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Tekin Dereli

Publications and source records attributed to Tekin Dereli.

At least 37 records · Page 2Linked to original sources
arXiv2021

$Λ$CDM cosmology with a quiescent anisotropy in a higher dimensional steady state universe

In this work, which follows a series of studies on the higher-dimensional steady state universe idea and prepared for Professor Tekin Dereli's Festschrift, we show the influence of the dynamical internal (unobservable) space on the evolution of the possible anisotropy of the external (observable) space. We obtain mathematically exactly the same Friedmann equation of the standard $Λ$CDM model for the external space, but with some remarkable physical differences. In particular, the higher-dimensional negative cosmological constant plays the role of the four-dimensional positive cosmological constant and the expansion anisotropy, viz., the shear scalar, of the external space mimics a negative cosmological constant; it would mimic a stiff fluid when allowed on top of the standard $Λ$CDM model. This latter feature gives us the opportunity to manipulate the CMB quadrupole temperature fluctuation, suggesting a possible answer to the fact that its observed value is lower than that predicted by the standard $Λ$CDM model.

gr-qc↗
arXiv2021

'Anti-Commutable' Pre-Leibniz Algebroids and Admissible Connections

The concept of algebroid is convenient for constructions of geometrical frameworks. For example, metric-affine and generalized geometries can be written on Lie and Courant algebroids, respectively. Furthermore, string theories might make use of many other algebroids such as metric algebroids, higher-Courant algebroids, or conformal Courant algebroids. Working on the possibly most general algebroid structure is fruitful as it creates a chance to study all of them at once. Local pre-Leibniz algebroids are such general ones in which metric-connection geometries are possible to construct. Yet, the existence of the locality operator necessitates the modification of torsion and curvature operators to achieve tensorial quantities. In this paper, this modification is explained from the point of view that the modification is applied to the bracket instead. This leads one to consider `anti-commutable' pre-Leibniz algebroids whose bracket satisfies a property defined with respect to a choice of an equivalence class of connections. These `admissible' connections are claimed to be the necessary ones for a geometry on algebroids because one can prove many desirable properties for them. For instance, we prove the first and second Bianchi identities, Ricci identity, Cartan structure equations, the construction of Levi-Civita connections, the decomposition of connection in terms of torsion and non-metricity. These all are possible because the modified bracket becomes anti-symmetric for an admissible connection so that one can apply the machinery of almost- or pre-Lie algebroids. We investigate various algebroid structures from the literature and show that they admit admissible connections which are metric-compatible in some generalized sense. Moreover, we prove that local pre-Leibniz algebroids that are not anti-commutable cannot be equipped with a torsion-free, and in particular Levi-Civita, connection.

math.DG↗
arXiv2021

Bloch waves and Non-commutative Tori of Magnetic Translations

We review the Landau problem of an electron in a constant uniform magnetic field. The magnetic translations are the invariant transformations of the free Hamiltonian. A Kähler polarization of the plane has been used for the geometric quantization. Under the assumption of quasi-periodicity of the wavefunction the magnetic translations in the Bravais lattice generate a non-commutative quantum torus. We concentrate on the case when the magnetic flux density is a rational number. The Bloch wavefunctions form a finite-dimensional module of the noncommutative torus of magnetic translations as well as of its commutant which is the non-commutative torus of magnetic translation in the dual Bravais lattice. The bi-module structure of the Bloch waves is shown to be the connecting link between two Morita equivalent non-commutative tori.

math-ph↗
arXiv2021

Non-Minimally Coupled Einstein-Yang-Mills Field Equations and Wu-Yang Monopoles in Bertotti-Robinson Spacetimes

Bertotti-Robinson spacetimes are topologically $AdS_2 \times S^2$ and described by a conformally flat metric. Together with the Coulomb electric potential, they provide a class of static, geodetically complete Einstein-Maxwell solutions. We show here that the Bertotti-Robinson metric together with Wu-Yang magnetic pole potentials give a class of static solutions of a system of non-minimally coupled Einstein-Yang-Mills equations that may be relevant for investigating vacuum polarization effects in a first order perturbative approach to quantum fields.

gr-qc↗
arXiv2020

The Variational Field Equations of Cosmological Topologically Massive Supergravity

