Article
Astronomy & Astrophysics
Masato Minamitsuji, Shinji Tsujikawa
Summary: We propose a new model of scalarized neutron stars (NSs) realized by a self-interacting scalar field phi nonminimally coupled to the Ricci scalar R of the form F(phi)R. The existence of scalarized NS solutions connecting phi v with phi c whose difference is significant allows sizable corrections to the Arnowitt-Deser-Misner mass and radius of NSs as well as the gravitational force around the NS surface. Unlike the original scenario of spontaneous scalarization induced by a negative nonminimal coupling, the catastrophic instability of cosmological solutions can be avoided.
Article
Astronomy & Astrophysics
Ryotaro Kase, Shinji Tsujikawa
Summary: In a theory where a scalar field has a nonminimal derivative coupling to the Einstein tensor, there are static and spherically-symmetric relativistic stars with scalar hair in their interiors. These hairy solutions are unstable to odd- and even-parity perturbations, with Laplacian instabilities for star compactness smaller than 1/3 and ghost instabilities for compactness greater than 1/3. The presence of a positive perfect-fluid pressure also leads to instability in the absence of a standard kinetic term.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Isaac Torres, Felipe de Melo Santos
Summary: The Nonminimal Derivative Coupling (NDC) is an alternative to General Relativity, which offers an inflationary mechanism in cosmology. The detection of gravitational waves has posed strict constraints on gravitational theories, leading to a revision of such theories to investigate their compatibility with observational data. This paper reviews the NDC and examines its compatibility with gravitational waves, finding that the existence of gravitational waves is limited in phase space and there are no accelerated solutions compatible with current data. This is partly due to the lack of detection of primordial gravitational waves.
Article
Astronomy & Astrophysics
Yakefu Reyimuajia, Xinyi Zhang
Summary: By introducing a very weak nonminimal coupling of the inflaton field to gravity, the tension between natural inflation model and recent Planck data can be alleviated, leading to good agreement between predictions and observations. The predictions of the extended model are consistent with observational data within certain parameter ranges.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Tiberiu Harko, Francisco S. N. Lobo, Emmanuel N. Saridakis
Summary: This study investigates the possibility of gravitationally generated particle production through the mechanism of nonminimal torsion-matter coupling. By using the formalism of irreversible thermodynamics in open systems, the physical and cosmological implications of the nonconservation of the energy-momentum tensor are explored, along with the particle creation rates and comoving entropy. The thermodynamics of open systems lead to a generalization of the standard Lambda CDM cosmological paradigm, where the particle creation rates and pressures are considered components of the cosmological fluid energy-momentum tensor.
Article
Astronomy & Astrophysics
Pavel E. Kashargin, Sergey V. Sushkov
Summary: In this study, we investigate neutron star configurations in the scalar-tensor theory of gravity, specifically focusing on the coupling between the kinetic term of a scalar field and the Einstein tensor. We construct neutron star configurations in a general case where the bare cosmological constant and standard kinetic term are non-zero, leading to an asymptotically anti-de Sitter black hole solution with a nontrivial scalar field. The resulting mass-radius diagrams differ significantly from those obtained in general relativity or the special case with vanishing bare cosmological constant and standard kinetic term. Additionally, we find that neutron stars in the theory of gravity with nonminimal derivative coupling are more compact compared to those in general relativity or the special model, and propose a method to estimate the parameter of nonminimal coupling.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Dhong Yeon Cheong, Koichi Hamaguchi, Yoshiki Kanazawa, Sung Mook Lee, Natsumi Nagata, Seong Chan Park
Summary: In axion models, the problem of gravitational violation of the Peccei-Quinn (PQ) symmetry due to axionic wormholes is addressed by introducing a nonminimal coupling of the axion field to gravity. The solution to the quality problem is found in the Palatini formulation, where the requested lower bound of the nonminimal coupling is slightly higher than that in the metric formulation.
Article
Astronomy & Astrophysics
Laur Jarv, Joosep Lember
Summary: Global phase portraits of inflationary dynamics in teleparallel gravity models with scalar fields nonminimally coupled to torsion scalar are constructed, focusing on distinguishing different asymptotic states and identifying the key role of heteroclinic orbits and asymptotic fixed points. Effective potential and effective mass provide a heuristic method for seeking asymptotic fixed points, applicable to any nonminimally coupled theories. Specific examples of positive quadratic nonminimal couplings with quadratic and quartic potentials are studied, showing qualitative differences from scalar-curvature counterparts.
