sissel Jentoft, Walter Salzburger
Summary: This text provides a detailed examination of the adaptive radiation of cichlid fishes in Lake Tanganyika, demonstrating empirical support for two theoretical predictions of how adaptive radiations proceed and revealing a positive correlation between species richness and per-individual heterozygosity. The study highlights the rapid morphological diversification through trait-specific pulses of accelerated evolution within the confines of the lake.
Article
Astronomy & Astrophysics
Ryan Endsley, Daniel P. Stark, Stephane Charlot, Jacopo Chevallard, Brant Robertson, Rychard J. Bouwens, Mauro Stefanon
Summary: Recent observations have shown strong Lyα emission in some of the most massive z = 7-9 galaxies, which is likely due to their enhanced ionizing photon production efficiency resulting from high specific star formation rates. Additionally, there is evidence to suggest that Lyα transmission declines less rapidly in massive galaxies compared to low-mass lensed systems in the redshift range of 6 < z < 7, implying that massive z ≈ 7 galaxies often reside in large ionized regions.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Physics, Particles & Fields
Oleksii Sokoliuk, Alexander Baransky
Summary: We study Morris-Thorne static traversable wormhole solutions in different modified theories of gravity and derive suitable shape functions by numerically solving the Einstein field equations. The stability of Morris-Thorne wormholes and energy conditions are analyzed, and general formulas for the extra force in MTOV due to the non-conserved stress-energy tensor are derived.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Computer Science, Software Engineering
Milan Jaros, Lubomir Riha, Petr Strakos, Matej Spetko
Summary: This study proposes a solution for path tracing of massive scenes on multiple GPUs, optimizing performance by analyzing memory access patterns and defining scene data distribution across CPUs. The approach is shown to be scalable and efficient, with only minor changes needed for application to other path tracers.
ACM TRANSACTIONS ON GRAPHICS
(2021)
Article
Astronomy & Astrophysics
Harvey B. Richer, Ilaria Caiazzo, Helen Du, Steffani Grondin, James Hegarty, Jeremy Heyl, Ronan Kerr, David R. Miller, Sarah Thiele
Summary: The study aimed to search for massive white dwarfs in the direction of young open star clusters using the Gaia DR2 database. WD candidates in each cluster were followed up with spectroscopy from Gemini to determine their membership status. Several white dwarfs previously included in the IFMR were found to be nonmembers based on astrometric data, leading to the development of a highly selected IFMR sample for high-mass WDs.
ASTROPHYSICAL JOURNAL
(2021)
Article
Astronomy & Astrophysics
Hao Wang, Chunhua Zhu, Helei Liu, Sufen Guo, Guoliang Lu
Summary: Neutrino losses are crucial for the evolution of massive stars, and their luminosity depends on factors such as stellar mass, rotation, and metallicity. Higher stellar mass and lower metallicity increase neutrino luminosity during hydrogen burning, while rapid rotation enhances it during main sequence, but decreases it during helium burning and FeCC stages.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Computer Science, Software Engineering
Xiaotong Li, Karel Adamek, Wes Armour
Summary: This paper presents a Python-based GPU-accelerated software for singular value thresholding (SVT) in matrix completion. The software achieves high accuracy and computational efficiency, and its potential applications are demonstrated through examples of image inpainting and traffic sensing.
Article
Physics, Particles & Fields
J. S. Goncalves, A. F. Santos
Summary: This paper examines the f(R, T, R μνP μν) gravity theory and its implications on causality. The analysis using Godel-type solutions shows that this model allows both causal and non-causal solutions, depending on the content of matter in the universe.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
Julio Cesar Vera-Hernandez
Summary: In this study, the deformation quantization formalism was applied to a HR cosmological model in the minisuperspace, with explicit quantization procedures performed for quantum cosmology. The stability analysis in the quantum minisuper phase space revealed a new interpretation of the graviton mass as an emergent cosmological constant and a measure of classical behavior deviation. Additionally, an interesting relation between curvature and graviton mass in a cusp catastrophe surface was discussed specifically in the Hartle-Hawking case.
GENERAL RELATIVITY AND GRAVITATION
(2021)
Article
Physics, Particles & Fields
Joao Luis Rosa, D. Bazeia, A. S. Lobao Jr
Summary: In this work, we study braneworld configurations in the scalar-tensor representation of the f (R, T) gravity theory in the presence of a Cuscuton term in the source field matter Lagrangian. We derive the scalar-tensor representation and obtain its equations of motion. We then investigate two different models for the source field in a flat braneworld model with one extra dimension for the general case of a f (R, T) theory. The solutions are obtained numerically due to the complexity of the field equations. The Cuscuton term effectively enhances the effect, increasing the height of the stability potential barrier and the depth of the minimum of the graviton zero-mode on the brane. Additionally, we examine the cases of F(R) + T and R + G (T), and demonstrate the essential role of the scalar field phi in the development of internal structure on the brane.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Physics, Multidisciplinary
R. Saleem, Aqsa Saleem
Summary: This article analyzes the baryogenesis of the universe in the framework of f (R, T, X) gravity under an isotropic universe model, exploring baryon asymmetry factor (BAF) in some recently proposed models of this gravity and concludes that this gravity is reliable for baryogenesis in some subspaces of the model parameter space.
EUROPEAN PHYSICAL JOURNAL PLUS
(2022)
Article
Physics, Multidisciplinary
V. K. Oikonomou
Summary: This study introduces a new theoretical framework of power-law f(R) gravity corrected canonical scalar field inflation and examines the inflationary dynamics of this new framework. The main characteristic is the dominance of a power-law f(R) gravity term, compared to the Einstein-Hilbert term, with good agreement with the latest Planck data for a wide range of free parameters. The resulting theory allows for viable inflationary theories which may otherwise be non-viable.
