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Mario Novello

Publications and source records attributed to Mario Novello.

16 recordsLinked to original sources
arXiv2025

Qualitative analysis of a quasi-magnetic universe

We investigate the cosmological dynamics induced by nonlinear electrodynamics in a homogeneous and isotropic universe, focusing on the role of primordial electromagnetic fields with random spatial orientations. Building upon a generalization of the Tolman-Ehrenfest averaging procedure, we derive a modified energy-momentum tensor consistent with the spacetime symmetries, incorporating the influence of the dual invariant G and its statistical contributions. A specific nonlinear electrodynamics model with quadratic corrections to Maxwell's Lagrangian is considered, giving rise to what we define as a quasi-magnetic universe, interpolating between purely magnetic and statistically null field configurations. We analyze the resulting cosmological dynamics through qualitative methods. By casting the equations into autonomous dynamical systems, we identify the equilibrium points, determine their stability, and study the behavior of solutions under various spatial curvatures. Our findings reveal the existence of bouncing and cyclic solutions, regions where energy conditions are violated, and scenarios of accelerated expansion. Special attention is given to two limiting cases, both of which exhibit qualitatively distinct phase portraits and energy-condition behavior. This work provides a comprehensive framework for understanding the influence of nonlinear electromagnetic fields in the early universe and opens avenues for exploring their observational consequences

gr-qc↗
arXiv2024

Qualitative analysis of viscous cosmology

We analyze the effect of higher order viscosity corrections in a spatially homogeneous and isotropic universe. The viscous effects are modeled by introducing a second-order term to the energy density in the equation of state and on the expansion factor. An autonomous dynamical system is then constructed to study the qualitative behavior of the energy density and expansion factor, employing the Raychaudhuri equation and the conservation of energy. We obtain the analytical expressions for the equilibrium points in terms of the model parameters, presenting several phase diagrams. The analysis of these diagrams reveals the presence of bifurcations, which manifest as changes in stability and qualitative behavior. Those changes lead to different configurations for the universe, such as bouncing, rebouncing and cyclic scenarios.

gr-qc↗
arXiv2024

Rotation in the Geometric Scalar Theory of Gravity

We present solutions corresponding to rotational configurations in the recently proposed Geometric Scalar Gravity (GSG) theory. The solutions obtained here have the important property that the associated closed time-like curves are always restricted to a compact domain of space-time. We compare this property with those of analogous solutions in General Relativity (GR).

gr-qc↗
arXiv2024

An overview of field theories of gravity

In the general relativity theory the basic ingredient to describe gravity is the geometry, which interacts with all forms of matter and energy, and as such, the metric could be interpreted as a true physical quantity. However the metric is not matter nor energy, but instead it is a new dynamical variable that Einstein introduced to describe gravity. In order to conciliate this approach to the more traditional ones, physicists have tried to describe the main ideas of GR in terms of standard conceptions of field theory. In this sense, curved metrics are seen as a dynamical variable emerging from a more fundamental field which lies upon a flat Minkowski spacetime. This was made by the hypothesis that the metric tensor may be written as $g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu}$ where the tensor $ h_{\mu\nu}$ was interpreted either in terms of a spin-2 or constructed in terms of other fields. We review some proposals that were suggested in the treatment of gravity in terms of scalar, spinor and tensor fields configurations.

gr-qc↗
arXiv2024

Friedmann's Universe Controlled by Gauss-Bonnet Modified Gravity

The accepted idea that the expansion of the universe is accelerating needs, for compatibility to general relativity, the introduction of some unusual forms of matter. However, several authors have proposed that instead of making weird hypothesis on some yet unobservable species of matter, one should follow the original idea of the first Einstein's paper on cosmology and consider that in the cosmic scene one has to modify the equations that controls the gravitational metric. This possibility led us to re-examine the evolution of the topological invariant containing two duals in a dynamical universe, the so called Gauss-Bonnet topological invariant. The particular interest on this invariant is due to the fact that in a homogeneous and isotropic universe this invariant drives the cosmic acceleration. In a decelerating scenario and as a necessary previous condition of an ulterior acceleration this invariant must have an extremum identified to its maximum value. We will examine the conditions for this to occur, and a description of the universe with epochs of accelerated and decelerated expansion.

gr-qc↗
arXiv2022

Mass generation and gravity

Despite the success of the Higgs mechanism to account for the generation of the masses of Standard Model (SM) elementary particles, the ultimate nature and origin of "mass" remain open questions in contemporary physics. From a foundational perspective, mass should be fundamentally related to the gravitational interaction and, according to Mach, to the structure of the Universe. In the present letter, a fully dynamical mass generation mechanism induced by higher-order corrections in the GR Lagrangian is discussed. Notably, the vacuum energy plays a key role in this process, which applies to both vector bosons and fermions. We show that, at the classical level, the Higgs mechanism can be thought of as a particular case of the present mechanism.

physics.gen-ph↗
arXiv2022

Photon propagation in a material medium on a curved spacetime

We consider a nonlinear dielectric medium surrounding a static, charged and spherically symmetric compact body which gravitational field is driven by General Relativity (GR). Considering the propagating waves on the dielectric medium, we describe the trajectory of light as geodesics on an effective geometry given by Hadamard's discontinuities. We analyze some consequences of the effective geometry in the propagation of light, with relation to the predictions of the background gravitational field, that includes corrections on the geometrical redshift and on the gravitational deflection of light. We show that the background electromagnetic field polarize the material medium, such that different polarizations of light are distinguished by different corrections on these quantities. As a consequence, we have two possible paths for the trajectory of light in such configuration, that coincide if we turn off the electromagnetic field or if the permittivity is constant. We show that the effective metric associated to the negative polarization, for a given dependence of the dielectric permittivity, is conformally flat.

