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Stefano Vignolo

Publications and source records attributed to Stefano Vignolo.

At least 19 recordsLinked to original sources
arXiv2026

Dirac fields in LRS III spacetimes: a dynamical systems analysis

Within the $1+1+2$ covariant formalism, we investigate Locally Rotationally Symmetric (LRS) class III spacetimes sourced by a self-gravitating Dirac field expressed in polar form. By recasting the covariant field equations into an autonomous dynamical system, we perform a phase-space analysis of their solutions. We show that the cosmological evolution asymptotically converges to a contracting Bianchi I spacetime, a result also supported by numerical integrations of the system.

gr-qc↗
arXiv2026

A covariant approach to the Dirac field in LRS space-times: the case of coplanar frames

We employ the polar decomposition of the Dirac field to describe it as an effective spinorial fluid. We then construct a $(1+1+2)$ covariant formalism for the Dirac field that avoids the introduction of tetrad fields and Clifford matrices. Within this framework, we analyze the conditions under which a self-gravitating Dirac field can be consistently embedded in Locally Rotationally Symmetric (LRS) space-times of types I, II, and III. In accordance with the LRS symmetry requirements, we extend a previous work by assuming that the velocity and spin vector fields of the Dirac field lie in the planes defined pointwise by the generators of the time-like and space-like congruences, which underlie the $(1+1+2)$ decomposition. We present some analytical and numerical solutions to illustrate the applicability of the proposed framework.

gr-qc↗
arXiv2026

A note on spinor fields in spherical symmetry

By employing the polar re-formulation, we show that there are no solutions of the Dirac equations in spherical symmetry when the spinor is required to satisfy the same symmetries as the space-time via the Lie derivative.

math-ph↗
arXiv2025

Quantized Dirac Fields in torsionful gravity: cosmological implications and links with the dark universe

We consider a classical field in square torsion theory as a source of torsion for a quantum fermion field in FLRW metric. In the framework of QFT, we obtain vacuum contributions to the energy-momentum tensor and to the axial current that modify the dynamics of the classical field and the field equations as back-reaction. These contributions lead to a modified classical field and therefore to a modified torsion term $L^\mu$ and expectation value of energy-momentum tensor $T^{\mu\nu}$ on the quantum vacuum, altering the field equations in an interative process. We consider the first step of this process and we find that the vacuum condensate could affect the inflationary phase of the Universe. Higher order terms could impact the dark Universe.

hep-th↗
arXiv2025

A covariant approach to the Dirac field in LRS space-times

We use the polar decomposition to describe the Dirac field in terms of an effective spinorial fluid. After reformulating all covariant equations in ``spinorial'' signature $(+ -- )$, we develop a $(1+1+2)$ covariant approach for the Dirac field that does not require the use of tetrad fields or Clifford matrices. By identifying the velocity and spin fields as the generators of time-like and space-like congruences, we examine the compatibility of a self-gravitating Dirac field with Locally Rotationally Symmetric space-times of types I, II, and III. We provide illustrative examples to demonstrate the effectiveness of our construction.

gr-qc↗
arXiv2025

Dirac Fields in Hydrodynamic Form and their Thermodynamic Formulation

We consider the theory of spinor fields written in polar form and we re-express it in terms of the so-called $1\!+\!1\!+\!2$ covariant splitting: after this is done for the basic kinematic variables, we proceed to decompose the dynamical equations, both for the case of the Dirac differential field equations and for the case of the energy density tensor. As an explicit example of a real physical application we deal with the hydrogen atom, superconductivity and an analogy with the van der Waals gas.

math-ph↗
arXiv2024

A note on the junction conditions in f(Q)-gravity

Using the notion of distribution-valued tensor, we discuss the junction conditions within the framework of f(Q)-gravity. We obtain the necessary and sufficient conditions for two distinct solutions of the field equations to be smoothly joined on a given separation hypersurface.

gr-qc↗
arXiv2023

Locally Rotationally Symmetric spacetimes of type II in $f(\mathcal{Q})$ gravity

We investigate the $1+1+2$ covariant formalism in the presence of nonmetricity. Focusing on Locally Rotationally Symmetric spacetimes, we show how nonmetricity affects all the kinematic quantities involved in the covariant $1+1+2$ decomposition. We apply the resulting geometrical framework to study both homogeneous solutions and static spherically symmetric solutions in the context of $f(\mathcal{Q})$ gravity. We obtain sufficient conditions for homogeneous solutions with flat spatial hypersurfaces and Schwarzschild-de Sitter type solutions in the $1+1+2$ formalism. We also explore an elementary gravastar solution utilizing covariant junction conditions.

gr-qc↗
arXiv2023

Polar form of Dirac fields: implementing symmetries via Lie derivative

We consider the Lie derivative along Killing vector fields of the Dirac relativistic spinors: by using the polar decomposition we acquire the mean to study the implementation of symmetries on Dirac fields. Specifically, we will become able to examine under what conditions it is equivalent to impose a symmetry upon a spinor or only upon its observables. For one physical application, we discuss the role of the above analysis for the specific spherical symmetry, obtaining some no-go theorem regarding spinors and discussing the generality of our approach.

math-ph↗
arXiv2023

Interacting quintessence cosmology from Noether symmetries: comparing theoretical predictions with observational data

In the framework of scalar-tensor gravity, we consider non-flat interacting quintessence cosmology where a scalar field is interacting with dark matter. Such a scalar field can be a standard or a phantom one. We use the Noether Symmetry Approach to obtain general exact solutions for cosmological equations and to select scalar-field self-interaction potentials. It turns out that the found solutions can reproduce the accelerated expansion of the Universe, and are compatible with observational dataset, as the SNeIa Pantheon data, gamma ray bursts Hubble diagram, and direct measurements of the Hubble parameter.

