Article
Astronomy & Astrophysics
M. Mousavi, K. Atazadeh
Summary: The study examines future cosmological singularities in the context of massive gravity and minimal massive bigravity theory. It identifies sudden and big brake singularities in different cosmological models, noting that the sudden singularity can be eliminated when model parameters approach zero.
PHYSICS OF THE DARK UNIVERSE
(2022)
Article
Astronomy & Astrophysics
R. E. Eaves
Summary: In standard cosmology, redshift is determined by the relationship between the scale factor and the speed of light. This relationship is also applied in varying speed of light cosmologies, assuming that the PLANCK CONSTANT OVER TWO PI is proportional to c. Measured redshift is compared not with the original emitted spectrum, but with a reference spectrum generated locally. Redshift can be decomposed into the change during the flight of a photon and the difference in physics at the time of emission and observation.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
Bao-Fei Li, Parampreet Singh
Summary: Contrary to conventional wisdom, it is not easy to achieve a cyclic evolution in certain nonsingular cosmological models. This research shows that a modified version of loop quantum cosmology, based on Thiemann's regularization, does not allow a cyclic evolution unless highly restrictive conditions are met. Furthermore, the necessary condition for a cyclic evolution turns out to be a violation of the weak energy condition.
Article
Physics, Particles & Fields
Ignatios Antoniadis, Spiros Cotsakis, Dimitrios Trachilis
Summary: We have proposed a general asymptotic solution for modified gravity near a sudden singularity. This solution involves a fluid source without an equation of state and possesses eleven independent arbitrary functions tied to the spatial coordinates, in accordance with the Cauchy problem of the theory. Additionally, we demonstrate that the solution exhibits a shock wave-like behavior near the sudden singularity, with the same number of free functions in both the Jordan and Einstein frame.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Astronomy & Astrophysics
Oem Trivedi
Summary: Interest in cosmological singularities has grown with the discovery of late-time acceleration of the universe and dark energy. Recent work classified singularities into strong and weak types, and discussed the formation conditions of type V singularities in different cosmologies.
Article
Astronomy & Astrophysics
Seokcheon Lee
Summary: The minimally extended VSL (meVSL) model proposed in the paper may have important implications in cosmology, offering new solutions to various problems of the standard ΛCDM model.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Seokcheon Lee
Summary: For the varying speed of light (VSL) models to be phenomenologically feasible, at least one dimensionless physical constant must change. Adiabaticity and the dependence of physical constants and quantities on cosmic time are also important. The minimally extended VSL (meVSL) model satisfies these conditions and the redshift-drift formula is the same as the standard model. However, the meVSL cannot be verified experimentally but can be tested using cosmological chronometers (CC).
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
R. E. Eaves
Summary: The study explores the relationship between the speed of light and gravitational constant, using experimental data to determine the range of variation in the speed of light over time. It is noted that constraints on the speed of light variation are based on data from within the Solar system rather than cosmological observations. Additionally, a relationship between the speed of light and the fine-structure constant is identified as potentially offering tighter constraints.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Multidisciplinary Sciences
Yijie Shen, Yaonan Hou, Nikitas Papasimakis, Nikolay Zheludev
Summary: Topology in electromagnetic fields can result in fascinating and unexpected phenomena. The authors introduce a new family of electromagnetic excitation, supertoroidal electromagnetic pulses, with skyrmionic structures and fractal-like energy distributions. These pulses are of interest for transient light-matter interactions, ultrafast optics, spectroscopy, and toroidal electrodynamics.
NATURE COMMUNICATIONS
(2021)
Article
Astronomy & Astrophysics
Yuting Liu, Shuo Cao, Marek Biesiada, Yujie Lian, Xiaolin Liu, Yilong Zhang
Summary: The paper proposes a new method to investigate the possible time variation of the speed of light by analyzing the Hubble diagram of high-redshift standard candles. The authors found that there is no strong evidence for deviation from a constant speed of light up to z similar to 2 based on the SNe Ia Pantheon sample and high-redshift quasars. They also discuss future improvements in measuring the speed of light at much higher redshifts using gravitational waves produced by binary neutron star mergers.