Article
Physics, Particles & Fields
Ekaterina O. Pozdeeva, Sergey Yu. Vernov
Summary: In this study, inflationary models involving a scalar field nonminimally coupled with both the Ricci scalar and the Gauss-Bonnet term are investigated. The approach of generalizing inflationary scenarios with the Gauss-Bonnet term and a scalar field minimally coupled with the Ricci scalar to scenarios with a scalar field nonminimally coupled with the Ricci scalar is proposed. By utilizing the effective potential, a set of models with identical scalar spectral index ns and scalar perturbation amplitude As but varying tensor-to-scalar ratio r values is constructed.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
Abdullah Guvendi, Soroush Zare, Hassan Hassanabadi
Summary: In this study, the relativistic dynamics of a two-body system with nonminimal couplings in a topological defect-generated 1 + 2-dimensional space-time are investigated using a fully-covariant two-body Dirac equation. The analysis reveals that the space-time background affects the dynamics of the considered composite structure, with different spin quantum states of the fermion-antifermion pair sensing the impact differently.
PHYSICS OF THE DARK UNIVERSE
(2022)
Article
Astronomy & Astrophysics
R. P. L. Azevedo, P. P. Avelino
Summary: The study shows that gravitational theories with nonminimal coupling to matter fields violate Etherington's distance-duality relation, and constraints on power-law and exponential NMC models are derived from existing measurements of type Ia supernovae and baryon acoustic oscillations. These constraints complement those derived from cosmic-microwave background radiation and big bang nucleosynthesis data.
Article
Physics, Multidisciplinary
Ruslan K. Muharlyamov, Tatiana N. Pankratyeva
Summary: The study proposes a reconstruction method for flat FRW spacetime in a subclass of Horndeski theory, and checks related conditions to determine the model's cosmological viability.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Astronomy & Astrophysics
Tatsuki Kodama, Tomo Takahashi
Summary: In this study, we investigate the predictions of inflation models with a nonminimal coupling to gravity for inflationary observables such as the spectral index and tensor-to-scalar ratio in a general setting. Depending on the relation between the Jordan frame potential and a function characterizing the nonminimal coupling, the models can be classified into three categories, each giving distinctive predictions for the inflationary observables. We derive general predictions for each class and investigate some specific models to understand the underlying features.
Article
Physics, Multidisciplinary
Ginestra Bianconi
Summary: In this study, a local theory is proposed by formulating a Dirac gauge theory on weighted and directed 3+1 dimensional networks. The commutators and anti-commutators of Dirac operators are shown to define the curvature tensor and magnetic field of the theory. The action associated with the theory comprises of a Dirac action and a metric action, and the gauge invariance of the action under both Abelian and non-Abelian transformations is described.
JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL
(2023)
Article
Physics, Multidisciplinary
E. E. Kangal, K. Sogut, M. Salti, O. Aydogdu
Summary: In this study, we discuss the exact solutions of the Dirac equation in a topologically trivial Godel-type space-time within the framework of rainbow gravity. We obtain the analytical solutions and energy spectrum for Dirac particles in the considered cosmological model and analyze the effects of rainbow scenarios on the energy spectrum compared to the solutions obtained with the usual scenario of general relativity theory.
Article
Mathematics, Applied
Luca Fabbri
Summary: The paper investigates the structure of general spinors in polar form and finds that Weyl and Majorana spinors can be interpreted as pure Goldstone states, without any real degree of freedom.
ADVANCES IN APPLIED CLIFFORD ALGEBRAS
(2022)
Article
Astronomy & Astrophysics
Stefano Vignolo, Sante Carloni, Roberto Cianci, Fabrizio Esposito, Luca Fabbri
Summary: This study presents a tetrad-affine approach to investigate the coupling between f(Q) gravity and spinor fields of spin-1/2. Field equations and the conservation law of spin density are derived. The study focuses on Bianchi type-I cosmological models and provides a general procedure to solve the corresponding field equations. Analytical solutions are obtained in the case of f(Q) = alpha Q(n), showing isotropization at late times and the possibility of accelerated expansion depending on the exponent n.
CLASSICAL AND QUANTUM GRAVITY
(2022)
Article
Astronomy & Astrophysics
Luca Fabbri
Summary: We use the fourth-order differential theory of gravitation to address the issue of singularity avoidance, specifically studying the short-distance behavior in the case of black holes and the big bang.
