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Bianchi type I in f(T) gravitational theories

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Chinese Physics B 2016年第5期25卷 58-64页

作者： M I Wanas G G L Nashed O A Ibrahim Astronomy Department Faculty of Science Cairo University Cairo Egypt Centre for theoretical Physics The British University in Egypt El-Sherouk City Egypt Mathematics Department Faculty of Science Ain Shams University Cairo Egypt Egyptian relativity group (ERG) Giza Egypt

A tetrad field that is homogeneous and anisotropic which contains two unknown functions A（t） and B（t） of cosmic time is applied to the field equations of f（T）, where T is the torsion scalar, T = T~μ_（νρ）S_μ^（νρ）. We calculate the equation of continuity and rewrite it as a product of two brackets, the first is a function of f（T） and the second is a function of the two unknowns A（t） and B（t）. We use two different relations between the two unknown functions A（t） and B（t） in the second bracket to solve it. Both of these relations give constant scalar torsion and solutions coincide with the de Sitter one. So,another assumption related to the contents of the matter fields is postulated. This assumption enables us to drive a solution with a non-constant value of the scalar torsion and a form of f（T） which represents ΛCDM.

关键词： f(T) gravitational theory Bianchi type I special solutions

Generic phase portrait analysis of finite-time singularities and generalized Teleparallel gravity

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Chinese Physics C 2017年第12期41卷 133-154页

作者： W. El Hanafy G.G.L. Nashed Centre for theoretical Physics The British University in Egypt P.O. Box 43 El Sherouk City Cairo 11837 Egypt Mathematics Department Faculty of Science Ain Shams University Cairo 11566 Egypt Egyptian relativity group (ERG) Cairo University Giza 12613 Egypt

We analyze the four common types of finite-time singularity using a generic framework of the phase portrait geometric approach. This technique requires the Friedmann system to be written as a one-dimensional autonomous system. We employ a scale factor that has been used widely in the literature to realize the four finite- time singularity types, then we give a detailed discussion for each case showing possible novel models. Moreover, we show how different singularity types can play essential roles in different cosmological scenarios. Among several modified gravity theories, we show that the f（T） cosmology is compatible with the phase portrait analysis, since the field equations include Hubble derivatives only up to first order. Therefore, we reconstruct the f（T） theory which generates these phase portraits. We also perform a complementary analysis using the effective equation of state. Furthermore, we investigate the role of the torsion fluid in realizing the cosmic singularities.

关键词： inflation bounce big brake ACDM teleparallel gravity

Slim Discs with Varying Accretion Rates

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Chinese Physics Letters 2009年第4期26卷 275-277页

作者：焦承亮卢炬甫 Department of Physics and Institute of theoretical Physics and astrophysics Xiamen University Xiamen 361005

It was revealed in our previous studies that there exists a maximal possible accretion rate for slim discs with constant accretion rates because the correctly calculated vertical gravitational force can only gather some limited amount of accreted matter. Here we show that when the accretion rate is not constant and instead decreases with decreasing radius because of outflows, such that the amount of accreted matter is adjusted to be within the allowed limit, global slim disc solutions can be constructed even for the case that accretion rates at large radii apparently exceed the maximal possible value. This result further demonstrates that outflows seem to be unavoidable for accretion flows with large accretion rates at large radii.

关键词： gamma-ray bursts gamma-rays relativity

Modelling Moran Process with Network Dynamics for the Evolution of Cooperation

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Chinese Physics Letters 2009年第6期26卷 302-305页

作者：杨冬平林海吴晨旭帅建伟 Department of Physics and Institute of theoretical Physics and astrophysics Xiamen University Xiamen 361005

We introduce a simple model based on the Moran process with network dynamics. Using pair approximation, the cooperation frequencies at equilibrium states are deduced for general interactions. Three usual social dilemmas are discussed in the framework of our model. It is found that they all have a phase transition at the same value of cost-to-benefit ratio. For the prisoner＇s dilemma game, notably it is exactly the simple rule reported in the literature [Nature 441 （2006） 502]. In our model, the simple rule results from the parent-offspring link. Thus the basic mechanism for cooperation enhancement in network reciprocity is in line with the Hamilton rule of kin selection. Our simulations verify the analysis obtained from pair approximation.

关键词： gamma-ray bursts gamma-rays relativity

Fundamental Physical Basis for Maxwell-Heaviside Gravitomagnetism

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Journal of Modern Physics 2015年第9期6卷 1207-1219页

作者： Golden Gadzirayi Nyambuya Department of Applied Physics Fundamental and Theoretical Astrophysics Group National University of Science and Technology Bulawayo Republic of Zimbabwe

Gravitomagnetism is universally and formally recognised in contemporary physics as being the linear first-order approximation of Einstein’s field equations emerging from the General Theory of relativity (GTR). Herein, we argue that, as has been done by others in the past, gravitomagnetism can be viewed as a fully-fledged independent theory of gravitomagnetism that can be divorced from Professor Einstein’s GTR. The gravitomagnetic theory whose exposition we give herein is exactly envisioned by Professor Maxwell and Dr. Heaviside. The once speculative Maxwell-Heaviside Gravitomagnetic theory now finds full justification as a fully fledged theory from Professor José Hera’s Existence Theorem which states that all that is needed for there to exist the four Max-well-type field equations is that a mass-current conservation law be obeyed. Our contribution in the present work, if any, is that we demonstrate conclusively that like electromagnetism, the gravitomagnetic phenomenon leads to the prediction of gravitomagnetic waves that travel at the speed of light. Further, we argue that for the gravitational phenomenon, apart from the Newtonian gravitational potential, there are four more potentials and these operate concurrently with the Newtonian potential. At the end of it, it is seen that the present work sets the stage for a very interesting investigation of several gravitational anomalies such as the ponderous Pioneer Anomaly, the vexing Flyby Anomalies, the mysterious Anomalous Rotation Curves of Spiral Galaxies and as well, the possibility of the generation of stellar magnetic fields by rotating gravitational masses.

