Article
Physics, Multidisciplinary
Ahmad Sheykhi, Maral Sahebi Hamedan
Summary: The thermodynamics-gravity conjecture suggests a strong connection between the gravitational field equations and the first law of thermodynamics, meaning any changes in entropy expression directly impact the field equations. By considering the modified Barrow entropy associated with the apparent horizon, the Friedmann equations are altered as well. This paper explores the implications of this modification on the holographic dark energy (HDE) model, highlighting changes in energy density and the Friedmann equations. The study also investigates the cosmological consequences of using the Hubble horizon and future event horizon as infrared cutoffs, including interactions between dark matter (DM) and dark energy (DE), and the impact of the Barrow exponent on the cosmological behavior of HDE, such as crossing the phantom line and shifting the universe phase transition time.
Article
Astronomy & Astrophysics
Leandros Perivolaropoulos, Ioannis Antoniou, Demetrios Papadopoulos
Summary: This study generalizes the result of Rindler-Ishak by considering the lensing deflection angle in a Schwarzschild-deSitter (SdS) space-time with a general spherically symmetric fluid. An analytical expression for the deflection angle to first post-Newtonian order is derived, which is further verified by numerical derivation and reduces to previous results in special cases. The density and pressure of the spherically symmetric fluid that induces this metric are derived in terms of constants, and observational constraints on these constants are obtained using data from distant clusters of galaxies.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
Alexander B. Balakin, Alexei S. Ilin
Summary: We propose a new self-consistent model that couples cosmic dark energy and dark matter using the rheological approach. This model represents the equations of state as integral operators of the Volterra type. We classify the exact solutions of the model based on the roots of the characteristic polynomial. The pressure, energy density, Hubble function, and acceleration parameter of dark energy and dark matter are expressed explicitly as functions of the dimensionless scale factor. We discuss two intriguing cosmological solutions that describe superexponential expansion and symmetric bounce, as well as new solutions corresponding to quasiperiodic behavior.
Review
Astronomy & Astrophysics
Veronica Motta, Miguel A. Garcia-Aspeitia, Alberto Hernandez-Almada, Juan Magana, Tomas Verdugo
Summary: The article summarizes the characteristics of dark energy models and some commonly used cosmological samples, discusses how to constrain model parameters using observational data, and finally provides an overview of the status of dark energy modeling.
Article
Astronomy & Astrophysics
George Lazarides, Rinku Maji, Rishav Roshan, Qaisar Shafi
Summary: Inspired by the recent determination of the W-boson mass, this study revisits an SO(10) axion model and finds that a scalar SU(2)(L) triplet field can significantly contribute to the W mass, thereby resolving the discrepancy between the Standard Model prediction and the experimental measurement. The model also satisfies gauge coupling unification and observable proton decay, and predicts the existence of axion and stable intermediate mass fermion as dark matter candidates. Furthermore, the model predicts the gravitational wave spectrum from topologically stable cosmic strings.
Article
Astronomy & Astrophysics
Mariana Jaber, Gustavo Arciniega, Luisa G. Jaime, Omar Abel Rodriguez-Lopez
Summary: This study explores the hypothesis of accelerated cosmic expansion by introducing additional fields or modifications to General Relativity. A parameterization form is proposed to reproduce the generic behavior of widely used physical models with infrared corrections. Different combinations of data are analyzed to constrain the specific cases embedded in the parameterization form.
PHYSICS OF THE DARK UNIVERSE
(2022)
Article
Astronomy & Astrophysics
Daniele Oriti, Xiankai Pang
Summary: The analysis of emergent cosmological dynamics in mean field hydrodynamics of quantum gravity condensates reveals accelerated expansion of the universe at both early and late times. Although it does not support a compelling inflationary scenario in the early universe, it naturally produces a phantom-like dark energy dynamics at late times, which crosses the phantom divide and avoids any Big Rip singularity, approaching a de Sitter universe asymptotically.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
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
Yuewei Wen, Eva Nesbit, Dragan Huterer, Scott Watson
Summary: Standard cosmological data analyses can't easily determine the presence of modified gravity. This study presents a quantitative mapping showing how modified gravity models appear within standard analyses, reporting specific biases in standard-parameter spaces. The implications for measurements of mass fluctuations are also discussed.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
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, 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)
Article
Astronomy & Astrophysics
Ekim Taylan Hanimeli, Isaac Tutusaus, Brahim Lamine, Alain Blanchard
Summary: This article discusses an alternative gravity model in which the contribution of dark energy naturally arises from the time-dependence of the gravitational constant, G. By introducing an additional tensor field and utilizing additional constraints, the model is able to explain the accelerated expansion of the universe without the need for a cosmological constant.
