20 papers found.

Use AND, OR, NOT, +word, -word, "long phrase", (parentheses) to fine-tune your search.

Use AND, OR, NOT, +word, -word, "long phrase", (parentheses) to fine-tune your search.

, Gower Street, London WC1E 6BT, UK
Correspondence should be addressed to **Tiberiu** **Harko**; ku.ca.lcu@okrah.t
Received 5 March 2018; Accepted 12 May 2018; Published 28 June 2018
Academic Editor: Luis

We consider the Palatini formulation of f(R, T) gravity theory, in which a non-minimal coupling between the Ricci scalar and the trace of the energy-momentum tensor is introduced, by considering the metric and the affine connection as independent field variables. The field equations and the equations of motion for massive test particles are derived, and we show that the...

We derive upper and lower limits for the mass–radius ratio of spin-fluid spheres in Einstein–Cartan theory, with matter satisfying a linear barotropic equation of state, and in the presence of a cosmological constant. Adopting a spherically symmetric interior geometry, we obtain the generalized continuity and Tolman–Oppenheimer–Volkoff equations for a Weyssenhoff spin fluid in...

If dark matter is composed of massive bosons, a Bose–Einstein condensation process must have occurred during the cosmological evolution. Therefore galactic dark matter may be in a form of a condensate, characterized by a strong self-interaction. We consider the effects of rotation on the Bose–Einstein condensate dark matter halos, and we investigate how rotation might influence...

Though not a part of mainstream physics, Salam’s theory of strong gravity remains a viable effective model for the description of strong interactions in the gauge singlet sector of QCD, capable of producing particle confinement and asymptotic freedom, but not of reproducing interactions involving SU(3) color charge. It may therefore be used to explore the stability and...

We consider the cosmological implications of a gravitational theory containing two vector fields coupled via a generalized Chern–Simons term. One of the vector fields is the usual Maxwell field, while the other is a constrained vector field with constant norm included in the action via a Lagrange multiplier. The theory admits a de Sitter type solution, with healthy cosmological...

We consider a vector–tensor gravitational model with terms quadratic in the Maxwell tensor derivatives, called the Bopp–Podolsky term. The gravitational field equations of the model and the equations describing the evolution of the vector field are obtained and their Newtonian limit is investigated. The cosmological implications of a Bopp–Podolsky type dark energy term are...

We numerically solve the equations of motion (EOM) for two models of circular cosmic string loops with windings in a simply connected internal space. Since the windings cannot be topologically stabilized, stability must be achieved (if at all) dynamically. As toy models for realistic compactifications, we consider windings on a small section of \(\mathbb {R}^2\), which is valid...

The study of the Brownian motion of a charged particle in electric and magnetic fields has many important applications in plasma and heavy ions physics, as well as in astrophysics. In the present paper we consider the electromagnetic radiation properties of a charged non-relativistic particle in the presence of electric and magnetic fields, of an exterior non-electromagnetic...

We investigate the cosmological implications of modified gravities induced by the quantum fluctuations of the gravitational metric. If the metric can be decomposed as the sum of the classical and of a fluctuating part, of quantum origin, then the corresponding Einstein quantum gravity generates at the classical level modified gravity models with a non-minimal coupling between...

Modified gravity theories have the potential of explaining the recent acceleration of the Universe without resorting to the mysterious concept of dark energy. In particular, it has been pointed out that matter–geometry coupling may be responsible for the recent cosmological dynamics of the Universe, and matter itself may play a more fundamental role in the description of the...

We study the stability of the cosmological scalar field models by using the Jacobi stability analysis, or the Kosambi-Cartan-Chern (KCC) theory. In this approach, we describe the time evolution of the scalar field cosmologies in geometric terms, by performing a “second geometrization” and considering them as paths of a semispray. By introducing a nonlinear connection and a...

**Tiberiu** **Harko** 0 5
Matthew J. Lake 3 4
0 Department of Physics, Babes-Bolyai University , Kogalniceanu Street, 400084 Cluj-Napoca , Romania
1 Theoretical Physics Group, Blackett Laboratory, Imperial

Due to their superfluid properties some compact astrophysical objects, like neutron or quark stars, may contain a significant part of their matter in the form of a Bose–Einstein condensate (BEC). Observationally distinguishing between neutron/quark stars and BEC stars is a major challenge for this latter theoretical model. An observational possibility of indirectly distinguishing...

We consider Kasner-type static, cylindrically symmetric interior string solutions in the \(f\left( R,L_m\right) \) theory of modified gravity. The physical properties of the string are described by an anisotropic energy-momentum tensor satisfying the condition \(T_t^t=T_z^z\); that is, the energy density of the string along the \(z\)-axis is equal to minus the string tension. As...

The existence of both a minimum mass and a minimum density in nature, in the presence of a positive cosmological constant, is one of the most intriguing results in classical general relativity. These results follow rigorously from the Buchdahl inequalities in four-dimensional de Sitter space. In this work, we obtain the generalized Buchdahl inequalities in arbitrary space–time...

We consider the possibility of a gravitationally induced particle production through the mechanism of a nonminimal curvature–matter coupling. An interesting feature of this gravitational theory is that the divergence of the energy–momentum tensor is nonzero. As a first step in our study we reformulate the model in terms of an equivalent scalar–tensor theory, with two arbitrary...

We consider a description of the stochastic oscillations of the general relativistic accretion disks around compact astrophysical objects interacting with their external medium based on a generalized Langevin equation with colored noise and on the fluctuation–dissipation theorems. The former accounts for the general memory and retarded effects of the frictional force. The...

**Tiberiu** **Harko**
2
Francisco S. N. Lobo
1
M. K. Mak
0
0
Department of Computing and Information Management, Hong Kong Institute of Vocational Education
, Chai Wan,
Hong Kong, People's Republic of

We analyse the general relativistic oscillations of thin accretion discs around compact astrophysical objects interacting with the surrounding medium through non‐gravitational forces. The interaction with the external medium (a thermal bath) is modelled via a friction force and a random force, respectively. The general equations describing the stochastically perturbed discs are...