关键词： Astrometry Celestial Mechanics Ephemerides Gravitomagnetism Quantum Gravity Unified Field Theory

Long Range, Long Lived and Gauge Invariant Massive Photons via Stückelberg Scalar Coupling

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Journal of Modern Physics 2016年第1期7卷 7-24页

作者： Golden Gadzirayi Nyambuya Fundamental theoretical and astrophysics group Department of Applied Physics Faculty of Applied Sciences National University of Science and Technology Bulawayo Zimbabwe

It is largely believed (or strongly assumed) that photons are massless particles and the most compelling evidence there—it is said—is found in the manifestations of photons being long ranged and long lived particles. As we have done before, albeit, with a much better and clear insight in the present than before;we argue herein that massive photons can still enjoy the special and rare privilege of travelling at the speed of light c while being long ranged, long lived and most of all, obeying the much desired gauge symmetry. This we achieve by breaking the traditional Lorenz gauge and in its place, we introduce a new Special Gauge Condition (SGC) that does the work of assuring the photon its longevity, long range-ness and that it [photon] propagates at the speed c. However, the most melancholic outcome of our investigation is that if the present scheme is what subtle Nature has chosen to endow the photon a non-zero mass so that it [photon] still obeys gauge invariance, is long ranged, long lived and travels at the sacred speed c;then, this non-zero photon mass may be very difficult, if not impossible to measure. We use the equations developed to investigate Lorentz violation in ?-ray bursts.

关键词： Lorentz Violation Lorenz Gauge: Violation Stueckelberg: Field Mechanism-Proca: Massive Photon Long Range Photon Long Lived Photon

Charged quark stars in f(R,T)gravity

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Chinese Physics C 2022年第11期46卷 308-317页

作者： Juan M.Z.Pretel Takol Tangphati Ayan Banerjee Anirudh Pradhan Centro Brasileiro de Pesquisas Físicas Rua Dr.Xavier Sigaud150 URCARio de Janeiro CEP 22290-180RJBrazil theoretical and Computational Physics group Theoretical and Computational Science Center(TaCS)Faculty of ScienceKing Mongkut’s University of Technology Thonburi126 Prachauthid Rd.Bangkok 10140Thailand astrophysics and Cosmology Research Unit School of MathematicsStatistics and Computer ScienceUniversity of KwaZulu-NatalPrivate Bag X54001Durban 4000South Africa Centre for Cosmology Astrophysics and Space ScienceGLA UniversityMathura-281406Uttar PradeshIndia

Recent advances in nuclear theory and new astrophysical observations have led to the need for specific theoretical models applicable to dense-matter physics *** chromodynamics(QCD)predicts the existence of non-nucleonic degrees of freedom at high densities in neutron-star matter,such as quark *** a confining quark matter model,which consists of homogeneous,neutral 3-flavor interacting quark matter with O(m_(s)^(4))corrections,we examine the structure of compact stars composed of a charged perfect fluid in the context of f(R,T)*** system of differential equations describing the structure of charged compact stars has been derived and numerically solved for a gravity model with f(R,T)=R+2β*** simplicity,we assumed that the charge density is proportional to the energy density,namely,ρ_(ch)=αρ.It is demonstrated that the matter-geometry coupling constant β and charge parameter α affect the total gravitational mass and the radius of the star.

关键词： charged stars f(R T)gravity

Cosmological applications in Kaluza-Klein theory

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Chinese Physics B 2012年第4期21卷 631-637页

作者： M. I. Wanas Gamal G. L. Nashed A. A. Nowaya Astronomy Department Faculty of Science Cairo University Giza Egypt Mathematics Department Faculty of Science Ain Shams University Cairo 11566 Egypt Center for theoretical Physics British University of Egypt Sherouk City 11837 P.O. Box 43 Egypt Mathematics Department Faculty of Science King Faisal University P.O. Box 380 Al-Ahsaa 31982 the Kingdom of Saudi Arabia Egyptian relativity group (ERG) Physics Department Faculty of Science Monofia University Monofia Egypt

The field equations of Kaluz-Klein （KK） theory have been applied in the domain of cosmology. These equations are solved for a fiat universe by taking the gravitational and the cosmological constants as a function of time t. We use Taylor＇s expansion of cosmological function, A（t）, up to the first order of the time t. The cosmological parameters are calculated and some cosmological problems are discussed.

关键词： Kaluz-Klein theory cosmology Taylor's expansion of cosmological function

THE GRAVITATIONAL COLLAPSE OF A HIGGS SCALAR FIELD

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Science in China,Ser.A 1984年第5期 521-530页

作者：吴忠超 Center for theoretical astrophysics University of Science and Technology of China Hefei

We investigate the spherical gravitational collapse of a Higgs scalar field in the frame-work of numerical relativity. This problem is stimulated by the inflationary universe *** is supposed that a sufficient fluctuation of the Higgs scalar field at the supercooled expan-sion stage causes gravitational collapse. We use an imitative model to give some estimate ofblack-hole formation at the reheating time. In order to form a black hole from a fluctuationwith a size of the same order as the cosmological horizon, the scalar field of the fluctuationmust evolve with a time delay of about one expansion time relative to its surroundings andthe mass of the black hole formed is of order 104g.

关键词： THE GRAVITATIONAL COLLAPSE OF A HIGGS SCALAR FIELD very