Cross-correlating Gravitational Waves with Large Scale Structure: from Cosmology to Astrophysics

Nowadays we can probe the Universe by means of different observables and through a large set of working or planned experiments. We can now rely on data coming from several ambitious experiments observing the Universe through the electromagnetic (EM) radiation emitted by celestial bodies and traveling through space. The Large Scale Structure (LSS) of the Universe has been mapped through various galaxy, weak lensing or intensity mapping surveys such as KiDS, DES or MeerKAT, and even a bigger amount of data will be provided by e.g., Euclid, SKAO, the Vera Rubin Observatory, the Nancy Grace Roman Space Telescope, EMU, SPHEREx and many others. In parallel, a load of information was delivered by looking at the Cosmic Microwave Background radiation (CMB), improving the knowledge on the history of the Universe by virtue of e.g., COBE, WMAP and Planck. The ΛCDM theoretical model is fitted very well by data, with its 6 main parameters being exquisitely constrained by Planck. Nonetheless, the quest for new physics is still open. The amount of knowledge that could be extracted by such large amount of data coming from EM observations was often found to be even more powerful when different observables were studied in cross-correlation together. Indeed, the study of the crosscorrelation between e.g., distinct galaxy types, intensity mapping (IM) of the 21 centimeter line or CMB either improved the constraining ability over the parameters or models that were being tested, helped in reducing systematics, or even opened new scientific paths that would not have been explorable with single-tracer experiments. Furthermore, on September 2015 the way we observe and study the cosmos changed once again, after the first detection (made by the LIGO/Virgo collaboration) of a Gravitational Wave (GW) signal emitted from a Binary Black Hole (BBH) system, leading to the beginning of the so called Gravitational Wave astronomy. Among several other reasons, the striking importance of this event consists on the fact that the Universe was then probable through a completely different and new observation channel. Before that breakthrough, many different types of observations had relied on the same physical observable: EM signals. With GWs, it was finally possible to retrieve data and test physics in a completely novel manner. Several detections were made in the subsequent years, and many more are expected thanks to experiments such as e.g., KAGRA, the Einstein Telescope, Cosmic Explorer and LISA. Due to the novelty of this alternative probe, multi-tracing analyses are expected to bring even more interesting and unexpected results, as some studies have started investigating, exploring the cross-correlation signal of GWs with observables based on EM signals, such as galaxies, line intensity mapping, CMB and so on. This thesis is based on my original scientific publications, which have the aim of expanding the knowledge in the GW×LSS cross-correlations field, by investigating how to better quantify this relation, by means of which observables and, mainly, which progresses in both Cosmology and Astrophysics the GW×EM cross-correlations are likely to be provided by forthcoming experiments and theoretical modeling. Chapter 1 of this thesis presents an overall introduction to fundamental topics for the description of the Universe, followed by some context of Gravitational Waves both under a theoretical and an experimental point of view. Finally, an overall introduction of GW×LSS cross-correlations is provided, along with the formal description of one of the most common tools to quantify it: the number counts angular power spectrum. This is followed by a sketched introduction to another observable often considered in this thesis: the intensity mapping of the 21 cm line. Finally, a presentation of the adopted Fisher formalism in provided. Chapter 2 is mainly based on Scelfo et al. (2020). In this chapter we study the measurable cross-correlation signal of galaxies and GWs with a refined characterization of both these tracers. Regarding the first one, we make use of a solid statistics of actively star-forming galaxies, based on high redshift far-IR/sub-mm observations. Regarding GWs, we treat events originated from Compact Objects (COs) mergers in the stellar mass range, adopting prescriptions consistently derived from the galaxy ones. Firstly, we aim at forecasting the detectability of such cross-correlation signal. Secondly, we make use of a proof-of-concept scenario to investigate the exploit-ability of GW×LSS cross-correlations to explore their utility in constraining astrophysical models. Chapter 3 is taken from Scelfo et al. (2022a). In this chapter we investigate, to our knowledge for the first time, several cosmological and astrophysical applications by exploring the cross-correlations of GWs with another relatively novel observable: the IM of the 21 cm line. The strongest advantage of the IM is given by the fact that this technique allows to perform a very well refined tomography, since the redshift information is known with great accuracy, unlike for GWs. We explore three main topics: (i) statistical inference of the observed redshift distribution of GWs events from BH-BH mergers; (ii) constraints on dynamical dark energy models as an example of cosmological studies; (iii) determination of the nature of the progenitors of merging binary black holes, distinguishing between primordial and astrophysical origin. Chapter 4 is mainly based on Scelfo et al. (2022b). Here we study how the crosscorrelation of GWs and EM sources (namely resolved galaxies and the 21 cm IM) can be indicative of possible signatures of Modified Gravity (MG) models, beyond General Relativity (GR). This investigation relies on the idea that, under a GR framework, GWs and EM signals are expected to behave in the same way under the effects of matter perturbations between the emitter and the observer. A different behaviour might be an imprint of alternative theories of gravity, which may be detectable with forthcoming experiments. Finally, in chapter 5 we draw our conclusions and discuss future perspectives for the GW×LSS cross-correlations domain.