Article
Physics, Mathematical
Nasr Ahmed, Tarek M. Kamel
Summary: This paper investigates the application of scale-invariant cosmological models in studying dark energy and cosmic transit by introducing three different models for analysis. Despite the presence of cosmic transits in all three models, the pressure remains negative throughout cosmic evolution, and there are singularities in the evolution of the equation of state parameter.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2021)
Article
Physics, Particles & Fields
Armin van de Venn, David Vasak, Johannes Kirsch, Juergen Struckmeier
Summary: The paper investigates the implications of metric compatible covariant canonical gauge theory of gravity on cosmological scales. The resulting equations of motion in a Friedmann-Lemaitre-Robertson-Walker Universe are derived for a totally anti-symmetric torsion tensor. The modifications of the Friedmann equations, equivalent to spatial curvature, are shown in the limit of a vanishing quadratic Riemann-Cartan term and are further investigated in the early and late times of the Universe's history.
EUROPEAN PHYSICAL JOURNAL C
(2023)
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
L. Golchin, S. Rahvar
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2020)
Article
Astronomy & Astrophysics
Sara Rezvanjou, Reza Saffari, Mozhgan Masoudi, Saheb Soroushfar
CLASSICAL AND QUANTUM GRAVITY
(2020)
Article
Astronomy & Astrophysics
Zahra Davari, Sohrab Rahvar
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2020)
Article
Astronomy & Astrophysics
Behzad Bojnordi Arbab, Sohrab Rahvar
Summary: The study analyzed the impact of close stellar encounters on planetary orbits and explored the expected number of habitable planets in circumstellar habitable zones. It found that close stellar encounters play a significant role in the expected number of continuously habitable planets in dense stellar environments, and the threatening stellar encounter rate follows the number density of stars. Further research is needed to investigate long-term multiple planetary systems and their effect on the overall expected value of habitable planets.
INTERNATIONAL JOURNAL OF MODERN PHYSICS D
(2021)
Article
Physics, Multidisciplinary
Bahareh Hoseini, Reza Saffari, Saheb Soroushfar
Summary: This paper examines the geodesic motion in the spacetime of an SU(2)-colored (A)dS black hole in conformal gravity, classifying solutions of the geodesic equations based on particle energy levels and angular momentums, and providing several examples of possible types of orbits. The analytical solutions for the equation of motion of test particles and light rays are derived using Weierstrass elliptic and Kleinian sigma functions.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Physics, Multidisciplinary
Mohammad Hossein Zhoolideh Haghighi, Sohrab Rahvar, Mohammad Reza Rahimi Tabar
Summary: In this study, the statistical mechanics of binary systems under the gravitational interaction of Modified Newtonian Dynamics (MOND) in three-dimensional space was investigated. The results showed a sharp phase transition in the microcanonical systems, unlike Newtonian gravity, and a smoother phase transition with a defined critical temperature in the canonical systems based on the physical parameters of the binary system.
Article
Astronomy & Astrophysics
Sohrab Rahvar
Summary: In the context of the existence of primordial black holes (PBHs), they may make up a fraction of the dark matter of the Universe. Assuming PBHs fill the dark content of the Milky Way Galaxy, the study calculates the probability of collisions with Earth and the potential consequences like heating. The research also compares the danger of PBH collision with asteroid impacts on Earth.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Astronomy & Astrophysics
Zeinab Kalantari, Alaa Ibrahim, Mohammad Reza Rahimi Tabar, Sohrab Rahvar
Summary: This study investigates the gravitational micro-lensing phenomenon in gamma-ray bursts, finding a candidate event through analysis of the light curve peaks, and estimating the effectiveness of the detection method through Monte Carlo simulations. The results indicate a density parameter for supermassive black holes, offering insights into the mass fraction of black holes in the universe.
