arXiv subjects

Aurel Bejancu

Publications and source records attributed to Aurel Bejancu.

4 recordsLinked to original sources

arXiv2016

On The splitting of the Einstein field equations with respect to a general $(1 + 3)$ threading of spacetime

Based on a general $(1+3)$ threading of the spacetime $(M,g)$, we obtain a new and simple splitting of a both the Einstein field equations (EFE) and the conservation laws in $(M,g)$. As an application we obtain the splitting of (EFE) in an almost FLRW universe with energy-momentum tensor of a perfect fluid. In particular, we state the perturbation Friedman equations in an almost FLRW universe.

math.DG↗

arXiv2015

Equations of Motion with Respect to the $(1+1+3)$ Threading of a $5D$ Universe

We continue our research work started in "Kinematic Quantities and Raychaudhuri Equations in a $5D$ Universe" (Eur. Phys. J. C, 2015), and obtain in a covariant form, the equations of motion with respect to the $(1+1+3)$ threading of a $5D$ universe $(\bar{M}, \bar{g})$. The natural splitting of the tangent bundle of $\bar{M}$ leads us to the study of three categories of geodesics: spatial geodesics, temporal geodesics and vertical geodesics. As an application of the general theory, we introduce and study what we call the $5D$ Robertson-Walker universe.

math.DG↗

arXiv2015

Kinematic Quantities and Raychaudhuri Equations in a $5D$ Universe

Based on some ideas emerged from the classical Kaluza-Klein theory, we present a $5D$ universe as a product bundle over the $4D$ spacetime. This enables us to introduce and study two categories of kinematic quantities (expansions, shear, vorticity) in a $5D$ universe. One category is related to the fourth dimension (time), and the other one comes from the assumption of the existence of the fifth dimension. The Raychaudhuri type equations that we obtain in the paper, lead us to results on the evolution of both the $4D$ expansion and $5D$ expansion in a $5D$ universe.

gr-qc↗

arXiv2015

On the (1+3) threading of spacetime with respect to an arbitrary timelike vector field

We develop a new approach on the (1+3) threading of spacetime $(M, g)$ with respect to a congruence of curves defined by an arbitrary timelike vector field. The study is based on spatial tensor fields and on the Riemannian spatial connection $\nabla^{\star}$, which behave as $3D$ geometric objects. We obtain new formulas for local components of the Ricci tensor field of $(M, g)$ with respect to the threading frame field, in terms of the Ricci tensor field of $\nabla^{\star}$ and of kinematic quantities. Also, new expressions for time covariant derivatives of kinematic quantities are stated. In particular, a new form of Raychaudhuri's equation enables us to prove Lemma 6.2, which completes a well known lemma used in the proof of Penrose-Hawking singularity theorems.Finally, we apply the new $(1+3)$ formalism to the study of the dynamics of a Kerr-Newman black hole.

math.DG↗