arXiv subjects

Nikolaos Dimakis

Publications and source records attributed to Nikolaos Dimakis.

12 recordsLinked to original sources

arXiv2026

Inhomogeneous Cosmologies with Bumpy Structure from Superfluid Pionic Vortices and the propagation of electromagnetic field

In this work we introduce exact cosmological solutions in General Relativity in (3+1) dimensions which take into account both the usual fluid component as well as a superfluid component describing superfluid Pionic vortices. Such superfluid vortices are source of inhomogeneities which can be treated exactly, no perturbation theory is required. These analytic solutions are relevant to obtain a non-perturbative description of inhomogeneities in cosmology. We discuss the peculiar effects of these hadronic inhomogeneities on the propagation of electromagnetic fields on such cosmological backgrounds analyzing possible observable effects. Finally, the properties that the effective superfluid satisfy for such solutions to exist are discussed, together with possible extensions of this family of solutions.

gr-qc↗

arXiv2026

Dirac-Bergmann algorithm and canonical quantization of $k$-essence cosmology

We develop a general canonical quantization scheme for $k$-essence cosmology in scalar-tensor theory. Utilizing the Dirac-Bergmann algorithm, we construct the Hamiltonian associated with the cosmological field equations and identify the first- and second-class constraints. The introduction of appropriate canonically conjugate variables with respect to Dirac brackets, allows for the canonical quantization of the model. In these new variables, the Hamiltonian constraint reduces to a quadratic function with no potential term. Its quantum realization leads to a Wheeler-DeWitt equation reminiscent of the massless Klein-Gordon case. As an illustrative example, we consider the action of a tachyonic field and investigate the conditions under which a phantom crossing can occur as a quantum tunneling effect. For the simplified constant potential case, we investigate the consequences of different boundary conditions on the singularity avoidance and to the mean expansion rate.

gr-qc↗

arXiv2025

Compact Stars in Symmetric Teleparallel Scalar-Tensor Gravity

We investigate the existence of static, spherically symmetric compact objects within the framework of symmetric teleparallel scalar-tensor gravity. This theory extends the Brans-Dicke and scalar-tensor models within the symmetric teleparallel formalism. We consider a nontrivial connection that allows for genuinely nontrivial solutions in the limit of General Relativity. The field equations admit a minisuperspace description and by applying the method of variational symmetries we construct the corresponding conservation laws in vacuum. The application of these conservation laws enables the reconstruction of analytic black-hole solutions. Finally, we study the interior structure of compact objects matched to an extremal Reissner-Nordstr\"{o}m exterior and show that the symmetric teleparallel scalar-tensor theory supports the existence of viable astrophysical objects.

gr-qc↗

arXiv2025

$f(R,\mathcal{G})$-cosmological dynamics in the FLRW background

We examine the cosmological dynamics of Einstein-Gauss-Bonnet gravity models in a four-dimensional spatially flat FLRW metric. These models are described by $f\left( R,\mathcal{G}\right) =f\left( R+\mu \mathcal{G}\right) $ theory of gravity. They are equivalent to models linear in the Ricci scalar $R$ and in the Gauss-Bonnet scalar $\mathcal{G}$ with one nonminimally coupled scalar field without kinetic term. We analyze the stability of the de Sitter solutions and construct the phase space of the field equations to investigate the cosmological evolution. We show that $f\left( R+\mu \mathcal{G}\right) $-theory provides a double inflationary epoch, this can be used to unify the early-time and late-time acceleration phases of the universe. Moreover, we discuss the initial value problem for theory to be cosmologically viable. Finally, the effects of the cold dark matter in cosmic evolution are discussed.

gr-qc↗

arXiv2025

Relativistic stars in $f(Q)$-gravity: Exact analytic solution for the power-law case $f(Q) = Q + b \: Q^\nu$

We investigate static spherically symmetric spacetimes within the framework of symmetric teleparallel $f(Q)$ gravity in order to describe relativistic stars. We adopt a specific ansatz for the background geometry corresponding to a singularity-free space-time. We obtain an expression for the connection, which allows the derivation of solutions for any $f(Q)$ theory in this context. Our approach aims to address a recurring error appearing in the literature, where even when a connection compatible with spherical symmetry is adopted, the field equation for the connection is systematically omitted and not checked if it is satisfied. For the stellar configuration, we concentrate on the power-law model $f(Q)=Q+\alpha Q_{0}\left( \frac{Q}{Q_{0}}\right) ^{\nu }$. The de Sitter-Schwarzschild geometry naturally emerges as an attractor beyond a certain radius, we thus utilize it as the external solution beyond the boundary of the star. We perform a detailed investigation of the physical characteristics of the interior solution, explicitly determining the mass function, analyzing the resulting gravitational fluid properties and deriving the angular and radial speed of sound.

gr-qc↗

arXiv2024

Quantum Mechanics from General Relativity and the Quantum Friedmann Equation

We demonstrate that the recently introduced linear equation, reformulating the first Friedmann equation, is the first-order WKB expansion of a quantum cosmological equation. This result shows a deeper underlying connection between General Relativity and Quantum Mechanics, pointing towards a unified framework. Solutions of this equation are built in terms of a scale factor encapsulating quantum effects on a free-falling particle. The quantum scale factor reshapes cosmic dynamics, resolving singularities at its vanishing points in several cases of interest. As an explicit example, we consider the radiation-dominated era and show that the quantum equation is dual to the one in Seiberg-Witten formulation, recently applied to black holes, and incorporates resurgence phenomena and complex metrics, as developed by Kontsevich, Segal, and Witten. This links to the invariance of time parametrization under $\Gamma(2)$ transformations of the dual wave function.

