Article
Physics, Particles & Fields
Chengjie Fu, Jing Liu, Tao Zhu, Hongwei Yu, Puxun Wu
Summary: The study reveals that in axion inflation, coupling the gravitational Chern-Simons term to a periodic function of the inflaton results in amplification of tensor perturbations with different polarizations in distinct ways, leading to characteristic features in the energy spectrum of gravitational waves.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
Tomohiro Inagaki, Masahiko Taniguchi
Summary: We study the quadrupole radiation of gravitational waves in F(R) modified gravity, which exhibits a massive scalar mode. The explicit expressions for the quadrupole radiation and energy current of the scalar mode in general F(R) gravity models are derived. Binary stars and bouncing stars are considered as astronomical sources, and the quadrupole radiation of scalar and tensor modes are calculated. Under spherically symmetric conditions, the scalar mode radiates, while the tensor modes do not. The mass of the scalar mode is estimated for typical energy scales, and the potential detection of the scalar mode in future gravitational wave observations is discussed.
Article
Physics, Multidisciplinary
Aisha Siddiqa, Rubab Manzoor, Arsal Kamal
Summary: This study analyzes the propagation of polar gravitational waves in the flat FRW cosmic background using the f(R, T-f) theory. The Regge-Wheeler polar perturbation scheme is used to study the perturbation of the flat geometry of the background spacetime and the corresponding scalar field source. Field equations for both the flat and perturbed geometries are constructed and used to evaluate perturbation parameters. The results show that polar fluctuations have an impact on both the geometry and scalar field, and the behavior of these fluctuations can be observed through numerical solutions and plots.
Article
Astronomy & Astrophysics
Adrian Casado-Turrion, Antonio Dobado, Alvaro de la Cruz-Dombriz
Summary: This study investigates gravitational collapse in the context of f(R) theories of gravity. By generalizing the Oppenheimer-Snyder model to fit within metric f(R) gravity, novel exterior solutions are found, and it is verified that certain vacuum metrics cannot represent spacetime outside a collapsing dust star in metric f(R) gravity. Additionally, it is discovered that the Oppenheimer-Snyder model is incompatible with the junction conditions of the Palatini formulation of f(R) gravity.
Article
Multidisciplinary Sciences
Vasilis K. Oikonomou, Fotis P. Fronimos, Olga Razina, Pyotr Tsyba
Summary: This work studies the phase space of f(R) gravity in the presence of a misalignment axion, and identifies four distinct and possibly unstable fixed points with physical significance. It demonstrates that the Chern-Simons terms do not affect the phase space.
Article
Astronomy & Astrophysics
Rubab Manzoor, Aisha Siddiqa, Arsal Kamal
Summary: This paper investigates the axial type gravitational waves within flat FRW universe and in the context of f(R, T-phi) gravity. The Regge-Wheeler axial perturbation scheme for geometry and the corresponding perturbations in the scalar field are considered. The field equations for both unperturbed and axially perturbed scenarios are derived, and the simultaneous solution leads to expressions for unknown perturbation parameters. The study concludes that the propagation of axial gravitational waves is influenced by the scalar field potential and the coupling of geometry and scalar field.
PHYSICS OF THE DARK UNIVERSE
(2022)
Article
Multidisciplinary Sciences
Carlos Herdeiro, Eugen Radu, D. H. Tchrakian
Summary: This study highlights that families of non-minimal coupling functions allowing scalarization naturally emerge in the context of Higgs-Chern-Simons gravity models. By studying a specific Einstein-GB-scalar field model, novel features and caveats are pointed out. The possibility of vectorization is also discussed, as this construction gives rise to vector fields non-minimally coupled to the GB invariant.
Article
Physics, Multidisciplinary
O. Aydogdu, M. Salti
Summary: In the context of f(R, T)-gravity, this study explores the propagation of gravitational waves (GWs) for even modes in the conformally flat Friedman-Lemaitre-Robertson-Walker (CFR) universe. The effects of polar perturbations on the four-velocity vector components and matter distribution are investigated, and the influence of rainbow functions on the shape of polar GWs is analyzed graphically. Additionally, the study examines whether the polar GWs satisfy the Huygens principle.