Article
Multidisciplinary Sciences
Rodolfo Jose Bueno Rogerio, Luca Fabbri
Summary: In this paper, a new class of spin-half mass-dimension one fermions is obtained using the machinery introduced in Ahluwalia (Ahluwalia 2020. Phys. Eng. Sci. 476, 20200249. (doi:10.1098/rspa.2020.0249) and EPL 131, 41001. (doi:10.1209/0295-5075/131/41001)). These spinors, after a suitable dual structure examination, can serve as expansion coefficients for a local field.
PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
(2022)
Article
Astronomy & Astrophysics
Fabrizio Esposito, Sante Carloni, Stefano Vignolo
Summary: By making use of the 1 + 3 covariant formalism, this study explicitly demonstrates the impact of nonmetricity on the dynamics of the Universe. Using the Dynamical System Approach, it analyzes the evolution of Bianchi type-I cosmologies within the framework of f(Q) gravity. The results show that different models of the function f(Q) can manifest isotropic eras of the Universe.
CLASSICAL AND QUANTUM GRAVITY
(2022)
Article
Mathematics, Applied
Luca Fabbri
Summary: In this article, we investigate tensorial connection for spinorial fields in polar form in various physical situations. We demonstrate that the tensorial connection is a useful tool in some cases, while in other cases, it is a necessary object. We compare and analyze different cases with a tensorial connection, exploring the analogies between space-time structures. Furthermore, we provide comments on quantum field theory and specific spinors.
ADVANCES IN APPLIED CLIFFORD ALGEBRAS
(2023)
Article
Astronomy & Astrophysics
Luca Fabbri
Summary: In this paper, we investigate the role of the torsion axial-vector in the dynamics of Dirac spinor fields. We demonstrate that the torsional correction leads to effects that regularize the otherwise singular distribution of spinorial matter fields. Finally, we provide comments on the implications for physics.
Article
Mathematics, Applied
Rodolfo Jose Bueno Rogerio, Luca Fabbri
Summary: In this paper, we investigate a versatile field propagator that can accommodate both newly-defined mass-dimension 1 fermions and spin-1/2 bosons. Our approach involves defining a mapping between spinors of different Lounesto classes and expressing the propagator in terms of the corresponding dual structures.
ADVANCES IN APPLIED CLIFFORD ALGEBRAS
(2023)
Article
Physics, Multidisciplinary
Luca Fabbri
Summary: We investigate the hydrodynamic formulation of quantum mechanics for relativistic spinor fields and find that the commonly defined velocity cannot accurately represent the tangent vector to particle trajectories. We propose an alternative definition for the tangent vector and hence particle trajectories, which we believe to be novel and unique. These findings are a crucial step towards addressing additional challenges such as defining trajectories for multi-particle systems or ensembles, as they have numerous applications and implications in quantum mechanics.
FOUNDATIONS OF PHYSICS
(2023)
Article
Nanoscience & Nanotechnology
Luca Fabbri, Camilla Bordoni, Pedro Barquinha, Jerome Crocco, Beatrice Fraboni, Tobias Cramer
Summary: The disordered microscopic structure of amorphous semiconductors leads to the formation of band tails in the density of states (DOS), which greatly affect charge transport properties. Kelvin Probe Force Microscopy (KPFM) is a powerful technique to measure the DOS, but lacks a model to interpret KPFM spectroscopy data on amorphous semiconductors of finite thickness. In this study, an analytical solution to the Poisson equation is provided for a metal-insulator-semiconductor junction interacting with the atomic force microscope tip, allowing for the fitting of experimental data and obtaining DOS parameters. This method was demonstrated on Indium-Gallium-Zinc Oxide (IGZO) thin-film transistors (IGZO-TFTs) and showed good agreement with values obtained from photocurrent spectroscopy.
Editorial Material
Astronomy & Astrophysics
Luca Fabbri
Article
Multidisciplinary Sciences
Luca Fabbri
Summary: This article discusses the re-formulation of relativistic spinor field theory in polar variables, allowing for interpretation in terms of fluid variables. The dynamics of spinor fields are then converted into a special type of spin fluid dynamics. The authors demonstrate that this conversion into dynamical spin fluid is not unique, but can be achieved through 19 different rearrangements, as evidenced by the 19 minimal systems of hydrodynamic equations equivalent to the Dirac equations.
Article
Physics, Multidisciplinary
Andre G. Campos, Luca Fabbri
Summary: In this study, the relativistic dynamical inversion technique is used to find analytical solutions to the Dirac equation in explicitly covariant form. The technique is demonstrated by making a change from Cartesian to spherical coordinates for a given Dirac spinor. The study also constructs a family of normalizable analytic solutions and identifies exact solutions for specific cases involving magnetic and electric fields.
PHYSICAL REVIEW RESEARCH
(2022)