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Giovanni Montani

Publications and source records attributed to Giovanni Montani.

At least 55 records · Page 3Linked to original sources

Metric $f(R)$ gravity with dynamical dark energy as a scenario for the Hubble tension

We introduce a theoretical framework to interpret the Hubble tension, based on the combination of a metric $f(R)$ gravity with a dynamical dark energy contribution. The modified gravity provides the non-minimally coupled scalar field responsible for the proper scaling of the Hubble constant, in order to accommodate for the local SNIa pantheon+ data and Planck measurements. The dynamical dark energy source, which exhibits a phantom divide line separating the low red-shift quintessence regime ($-1<w<-1/3$) from the phantom contribution ($w<-1$) in the early Universe, guarantees the absence of tachyonic instabilities at low red-shift. The resulting $H_0(z)$ profile rapidly approaches the Planck value, with a plateau behaviour for $z\gtrsim 5$. In this scenario, the Hubble tension emerges as a low red-shift effect, which can be in principle tested by comparing SNIa predictions with far sources, like QUASARS and Gamma Ray Bursts.

gr-qc↗

On the Effects of Tokamak Plasma Edge Symmetries on Turbulence Relaxation

The plasma edge of a tokamak configuration is characterized by turbulent dynamics leading to enhanced transport. We construct a simplified 3D Hasegawa--Wakatani model reducing to a single partial differential equation for the turbulent electric potential dynamics. Simulations demonstrate how the 3D turbulence relaxes on a 2D axisymmetric profile, corresponding to the so-called interchange turbulence. The spectral features of this regime are found to be strongly dependent on the initialization pattern. We outline that the emergence of axisymmetric turbulence is also achieved when the corresponding mode amplitude is not initialized. Then, we introduce the symmetries of the magnetic X-point of a tokamak configuration. We linearize the governing equation by treating the poloidal field as a small correction. We show that it is not always possible to solve the electric potential dynamics following a perturbative approach. This finding, which is due to resonance between the modes of the background and the poloidal perturbation, confirms that the X-point symmetries can alter the properties of turbulent transport in the edge region.

physics.plasm-ph↗

Emergent Universe Model from Modified Heisenberg Algebra

We provide an Emergent Universe picture in which the fine-tuning on the initial conditions is replaced by cut-off physics, implemented on a semiclassical level when referred to the Universe dynamics and on a purely quantum level for the quantum fluctuations of the inflaton field. The adopted cut-off physics is inspired by Polymer Quantum Mechanics but expanded in the limit of a small lattice step. On a quasi-classical level, this results in modified Poisson Brackets for the Hamiltonian Universe dynamics similar to a Generalized Uncertainty Principle algebra. The resulting Universe is indeed asymptotically Einstein-static, emerging from a finite volume configuration in the distant past and then properly reconnecting with the most relevant Universe phases. The calculation of the modifications of the primordial inflaton spectrum is then performed by treating new physics as a small correction on the standard Hamiltonian of each Fourier mode of the field. The merit of this study is to provide a new paradigm for a non-singular Emergent Universe, which is associated with a precise fingerprint on the temperature distribution of the microwave background, in principle observable by future experiments.

gr-qc↗

Is the diagonal case a general picture for Loop Quantum Cosmology?

The correct implementation of the Loop Quantum Gravity to the early homogeneous Universe has been the subject of a long debate in the literature because the SU(2) symmetry cannot be properly retained. The role of this symmetry is expressed by the Gauss constraint. Here, a non-vanishing Gauss constraint is found. However, we show that using suitable variables, it can be recast into three Abelian constraints, justifying the absence of such a symmetry in Loop Quantum Cosmology.

gr-qc↗

Extended GUP formulation and the role of momentum cut-off

We analyze the extension of the GUP theory deriving from the modified uncertainty principle in agreement with the string low energy limit, which represents one of the most general formulations satisfying the Jacobi identity, in the context of the associative algebras. After providing some physical insights on the nature of the considered approaches exploiting the cosmological arena, first, we show how a natural formulation of the theory in an infinite momentum space does not lead to the emergence of a nonzero minimal uncertainty in position, then we construct a truncated formulation of the theory in momentum space, proving that only in this case we can recover the desired feature of the presence of a nonzero minimal uncertainty in position, which - as usual in these theories - can be interpreted as a phenomenological manifestation of cut-off physics effects. Both quantization schemes are completely characterized and finally applied to study wave packets' behavior and their evolution in time. The obtained results can shed light on which generalizations of the GUP theory are more coherent with the string low energy limit, in view of the existence of a minimum length in the form of a minimal uncertainty in position.