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: In this work, a refined Einstein-Gauss-Bonnet inflationary theoretical framework is provided, which is compatible with observational constraints on the gravitational wave speed. By introducing new assumptions and conditions, much simpler expressions for the slow-roll indices, tensor and scalar spectral indices, and tensor-to-scalar ratio are successfully derived. A particular model with a blue-tilted tensor spectral index is exemplified, showing compatibility with the latest Planck data and potential to describe the NANOGrav result according to recent literature.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: This study focuses on static neutron stars within a specific class of non-minimally coupled inflationary potentials, known as universal attractors. The analysis reveals that all studied cases predict larger maximum masses for neutron stars and are consistent with the GW170817 constraints on neutron star radii.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Physics, Multidisciplinary
A. Astashenok, S. Capozziello, S. D. Odintsov, V. K. Oikonomou
Summary: Novel implications on neutron stars come from extended gravity. Specifically, new research suggests the possibility of large mass stars in the mass gap region, which may require extensions of General Relativity to successfully describe this phenomenon and speculate on the largest allowed neutron star mass in a quantitative manner.
Article
Physics, Particles & Fields
G. G. L. Nashed, S. D. Odintsov, V. K. Oikonomou
Summary: This paper investigates spherically symmetric spacetime solutions that describe the interior of compact stellar objects within the context of higher-order curvature theory of the f(R) type. By deriving the non-vacuum field equations and solving the system of nonlinear differential equations, the study successfully identifies a specific solution for compact stars. Furthermore, the model is shown to be more stable and different from those in general relativity, with specific implications for the considered compact star Her X-1.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Physics, Mathematical
V. K. Oikonomou, Achilles Gitsis, Maria Mitrou
Summary: This study focuses on the phase space singularities of interactive quintessence model in the presence of matter fluid. The presence of matter fluid affects the dynamical system, making the singularities depend on initial conditions rather than occurring for general initial conditions.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2021)
Biographical-Item
Multidisciplinary Sciences
Sergei D. Odintsov
Review
Multidisciplinary Sciences
Sergei D. Odintsov, Vasilis K. Oikonomou, Ratbay Myrzakulov
Summary: In this work, the authors exhaustively study the effects of modified gravity on the energy spectrum of the primordial gravitational waves background. They provide calculations for various cases of modified gravity, focusing on specific forms of interest. They also present the exact form of a parameter involved in the calculation of the effects of modified gravity on the energy spectrum for the first time.
Article
Astronomy & Astrophysics
Sergei D. Odintsov, Tanmoy Paul
Summary: This work explores the influence of a Type-IV singularity on the dynamics of a bouncing universe in the context of a ghost-free Gauss-Bonnet theory of gravity. It is found that when the singularity has a global effect on spacetime, the scalar power spectrum becomes red-tilted and the tensor-to-scalar ratio is incompatible with observational data. However, if the singularity only locally affects spacetime around the time of occurrence, the observable quantities are compatible with Planck data.
Editorial Material
Multidisciplinary Sciences
Sergei D. D. Odintsov
Editorial Material
Multidisciplinary Sciences
Sergei D. Odintsov
Article
Multidisciplinary Sciences
Artyom V. Astashenok, Sergey D. Odintsov, Vasilis K. Oikonomou
Summary: This study investigates the Chandrasekhar mass limit of white dwarfs in various models of f (R) gravity. Two equations of state for stellar matter are used: the simple relativistic polytropic equation and the realistic Chandrasekhar equation of state. The solution for any model of f (R) = R + beta R-m gravity shows that the stellar mass decreases compared to standard General Relativity. For realistic equations of state, there is a value of the central density for which the mass of a white dwarf peaks. This implies that in modified gravity, there exists a minimum radius for stable white dwarfs, which is greater than in General Relativity. The behavior of the Chandrasekhar mass limit in f (R) gravity is also investigated.
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: In this study, we analyze the scenario where the effective gravitational Lagrangian of a minimally coupled scalar field becomes a rescaled Einstein-Hilbert gravity at large curvatures, leading to changes in the inflationary phenomenology and satisfaction of swampland criteria. Two inflation models, fiber inflation and supergravity alpha-attractor models, are discussed to show their viability and satisfaction of swampland criteria. The late-time phenomenology of the fiber inflation potential in the presence of full f(R) gravity is examined, showing the production of a viable dark energy era similar to the Lambda-cold-dark-matter model. The modified gravity model presents a universe described by a rescaled Einstein-Hilbert gravity at early times, with the scalar field mainly controlling dynamics synergistically with f(R) gravity at late times.
Article
Astronomy & Astrophysics
S. D. Odintsov, V. K. Oikonomou, F. P. Fronimos
Summary: This paper investigates the late-time dynamics of scalar coupled f(R, G) gravity through numerical analysis, exploring various models in modified gravity theories. It is found that the contribution of Gauss-Bonnet related terms is minor when f(R) gravity terms are present, with the f(R) sector dominating at late times. The Einstein-Gauss-Bonnet models considered produce a dark energy era free from oscillations, with constraints on scalar coupling functions for compatibility with observational data.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
V. K. Oikonomou
Summary: This article introduces a theoretical model of F(R) gravity that can describe inflation, early and late dark energy eras in the presence of a light axion particle as the dark matter component of the Universe. The model exhibits early and late-time dark energy eras, with an early dark energy era followed by a deceleration era and then a late-time dark energy era. A notable feature is the absence of dark energy oscillations in the dark energy era within the redshift interval z = [0, 10]. The model also shows a sudden jump in the value of the Hubble rate within the redshift interval z ≈ [2, 2.6], increasing and then decreasing until z ≈ 0.