ASTROPHYSICAL JOURNAL
(2021)
Article
Physics, Multidisciplinary
Solmaz Asgari, Reza Saffari
Summary: In this article, a new metric with an additional velocity-based term in spacetime is introduced. Despite the indication of broken Lorentz symmetry, the change in the speed of light is found to be minimal. The study suggests that multiple versions of the velocity function can explain the universe's initial explosion and expansion behavior, offering a potential alternative to dark matter.
EUROPEAN PHYSICAL JOURNAL PLUS
(2021)
Article
Astronomy & Astrophysics
Sohrab Rahvar
Summary: MOG, a modified gravity theory designed to replace dark matter, introduces a massive vector field to describe gravity, with each particle carrying a charge proportional to its inertial mass. In this study, the Hamiltonian formalism is used to analyze the dynamics of particles in this theory, revealing that massive particles deviate from geodesics while photons follow them. The dynamics of particles in the Newtonian and post-Newtonian regimes are also investigated for observational purposes, leading to the interesting finding that the deflection angle from lensing by stellar-sized objects is larger than predicted by general relativity. This result suggests the need for a screening mechanism to modify the effective gravitational constant near compact objects like stars.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
Sedighe Sajadian, Sohrab Rahvar, Fatemeh Kazemian
Summary: This study uses Gaia archived data to find a relationship between velocity dispersion and mass, as well as the age of stars. It confirms the known age-velocity relation for early-type and massive stars and reveals a dependence of stellar velocity dispersion on mass for low-mass and late-type stars. By considering this correlation, the study simulates short-duration microlensing events caused by brown dwarfs and concludes that lens masses are underestimated by about 2.5%-5.5% when modeling such events using Bayesian analysis.
ASTRONOMICAL JOURNAL
(2022)
Article
Physics, Multidisciplinary
Sara Dastan, Reza Saffari, Saheb Soroushfar
Summary: This paper analyzes the shadow of a charged rotating black hole in f(R) gravity and examines the effects of spin, charge, and modification of gravity on the shadow's shape, as well as the influence of the presence of plasma. The results show that the shadow becomes smaller with increasing electric charge, and increasing rotation parameters leads to a change in the circular symmetry of the black hole's shadow image. Additionally, the plasma parameter affects the size of the shadow.
EUROPEAN PHYSICAL JOURNAL PLUS
(2022)
Article
Astronomy & Astrophysics
Bahram Mashhoon, Sohrab Rahvar
Summary: In this paper, the polarization of gravitational radiation is discussed within the standard framework of linearized general relativity. The recent discovery of gravitational waves motivates a reexamination of the spin-rotation-gravity coupling and its implications for polarized gravitational radiation; therefore, the coupling of the helicity of gravitational waves to the rotation of an observer or the gravitomagnetic field of a rotating astronomical source is considered. Possible observational effects dependent on polarization in connection with future gravitational wave detectors are briefly explored.
Article
Astronomy & Astrophysics
Zahra Davari, Sohrab Rahvar
Summary: By comparing the MOG theory and the ΛCDM cosmology, the study explored their differences in background cosmological properties and structure formation, showing that the MOG model is consistent with the ΛCDM model in statistical analysis.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Astronomy & Astrophysics
Elahe Khalouei, Sedighe Sajadian, Sohrab Rahvar
Summary: This study investigates potential follow-up polarimetric microlensing observations to study the stellar atmospheres of distant stars. It shows that the VLT's accuracy allows for constraining the atmospheres of cool red giant branch stars. By combining polarimetric and icinformation, the scattering optical depth of the atmosphere and the inner radius of the stellar envelope of RGB stars can be measured.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
