arXiv subjects

Konstantinos F. Dialektopoulos

Publications and source records attributed to Konstantinos F. Dialektopoulos.

39 records · Page 3Linked to original sources

arXiv2016

Noether Symmetries in Gauss-Bonnet-teleparallel cosmology

A generalized teleparallel cosmological model, $f(T_\mathcal{G},T)$, containing the torsion scalar $T$ and the teleparallel counterpart of the Gauss-Bonnet topological invariant $T_{\mathcal{G}}$, is studied in the framework of the Noether Symmetry Approach. As $f(\mathcal{G}, R)$ gravity, where $\mathcal{G}$ is the Gauss-Bonnet topological invariant and $R$ is the Ricci curvature scalar, exhausts all the curvature information that one can construct from the Riemann tensor, in the same way, $f(T_\mathcal{G},T)$ contains all the possible information directly related to the torsion tensor. In this paper, we discuss how the Noether Symmetry Approach allows to fix the form of the function $f(T_\mathcal{G},T)$ and to derive exact cosmological solutions.

gr-qc↗

arXiv2016

Large scale structures and the cubic galileon model

The maximum size of a bound cosmic structure is computed perturbatively as a function of its mass in the framework of the cubic galileon, proposed recently to model the dark energy of our Universe. Comparison of our results with observations constrains the matter-galileon coupling of the model to $0.033\lesssim α\lesssim 0.17$, thus improving previous bounds based solely on solar system physics.

gr-qc↗

arXiv2015

Can Brans-Dicke theory with $Λ>0$ describe stars?

A step-by-step approach is followed to study cosmic structures in the context of Brans-Dicke theory with positive cosmological constant $Λ$ and parameter $ω$. First, it is shown that regular stationary black-hole solutions not only have constant Brans-Dicke field $ϕ$, but can exist only for $ω=\infty$, which forces the theory to coincide with the General Relativity. Generalizations of the theory in order to evade this black-hole no-hair theorem are presented. It is also shown that in the absence of a stationary cosmological event horizon in the asymptotic region, a stationary black hole horizon can support a non-trivial Brans-Dicke hair. Even more importantly, it is shown next, that the presence of a stationary cosmological event horizon rules out any regular stationary solution, appropriate for the description of a star. Thus, to describe a star one has to assume that there is no such stationary horizon in the faraway asymptotic region. Under this implicit assumption generic spherical cosmic structures are studied perturbatively and is shown that only for $ω>0$ or $ω\lesssim -5$ their predicted maximum sizes are consistent with observations. We also point out how, many of the conclusions of this work differ qualitatively from the $Λ=0$ spacetimes.

gr-qc↗