hep-th↗

arXiv2024

Static, spherically symmetric solutions in $f(Q)$-gravity and in nonmetricity scalar-tensor theory

We solve the gravitational field equations for a static, spherically symmetric spacetime within the framework of the symmetric teleparallel theory of gravity. Specifically, we derive new solutions within the context of power-law $f(Q)$ gravity and the nonmetricity scalar-tensor theory. For the connection in the non-coincidence gauge, we present the point-like Lagrangian that describes the employed field equations. Furthermore, we construct two conservation laws, and for different values of these conserved quantities, we analytically solve the gravitational field equations. New solutions are obtained, we investigate their physical properties and their general relativistic limit. Finally, we discuss the algebraic properties for the derived spacetimes.

gr-qc↗

arXiv2024

The solution space of a five-dimensional geometry: Kundt spacetimes and cosmological time-crystals

We uncover the solution space of a five dimensional geometry which we deem it as the direct counterpart of the Bianchi Type V cosmological model. We kinematically reduce the scale factor matrix and then, with an appropriate scaling and choice of time, we cast the spatial equations into a simple "Kasner" like form; thus revealing linear integrals of motion. Their number is enough so that, along with the quadratic constraint, it suffices to scan the entire space of solutions. The latter is revealed to be quite rich, containing cosmological solutions, some of which admit dimensional reduction asymptotically to four dimensions, Kundt spacetimes with vanishing type I (polynomial) curvature scalars and solutions describing periodic universes which behave like cosmological time crystals.

gr-qc↗

arXiv2023

Mapping Solutions in Nonmetricity Gravity: Investigating Cosmological Dynamics in Conformal Equivalent Theories

We investigate the impact of conformal transformations on the physical properties of solution trajectories in nonmetricity gravity. Specifically, we explore the phase-space and reconstruct the cosmological history of a spatially flat Friedmann-Lema\^{\i}tre-Robertson-Walker universe within scalar-nonmetricity theory in both the Jordan and Einstein frames. A detailed analysis is conducted for three different connections defined in both the coincident and non-coincident gauges. Our findings reveal the existence of a unique one-to-one correspondence for equilibrium points in the two frames. Furthermore, we demonstrate that solutions describing accelerated universes remain invariant under the transformation that relates these conformally equivalent theories.

gr-qc↗

arXiv2021

Crossing the phantom divide line as an effect of quantum transitions

We consider the Chiral cosmological model consisting of two scalar fields minimally coupled to gravity. In the context of a Friedmann--Lema\^ıtre--Robertson--Walker (FLRW) space-time, and for massless fields in the presence of a cosmological constant, we present the general solution of the field equations. The minisuperspace configuration that possesses maximal symmetry leads to scenarios which - depending on the admissible value of the parameters - correspond to a quintessence, quintom or phantom case. The canonical quantization of the model retrieves this distinction as different families of quantum states. The crossing of the phantom line is related to the existence of free or bound states for the Casimir operator of the symmetry algebra of the fields. The classical singularity, which is present in the quintessence solution, is also resolved at the quantum level.

gr-qc↗

arXiv2019

Bianchi IX cosmologies in the Einstein-Skyrme system in a sector with non-trivial topological charge

The dynamics of the most general Bianchi IX cosmology with three time dependent scale factors for the Einstein-Skyrme system is analyzed. For the Skyrmion, a generalized hedgehog ansatz with unit baryon charge is introduced. The most remarkable feature of this ansatz is that, in the above topologically non-trivial sector with unit topological charge, the Skyrme field equations are identically satisfied on any Bianchi IX metric. We will show that due to this feature the complete set of coupled Einstein-Skyrme field equations can be deduced from a suitable minisuperspace Lagrangian. The latter allows to perform a systematic study of the integrability properties of the Einstein-Skyrme system for the Bianchi IX cosmology. Moreover, some analytic and algebraic solutions for the Einstein-Skyrme model are derived. Another remarkable consequence of the present formalism is that it is possible to derive the Wheeler de-Witt equation for the Bianchi IX metric in the Einstein-Skyrme cosmology in which all the effects of the Skyrmion are encoded in an effective potential of the minisuperspace Lagrangian.

gr-qc↗

arXiv2017

Topologically non-trivial configurations in the 4d Einstein--nonlinear $σ$-model system

We construct exact, regular and topologically non-trivial\ configurations of the coupled Einstein-nonlinear sigma model in (3+1) dimensions. The ansatz for the nonlinear $SU(2)$ field is regular everywhere and circumvents Derrick's theorem because it depends explicitly on time, but in such a way that its energy-momentum tensor is compatible with a stationary metric. Moreover, the $SU(2)$ configuration cannot be continuously deformed to the trivial Pion vacuum as it possesses a non-trivial winding number. We reduce the full coupled 4D Einstein nonlinear sigma model system to a single second order ordinary differential equation. When the cosmological constant vanishes, such master equation can be further reduced to an Abel equation. Two interesting regular solutions correspond to a stationary traversable wormhole (whose only \textquotedblleft exotic matter" is a negative cosmological constant) and a (3+1)-dimensional cylinder whose (2+1)-dimensional section is a Lorentzian squashed sphere. The Klein-Gordon equation in these two families of spacetimes can be solved in terms of special functions. The angular equation gives rise to the Jacobi polynomials while the radial equation belongs to the Poschl-Teller family. The solvability of the Poschl-Teller problem implies non-trivial quantization conditions on the parameters of the theory.

hep-th↗