We discuss the formulation of cosmological topologically massive (simple) supergravity theory in three-dimensional Riemann-Cartan space-times. We use the language of exterior differential forms and the properties of Majorana spinors on 3-dimensional space-times to explicitly demonstrate the local supersymmetry of the action density involved. Coupled field equations that are complete in both of their bosonic and fermionic sectors are derived by a first order variational principle subject to a torsion-constraint imposed by the method of Lagrange multipliers. We compare these field equations with the partial results given in the literature using a second order variational formalism.

gr-qc↗
arXiv2020

Electric Charge Screening By Non-Minimal Couplings Of Electromagnetic Fields To Gravity

A case of non-minimal couplings between gravity and electromagnetic fields is presented. The field equations are worked out in the language of exterior differential forms. An exact charge screening class of solutions is given with a specific discussion on the polarisation and magnetisation of spacetime. The consequences of non-minimal couplings to gravity are examined.

gr-qc↗
arXiv2020

Metric-Connection Geometries on Pre-Leibniz Algebroids: A Search for Geometrical Structure in String Models

The metric-affine and generalized geometries, respectively, are arguably the appropriate mathematical frameworks for Einstein's theory of gravity and the low-energy effective massless oriented closed bosonic string field theory. In fact, mathematical structures in a metric-affine geometry are written on the tangent bundle, which is itself a Lie algebroid; whereas those in generalized geometries introduced as the basis of double field theories, are written on Courant algebroids. The Lie, Courant and the higher Courant algebroids used in exceptional field theories, are all special cases of pre-Leibniz algebroids. Provided with some additional ingredients, the construction of such geometries can all be carried over to regular pre-Leibniz algebroids. We define below the notions of locality structures and locality projectors, which are some such necessary ingredients. In terms of these structures, $E$-metric-connection geometries are constructed with (possibly) a minimum number of assumptions. Certain small gaps in the literature are also filled as we go along. $E$-Koszul connections, as a generalization of Levi-Civita connections, are going to be defined and shown to be helpful for some results including a simple generalization of the fundamental theorem of Riemannian geometry. We also show that metric-affine geometries can be constructed in a unique way as special cases of $E$-metric-connection geometries. Moreover, generalized geometries are shown to follow as special cases, and various properties of linear generalized-connections are proven in the present framework. Similarly, uniqueness of the locality projector in the case of exact Courant algebroids is proven; a result that explains why the curvature operator, defined with a projector in the double field theory literature is a necessity.

hep-th↗
arXiv2020

A non-minimally coupled, conformally extended Einstein-Maxwell theory of pp-waves

A non-minimal coupling of Weyl curvatures to electromagnetic fields is considered in Brans-Dicke-Maxwell theory. The gravitational field equations are formulated in a Riemannian spacetime where the spacetime torsion is constrained to zero by the method of Lagrange multipliers in the language of exterior differential forms. The significance and ramifications of non-minimal couplings to gravity are examined in a pp-wave spacetime.

gr-qc↗
arXiv2019

Gravitational Plane Waves in a Non-Riemannian Description of Brans-Dicke Gravity

The variational field equations of Brans-Dicke scalar-tensor theory of gravitation are given in a non-Riemannian setting in the language of exterior differential forms over 4-dimensional spacetime. The class of pp-wave metrics together with the Brans-Dicke scalar field are used to derive the geodesic and autoparallel equations of motion for non-spinning test masses in Brinkmann and Rosen coordinates. The effects of the gradient of the scalar field on the auto-parallel equations of motion and Jacobi deviation equation are discussed.

gr-qc↗
arXiv2019

Dark range of $ω$ in Brans-Dicke gravity

The variational field equations of Brans-Dicke scalar-tensor theory of gravitation are presented in a Riemannian and non-Riemannian setting in the language of exterior differential forms over 4-dimensional spacetime. In Rosen coordinates, the equations of motion of non-spinning test masses are considered in a gravitational plane wave setup and detailed to interpret a scalar field $α$ as a candidate for dark energy for negative values of the Brans-Dicke parameter $-3/2 < ω< 0$ .

gr-qc↗
arXiv2019

Kaluza-Klein Reduction of the 6 Dimensional \\ Dirac Equation on $\mathbb{S}^3 \cong SU(2)$ and \\ Non-abelian Topological Insulators