gr-qc↗
arXiv2023

Integrability of Dirac equations in static spherical space-times

We consider the Dirac equations in static spherically-symmetric space-times, and we present a type of spinor field whose structure allows the separation of elevation angle and radial coordinate in very general situations. We demonstrate that after such a separation of variables the Dirac equations reduce to two equations that can always be integrated, at least in principle. To prove that ours is a fully-working method, we find an explicit exact solution in the special case of the de Sitter universe.

gr-qc↗
arXiv2022

Bianchi type-I cosmological dynamics in $f(\mathcal{Q})$ gravity: a covariant approach

Making use of the $1 + 3$ covariant formalism, we show explicitly the effect that nonmetricity has on the dynamics of the universe. Then, using the Dynamical System Approach, we analyze the evolution of Bianchi type-I cosmologies within the framework of $f(\mathcal{Q})$ gravity. We consider several models of function $f(\mathcal{Q})$, each of them manifesting isotropic eras of the universe, whether transitional or not. In one case, in addition to the qualitative analysis provided by the dynamical system method, we also obtain analytical solutions in terms of the average length scale $l$.

gr-qc↗
arXiv2022

Reconstructing isotropic and anisotropic $f(\mathcal{Q})$ cosmologies

We present a reconstruction algorithm for cosmological models based on $f(\mathcal{Q})$ gravity. We specifically focus on obtaining exact Bianchi Type-I and Friedmann-Lema\^ıtre-Robertson-Walker solutions, finding solutions that might have application in a variety of scenarios such as spontaneous isotropization of Bianchi Type-I models, dark energy, inflation as well as pre-Big Bang cosmologies.

gr-qc↗
arXiv2021

Spinor fields in $f(\mathcal{Q})$-gravity

We present a tetrad-affine approach to $f(\mathcal{Q})$ gravity coupled to spinor fields of spin-1/2. After deriving the field equations, we derive the conservation law of the spin density, showing that the latter ensures the vanishing of the antisymmetric part of the Einstein-like equations, just as it happens in theories with torsion and metricity. We then focus on Bianchi type-I cosmological models proposing a general procedure to solve the corresponding field equations and providing analytical solutions in the case of gravitational Lagrangian functions of the kind $f(\mathcal{Q})=α\mathcal{Q}^n$. At late time such solutions are seen to isotropize and, depending on the value of the exponent $n$, they can undergo an accelerated expansion of the spatial scale factors.

gr-qc↗
arXiv2020

The mass-radius relation for neutron stars in $f(R)=R+αR^2$ gravity: a comparison between purely metric and torsion formulations

Within the framework of $f(R)=R+αR^2$ gravity, we study realistic models of neutron stars, using equations of state compatible with the LIGO constraints. i.e. APR4, MPA1, SLy, and WW1. By numerically solving modified Tolman-Oppenheimer-Volkoff equations, we investigate the Mass--Radius relation in both metric and torsional $f(R)=R+αR^2$ gravity models. In particular, we observe that torsion effects decrease the compactness and total mass of neutron star with respect to the General Relativity predictions, therefore mimicking the effects of a repulsive massive field. The opposite occurs in the metric theory, where mass and compactness increase with $α$, thus inducing an excess of mass that overtakes the standard General Relativity limit. We also find that the sign of $α$ must be reversed whether one considers the metric theory (positive) or torsion (negative) to avoid blowing up solutions. This could draw an easy test to either confirm or discard one or the other theory by determining the sign of parameter $α$.

astro-ph.HE↗
arXiv2020

Axially-Symmetric Exact Solutions for Flagpole Fermions with Gravity

The Lounesto classification splits spinors in six classes: I, II, III are those for which at least one among scalar and pseudo-scalar bi-linear spinor quantities is non-zero, its spinors are called regular, and among them we find the usual Dirac spinor. IV, V, VI are those for which the scalar and pseudo-scalar bi-linear spinor quantities are identically zero, its spinors are called singular, and they are split in further sub-classes: IV has no further restrictions, its spinors are called flag-dipole; V is the one for which the spin axial-vector vanishes, its spinors are called flagpole, and among them we find the Majorana spinor; VI is the one for which the momentum antisymmetric-tensor vanishes, its spinors are called dipole, and among them we find the Weyl spinor. In the quest for exact solutions of fully-coupled systems of spinor fields in their own gravity, we have already given examples in the case of Dirac fields and Weyl fields but never in the case of Majorana or more generally flagpole spinor fields. Flagpole spinor fields in interaction with their own gravitational field, in the case of axial symmetry, will be considered. Exact solutions of the field equations will be given.

gr-qc↗
arXiv2018

Dirac spinors and their application to Bianchi-I space-times in 5 dimensions

We consider a five-dimensional Einstein--Cartan spacetime upon which Dirac spinor fields can be defined. Dirac spinor fields in five and four dimensions share many features, like the fact that both are described by four-component spinor fields, but they are also characterized by strong differences, like the fact that in five dimensions we do not have the possibility to project on left-handed and right-handed chiral parts unlike what happens in the four-dimensional instance: we conduct a polar decomposition of the spinorial fields, so to highlight all similarities and discrepancies. As an application of spinor fields in five dimensions, we study Bianchi-I spacetimes, verifying whether the Dirac fields in five dimensions can give rise to inflation or dark-energy dominated cosmological eras or not.

gr-qc↗