Irfan Acikgoz

Extended scalar–tensor theory and thermodynamics in teleparallel framework

We present here a new modified gravitation theory for the galactic dark energy effect by using a general Lagrangian density which is represented by an arbitrary function f(T, ϕ, X) where T describes the torsion scalar in teleparallel gravity while X shows the kinetic scalar field energy. While the function is in general form, once reduced, the model can be transformed into some of the other well-known gravitation theories. After deriving the corresponding field equations and considering the flat Friedmann–Robertson–Walker type universe which is filled with ordinary cosmic matter, we discuss both the non-equilibrium and equilibrium profiles of galactic thermodynamics. We find that there exists an equilibrium picture of thermodynamics. Additionally, we also generalize ordinary f(T, ϕ, X) model’s action to the case in which there exists an interaction between the chameleon and scalar fields.

A unified picture of cosmological entropy on apparent horizon in F(R, G) gravity

In this study, the validity of the generalized second law of thermodynamics (GSLT) has been investigated in F(R, G) gravity. We consider that the boundary of the universe is surrounded by an apparent horizon in the spatially flat Friedmann–Robertson–Walker (FRW) universe, and we take into account the Hawking temperature on the horizons. The unified solutions of the field equations corresponding to gravity theory have been applied to the validity of the GSLT frame, and in this way, both the solutions have been verified and all the expansion history of the universe has been shown in a unified picture.

Galactic entropy in extended Kaluza–Klein cosmology

We use a Kaluza–Klein model with variable cosmological and gravitational terms to discuss the nature of galactic entropy function. For this purpose, we assume a universe filled with dark fluid and consider five-dimensional (5D) field equations using the Gamma law equation. We mainly discuss the validity of the first and generalized second laws of galactic thermodynamics for viable Kaluza–Klein models.

Polytropic Gas and Gravitational Thermodynamics

We mainly assume that our universe is locally rotationally symmetric (LRS) Bianchi-type II and is filled with a combination of a polytropic gas and baryonic matter. Also, we consider the polytropic gas scenario as the unification of dark matter and dark energy. By making use of these assumptions, we investigate the first and second laws of gravitational thermodynamics on the apparent horizon.

Finding Dirac Spin Effect in NUT Spacetime

In the present study, we are interested in finding the spin precession of a Dirac particle in expanding and rotating NUT spacetime. A tetrad with two functions to be determined is applied to the field equation of the teleparallel theory of gravity via a coordinate transformation. The vector, the axial-vector and the tensor parts of the torsion tensor are obtained. We found that the vector parts are in the radial and -directions. The axial-vector torsion is along r-direction while its other components along θ and -directions vanish everywhere. The vector connected with Dirac spin has been evaluated as well.

Energy Associated with the Gibbons-Maeda Dilaton Spacetime

In order to obtain energy and momentum (due to matter and fields including gravitation) distributions of the Gibbons-Maeda dilaton spacetime, we use the Møller energy-momentum prescription both in Einsteins theory of general relativity and teleparallel gravity. We find the same energy distribution for a given metric in both of these different gravitation theories. Under two limits, we also calculate energy associated with two other models such as the Garfinkle-Horowitz-Strominger dilaton spacetime and the Reissner-Nordstrom spacetime. The energy obtained is also independent of the teleparallel dimensionless coupling constant, which means that it is valid in any teleparallel model. Our result also sustains (a) the importance of the energy-momentum definitions in the evaluation of the energy distribution for a given spacetime and (b) the viewpoint of Lessner that the Møller energy-momentum complex is a powerful concept of energy and momentum (c) the hypothesis of Vagenas that there is a connection between the coefficients of the energy-momentum expression of Einstein and those of Møller.

f(T,R) theory of gravity

We mainly focus on the idea that the dynamics of the whole universe may be understood by making use of torsion T and curvature R at the same time. The f(T,R)-gravity can be considered as a fundamental gravitational theory describing the evolution of the universe. The model can produce the unification of the general relativity (GR), teleparallel gravity (TPG), f(R)-gravity and f(T)-gravity theories. For this purpose, the corresponding Lagrangian density is written in terms of an arbitrary function of the torsion and curvature scalars. Furthermore, we use the absence/existence puzzle of relativistic neutron stars and thermodynamical laws as constraining tools for the new proposal.

Black holes, wormholes and the Hamiltonian approach in the framework of teleparallel gravity

In order to elaborate the conserved energy distribution (due to matter and fields including gravity) associated with some black hole and wormhole models we investigate a general space–time in the framework of teleparallel gravity using the Hamiltonian approach. After performing the calculations to obtain the energy distributions generally, we consider four different specific space–times (two of which are black holes and the others are wormholes). Since, the general relativity versions of the calculations for the Vaidya black holes and conformal scalar dyon black holes have been done previously in the literature, we compare our results with both. We see that the results are the same and agree with each other. In this way, the calculations give consistency with the results of different formulations, both in general relativity and teleparallel gravity.

Effective Hamiltonian for non-minimally coupled scalar fields

In the post Newtonian limit, a non-relativistic Hamiltonian is derived for scalar fields with quartic self-interaction and non-minimal coupling to the curvature scalar of the background spacetime. These effects are found to contribute to the non-relativistic Hamiltonian by adding nonlinearities and by modifying the gravitational Darwin term. As we discuss briefly in the text, the impact of these novel structures can be sizable in dense media like neutron star core, and can have observable signatures in phase transitions, for example.

Gödel-type spacetimes in f(R)-gravity

We focus on one of the famous problems in theoretical physics today: the problem of energy-momentum localization. Although many authors have endeavoured to solve this problem, it has remained unsolved until now. In this work, we consider the generalized version of the Landau-Lifshitz definition in f(R)-Gravity to discuss the energy-momentum localization problem in Gödel-type metrics. We also take into account five popular f(R) models to obtain specific results.