Article
Physics, Multidisciplinary
M. Ilyas, Aftab Ahmad, Fawad Khan, M. Wasif
Summary: In this paper, the authors investigate the effects of perfect fluid models and extended f(R, G, T) gravity on the flat Friedmann-Lematre-Robertson-Walker metric. They focus on exploring finite-time future singularities in late-time cosmic accelerating phases using specific f(R, G, T) models. The authors also consider the most recent estimated values for various parameters to examine the viability and bounds of the models induced by different energy conditions.
Article
Physics, Multidisciplinary
M. Farasat Shamir, G. Mustafa, Saad Waseem, Mushtaq Ahmad
Summary: This manuscript explores traversable wormhole solutions in modified f (R, G) gravity, showing that null energy bounds for the effective energy-momentum tensor are generally violated in the presence of ordinary matter. However, it is found that near the wormhole throat, non-exotic wormhole geometries threaded by matter may exist.
COMMUNICATIONS IN THEORETICAL PHYSICS
(2021)
Article
Physics, Particles & Fields
Beatrice Murorunkwere, Joseph Ntahompagaze, Edward Jurua
Summary: In this study, the 1+3 covariant approach and equivalence between f(R) gravity and scalar-tensor theory were applied to analyze cosmological perturbations. Focus was on dust dominated area with perturbation equations applied to a background solution of alpha R+beta R-n model, and the evolution of energy-density perturbations in a dust-dominated universe was found to decay with increasing redshift through numerical solutions.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Physics, Nuclear
M. Ilyas
Summary: This work introduces a new modified gravitational theory named f(R, G, T), investigates physical features of relativistic compact stars, and analyzes properties such as energy density, pressures, stability, of these stars.
INTERNATIONAL JOURNAL OF MODERN PHYSICS A
(2021)
Article
Astronomy & Astrophysics
Nadeem Azhar, Abdul Jawad, Shamaila Rani
Summary: The matter-antimatter asymmetry through baryogenesis mechanism in different gravity models with generalized baryogenesis interaction is being examined, and the baryon to entropy ratios in different models are found to be consistent with observational data.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Astronomy & Astrophysics
Bita Farsi, Ahmad Sheykhi
Summary: This study reveals the effects of an extra longitudinal degree of freedom on the evolution of perturbations in the context of mimetic gravity. By analyzing linear perturbations, it is found that the mimetic potential can act as dark energy and influence the dynamics of matter perturbations and cosmological parameters.
Article
Physics, Mathematical
W. U. Rahman, M. Ilyas, Z. Yousaf, S. Ullah, F. Khan, R. Khan
Summary: This article investigates the potential role of modified gravity theories in describing the rapid expansion of the universe, with a focus on the physical behavior of compact stars. The radial and tangential pressures, as well as energy density, are calculated for six stars. The study dissects the modified theory of gravity into different models and analyzes the stability, energy conditions, and anisotropic measurements of compact stars. The results indicate that all six stars behave normally for certain parameters of the model.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2022)
Article
Physics, Multidisciplinary
Uday Chand De, Dipankar Hazra
Summary: This article discusses the characterization of spacetime and modified gravity, along with f R,G-gravity and f R,L-m-gravity equipped with the m-projective curvature tensor. It demonstrates the properties of perfect fluid spacetime satisfying Einstein's field equations and establishes their connection to dark energy. The article also studies the relationship between perfect fluid spacetime and f R,G and f R,L-m-gravity theories, and explores the energy conditions associated with these models.
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
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
Physics, Mathematical
A. R. Athar, M. Ilyas, Bilal Masud
Summary: This paper elaborates on the matter distribution and different characteristics of anisotropic compact stars in f (R, G) gravity background. By calculating physical quantities such as energy density, radial pressure, and tangential pressure using different models, the physical behavior of compact stars is explored. The results of measurements on anisotropy, energy conditions, and stability aspects of the models are presented using graphical techniques, and the behavior of the six stars under the (G) model parameter is discussed.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Astronomy & Astrophysics
Mariaveronica De Angelis, Laria Figurato, Giovanni Montani
Summary: In this study, the canonical quantum dynamics of the isotropic universe in metric f(R) gravity theory is analyzed, with investigations into Schrodinger evolution in both vacuum and scalar field cases. The results show nonspreading wave packet profiles in vacuum and increased localization of universe volume when an external scalar field is present. This suggests a spontaneous mechanism for Universe classicalization in metric f(R) gravity, with implications for the transition from a Planckian Universe to a classical de-Sitter phase.
