arXiv subjects

Dimitrios Trachilis

Publications and source records attributed to Dimitrios Trachilis.

9 recordsLinked to original sources

arXiv2026

Quasi-isotropic Asymptotic Expansions in Varying Speed of Light Cosmologies

We generalize the quasi-isotropic solution of the Einstein equations near a cosmological initial singularity to models with a varying speed of light and/or a varying gravitational 'constant'. We construct a formal asymptotic series expansion in a synchronous reference system, taking into account the additional degrees of freedom introduced by the two varying 'constants'. By applying the Landau-Lifshitz quasi-isotropic method, we go beyond the standard results of previous studies that based on the Friedmann-Lema\^itre-Robertson-Walker (FLRW) model, providing a more general asymptotic analysis of varying speed of light cosmologies. We show that the resulting solutions do not contain the required number of arbitrary functions to qualify as general solutions of these theories.

gr-qc↗

arXiv2026

Asymptotic analysis of varying light speed cosmologies

We investigate potential singularities in isotropic cosmological models featuring a time-varying speed of light and the gravitational constant. The governing field equations generally simplify into two-dimensional systems, subject to analysis through dynamical systems techniques within phase space and the application of asymptotic splitting methods. In the broader context, our findings reveal the existence of initially expanding closed models that undergo subsequent recollapse towards a future singularity, and open universes exhibiting perpetual expansion into the future. The specific characteristics of these singularities are also expounded upon.

gr-qc↗

arXiv2023

Sudden Shock Waves in modified gravity

We construct a generic asymptotic solution for modified gravity near a sudden singularity. This solution contains a fluid source with no equation of state and is function-counting stable, that is it has eleven independent arbitrary functions of the spatial coordinates as dictated by the Cauchy problem of the theory. We further show that near the sudden singularity the solution has a shock wave character with the same number of free functions in the Jordan and Einstein frame.

gr-qc↗

arXiv2022

The conformal cosmological potential

We discuss qualitative features of the conformal relation between certain classes of gravity theories and general relativity, common to different themes such as $f(R)$, Brans-Dicke-type, and string theories. We focus primarily on the frame relations of the fields involved, slice energy, traceless and Palatini extensions, and selected cosmological applications.

gr-qc↗

arXiv2021

The radiation instability in modified gravity

We study the problem of the instability of inhomogeneous radiation universes in quadratic lagrangian theories of gravity written as a system of evolution equations with constraints. We construct formal series expansions and show that the resulting solutions have a smaller number of arbitrary functions than that required in a general solution. These results continue to hold for more general polynomial extensions of general relativity.

gr-qc↗

arXiv2020

The Generic Sudden Singularity in Brans-Dicke Theory

We construct a formal asymptotic series expansion for a general solution of the Brans-Dicke equations with a fluid source near a sudden singularity. This solution contains eleven independent arbitrary functions of the spatial coordinates as required by the Cauchy problem of the theory. We show that the solution is geodesically complete and has the character of a shock wave in the sudden asymptotic region. This solution is weak in the senses of Tipler and Krolak as in the corresponding case of general relativity.

gr-qc↗

arXiv2019

The Generic Expansion in Analytic Modified Gravity

In this Thesis, we treat the problem of the existence of generic perturbations of the regular and singular state in higher-order gravity in cases of vacuum and radiation models that derives from the lagrangian $R+εR^2$. We show that there is a regular state of the theory in vacuum in the form of a formal series expansion having the same number of free functions as those required for a general solution of the theory, while this is not true for the case of radiation. This means that there exists an open set in the space of analytic initial data of the theory in vacuum that leads to a regular solution having the correct number of free functions to qualify as a general solution. Further, we show that a singular state of the theory in vacuum cannot be admitted, while in the case of radiation we obtain a particular solution.

gr-qc↗

arXiv2016

The regular state in higher-order gravity

We consider the higher-order gravity theory derived from the quadratic lagrangian $R+εR^2$ in vacuum as a first-order (ADM-type) system with constraints, and build time developments of solutions of an initial value formulation of the theory. We show that all such solutions, if analytic, contain the right number of free functions to qualify as general solutions of the theory. We further show that any regular analytic solution which satisfies the constraints and the evolution equations can be given in the form of an asymptotic formal power series expansion.

gr-qc↗

arXiv2013

Generic regular universes in higher order gravity theories

We review recent results on the Cauchy-Kowalevsky structure of theories with higher derivatives in vacuum. We prove genericity of regularity of solutions under the assumption of analyticity. Our approach is framed in the general context of formal series expansions of the metric around a regular point.

gr-qc↗