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Cross-correlating Gravitational Waves with Large Scale Structure: from Cosmology to Astrophysics
Full text linkNowadays 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
Lyman- forest constraints on Primordial Black Holes as Dark Matter
Full text linkThe renewed interest in the possibility that primordial black holes (PBHs) may constitute a significant part of dark matter has provided motivation for revisiting old observational constraints, as well as developing new ones. We present new limits on the PBH abundance, from a comprehensive analysis of high-resolution high-redshift Lyman-α forest data. Poisson fluctuations in the PBH number density induce a small-scale power enhancement which departs from the standard cold dark matter prediction. Using a grid of hydrodynamic simulations exploring different values of astrophysical parameters, we obtain a marginalized upper limit on the PBH mass of fPBHMPBH∼60M⊙ at 2σ, when a Gaussian prior on the reionization redshift is imposed, preventing its posterior distribution from peaking on very high values, which are disfavored by the most recent estimates obtained both through cosmic microwave background and intergalactic medium observations. Such a bound weakens to fPBHMPBH∼170M⊙ when a conservative flat prior is instead assumed. Both limits significantly improve on previous constraints from the same physical observable. We also extend our predictions to nonmonochromatic PBH mass distributions, ruling out large regions of the parameter space for some of the most viable PBH extended mass functions.The renewed interest in the possibility that primordial black holes (PBHs) may constitute a significant part of the dark matter has motivated revisiting old observational constraints, as well as developing new ones. We present new limits on the PBH abundance, from a comprehensive analysis of high-resolution, high-redshift Lyman- forest data. Poisson fluctuations in the PBH number density induce a small-scale power enhancement which departs from the standard cold dark matter prediction. Using a grid of hydrodynamic simulations exploring different values of astrophysical parameters, {we obtain a marginalized upper limit on the PBH mass of at , when a Gaussian prior on the reionization redshift is imposed, preventing its posterior distribution to peak on very high values, which are disfavoured by the most recent estimates obtained both through Cosmic Microwave Background and Inter-Galactic Medium observations. Such bound weakens to , when a conservative flat prior is instead assumed. Both limits significantly improves previous constraints from the same physical observable.} We also extend our predictions to non-monochromatic PBH mass distributions, ruling out large regions of the parameter space for some of the most viable PBH extended mass functions
Gravitational waves HI intensity mapping: cosmological and astrophysical applications
No full textTwo of the most rapidly growing observables in cosmology and astrophysics are gravitational waves (GW) and the neutral hydrogen (HI) distribution. In this work, we investigate the cross-correlation between resolved gravitational wave detections and HI signal from intensity mapping (IM) experiments. By using a tomographic approach with angular power spectra, including all projection effects, we explore possible applications of the combination of the Einstein Telescope and the SKAO intensity mapping surveys. We focus on three main topics: (i) statistical inference of the observed redshift distribution of GWs; (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. Our results show that: (i) the GW redshift distribution can be calibrated with good accuracy at low redshifts, without any assumptions on cosmology or astrophysics, potentially providing a way to probe astrophysical and cosmological models; (ii) the constrains on the dynamical dark energy parameters are competitive with IM-only experiments, in a complementary way and potentially with less systematics; (iii) it will be possible to detect a relatively small abundance of primordial black holes within the gravitational waves from resolved mergers. Our results extend towards GW × IM the promising field of multi-tracing cosmology and astrophysics, which has the major advantage of allowing scientific investigations in ways that would not be possible by looking at single observables separately.Two of the most rapidly growing observables in cosmology and astrophysics are gravitational waves (GW) and the neutral hydrogen (HI) distribution. In this work, we investigate the cross-correlation between resolved gravitational wave detections and HI signal from intensity mapping (IM) experiments. By using a tomographic approach with angular power spectra, including all projection effects, we explore possible applications of the combination of the Einstein Telescope and the SKAO intensity mapping surveys. We focus on three main topics: \textit{(i)} statistical inference of the observed redshift distribution of GWs; \textit{(ii)} constraints on dynamical dark energy models as an example of cosmological studies; \textit{(iii)} determination of the nature of the progenitors of merging binary black holes, distinguishing between primordial and astrophysical origin. Our results show that: \textit{(i)} the GW redshift distribution can be calibrated with good accuracy at low redshifts, without any assumptions on cosmology or astrophysics, potentially providing a way to probe astrophysical and cosmological models; \textit{(ii)} the constrains on the dynamical dark energy parameters are competitive with IM-only experiments, in a complementary way and potentially with less systematics; \textit{(iii)} it will be possible to detect a relatively small abundance of primordial black holes within the gravitational waves from resolved mergers. Our results extend towards the promising field of multi-tracing cosmology and astrophysics, which has the major advantage of allowing scientific investigations in ways that would not be possible by looking at single observables separately
Intensity and anisotropies of the stochastic gravitational wave background from merging compact binaries in galaxies
No full textWe investigate the isotropic and anisotropic components of the Stochastic Gravitational
Wave Background (SGWB) originated from unresolved merging compact binaries in
galaxies. We base our analysis on an empirical approach to galactic astrophysics that allows
to follow the evolution of individual systems. We then characterize the energy density of
the SGWB as a tracer of the total matter density, in order to compute the angular power
spectrum of anisotropies with the Cosmic Linear Anisotropy Solving System (CLASS) public
code in full generality. We obtain predictions for the isotropic energy density and for the
angular power spectrum of the SGWB anisotropies, and study the prospect for their observations
with advanced Laser Interferometer Gravitational-Wave and Virgo Observatories
and with the Einstein Telescope. We identify the contributions coming from different type
of sources (binary black holes, binary neutron stars and black hole-neutron star) and from
different redshifts. We examine in detail the spectral shape of the energy density for all types
of sources, comparing the results for the two detectors. We find that the power spectrum of
the SGWB anisotropies behaves like a power law on large angular scales and drops at small
scales: we explain this behavior in terms of the redshift distribution of sources that contribute
most to the signal, and of the sensitivities of the two detectors. Finally, we simulate a high
resolution full sky map of the SGWB starting from the power spectra obtained with CLASS
and including Poisson statistics and clustering properties
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
La radiazione cosmica di fondo nelle microonde: anisotropie di intensità e loro significato
Full text linkLo studio delle fluttuazioni in temperatura dello spettro della Radiazione Cosmica di Fondo fornisce forti limiti sulle stime di numerosi parametri cosmologici, nonchè la possibilità di confutare o avvalorare teorie
cosmologiche. L'elaborato presta particolare attenzione alla relazione tra anisotropie, cause fisiche e stima di alcuni parametri cosmologici, con particolare riferimento ai risultati ottenuti nel Febbraio 2015 dalla missione Planck
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Determining the progenitors of merging black-hole binaries with general relativistic galaxy correlations
Full text linkThe detection of binary black hole mergers through gravitational waves by the LIGO-Virgo instrument sparked the discussion on whether they have astrophysical or primordial origin. According to a currently popular model, primordial black holes of stellar mass could constitute a relevant fraction of the dark matter. This work aims to forecast the possibility to infer the nature of the binary black hole progenitors through cross-correlations between galaxy catalogs and gravitational wave maps.ope
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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