Article
Physics, Nuclear
Hassan Shah, Suhail Khan, Dianchen Lu, Zahid Ahmad, Sardar Muhmmad Hussain, Hasrat Hussain Shah
Summary: This investigation examines the dynamics of spherical collapse of dissipative perfect fluid in f(R, G) gravity theory. The dynamical equation of the collapsing stellar fluid is constructed using the Misner-Sharp technique. Additionally, the heat transport equation is derived and coupled with the dynamical equation to obtain the collapsing rate. It is observed that the contribution of extra terms of f(R, G) in Einstein's field equations increases or decreases the collapse rate depending on the cases.
INTERNATIONAL JOURNAL OF MODERN PHYSICS A
(2022)
Article
Physics, Mathematical
M. Ilyas, Rohna Iqbal
Summary: The main purpose of this research is to study the interior configurations of stable anisotropic objects in the form of spherical charged stars within the gravity region described by f(R,G). The study examines the formation of these charged stars using solutions found by Karori and Barua, and explores various physical components through graphs. The feasibility of the model is evaluated against different energy conditions, and topics such as density modification, stress evolution, star stability, anisotropy measurement, and charge distribution are discussed.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Physics, Mathematical
Bhupendra Kumar Shukla, R. K. Tiwari, D. Sofuoglu
Summary: In this article, the dynamical behavior of the universe is examined in the context of the f(R, G) theory of gravity, which incorporates the Ricci scalar and Gauss-Bonnet invariant. The best fit values of model parameters are predicted to be in agreement with recent observational datasets, using the CC, Pantheon and BAO datasets, Bayesian analysis, likelihood function, and MCMC method. The obtained f(R, G) model exhibits a transition from decelerating to accelerating expansion of the universe, explaining the late accelerating expansion without the need for dark energy.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Astronomy & Astrophysics
Antonio De Felice, Andreas Doll, Francois Larrouturou, Shinji Mukohyama
Summary: This study investigates spherically symmetric vacuum solutions in the context of the recently proposed type-II minimally modified gravity theory. Despite local differences from the traditional metric, the solutions can be recovered to the standard metric by imposing suitable asymptotic conditions.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Antonio De Felice, Shinji Mukohyama, Masroor C. Pookkillath
Summary: The study proposes a theory of minimally modified gravity called VCDM to reduce the H-0 tension puzzle, and after analyzing data, a transition in the expansion history of the universe is found at low-redshift z around 0.3. By using low redshift Pantheon data, the local Hubble expansion rate today within this theory is inferred to be 73.69, leading to a reduced tension within the VCDM theory.
Article
Astronomy & Astrophysics
Robert Hagala, Antonio De Felice, David F. Mota, Shinji Mukohyama
Summary: Research indicates that the minimal theory of massive gravity shows small deviations from general relativity in halo density profiles and matter power spectra. However, there is a clear difference between MTMG and GR in terms of void density. While measuring void profiles is complex from an observational perspective, studying the abundance of dark matter halos is a better way to probe MTMG.
ASTRONOMY & ASTROPHYSICS
(2021)
Article
Astronomy & Astrophysics
Antonio De Felice, Francois Larrouturou, Shinji Mukohyama, Michele Oliosi
Summary: MTBG is a stable and complete theory of alternative gravity, with cosmological solutions supporting two different branches, differing from GR in the scalar sector but retaining HRBG features in the tensor sector.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Antonio De Felice, Shinji Mukohyama
Summary: This paper introduces a new cosmological framework integrating dark matter into a minimally modified gravity model, maintaining the same number of gravitational degrees of freedom through a series of transformations. The framework includes two time-dependent free functions to achieve desired evolutions of Hubble expansion rate and effective gravitational constant for dark matter.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Physics, Multidisciplinary
Antonio De Felice, Shinji Mukohyama, Kazufumi Takahashi
Summary: In modified gravity theories, the strong coupling problem of stealth solutions can be avoided with the use of U-DHOST theories. Additionally, the existence of a nonpropagating mode in U-DHOST theories is clarified.