quant-ph↗

Quantum Big Bounce of the isotropic Universe via a relational time

We analyze the canonical quantum dynamics of the isotropic Universe in a metric approach by adopting a self-interacting scalar field as relational time. When the potential term is absent we are able to associate the the expanding and collapsing dynamics of the Universe with the positive and negative frequency modes that emerge in the Wheeler-DeWitt equation. On the other side, when the potential term is present a non-zero transition amplitude from positive to negative frequency states arises, as in the standard relativistic scattering theory below the particle creation threshold. In particular, we are able to compute the transition probability for an expanding Universe that emerges from a collapsing regime both in the standard quantization procedure and in the polymer formulation. The probability distribution results similar in the two cases and its maximum takes place when the mean values of the momentum essentially coincide in the in-going and out-going wave packets, as it would take place in a semiclassical Big Bounce dynamics.

gr-qc↗

$f(R)$ gravity in the Jordan Frame as a Paradigm for the Hubble Tension

We analyse the $f(R)$ gravity in the so-called Jordan frame, as implemented to the isotropic Universe dynamics. The goal of the present study is to show that, according to recent data analyses of the supernovae Ia Pantheon sample, it is possible to account for an effective redshift dependence of the Hubble constant. This is achieved via the dynamics of a non-minimally coupled scalar field, as it emerges in the $f(R)$ gravity. We face the question both from an analytical and purely numerical point of view, following the same technical paradigm. We arrive to establish that the expected decay of the Hubble constant with the redshift $z$ is ensured by a form of the scalar field potential, which remains essentially constant for $z\lesssim0.3$, independently if this request is made a priori, as in the analytical approach, or obtained a posteriori, when the numerical procedure is addressed. Thus, we demonstrate that an $f(R)$ dark energy model is able to account for an apparent variation of the Hubble constant due to the rescaling of the Einstein constant by the $f(R)$ scalar mode.

gr-qc↗

On the emergence of a classical Isotropic Universe from a Quantum $f(R)$ Bianchi Cosmology in the Jordan Frame

We demonstrate a spontaneous tendency of quantum wave packets to become quasi-classical, providing a classical limit for the Universe dynamics. However, this limit is not maintained in the future (after a critical value of the relational time) and a spreading process is turned on. We show that the onset of an inflationary scenario is not able to make this localization stable of the wave packets for the Bianchi I model. Instead, when we implement a perturbative inflationary scenario for the isotropic Universe a mechanism of stable classicalization of the Universe emerges. This result outlines a sharp difference between the standard relativistic cosmology and a modified $f(R)$ paradigm.

gr-qc↗

Study of the Inflationary Spectrum in the Presence of Quantum Gravity Corrections

After a brief review of the different approaches to predict the possible quantum gravity corrections to quantum field theory, we discuss in some detail the formulation based on a Gaussian reference frame fixing. Then, we implement this scenario to the determination of the inflationary spectrum of primordial perturbations. We consider the quantization of an inhomogeneous free massless scalar field on a quasi-classical isotropic Universe, developing a WKB expansion of the dynamics at the next order in the Planckian parameter, with respect to the one at which standard QFT emerges. The quantum gravity corrections to the scale invariant spectrum are discussed in a specific primordial cosmological setting and then in a general minisuperspace formalism, showing that there is no mode-dependent effect and thus the scale invariant inflationary spectrum is preserved. Such result is discussed in connection to the absence of a matter backreaction on the gravitational background in the considered paradigm.

gr-qc↗

QFT in Curved Spacetime from Quantum Gravity: proper WKB decomposition of the gravitational component

