Article
Astronomy & Astrophysics
Tyler McMaken, Andrew J. S. Hamilton
Summary: In this paper, the Hawking radiation detected by an inertial observer in a Reissner-Nordstrom spacetime is analyzed, with a focus on the asymptotic behavior of the Hawking spectrum as the observer approaches the inner or outer horizon. Two different methods are used to analyze the Hawking flux: an effective temperature is calculated to quantify the redshift experienced by the observer, and the full Bogoliubov graybody spectrum is computed in analytically solvable regions. The results show that the effective Hawking temperature is finite and becomes negative at the event horizon for certain conditions, while it is always negative and infinite at the inner horizon.
Article
Astronomy & Astrophysics
Arshad Ali, Yungui Gong, Yizhou Lu
Summary: The paragraph discusses the results of studying scalar-induced tensor perturbations during matter domination in various gauges, revealing that the kernel functions in synchronous and comoving gauges are the same and identifying oscillating terms in the perturbations as the secondary gravitational waves induced by scalars. Additionally, it is found that the energy density of these waves is gauge independent and behaves like radiation in the matter-dominated era.
Article
Astronomy & Astrophysics
Paulo M. Sa
Summary: Our research shows that within a cosmological model where dark matter and dark energy are identified with two interacting scalar fields, there are multiple viable cosmological solutions. These solutions can be categorized into everlasting accelerated expansion and temporary accelerated expansion, with distinct behaviors regarding the dominance of dark energy and dark matter energy densities.
Article
Physics, Particles & Fields
Vsevolod R. Ivanov, Sergey Yu. Vernov
Summary: The study explores modified gravity cosmological models that can be transformed into two-field chiral cosmological models through conformal metric transformation. General solutions have been obtained in the spatially flat FLRW metric for the R-2 gravity model with an additional scalar field and the corresponding two-field model with nonstandard kinetic part of the action and a cosmological constant. The analysis focuses on the correspondence of cosmic time solutions obtained and the possible evolutions of Hubble parameters in the Einstein and Jordan frames.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
Richa Arya, Arvind Kumar Mishra
Summary: This study discusses how large curvature perturbations generated in a warm inflation model can induce second-order tensor perturbations through scalar-tensor mode coupling. These perturbations may also lead to the formation of primordial black holes. The spectrum of these secondary gravitational waves from our warm inflationary model is calculated, and it is found that scalar induced gravitational waves (SIGW) are generated in a frequency range of (1-106) Hz. The possibilities of detecting these SIGWs are discussed, taking into account the sensitivity of different ongoing and future gravitational wave experiments.
PHYSICS OF THE DARK UNIVERSE
(2022)
Review
Astronomy & Astrophysics
Guillem Domenech
Summary: This review provides an overview of gravitational waves induced by primordial fluctuations, focusing on the intuitive physics and general analytical formulation behind induced gravitational waves. It suggests the open possibility of a different expansion history in the primordial universe than the commonly assumed radiation dominated cosmology, and aims to raise awareness of current advances in the cosmology of induced gravitational waves.
Article
Astronomy & Astrophysics
Francesco Di Filippo, Raul Carballo-Rubio, Stefano Liberati, Costantino Pacilio, Matt Visser
Summary: This short work reviews the physical mechanism leading to the instability of the central core in regular black holes and argues for the importance of taking into account non-perturbative backreaction for a meaningful description of physical black holes.
Article
Physics, Particles & Fields
Saikat Chakraborty, Esteban Gonzalez, Genly Leon, Bin Wang
Summary: This paper studies a cosmological model inspired by axionic matter, involving two scalar fields interacting through a added potential term. By introducing new variables and a dimensionless time scale, the resulting dynamics are explored. The analysis of oscillations using methods from the theory of averaging nonlinear systems is motivated by difficulties in standard approaches, ultimately proving the consistency of time-dependent systems and their time-averaged counterparts in late-time dynamics.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
Shyam Balaji, Guillem Domenech, Gabriele Franciolini
Summary: Pulsar timing arrays have gathered evidence of a gravitational wave background at nHz frequencies. By studying the relationship between the gravitational wave background and primordial black holes, this phenomenon can be explained. Future gravitational wave detectors can be used to test this model.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Jonathan Tot, Balkar Yildirim, Alan Coley, Genly Leon
Summary: We present a comprehensive (compactified) dynamical systems analysis of the Quintom model consisting of an interacting quintessence scalar field and a phantom. By finding a range of parameters Kappa and lambda, we identify expanding Quintom cosmologies that undergo two inflationary periods, unaffected by spatial curvature. We also discuss a class of bouncing cosmologies and examine the linear cosmological perturbations.
PHYSICS OF THE DARK UNIVERSE
(2023)
Article
Astronomy & Astrophysics
Lukas T. Witkowski, Guillem Domenech, Jacopo Fumagalli, Sebastien Renaux-Petel
Summary: This study investigates how the oscillatory frequency profile of the scalar-induced contribution to the stochastic gravitational wave background is influenced by the expansion history of the post-inflationary universe. The results show that the frequencies and amplitudes of the oscillatory contributions are affected by the equation of state, and provide information about both inflationary physics and the post-inflationary expansion history.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Thermodynamics
Paul Clavin
Summary: This paper revisits the critical condition for deflagration to detonation transition (DDT) on the tip of elongated flames in tubes. Previous experiments and numerics have shown that a train of successive shock waves is produced by the self-accelerating flame front. However, the abrupt transition remained unexplained. Using a simplified one-dimensional model, the objective of this theoretical analysis is to identify the pre-conditioned state just prior to the spontaneous formation of a flow singularity on the flame front. The focus is on the unsteady compression waves emitted in the unreacted gas by the self-accelerating flame front, which were overlooked by previous theoretical analyses.
COMBUSTION AND FLAME
(2022)
Article
Astronomy & Astrophysics
Pedro Canate
Summary: The concept of black bounce spacetimes presents a globally regular modification of ordinary black holes to avoid the singularity problem at their centers, creating a globally regular black hole spacetime.
Article
Astronomy & Astrophysics
Jia-Xi Feng, Fengge Zhang, Xian Gao
Summary: In this study, we investigate the polarized properties of scalar induced gravitational waves (SIGWs) in the Chern-Simons gravity with a dynamical scalar field. We find that the correction from the parity-violating Chern-Simons term is negligible on large scales, indicating a very small degree of circular polarization for these gravitational waves.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Physics, Particles & Fields
G. G. L. Nashed, Shin'ichi Nojiri
Summary: It is suggested that the EGBS theory combined with a scalar field can provide physically viable models of celestial phenomena with the scalar field effect active in four dimensions. By considering the spherically symmetric and static configuration of compact stars, the consequences of the EGBS theory in stellar modeling are explained. The mass-radius relation alone is not enough to constrain the model due to the degeneracy between the equation of state (EoS) and the functions characterizing the model, indicating the need for additional considerations.
EUROPEAN PHYSICAL JOURNAL C
(2023)
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)
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)