In this work, the Kaluza-Klein reduction of the Dirac equation on a 6 dimensional spacetime $\mathbb{M}^{1+5} := \mathbb{M}^{1+2} \times \mathbb{S}^3$ is studied. Because of the group structure on $\mathbb{S}^3$, $\mathbb{M}^{1+5}$ can be seen as a principal $SU(2)$ bundle over the model Lorentzian spacetime $\mathbb{M}^{1+2}$. The dimensional reduction induces non-minimal $SU(2)$ couplings to the theory on $\mathbb{M}^{1+2}$. These interaction terms will be investigated by comparing with a minimally $SU(2)$ coupled Dirac equation on $\mathbb{M}^{1+2}$. We hope that these results may help us to understand non-abelian interactions of topological insulators.

math-ph↗
arXiv2019

Acoustic Superradiance from a Bose-Einstein Condensate Vortex with a Self-Consistent Background Density Profile

The axisymmetric acoustic perturbations in the velocity potential of a Bose-Einstein condensate in the presence of a single vortex behave like minimally coupled massless scalar fields propagating in a curved (1+1) dimensional Lorentzian space-time, governed by the Klein-Gordon wave equation. Thus far, the amplified scattering of these perturbations from the vortex, as a manifestation of the acoustic superradiance, has been investigated with a constant background density. This paper goes beyond by employing a self-consistent condensate density profile that is obtained by solving the Gross-Pitaevskii equation for an unbound BEC. Consequently, the loci of the event horizon and the ergosphere of the acoustic black hole are modified according to the radially varying speed of sound. The superradiance is investigated both for transient features in the time-domain and for spectral features in the frequency domain. In particular, an effective energy-potential function defined in the spectral formulation correlates with the existence and the frequency dependence of the acoustic superradiance. The numerical results indicate that the constant background density approximation underestimates the maximum superradiance and the frequency at which this maximum occurs.

cond-mat.quant-gas↗
arXiv2018

Non-Riemannian Description of Robinson-Trautman Spacetimes in Brans-Dicke Theory Of Gravity

The variational field equations of Brans-Dicke scalar-tensor theory of gravitation are given in a non-Riemannian setting in the language of exterior differential forms over 4-dimensional spacetimes. A conformally re-scaled Robinson-Trautman metric together with the Brans-Dicke scalar field are used to characterise algebraically special Robinson-Trautman spacetimes. All the relevant tensors are worked out in a complex null basis and given explicitly in an appendix for future reference. Some special families of solutions are also given and discussed.

gr-qc↗
arXiv2018

Analog Black Holes and Energy Extraction by Super-Radiance from Bose Einstein Condensates (BEC) with Constant Density

This paper presents a numerical study of the acoustic superradiance from the single vortex state of a Bose-Einstein condensate (BEC). The draining bathtub model of an incompressible barotropic fluid is adopted to describe the vortex. The propagation of the velocity potential fluctuations are governed by the massless scalar Klein-Gordon wave equation, which establishes the rotating black-hole analogy. Hence, the amplified scattering of these fluctuations from the vortex comprise the superradiance effect. Particular to this study, a coordinate transformation is applied which enables the identification of the event horizon and the ergosphere termwise in the metric. Thus, the respective spectral solutions can be obtained asymptotically at either boundary. Further, the time-domain calculations of the energy of the propagating perturbations and the independently performed reflection coefficient calculations from the asymptotic solutions of the propagating perturbations are shown to be in very good agreement. While the former solution provides the full dynamical behavior of the superradiance, the latter method gives the frequency spectrum of the superradiance as a function of the rotational frequency of the vortex. Hence, a comprehensive analysis of the superradiance effect can be conducted within this workframe.

cond-mat.other↗
arXiv2018

Hamilton-Jacobi Formulation of the Thermodynamics of Einstein-Born-Infeld-AdS Black Holes

A Hamilton-Jacobi formalism for thermodynamics was formulated by Rajeev [Ann. Phys. 323, 2265 (2008)] based on the contact structure of the odd dimensional thermodynamic phase space. This allows one to derive the equations of state of a family of substances by solving a Hamilton-Jacobi equation (HJE). In the same work it was applied to chargeless non-rotating black holes, and the use of Born-Infeld electromagnetism was proposed to apply it to charged black holes as well. This paper fulfills this suggestion by deriving the HJE for charged non-rotating black holes using Born-Infeld theory and a negative cosmological constant. The most general solution of this HJE is found. It is shown that there exists solutions which are distinct from the equations of state of the Einstein-Born-Infeld-AdS black hole. The meaning of these solutions is discussed.

gr-qc↗