PHYSICAL REVIEW LETTERS
(2022)
Article
Astronomy & Astrophysics
Joshua A. Kable, Giampaolo Benevento, Noemi Frusciante, Antonio De Felice, Shinji Tsujikawa
Summary: This study investigates the physics of late-time cosmic acceleration using the cross-correlation power spectrum of the integrated Sachs-Wolfe effect and galaxy fluctuations in the cosmic microwave background temperature anisotropy. Three dark energy models belonging to a subclass of Horndeski theories are considered: Galileon Ghost Condensate (GGC), Generalized Cubic Covariant Galileon (GCCG), and K-mouflage. The GGC and GCCG models predict negative ISW-galaxy cross-correlations, while the K-mouflage model predicts a positive correlation similar to the A-cold-dark-matter (ACDM) model. By fitting the best-fit models to the ISW-galaxy cross-correlation power spectrum extracted from photometric redshift surveys, it is found that the GGC and GCCG models have a poorer fit compared to the ACDM model, attributed to the suppressed ISW tails caused by the cubic-order scalar self-interaction.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Antonio De Felice, Kei-ichi Maeda, Shinji Mukohyamaa, Masroor C. Pookkillatha
Summary: We study the collapse of a spherically symmetric dust cloud in the VCDM gravitational theory and find that it corresponds to a specific foliation of the Oppenheimer-Snyder solution in general relativity. The resulting collapse leads to a static configuration with a vanishing lapse function at a radius inside the apparent horizon. This implies the need for a UV completion to describe the physics inside the resulting black hole.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Antonio De Felice, Shinji Mukohyama, Kazufumi Takahashi
Summary: We investigate a generic quadratic higher-order scalar-tensor theory with a scor-datura term and show that exact stealth solutions cannot be realized in theories beyond DHOST. However, approximate stealth solutions exist, where the deviation from the exact stealth configuration is controlled by the mass scale M. The accretion of the scalar field leads to time-dependent approximately stealth solutions, which can be considered stealth at astrophysical scales for all practical purposes.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Antonio De Felice, Shinji Tsujikawa
Summary: Einsteinian cubic gravity is a higher-order gravitational theory that allows the existence of a static and spherically symmetric black hole solution with a modified Schwarzschild geometry induced by cubic curvature terms. However, this theory is plagued by ghost and Laplacian instabilities, with at least one of the dynamical perturbations behaving as a ghost mode. Additionally, one dynamical degree of freedom shows a negative sound speed squared -1/2 for high angular momentum modes.
Article
Astronomy & Astrophysics
Masato Minamitsuji, Antonio De Felice, Shinji Mukohyama, Michele Oliosi
Summary: We investigate the dynamical properties of static and spherically symmetric systems in the self-accelerating branch of the minimal theory of bigravity (MTBG). In the first part, we study the gravitational collapse of pressureless dust and find special solutions that represent gravitational collapse in MTBG. In the latter part, we study odd-parity perturbations of the Schwarzschild-de Sitter solutions and find that for higher-multipole modes, the system reduces to four physical modes with two dynamical modes and two shadowy modes.
Article
Astronomy & Astrophysics
Masato Minamitsuji, Antonio De Felice, Shinji Mukohyama, Michele Oliosi
Summary: In this study, static and spherically symmetric solutions are investigated in the minimal theory of bigravity (MTBG). It is found that a pair of Schwarzschild-de Sitter spacetimes with different cosmological constants and black hole masses is a solution in the self-accelerating branch of MTBG, but not in the normal branch. It is also shown that the Schwarzschild-de Sitter solutions can become compatible with the normal branch by using different coordinates. Furthermore, it is confirmed that the self-accelerating branch of MTBG admits static and spherically symmetric general relativity solutions with matter written in the spatially flat coordinates, including neutron stars with arbitrary matter equations of state. Finally, it is demonstrated that nontrivial solutions in the self-accelerating branch are given by the Schwarzschild-de Sitter metrics written in nonstandard coordinates.
Article
Astronomy & Astrophysics
Antonio De Felice, Shinji Mukohyama, Masroor C. Pookkillath
Summary: In this work, a class of extended minimal theories of massive gravity is introduced, which does not require the same cosmological solutions as the de Rham-Gabadadze-Tolley massive gravity. The theory is constructed to have only two degrees of freedom in the gravity sector, and the unwanted degrees of freedom are removed by adding constraints. The gravitational waves are the only propagating gravitational degrees of freedom that acquire a nontrivial mass. The theory's effective gravitational force is investigated by studying cosmological scalar perturbations. The class of models is further restricted by imposing certain conditions, leading to finite contributions of the Integrated Sachs-Wolfe (ISW) effect and the possibility for nontrivial modifications of gravity.
Article
Astronomy & Astrophysics
Antonio De Felice, Shinji Mukohyama, Masroor C. Pookkillath
Summary: Static, spherically symmetric solutions representing stars made of barotropic perfect fluid are studied in the context of two theories of type-II minimally modified gravity, VCDM and VCCDM. It is found that the solutions in V(C)CDM exactly coincide with the ones in GR.
Article
Astronomy & Astrophysics
Jose C. N. de Araujo, Antonio De Felice, Suresh Kumar, Rafael C. Nunes
Summary: The authors investigated the minimal theory of massive gravity (MTMG) with the Planck-cosmic microwave background (CMB) data for the first time and found that the MTMG model can provide solutions to tensions in observational data within the A cold dark matter cosmology. They also showed that the graviton in the MTMG scenario has a small, positive, non-zero mass and that the theory remains consistent with CMB observations, positioning it as a viable candidate among other modified gravity theories.
