arXiv subjects

Flavio Bombacigno

Publications and source records attributed to Flavio Bombacigno.

23 records · Page 2Linked to original sources

arXiv2019

Big Bounce cosmology for Palatini $R^2$ gravity with a Nieh-Yan term

We analyze the cosmological implementation of Palatini $f(R)$ theories, constructed with a Nieh-Yan term and solved with respect to the torsion. We consider the relevant case of the quadratic correction to the Hilbert-Palatini action in the Ricci scalar, mimicking the Starobinsky model of the metric formulation. We point out the emergence of peculiar cosmological scenarios, depending on the sign of such correction, able to reproduce bouncing settings and to restore the standard Universe dynamics in the late asymptotic limit. Furthermore, we outline the settling of Little-Rip dynamics, which calls for a deeper investigation in order to be regularized via matter creation. Finally, we also show that in our model the Immirzi field is asymptotically frozen in time, resembling the morphology of Loop Quantum Gravity standard formulation.

gr-qc↗

arXiv2019

Semi-classical and quantum analysis of the isotropic Universe in the polymer paradigm

We analyse the semi-classical and quantum dynamics of the isotropic Universe in the framework of the Polymer Quantum mechanics, in order to implement a cut-off physics on the initial singularity. We first identify in the Universe cubed scale factor (i.e. the spatial volume) the suitable configuration variable, providing a constant critical energy density, such that the Bounce arises as intrinsic geometric feature. We then investigate the obtained semi-classical Bounce dynamics for the primordial Universe, and we outline its impact on the resolution of cosmological paradoxes, as soon as the semi-classical evolution is extended (in the spirit of the Ehrenfest theorem) to the collapsing pre-Bounce Universe. Finally, we validate the use of the semi-classical effective dynamics by investigating the behaviour of the expectation values of a proper semiclassical states. The present analysis has the merit to enforce the equivalence between the Polymer quantization paradigm in the Minisuperspace and the Loop Quantum Cosmology approach. In fact, our study allows to define a precise correspondence between the Polymer cut-off scale and the discrete geometric structure of LQG.

gr-qc↗

arXiv2019

Implications of the Holst term in a $f(R)$ theory with torsion

We analyze a modified $f(R)$ theory of gravity in the Palatini formulation, when an Holst term endowed with a dynamical Immirzi field is included. We study the basic features of the model, especially in view of liminating the torsion field via the Immirzi field and the scalar-tensor degrees of freedom of the $f(R)$ model. The main task of this study is the investigation of the morphology of the gravitational wave polarization when their coupling to a circle of test particles is considered. We first observe that the dynamics of the scalar mode of the $f(R)$ Lagrangian is frozen out, since its first order term identically vanishes. This allows a detailed characterization of the linearized theory, which outlines the emergence of a modified Newtonian potential in the static limit, and when time independence is relaxed a standard gravitational wave plus the scalar wave associated to the Immirzi field. Investigating the effect of the coupling of this scalar-tensor wave on a circle of test particles, we arrive to define two effective gravitational polarizations, corresponding to an equivalent phenomenological wave, whose morphology is anomalous with respect the standard case of General Relativity. In fact, the particle circle suffers modifications as it was subjected to modified plus and cross modes, whose specific features depend on the model free parameters and are, in principle, detectable via a data analysis procedure.

gr-qc↗

arXiv2018

$f(R)$ gravity with Torsion and the Immirzi field: signature for GW detection

We propose a Nieh-Yan extension of f (R) models in the presence of Immirzi field and the resulting theory is studied in the context of first order formalism. We claim as in our model the ambiguities related to the Palatini vacuum case are solved, allowing us to extend the range of applicability of the method proposed in [4] to a wider class of f (R) theories. In particular we analyze within such a theoretical framework the propagation of gravitational waves in vacuum and outline as a dynamical Immirzi field could be regarded as a propagating torsion field with non-trivial and detectable effects on standard gravitational polarizations.

gr-qc↗

arXiv2016

Big-Bounce cosmology in the presence of Immirzi field

The Immirzi parameter is promoted to be a scalar field and the Hamiltonian analysis of the corresponding dynamical system is performed in the presence of gravity. We identified some SU(2) connections, generalizing Ashtekar-Barbero variables, and we rewrite the constraints in terms of them, setting the classical formulation suitable for loop quantization. Then, we consider the reduced system obtained when restricting to a flat isotropic cosmological model. By mimicking loop quantization via an effective semiclassical treatment, we outline how quantum effects are able to tame the initial singularity both in synchronous time and when the Immirzi field is taken as a relational time.

gr-qc↗