Article
Physics, Particles & Fields
Adnan Malik, Tayyaba Naz, Abdul Qadeer, M. Farasat Shamir, Zeeshan Yousaf
Summary: The objective of this manuscript is to investigate wormhole solutions in the f (R, F) theory of gravity, using the Karmarkar criterion to create a suitable wormhole shape function. The suggested shape function meets necessary requirements and demonstrates wormhole configurations in the embedding diagram. By choosing suitable parameters, the proposed shape function provides wormhole solutions with less exotic matter, although energy conditions are violated indicating the existence of exotic matter and wormhole geometry. The Karmarkar technique yields validated wormhole configurations with even less exotic matter for the chosen f (R, F) gravity models.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
Joao Luis Rosa, Matheus A. Marques, Dionisio Bazeia, Francisco S. N. Lobo
Summary: This work investigates the stability of thick braneworld systems in the context of f(R,T) gravity, considering models with and without matter fields. It shows that gravitational sector is stable against small metric perturbations and the shape of the graviton zero-mode can develop internal structure under certain parameter choices.
EUROPEAN PHYSICAL JOURNAL C
(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
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
Astronomy & Astrophysics
Giorgos Anastasiou, Ignacio J. Araya, Mairym Busnego-Barrientos, Cristobal Corral, Nelson Merino
Summary: We investigate a conformally coupled scalar-tensor theory with a quartic potential and local conformal symmetry. By restoring the full local conformal symmetry, we determine the counterterms that make the on-shell action finite. The resulting action contains a conformally covariant tensor constructed from the metric and scalar field, which has the same conformal weight as the Weyl tensor. Adding conformal gravity and the Einstein-AdS action to the theory, we find that the action can still be renormalized even with broken conformal symmetry.
Article
Astronomy & Astrophysics
P. J. Pompeia
Summary: The analysis of f(T, del(mu 1) T, . . . ,del(mu n) . . . del(mu 1) T) gravity in the Jordan and Einstein frames was presented. The equivalence with a scalar-multitensor theory was proved for systems with a regular Hessian matrix in both frames. Introducing an auxiliary tensor of the same order for each derivative reduced the differential equation for the tetrad field to second order, but led to a system of coupled equations for the auxiliary fields.
Article
Astronomy & Astrophysics
Yermek Aldabergenov
Summary: In this study, old minimal R + R(2) supergravity is extended by gauging the U(1) R symmetry that rotates the chiral curvature superfield. New interactions involving the gauge multiplet in the context of inflation and supersymmetry breaking are studied. Models are constructed where supersymmetry and the R symmetry undergo spontaneous breaking during and after Starobinsky inflation, and one-loop gauge anomalies are cancelled by the Green-Schwarz mechanism. The hierarchy between auxiliary fields leads to a split mass spectrum with chiral multiplet masses around the inflationary scale, while gauge multiplet masses can be arbitrarily small.
Article
Astronomy & Astrophysics
Ping Li, Rui Jiang, Jian Lv, Xianghua Zhai
Summary: In this paper, the perturbations of charged static spherically symmetric black holes in the f(R)=R-2 alpha root R model by a scalar field are studied. The results show the stability of the black hole and the properties of the superradiant modes.
Article
Astronomy & Astrophysics
E. Klempt, A. Sarantsev
Summary: The mixing angles between scalar isoscalar resonances and a scalar glueball are determined from their decays into two pseudoscalar mesons. Results show that a small glueball component is admitted by some scalar isoscalar resonances, while significant glueball fractions are required for other resonances. The summation of all observed glueball fractions yields approximately 78%.
Article
Physics, Mathematical
Beatrice Murorunkwere, Fidele Twagirayezu, Albert Munyeshyaka, Joseph Ntahompagaze, Abraham Ayirwanda
Summary: This paper revisits the equivalence between f(R) and scalar tensor theories and explores different f(R) models. The potentials V(phi) for each f(R) model are derived, focusing on the early Universe and the inflation epoch. The expressions for the spectral index n(s) and tensor-to-scalar ratio r are obtained using the slow-roll approximation approach. Numerical values associated with each f(R) model are determined, some of which are consistent with the Planck survey results and others fall within the suggested range by Planck data. The Klein-Gordon equations (KGEs) of each f(R) model are also constructed, and numerical solutions considering different values of free parameters and initial conditions are found, indicating a decrease in the scalar field in the late Universe.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2022)
Article
Astronomy & Astrophysics
Ivan De Martino
Summary: We investigated the ability of the Scalar-Tensor-Vector Gravity (STVG) theory to explain the kinematics of stars in dwarf spheroidal galaxies. STVG modifies general relativity by introducing scalar and vector fields, aiming to replace dark matter. By solving the spherical Jeans equation using the modified gravitational potential from STVG, we predicted the line-of-sight velocity dispersion profiles of eight dwarf spheroidal galaxies and compared them to data. The results showed tensions on one of the parameters, α, but demonstrated the effectiveness of STVG in replacing dark matter.
Article
Astronomy & Astrophysics
Umesh Kumar Sharma, Mukesh Kumar, Gunjan Varshney
Summary: This research investigates Barrow holographic dark energy with an energy density of ρλ=CH2-δ, considering the Hubble horizon as the IR cut-off in the f(R,T) gravity framework. The study shows that under certain limitations, Barrow holographic quintessence, k-essence, and dilation scalar field models can achieve accelerated expansion.