ASTROPHYSICAL JOURNAL
(2023)
Article
Multidisciplinary Sciences
Iver Brevik, Alexander V. Timoshkin
Summary: This article investigates the late-time accelerated universe with non-zero curvature in the FRW spacetime, considering an inhomogeneous and viscous (bulk viscous) cosmic fluid coupled with dark matter. The influence of thermal effects caused by Hawking radiation on the formation of various types of singularities within a finite time is explored. It is found that the presence of Hawking radiation can alter the time of singularity formation and the nature of the singularity itself. Additionally, the combined effects of radiation, viscosity, and space curvature result in a singularity-free universe. The assumption of spatial isotropy is crucial in this theory, which is reflected in the use of bulk viscosity instead of shear viscosity.
Article
Astronomy & Astrophysics
Teodor Borislavov Vasilev, Mariam Bouhmadi-Lopez, Prado Martin-Moruno
Summary: We revise the future fate of kinetic gravity braiding models, a group of scalar-tensor theories. These models can drive the expansion of the universe towards a future de Sitter state if the Lagrangian is invariant under constant shifts in the scalar field. However, we show that future cosmic singularities characterized by a divergence of the energy density can also appear in this framework. We present an explicit example where a big rip singularity is the only possible future fate of the cosmos.
Article
Astronomy & Astrophysics
Tonghua Liu, Shuo Cao, Marek Biesiada, Yuting Liu, Yujie Lian, Yilong Zhang
Summary: The study introduces a new model-independent method to test the invariance of the speed of light in the distant Universe. Results show no significant deviation of the speed of light from the constant value within the uncertainty of current observations. The proposed approach may achieve higher precision in testing the deviation of the speed of light compared to previous methods.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Physics, Particles & Fields
Meng-Yao Zhang, Hao Chen, Hassan Hassanabadi, Zheng-Wen Long, Hui Yang
Summary: The classification of critical points of charged topological black holes in anti-de Sitter spacetime under the Power Maxwell Invariant (PMI)-massive gravity is studied in the framework of black hole chemistry. It is found that 4-dimensional black holes have only one topological class, while 5-dimensional and higher dimensional black holes belong to two different topology classes. The traditional critical point corresponds to the maximum extreme point of temperature with negative topological charge, while the novel critical point corresponds to the minimum extreme point of temperature with opposite topological charge.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Astronomy & Astrophysics
Subinoy Das, Anshuman Maharana, Vivian Poulin, Ravi Kumar Sharma
Summary: This paper investigates the discrepancy between the predicted and directly estimated values of S-8 and explores the possibility of power suppression due to the contribution of nonthermal hot dark matter to the energy density of the Universe. It is found that Planck data can decrease the tension, but does not support a nonzero contribution. Combining with other measurements, the existence of a particle with specific mass and contribution is suggested. The implications for other nonthermal HDM models are discussed, as well as the potential connection to the short baseline oscillation anomaly.
Article
Physics, Nuclear
R. Moti, A. Shojai
INTERNATIONAL JOURNAL OF MODERN PHYSICS A
(2020)
Article
Physics, Multidisciplinary
H. Khodabakhshi, F. Shojai
Article
Astronomy & Astrophysics
Mahsa Sanati, Yves Revaz, Jennifer Schober, Kerstin E. Kunze, Pascale Jablonka
ASTRONOMY & ASTROPHYSICS
(2020)
Article
Astronomy & Astrophysics
R. Saadati, F. Shojai
Summary: The study introduces new aspects of the Kiselev black hole and explores null and timelike thin shell collapse in this space-time. It demonstrates that the Kiselev black hole can be matched to a de Sitter core with a thin timelike dust shell to create a non-singular space-time. The stability of the shell's stationary solutions is discussed, and suitable parameter ranges for constructing a stable stationary black hole are identified within the Kiselev geometry.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
Raihaneh Moti, Ali Shojai
Summary: The paper explores the effects of improving pure affine theories of gravity using the running gravitational coupling obtained from asymptotically safe gravity. By comparing with other improvement methods, a general approach for affine improvement is presented in the framework of Schrodinger-Eddington affine theory.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Astronomy & Astrophysics
S. M. M. Rasouli, F. Shojai
Summary: This paper explores the geodesic deviation equation within the context of the Brans-Dicke theory in D dimensions, focusing on cosmological models and exact solutions. By studying cosmological models in both general relativity and the Brans-Dicke theory, the authors provide settings for dynamical systems and investigate well-known cosmological models with modified and corrected solutions. They demonstrate the potential advantages of the modified Brans-Dicke theory in describing the universe according to observational data and theoretical viewpoints.