Article
Astronomy & Astrophysics
Djuna Croon, Samuel D. McDermott, Jeremy Sakstein
Summary: This study explores the black hole mass gap as a tool for constraining new particles, showing that new particles can affect the masses of astrophysical black holes. The findings suggest that the solar axion explanation for the XENON1T excess leads to the formation of black holes with masses around 56 solar masses.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Astronomy & Astrophysics
Fabrizio Di Giovanni, Davide Guerra, Simone Albanesi, Miquel Miravet-Tenes, Dimitra Tseneklidou
Summary: The study constructs spherically symmetric static solutions describing gravitationally bound composites of fermions and axions, known as fermion-axion stars. Through numerical simulations, it is found that there may exist multiple stable branches and multiple islands of stability in the existence domain.
Article
Astronomy & Astrophysics
Ogan Ozsoy
Summary: The model based on string theory inspiration can generate gravitational wave signals that satisfy current limits on scalar perturbations at CMB scales. Due to the gravitational coupling of gauge fields and the localized nature of particle production, the characteristics of gravitational wave signals can be probed in future experiments.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Physics, Particles & Fields
Wittaya Thipaksorn, Khamphee Karwan
Summary: We study cosmic evolution based on the fixed points in the dynamical analysis of the degenerate higher-order scalar-tensor (DHOST) theories. We find stable scaling fixed points and field-dominated fixed points in the DHOST theory, and the cosmic evolution can transition from the phi MDE regime to the field-dominated regime. Additionally, we observe similarities between the DHOST theory and coupled dark energy models in Einstein gravity.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Physics, Multidisciplinary
A. S. Agrawal, S. K. Tripathy, B. Mishra
Summary: In this study, gravitational baryogenesis in f(R) theory of gravity with an anisotropic Bianchi I space-time is investigated. Two models related to specific forms of Ricci scalar are presented, and the baryon-to-entropy ratio is derived in an anisotropic background. The gravitational baryogenesis is examined and its behaviors are studied.
CHINESE JOURNAL OF PHYSICS
(2021)
Article
Physics, Multidisciplinary
Daniel G. Figueroa, Joanes Lizarraga, Ander Urio, Jon Urrestilla
Summary: This study focuses on the nonlinear dynamics of axion inflation, highlighting the importance of considering inhomogeneity effects. The results reveal new insights into the behavior of axion inflation and have significant implications for its phenomenology and observability.
PHYSICAL REVIEW LETTERS
(2023)
Article
Astronomy & Astrophysics
V. K. Oikonomou, F. P. Fronimos, Pyotr Tsyba, Olga Razina
Summary: Under the assumption that the axion scalar field forms the dark matter in the Universe, this paper extends the formalism of kinetic axion R2 gravity to include non-minimally coupled Gauss-Bonnet terms. It is shown that this Gauss-Bonnet term has significant effects on inflationary phenomenology and the kinetic axion scenario. Unlike in kinetic axion R2 gravity, the Gauss-Bonnet corrected kinetic axion R2 gravity leads to a non-extended inflationary era and immediate commencement of reheating driven by R2 fluctuations.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Multidisciplinary Sciences
Asmaa G. Shalaby, Vasilis K. Oikonomou, Gamal G. L. Nashed
Summary: The study explores modified cosmological models based on f(T) gravitational theory and non-extensive thermodynamics framework, with effects captured by the parameter delta. Detailed examination of cosmological evolution with collisionless non-relativistic matter was conducted, and constraints on the non-extensive thermodynamics parameter delta were imposed using Planck 2018 data.