Starting from a re-analysis of previous work, we construct the proper low energy quantum field theory (QFT) limit of a full quantum gravity theory in the Born-Oppenheimer approach. We separate the gravitational sector into a classical background, given by a vacuum diagonal Bianchi I cosmology, and its quantum perturbations represented by the two graviton degrees of freedom; we further include quantum matter in the form of a test scalar field. We then implement a Born-Oppenheimer separation, where the gravitons and matter play the role of "slow" and "fast" quantum components respectively, and perform a Wentzel-Kramers-Brillouin (WKB) expansion in a Planckian parameter. The functional Schrödinger evolution for matter is recovered after averaging over quantum gravitational effects, provided that a condition is imposed on the gravitons' wave functional. Such a condition fixes the graviton dynamics and is equivalent to the purely gravitational Wheeler-DeWitt constraint imposed in previous approaches. The main accomplishment of the present work is to clarify that QFT in curved spacetime can be recovered in the low energy limit of quantum gravity only after averaging over the graviton degrees of freedom, in the spirit of effective field theory. Furthermore, it justifies a posteriori the implementation of the gravitational Wheeler-DeWitt equation on the "slow" gravitons' wave functional rather than assuming its validity a priori.

gr-qc↗

Linear analysis of the gravitational beam-plasma instability

We investigate the well-known phenomenon of the beam-plasma instability in the gravitational sector, when a fast population of particles interacts with the massive scalar mode of an Horndeski theory of gravity, resulting into the linear growth of the latter amplitude. Following the approach used in the standard electromagnetic case, we start from the dielectric representation of the gravitational plasma, as introduced in a previous analysis of the Landau damping for the scalar Horndeski mode. Then, we set up the modified Vlasov-Einstein equation, using at first a Dirac delta function to describe the fast beam distribution. This way, we provide an analytical expression for the dispersion relation and we demonstrate the existence of non-zero growth rate for the linear evolution of the Horndeski scalar mode. A numerical investigation is then performed with a trapezoidal beam distribution function, which confirms the analytical results and allows to demonstrate how the growth rate decreases as the beam spread increases.

gr-qc↗

The role of spatial curvature in constraining the Universe anisotropies across a Big Bounce

We study the implementation of Polymer Quantum Mechanics (PQM) to a system decomposed into a quasi-classical background and a small quantum subsystem, according to the original Vilenkin proposal. We develop the whole formalism in the momentum representation that is the only viable in the continuum limit of the polymer paradigm and we generalize the fundamental equations of the original Vilenkin analysis in the considered context. Then, we provide a Minisuperspace application of the theory, first considering a Bianchi I cosmology and then extending the analysis to a Bianchi IX model in the limit of small anisotropies. In both these cases, the quasi-classical background is identified with an isotropic bouncing Universe whereas the small quantum subsystem contains the anisotropic degrees of freedom. When the Big Bounce scenario is considered, we obtain that in the Bianchi I model the anisotropies standard deviation is regular at $t=0$ but still increases indefinitely, whereas in the presence of the harmonic Bianchi IX potential such same quantity is bounded and oscillate around a constant value. As a consequence, we demonstrate that the picture of a semiclassical isotropic Bounce can be extended to more general cosmological settings if the spatial curvature becomes relevant when the anisotropic degrees of freedom are still small quantum variables.

gr-qc↗

On the 3D turbulence regime in a Tokamak plasma edge

We derive a reduced model for the electrostatic turbulence in a Tokamak edge, when dealing with a resistive plasma and neglecting the spatial gradient of the background density which triggers the linear drift wave response. The obtained dynamics, de facto equivalent to a Hasegawa-Wakatani model, is characterized by a constitutive relation between the electric potential Laplacian and the density, which allows to deal with a single 3D equation governing the electric potential fluctuations. We study the evolution of the model, by separating the $n=0$ ($n$ indicating toroidal-like number) mode from all the other ones. Then, we linearize the dynamics of the $n\neq 0$ modes around the steady 2D spectrum, which describes the spectral features of the electrostatic 2D turbulence. We theoretically and numerically demonstrate the existence of decaying branch of the 3D turbulence, having such a 2D steady spectrum as a natural attractor. This result suggests that the basic constituent of the self-sustained non-linear drift response in the plasma edge has to be individualized in the non-linear 2D electrostatic turbulence.