gr-qc↗
arXiv2021

Galaxy rotation curves in the light of the Spinor Theory of Gravity

We analyze the recently obtained static and spherically symmetric solution of the Spinor Theory of Gravity (STG) which, in the weak field limit, presents an effective Newtonian potential that contains an extra logarithmic behavior. We apply this solution to the description of the galaxy rotation curves finding an interesting analogy with the dark matter halo profile proposed by Navarro, Frenk and White.

gr-qc↗
arXiv2021

New dynamical features of pure k-essential cosmologies

We come back on the dynamical properties of k-essential cosmological models and show how the interesting phenomenological features of those models are related to the existence of boundaries in the phase surface. We focus our attention to the branching points where the energy density has an extremum and the effective speed of sound diverges. We discuss the behaviour of solutions of a general class of cosmological models exhibiting such curves and give two possible interpretations; the most interesting possibility regards the arrow of time that is reversed in trespassing the branching curve. This study teaches to us something new about general FLRW cosmologies where the fluids driving the cosmic evolution have equations of state that are multivalued functions of the energy density and other thermodynamical quantities.

gr-qc↗
arXiv2021

Gravitational waves in the Spinor Theory of Gravity

We analyze the gravitational waves within the Spinor Theory of Gravity and compare it with the General Relativity proposal. In the case of STG a gravitational wave may occur if the effective gravitational metric induced by the spinorial field is Ricci flat.

gr-qc↗
arXiv2021

Beyond the Equivalence Principle: Gravitational Magnetic Monopoles

We review the hypothesis of the existence of gravitational magnetic monopoles (H-pole for short) defined in analogy with the Dirac's hypothesis of magnetic monopoles in electrodynamics. These hypothetical dual particles violate the equivalence principle and are accelerated by a gravitational field. We propose an expression for the gravitational force exerted upon an H-pole. According to GR ordinary matter (which we call E-poles) follows geodesics in a background metric. The dual H-poles follows geodesics in an effective metric.

gr-qc↗
arXiv2019

Gravitational baryogenesis without CPT violation

We show that in theories with nonminimal curvature-matter couplings up to first order in the curvature, baryon number (B) conservation and CP are automatically violated, provided that the curvature scalar and its gradient do not vanish. Together with the associated gravitationally induced particle creation in a cosmological framework, which is an essentially out-of-equilibrium process, this implies that all of Sakharov's conditions are satisfied in the simplest extension of General Relativity, without the need of CPT violation. Here, derivatives of the curvature can be disregarded, thus evading the potential problems associated with the Gravitational Baryogenesis (GB) model in a cosmological context. Thanks to the particle creation pressure and the trace anomaly, the curvature scalar is nonzero even in a radiation-dominated Universe. Moreover, the trace anomaly or the energy density of a massive scalar field can amplify the baryon asymmetry at very high temperatures, even when the particle creation rate is very slow. As a result, a baryon asymmetry compatible with observations and a huge CP violation can be generated in the early Universe.

gr-qc↗
arXiv2016

Time-delayed electromagnetic radiation reaction

The Lorentz-Abraham-Dirac (LAD) equation has proved valuable in describing the motion of radiating electric charges but suffers from runaway, pre-acceleration and other ambiguities. The usual scheme is problematic because of locality, which leads to self-interaction with the propagating radiation (i.e. real photons). Instead, the present heuristic model relies on an infinitesimal time delay between the action of external forces and the inertial reaction by the charge. This yields a new and pathology-free equation of motion whereas the radiated energy-momentum is expressed as an infinite series that generalises Larmor's formula and leads to testable predictions using current and near future ultra-intense lasers. The time-delay hypothesis is to be put in parallel with recently observed delays of order $10^{-18}$s (attosecond) in photoemission by atoms and small molecules. Such behaviour is extended here to elementary charges which are supposed to exhibit delays given by the time taken by light to cross the charge's classical radius, which gives $\approx 10^{-23}$s for an electron.

physics.class-ph↗
arXiv2006

The nature of $Λ$ and the mass of the graviton: A critical view

The existence of a non-zero cosmological constant $Λ$ gives rise to controversial interpretations. Is $Λ$ a universal constant fixing the geometry of an empty universe, as fundamental as the Planck constant or the speed of light in the vacuum? Its natural place is then on the left-hand side of the Einstein equation. Is it instead something emerging from a perturbative calculus performed on the metric $g\_{μν}$ solution of the Einstein equation and to which it might be given a material status of (dark or bright) "energy"? It should then be part of the content of the right-hand side of the Einstein equations. The purpose of this paper is not to elucidate the fundamental nature of $Λ$, but instead we aim to present and discuss some of the arguments in favor of both interpretations of the cosmological constant. We conclude that if the fundamental of the geometry of space-time is minkowskian, then the square of the mass of the graviton is proportional to $Λ$; otherwise, if the fundamental state is deSitter/AdS, then the graviton is massless in the deSitterian sense.

gr-qc↗
arXiv2002

Possible time variations of G in scalar-tensor theories of gravity

We estimate the possible variations of the gravitational constant G in the framework of a generalized (Bergmann-Wagoner-Nordtvedt) scalar-tensor theory of gravity on the basis of the field equations, without using their special solutions. Specific estimates are essentially related to the values of other cosmological parameters (the Hubble and acceleration parameters, the dark matter density etc.), but the values of G-dot/G compatible with modern observations do not exceed 10^{-12}.

gr-qc↗