PHYSICS OF THE DARK UNIVERSE
(2021)
Article
Astronomy & Astrophysics
F. Shojai, A. Sadeghi, R. Hassannejad
Summary: This article examines the formation of a class of regular black holes from the gravitational collapse of massive stars and presents a mathematical model and results.
CLASSICAL AND QUANTUM GRAVITY
(2022)
Article
Astronomy & Astrophysics
Raihaneh Moti, Ali Shojai
Summary: In this paper, we demonstrate the possibility of a pure quantum gravastar solution to the quantum improved Einstein equations. This object is essentially a gravastar with the thin layer of matter replaced by quantum effects. The metric of a pure quantum gravastar is obtained and its stability is discussed.
INTERNATIONAL JOURNAL OF MODERN PHYSICS D
(2022)
Article
Astronomy & Astrophysics
Sh. Khodabakhshi, M. Farhang, A. Shojai, M. S. Esmaeilian, R. Moti
Summary: The de Sitter-Godel-de Sitter phase transition (dGd) is a possible geometrical phase transition in the very early universe, leaving observable traces on matter and radiation fields. A simulation based on dGd reveals no signs of the transition, and instead shows consistency with pure inflationary power spectrum. Future large-scale surveys could provide more precise constraints on the physics of the early universe.
GENERAL RELATIVITY AND GRAVITATION
(2022)
Article
Astronomy & Astrophysics
R. Hassannejad, A. Sadeghi, F. Shojai
Summary: In this paper, the 4-dimensional Einstein-Gauss-Bonnet (EGB) gravity is treated as general relativity with an effective stress-energy tensor. The modified Oppenheimer-Snyder-Datt model of the gravitational collapse of a star in a 4-dimensional EGB black hole is studied. The interior geometry of the star is described by the spatially flat Friedmann-Robertson-Walker metric, and the matter is uniformly distributed without any assumption about its equation of state. The time evolution of event and apparent horizons is also studied.
CLASSICAL AND QUANTUM GRAVITY
(2023)
Article
Physics, Multidisciplinary
S. Cheraghchi, F. Shojai, M. H. Abbassi
Summary: Apart from assuming the inflation started at an infinite time in the past, a more realistic initial state of quantum fluctuations is imposed at a finite initial time. The α-vacuum is an important non-trivial vacuum specified by a momentum cutoff. Therefore, different k-modes have different initial time conditions, which modify the amplitude of quantum fluctuations and the corresponding power spectra. This paper investigates the impact of the α-vacuum state on the power spectrum of scalar perturbations in f(R) gravity by considering an ultraviolet cutoff. The results show that the leading order corrections to the scalar power spectra in f(R) gravity exhibit an oscillatory behavior similar to general relativity, and are sufficiently consistent with the ΛCDM model.
Article
Astronomy & Astrophysics
R. Moti, A. Shojai
Article
Astronomy & Astrophysics
H. Khodabakhshi, A. Shirzad, F. Shojai, Robert B. Mann
Article
Astronomy & Astrophysics
R. Moti, A. Shojai
Article
Astronomy & Astrophysics
R. Saadati, F. Shojai