physics.plasm-ph↗

Loop Quantum Cosmology of non-diagonal Bianchi models

The non-diagonal Bianchi models are studied in the loop framework for their classical and quantum formulation. The expressions of the Ashtekar-Barbero-Immirzi variables and their properties are found to provide a loop quantization of these models. In the special case of Bianchi I Universe, it is shown that the geometrical operators result invariant from the diagonal description. Hence, the kinematical Hilbert space of the non-diagonal Bianchi I model has similar features to the diagonal one.

gr-qc↗

The Hubble constant tension: current status and future perspectives through new cosmological probes

The Hubble constant ($H_0$) tension is one of the major open problems in modern cosmology. This tension is the discrepancy, ranging from 4 to 6 $σ$, between the $H_0$ value estimated locally with the combination of Supernovae Ia (SNe Ia) + Cepheids and the cosmological $H_0$ obtained through the study of the Cosmic Microwave Background (CMB) radiation. The approaches adopted in Dainotti et al. 2021 (ApJ) and Dainotti et al. 2022 (Galaxies) are introduced. Through a binning division of the Pantheon sample of SNe Ia (Scolnic et al. 2018), the value of $H_0$ has been estimated in each of the redshift-ordered bins and fitted with a function lowering with the redshift. The results show a decreasing trend of $H_0$ with redshift. If this is not due to astrophysical biases or residual redshift evolution of the SNe Ia parameters, it can be explained in light of modified gravity theories, e.g., the $f(R)$ scenarios. We also briefly describe the possible impact of high-$z$ probes on the Hubble constant tension, such as Gamma-ray bursts (GRBs) and Quasars (QSOs), reported in Dainotti et al. 2022 (Galaxies) and Lenart et al. 2022 (ApJ), respectively.

astro-ph.CO↗

Inflationary Quantum Spectrum of the Quasi-Isotropic Universe

We investigate the quantum dynamics of the quasi-isotropic inflationary solution. This is achieved by deriving the Lagrangian and Hamiltonian for both the FLRW background and the inhomogeneous correction, via an expansion of the Einstein-Hilbert action up to second order in the perturbation amplitudes. We do this in a gauge invariant fashion adopting the Mukhanov-Sasaki variable for the scalar degree of freedom. Then we construct the quantum dynamics in terms of a semiclassical WKB scenario for which the inhomogeneous component of the Universe is treated as a "small" quantum subsystem, evolving on the classical isotropic background. Starting from the Wheeler-DeWitt equation, we recover a Schrödinger dynamics for the perturbations, in which the time dependence of the wave function emerges thanks to the classicality of the background, and we solve it for a de Sitter inflationary phase. The main result of this paper is that the resulting scalar component of the power spectrum has the standard scale invariant profile, while the tensor one results to be not constrained by the inflationary expansion (modulo an overall normalization factor), therefore preserving the spatial distribution of the quasi-isotropic correction to the metric.

gr-qc↗

Diffusive time evolution of the Grad-Shafranov Equation for a Toroidal Plasma

We describe the evolution of a plasma equilibrium having a toroidal topology in the presence of constant electric resistivity. After outlining the main analytical properties of the solution, we illustrate its physical implications by reproducing the essential features of a scenario for the upcoming Italian experiment Divertor Tokamak Test Facility, with a good degree of accuracy. Although we find the resistive diffusion timescale to be of the order of $10^4\,$s, we observe a macroscopic change in the plasma volume on a timescale of $10^2\,$s, comparable to the foreseen duration of the plasma discharge by design. In the final part of the work, we compare our self-consistent solution to the more common Solov'ev one, and to a family of nonlinear configurations.

physics.plasm-ph↗

Energetic particle transport: diffusion vs convection and phase-space barriers

Energetic particle redistribution in the presence of multiple Alfvén eigenmodes is analyzed in [PPCF 58, 014019 (2016)] for the ITER 15MA baseline scenario: non-linear hybrid simulations (within their well known limits) point out that transport can be dominated by avalanches under certain conditions. These phenomena are properly reproduced by the 1D reduced description of [PPCF 64, 035010 (2022)]. Here, using this simplified 1D model, we define the transport character (convective/diffusive) of self-consistent energetic particle redistribution. Transport barriers in phase space are studied using the Lagrangian Coherent Structures technique.

physics.plasm-ph↗