An introduction to scalar-tensor gravity: theory and experiment in the Brans-Dicke model

Le teorie della gravità scalare-tensore sono una classe di teorie alternative alla rel- atività generale in cui l’interazione gravitazionale è descritta sia dalla metrica, sia da un campo scalare. Ne costituisce un esempio caratteristico la teoria di Brans-Dicke, in- trodotta come estensione della relatività generale in modo da renderla conforme con il principio di Mach. Il presente lavoro di tesi è volto a presentare un’analisi di questa teoria nei suoi aspetti principali, studiandone i fondamenti teorici e il modello cosmologico derivante, sottolineandone inoltre i limiti e le criticità; in seguito vengono esposti i risultati degli esperimenti fino ad ora svolti per verificare fondamenti e previsioni del modello.

Cosmological predictions for a scalar tensor dark energy model by a dedicated Einstein-Boltzmann code

We have extended the Boltzmann code CLASS and studied a specific scalar tensor dark energy model: Induced Gravity

Study of a wind-wave numerical model and its integration with ocean and oil-spill numerical models

The ability to represent the transport and fate of an oil slick at the sea surface is a formidable task. By using an accurate numerical representation of oil evolution and movement in seawater, the possibility to asses and reduce the oil-spill pollution risk can be greatly improved. The blowing of the wind on the sea surface generates ocean waves, which give rise to transport of pollutants by wave-induced velocities that are known as Stokes’ Drift velocities. The Stokes’ Drift transport associated to a random gravity wave field is a function of the wave Energy Spectra that statistically fully describe it and that can be provided by a wave numerical model. Therefore, in order to perform an accurate numerical simulation of the oil motion in seawater, a coupling of the oil-spill model with a wave forecasting model is needed. In this Thesis work, the coupling of the MEDSLIK-II oil-spill numerical model with the SWAN wind-wave numerical model has been performed and tested. In order to improve the knowledge of the wind-wave model and its numerical performances, a preliminary sensitivity study to different SWAN model configuration has been carried out. The SWAN model results have been compared with the ISPRA directional buoys located at Venezia, Ancona and Monopoli and the best model settings have been detected. Then, high resolution currents provided by a relocatable model (SURF) have been used to force both the wave and the oil-spill models and its coupling with the SWAN model has been tested. The trajectories of four drifters have been simulated by using JONSWAP parametric spectra or SWAN directional-frequency energy output spectra and results have been compared with the real paths traveled by the drifters.

Bianchi type II cosmology in Hořava–Lifshitz gravity

In this work a Bianchi type II space-time within the framework of projectable Horava Lifshitz gravity was investigated; the resulting field equations in the infrared limit λ = 1 were analyzed qualitatively. We have found the analytical solutions for a toy model in which only the higher curvature terms cubic in the spatial Ricci tensor are considered. The resulting behavior is still described by a transition among two Kasner epochs, but we have found a different transformation law of the Kasner exponents with respect to the one of Einstein's general relativity.

Modelling the Halo mass number counts in modified gravity cosmological scenarios

In this Thesis work we investigate some of different cosmological background scenarios using one of the main probes used in cosmology: the halo mass function. The observed abundance of galaxy clusters (or similarly DM haloes) can indeed be compared to its theoretical predictions to derive fundamental constrains on the cosmological scenario assumed. Given the importance of exploring and constraining models degenerate with the ΛCDM one, we test the applicability of some notable halo mass function models to these scenarios. To this purpose, we made use of the DUSTGRAIN-pathfinder N-body simulations, which assume cosmological scenarios that include modified gravity in the form of f(R) models and massive neutrinos. We carried on the analysis of 3 simulation snapshots at different redshifts, z = 0, 0.5, 1, building multiple samples of dark matter haloes by applying different overdensity thresholds during the procedure of halo identification. We started our analysis by considering the halo mass function model introduced by Despali et al. (2016), who proposed a parametrization that encapsulates the effect of the different halo mass definitions and the relative evolution with the redshift. We calibrated the main parameters of this relation by using the ΛCDM halo catalogues extracted from the DUSTGRAIN-pathfinder simulations, fitting the measured halo abundances at all redshifts and density thresholds. Afterwards we tested the same model parametrization with halo catalogues extracted from the simulations implementing both modified gravity and massive neutrinos. We repeated therefore the calibration procedure on these data to search for discrepancies with respect to the ΛCDM model. Finally we focused the analysis on the cosmological models implementing modified gravity only. We took our ΛCDM calibrated halo mass function and we modified it with the additional f (R) gravity form proposed by Gupta et al. (2022).

Modified gravity with tensor computer algebra: a case study

Il modello ΛCDM è il modello cosmologico più semplice, ma finora più efficace, per descrivere l'evoluzione dell'universo. Esso si basa sulla teoria della Relatività Generale di Einstein e fornisce una spiegazione dell'espansione accelerata dell'universo introducendo la costante cosmologica Λ, che rappresenta il contributo della cosiddetta energia oscura, un'entità di cui ben poco si sa con certezza. Sono stati tuttavia proposti modelli teorici alternativi che descrivono gli effetti di questa quantità misteriosa, introducendo ad esempio gradi di libertà aggiuntivi, come nella teoria di Horndeski. L'obiettivo principale di questa testi è quello di studiare questi modelli tramite il tensor computer algebra xAct. In particolare, il nostro scopo sarà quello di implementare una procedura universale che permette di derivare, a partire dall'azione, le equazioni del moto e l'evoluzione temporale di qualunque modello generico.

One-loop quantum gravity in arbitrary dimensions

In the context of perturbative quantum gravity, the first three Seeley-DeWitt coefficients represent the counterterms needed to renormalize the graviton one-loop effective action in $D=4$ dimensions. A standard procedure to compute them is by means of the traditional heat kernel method. However, these coefficients can be studied also from a first quantization perspective through the so-called $\mathcal{N} = 4$ spinning particle model. It relies on four supersymmetries on the worldline and a set of worldline gauge invariances. In the present work, a different worldline model, able to reproduce correctly the Seeley-DeWitt coefficients in arbitrary dimensions, is developed. After a covariant gauge-fixing procedure of the Einstein-Hilbert action with cosmological constant, a worldline representation of the kinetic operators identified by its quadratic approximation is found. This quantum mechanical representation can be presented in different but equivalent forms. Some of these different forms are discussed and their equivalence is verified by deriving the gauge invariant counterterms needed to renormalize quantum gravity with cosmological constant at one-loop.