Article
Physics, Particles & Fields
Christian G. Bohmer, Erik Jensko, Ruth Lazkoz
Summary: When considering a homogeneous and isotropic cosmological model, the field equations of modified gravity theories can be transformed into autonomous differential equations, which is a successful approach to study such models. We propose a perspective that is applicable to many different modified gravity models and relies on the standard cosmological density parameters, allowing for the extraction of generic information before considering a specific model.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
Hamid Shabani, Hooman Moradpour, Amir Hadi Ziaie
Summary: In this work, we studied the phase-space analysis of generalized Rastall gravity (GRG), which is a modification of the original version of Rastall gravity (RG). We found that GRG allows for a stable critical point corresponding to the late-time accelerated expansion of the Universe. By considering dark matter (DM) and an ultra-relativistic perfect fluid (radiation) as cosmic ingredients, we showed that the model exhibits a viable cosmic evolution sequence, transitioning from a radiation-dominated era to a DM-dominated era and finally reaching a late-time dark energy (DE)-dominated era. Additionally, taking into account the contribution of spatial curvature leads to a growing mode that is consistent with recent observations.
PHYSICS OF THE DARK UNIVERSE
(2022)
Article
Physics, Nuclear
Mihai Marciu, Dana Maria Ioan
Summary: This paper extends the basic Einstein-Hilbert action by introducing a non-minimal coupling between the space-time geometry and matter fields. It formulates a cosmological model based on specific associations between the matter energy-momentum tensor and the Einstein tensor. The dynamics of the model and its physical implications are explored through a dynamical system analysis.
Article
Astronomy & Astrophysics
Giovanni Acquaviva, Nihan Katirci
Summary: We perform a dynamical system analysis of a cosmological model in the energy-momentum squared gravity (EMSG) and show that a future attractor solution is obtained. The model includes new terms that yield constant inertial mass density and dynamical dark energy passing below zero. Additionally, the second law of thermodynamics requires a certain condition for the screening mechanism of Λ.
PHYSICS OF THE DARK UNIVERSE
(2022)
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)
Article
Astronomy & Astrophysics
Wompherdeiki Khyllep, Jibitesh Dutta, Emmanuel N. Saridakis, Kuralay Yesmakhanova
Summary: Motivated by the success of f(Q) gravity in fitting observational data, we analyze the behavior of two studied f(Q) models, power-law and exponential, through dynamical system analysis. We find that both models have a matter-dominated saddle point followed by a stable dark-energy-dominated accelerated universe. The models fit observational data well and can be considered as promising alternatives to the ACDM concordance model.
Article
Astronomy & Astrophysics
Masato Minamitsuji
Summary: The analysis shows that the difference between comoving curvature perturbations in the original and new frames on superhorizon scales is determined by a combination of time derivatives, intrinsic entropy perturbations of the scalar field, and their time derivatives. Comoving curvature perturbations can remain invariant under disformal transformations on superhorizon scales under certain conditions, while tensor perturbations can also remain disformally invariant if conserved over time in the original frame.
Article
Astronomy & Astrophysics
Pavel Jirousek, Keigo Shimada, Alexander Vikman, Masahide Yamaguchi
Summary: In this study, promoting the trace part of the Einstein equations to a trivial identity leads to the Newton constant becoming an integration constant, making it a global dynamical degree of freedom subject to quantization and quantum fluctuations. By introducing a constrained variational formulation of the modified Einstein equations and constructing different general-covariant actions, the relationship between the inverse of dynamical Newton constant and the Ricci scalar integrated over spacetime is revealed. Surprisingly, an equivalent theory can be formulated using a dynamical Planck constant instead of the dynamical Newton constant, and it is shown that an axion-like field can play a role in the gravitational or quantum constants.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Physics, Particles & Fields
Maria Ortiz-Banos, Mariam Bouhmadi-Lopez, Ruth Lazkoz, Vincenzo Salzano
Summary: This study explores a modified theory of gravity with non-minimal coupling to matter, showing that gravitational coupling to matter can drive accelerated expansion of the universe. Analytical solutions for the functions f1 and f2 were calculated for different energy density contents, supporting the conclusion. Cosmographic analysis was also performed to further understand the impact of gravitational coupling.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Physics, Mathematical
Carlos Araoz Alvarado, Celia Escamilla-Rivera
Summary: In this paper, the authors extend the study of the thermodynamics of structure formation in the large-scale Universe using nonlocal gravity formalism and standard statistical mechanics. By deriving the grand partition function in a modified version of the Yang-Lee theory, they obtain thermodynamic properties consistent with a Bose-Einstein dark matter framework and derive its gravitational phase transition.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2022)
Article
Astronomy & Astrophysics
G. G. L. Nashed, S. Capozziello
Summary: This study investigates the possible modifications of general relativity at ultraviolet and infrared scales, particularly considering the possibility of parity violation in strong gravity regime. The Chern-Simons gravity theory is used to account for the parity violation. The study shows that for certain spacetime structures, Chern-Simons gravity is equivalent to general relativity, but it is not applicable to (A)dS-Kerr/Kerr black holes. The thermodynamic quantities and geodesic equation of the black hole are also calculated.
Article
Physics, Particles & Fields
L. N. Granda
Summary: New corrections to General Relativity are proposed within the context of modified f(R) gravity that satisfy cosmological and local gravity constraints. The proposed models behave asymptotically as R - 2 Lambda at large curvature and show the vanishing of the cosmological constant at the flat spacetime limit. The steepness of the deviation parameter in late times leads to measurable signal of scalar tensor regime in matter perturbations, allowing departures from the ΛCDM model to be detected.
JOURNAL OF HIGH ENERGY PHYSICS
(2021)
Article
Physics, Fluids & Plasmas
Carlo Maria Scandolo, Gilad Gour, Barry C. Sanders
Summary: We develop a rigorous theory of external influences on finite discrete dynamical systems, going beyond the perturbation paradigm. The theory of covariant influences is developed using resource theories, for both deterministic and stochastic evolutions. We provide necessary and sufficient conditions for state transitions under deterministic covariant influences and necessary conditions in the presence of stochastic covariant influences, predicting forbidden transitions between states. Our approach, employing the framework of resource theories, unifies the behavior of different types of finite discrete dynamical systems.
Article
Physics, Particles & Fields
J. K. Singh, Harshna Balhara, Kazuharu Bamba, J. Jena
Summary: This study explores a bouncing scenario of a flat homogeneous and isotropic universe using the reconstruction technique for the power-law parametrization of the Hubble parameter in a modified gravity theory with higher-order curvature and trace of the energy-momentum tensor terms. It is demonstrated that the bouncing criteria are satisfied, avoiding the cosmological initial singularity. Additionally, the equation of state parameter crosses the line of the phantom divide, indicating a highly unstable universe near the bouncing point. Extreme acceleration is also found to occur at the bouncing point.
JOURNAL OF HIGH ENERGY PHYSICS
(2023)
Article
Physics, Particles & Fields
H. R. Fazlollahi
Summary: This study investigates the conservation of stress-energy tensor in curved spacetime and examines the application of modified gravity. The spherically symmetric solutions and evolution of the Universe for early and late time are explored, showing that there are no new results compared to Einstein gravity in vacuum solutions. However, during inflation, the scalar spectra index deviates from the standard model. Late-time and constraint models are also considered using the MCMC algorithm.
EUROPEAN PHYSICAL JOURNAL C
(2023)
