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Cetin Senturk

Publications and source records attributed to Cetin Senturk.

7 recordsLinked to original sources
arXiv2024

Minimal Einstein-Aether Theory

We show that there is a phenomenologically and theoretically consistent limit of the generic Einstein-Aether theory in which the Einstein-Aether field equations reduce to Einstein field equations with a perfect fluid distribution sourced by the aether field. This limit is obtained by taking three of the coupling constants of the theory to be zero but keeping the expansion coupling constant to be nonzero. We then consider the further reduction of this limited version of Einstein-Aether theory by taking the expansion of the aether field to be constant (possibly zero), and thereby we introduce the Minimal Einstein-Aether theory that supports the Einstein metrics as solutions with a reduced cosmological constant. The square of the expansion of the unit-timelike aether field shifts the bare cosmological constant and thus provides, via local Lorentz symmetry breaking inherent in the Einstein-Aether theories, a novel mechanism for reconciling the observed, small cosmological constant (or dark energy) with the large theoretical prediction coming from quantum field theories. The crucial point here is that minimal Einstein-Aether theory does not modify the well-tested aspects of General Relativity such as solar system tests and black hole physics including gravitational waves.

gr-qc↗
arXiv2021

Kerr-Schild-Kundt Metrics in Generic Gravity Theories with Modified Horndeski Couplings

The Kerr-Schild-Kundt (KSK) metrics are known to be one of the universal metrics in general relativity, which means that they solve the vacuum field equations of any gravity theory constructed from the curvature tensor and its higher-order covariant derivatives. There is yet no complete proof that these metrics are universal in the presence of matter fields such as electromagnetic and/or scalar fields. In order to get some insight into what happens when we extend the "universality theorem" to the case in which the electromagnetic field is present, as a first step, we study the KSK class of metrics in the context of Modified Horndeski theories with Maxwell's field. We obtain exact solutions of these theories representing the $pp$-waves and AdS-plane waves in arbitrary $D$ dimensions.

gr-qc↗
arXiv2019

NAT Black Holes

We study some physical properties of black holes in Null Aether Theory (NAT)--a vector-tensor theory of gravity. We first review the black hole solutions in NAT and then derive the first law of black hole thermodynamics. The temperature of the black holes depends on both the mass and the NAT \textquotedblleft charge" of the black holes. The extreme cases where the temperature vanishes resemble the extreme Reissner-Nordström black holes. We also discuss the contribution of the NAT charge to the geodesics of massive and massless particles around the NAT black holes.

gr-qc↗
arXiv2019

A Modified Gravity Theory: Null Aether

General quantum gravity arguments predict that Lorentz symmetry might not hold exactly in nature. This has motivated much interest in Lorentz breaking gravity theories recently. Among such models are vector-tensor theories with preferred direction established at every point of spacetime by a fixed-norm vector field. The dynamical vector field defined in this way is referred to as the \textquotedblleft aether." In this paper, we put forward the idea of a \textit{null} aether field and introduce, for the first time, the Null Aether Theory (NAT)--a vector-tensor theory. We first study the Newtonian limit of this theory and then construct exact spherically symmetric black hole solutions in the theory in four dimensions, which contain Vaidya-type non-static solutions and static Schwarzschild-(A)dS type solutions, Reissner-Nordström-(A)dS type solutions and solutions of conformal gravity as special cases. Afterwards, we study the cosmological solutions in NAT: We find some exact solutions with perfect fluid distribution for spatially flat FLRW metric and null aether propagating along the $x$ direction. We observe that there are solutions in which the universe has big-bang singularity and null field diminishes asymptotically. We also study exact gravitational wave solutions--AdS-plane waves and $pp$-waves--in this theory in any dimension $D\geq3$. Assuming the Kerr-Schild-Kundt class of metrics for such solutions, we show that the full field equations of the theory are reduced to two, in general coupled, differential equations when the background metric assumes the maximally symmetric form. The main conclusion of these computations is that the spin-0 aether field acquires a \textquotedblleft mass" determined by the cosmological constant of the background spacetime and the Lagrange multiplier given in the theory.

gr-qc↗
arXiv2016

Godel Type Metrics in Einstein-Aether Theory II: Nonflat Background in Arbitrary Dimensions

It was previously proved that the Gödel-type metrics with flat three-dimensional background metric solve exactly the field equations of the Einstein-Aether theory in four dimensions. We generalize this result by showing that the stationary Gödel-type metrics with nonflat background in $D$ dimensions solve exactly the field equations of the Einstein-Aether theory. The reduced field equations are the $(D-1)$-dimensional Euclidean Ricci-flat and the $(D-1)$-dimensional source-free Maxwell equations, and the parameters of the theory are left free except $c_{1}-c_{3}=1$. We give a method to produce exact solutions of the Einstein-Aether theory from the Gödel-type metrics in $D$ dimensions. By using this method, we present explicit exact solutions to the theory by considering the particular cases: ($D-1$)-dimensional Euclidean flat, conformally flat, and Tangherlini backgrounds.

gr-qc↗
arXiv2012

Energy and Angular Momentum in Generic F(Riemann) Theories

We construct the conserved charge of generic gravity theories built on arbitrary contractions of the Riemann tensor (but not on its derivatives) for asymptotically (anti)-de Sitter spacetimes. Our construction is a generalization of the ADT charges of linear and quadratic gravity theories in cosmological backgrounds. As an explicit example we find the energy and angular momentum of the BTZ black hole in the 2+1 dimensional Born-Infeld gravity.

hep-th↗