Article
Physics, Particles & Fields
Joao Luis Rosa, A. S. Lobao Jr, D. Bazeia
Summary: In this work, we studied braneworld models in generalized f(R, T) gravity theories and used the scalar-tensor representation theory. By introducing auxiliary fields and relating them to the source field, we analyzed four possibilities regarding compactification and asymmetry of the brane. We found that in certain models, the solutions of the auxiliary fields are significantly influenced by the source field, exhibiting different behaviors.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
Kota Numajiri, Yong -Xiang Cui, Taishi Katsuragawa, Shin'ichi Nojiri
Summary: We reexamine the static and spherical symmetric compact star configuration in the R2 model of the F(R) gravity theory. With asymptotic solutions for the additional scalar degrees of freedom, we refine analysis on the external geometry and settle the scalar-hair problem argued in previous works. Performing the numerical integration of the modified Tolman-Oppenheimer-Volkoff equations as a two-boundaries-value problem, we further discuss the scalar-field distribution inside the compact stars and its influence on the mass-radius relation. We show that the chameleon potential plays an essential role in determining the scalar field inside the star. The scalar field often behaves as a quintessential field that effectively decreases the mass of compact stars with lower central energy density.
Article
Astronomy & Astrophysics
Tiago B. Goncalves, Joao Luis Rosa, Francisco S. N. Lobo
Summary: In this study, cosmological solutions are obtained in the scalar-tensor representation of f(R, T) gravity using reconstruction methods. By assuming matter as an isotropic perfect fluid and the spacetime as homogeneous and isotropic (Friedmann-Lemattre-Robertson-Walker universe), the energy density, pressure, and scalar field are expressed as general functions of the scale factor. Three particular forms of the scale factor are selected, and a comprehensive analysis is conducted for different curvature and equation of state parameters. The explicit forms of the functions f(R, T) associated with the scalar-field potentials are derived.
Article
Physics, Particles & Fields
Yermek Aldabergenov, Sergei V. Ketov
EUROPEAN PHYSICAL JOURNAL C
(2017)
Article
Astronomy & Astrophysics
Yermek Aldabergenov, Sergei Ketov, Rob Knoops
Article
Astronomy & Astrophysics
Hiroyuki Abe, Yermek Aldabergenov, Shuntaro Aoki, Sergei V. Ketov
CLASSICAL AND QUANTUM GRAVITY
(2019)
Review
Multidisciplinary Sciences
Sergei V. Ketov, Maxim Yu. Khlopov
Article
Physics, Multidisciplinary
Sergei Ketov
JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL
(2020)
Review
Multidisciplinary Sciences
Sergei V. Ketov
Article
Astronomy & Astrophysics
Sergei V. Ketov
Summary: This study reviews the models that unify inflation and Primordial Black Hole (PBH) formation based on modified supergravity, including single-field and two-field models. It shows that the two-field models, driven by Starobinsky's scalaron, lead to an enhancement in the scalar power spectrum in the ultra-slow-roll regime, which efficiently produces PBH in agreement with cosmological observations.
Article
Astronomy & Astrophysics
Ryotaro Ishikawa, Sergei Ketov
Summary: This study examines the parameter space of cosmological inflation and primordial black hole formation in modified supergravity models. The research finds that in these models, primordial black holes can account for a significant part or all of dark matter, but fine-tuning of parameters is necessary to achieve agreement with the observed value of the scalar tilt.
CLASSICAL AND QUANTUM GRAVITY
(2022)
Article
Astronomy & Astrophysics
Sergei Ketov
Summary: A novel gravitational theory, inspired by string theory, is proposed in four spacetime dimensions. This theory combines the modified (R+alpha R-2) gravity motivated by the Starobinsky inflation with the Bel-Robinson-tensor-squared correction from superstrings/M-theory compactified down to four dimensions. The origin of this theory from higher dimensions is also revealed. The proposed theory, called the Starobinsky-Bel-Robinson action, has only two free parameters, making it suitable for verifiable applications in black hole physics, cosmological inflation, and Hawking radiation.
Article
Astronomy & Astrophysics
Shuntaro Aoki, Ryotaro Ishikawa, Sergei Ketov
Summary: We extend the Cecotti-Kallosh model of Starobinsky inflation in supergravity by adding a holomorphic function to the superpotential to generate a large peak in the power spectrum of scalar perturbations. The non-canonical kinetic terms are responsible for inflation while the superpotential generates PBH production. We find that efficient production of PBH consistent with CMB measurements is only possible in the case of a quadratic holomorphic function.
CLASSICAL AND QUANTUM GRAVITY
(2023)
Article
Astronomy & Astrophysics
Sergei Ketov, Ekaterina O. Pozdeevad, Sergey Yu. Vernovd
Summary: This paper investigates the influence of quantum corrections inspired by superstring theory on the Starobinsky inflation model by adding the Bel-Robinson tensor term. The derived physical bounds on the parameter reveal the constraints imposed by demanding the absence of ghosts and consistency with measurements of the cosmic microwave background radiation.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Daniel Frolovsky, Sergei Ketov, Sultan Saburov
Summary: In this paper, the researchers adapted the Appleby-Battye-Starobinsky model to study the double cosmological inflation and formation of primordial black holes. They found that the power spectrum of scalar curvature perturbations can be enhanced, allowing the resulting primordial black holes to survive in the present universe.
MODERN PHYSICS LETTERS A
(2022)
Article
Astronomy & Astrophysics
Anirudh Gundhi, Sergei Ketov, Christian F. Steinwachs
Summary: The study investigates the production of primordial black holes in a dilaton two-field extension of Starobinsky's quadratic f(R) model, which features a multi-field amplification mechanism leading to a sharp peak in the inflationary power spectrum at small wavelengths. This mechanism is significantly different from single-field models and successfully explains the observed cold dark matter content in the Universe through the formation of primordial black holes.
Article
Astronomy & Astrophysics
Yermek Aldabergenov, Shuntaro Aoki, Sergei Ketov
Article
Astronomy & Astrophysics
Sergei V. Ketov, Hiroshi Nakada