Article
Physics, Particles & Fields
S. D. Odintsov, V. K. Oikonomou, F. P. Fronimos
Summary: In this paper, we study the inflationary phenomenology of a k-inflation corrected Einstein-Gauss Bonnet theory by adding Gauss Bonnet string corrections to achieve a ghost free description. The scalar functions of the theory are revealed to be interconnected, and assuming a specific form for one immediately specifies the other. By deriving the scalar potential from the equations of motion, a relatively tractable phenomenology is produced for each term entering the equations of motion. The predicted amount of non-Gaussianities is significantly enhanced compared to the k-inflation free Einstein-Gauss-Bonnet theory under certain conditions.
Article
Physics, Multidisciplinary
V. K. Oikonomou
Summary: This study focuses on k-inflation theories with non-minimal coupling of the scalar field to gravity, investigating the evolution of the scalar field under slow-roll or constant-roll conditions when a scalar potential is present or absent. The results show that the theory is compatible with the latest Planck data in the slow-roll models with scalar potential, but less appealing in models without potential. Additionally, it is suggested that the Einstein frame counterpart of non-minimal k-inflation models with scalar potential may be a viable theory due to the conformal invariance of the observational indices.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Multidisciplinary Sciences
Polina Petriakova, Arkady Popov, Sergey Rubin
Summary: Studying the evolution of the Universe from sub-Planckian scale to present times reveals the requirement for exponential expansion of space and significant restrictions on parameter values of a function f(R). The initial metric of the Universe is assumed to be maximally symmetric with positive curvature.
Article
Astronomy & Astrophysics
A. V. Astashenok, S. Capozziello, S. D. Odintsov, V. K. Oikonomou
Summary: Investigated the causal limit of maximum mass for stars in the framework of f(R) gravity, numerically examining the combined effect of causal equation of state and sound speed on maximum mass of static neutron stars. This provides the most extreme upper bound for neutron star masses in extended gravity.
Article
Astronomy & Astrophysics
S. D. Odintsov, V. K. Oikonomou
Summary: This work examines the implications of a subclass of E-models cosmological attractors, known as a-attractors, on hydrodynamically stable slowly rotating neutron stars. By numerically solving the Tolman-Oppenheimer-Volkoff equations in the Einstein frame, the study finds that the masses and radii of neutron stars vary with the parameter a characterizing the a-attractors. Results indicate a complex relationship between non-minimal inflationary attractors and neutron star phenomenology in scalar-tensor theory.
PHYSICS OF THE DARK UNIVERSE
(2021)
Biographical-Item
Multidisciplinary Sciences
Sergei D. Odintsov
Article
Physics, Particles & Fields
S. Nojiri, S. D. Odintsov, V. K. Oikonomou, Arkady A. Popov
Summary: In this work, we address the ghost issue in F(R, G) gravity and propose a technique to eliminate ghost degrees of freedom. By introducing auxiliary scalar fields and employing the Lagrange multiplier technique, the theory can be ghost-free in the Einstein frame and used as a reconstruction technique for various cosmological evolutions of interest. We demonstrate how this reconstruction technique can realize several cosmologically interesting phenomenologies.
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
Article
Multidisciplinary Sciences
Dmitriy Lisenkov, Arkady Popov
Summary: This paper investigates the vacuum polarization of a scalar field on the background of a short throat wormhole. The scalar field is assumed to be massless, with an arbitrary coupling to the scalar curvature of spacetime. Additionally, it is supposed that the field is in a thermal state with an arbitrary temperature.
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
Physics, Particles & Fields
Polina Petriakova, Arkady A. Popov, Sergey G. Rubin
Summary: This paper discusses the origin of small parameters and aims to explain the Hierarchy problem. Flexible extra dimensions play a crucial role in the formation of physical parameters. The evolution of multidimensional metric starts from the Planck scale and ends with a static extra-dimensional metric and a 4-dimensional de Sitter space at high energies, leading to exponentially produced causally disconnected universes. Quantum fluctuations independently distort the metric within these universes, causing inflationary processes. Some universes asymptotically tend towards states characterized by small Hubble parameters. The explanation for the effective parameter reduction in the Higgs sector of the Standard Model lies in the presence of small-amplitude distributions of a scalar field in a fraction of these universes.
EUROPEAN PHYSICAL JOURNAL C
(2023)