The first direct detection of gravitational waves opened the possibility of mapping the Universe via this new and independent messenger. Indeed, during their propagation, gravitational waves pick... Show moreThe first direct detection of gravitational waves opened the possibility of mapping the Universe via this new and independent messenger. Indeed, during their propagation, gravitational waves pick up information about the spacetime as they are affected by its expansion and by the matter structures along the propagation path. The aim of this Thesis is to investigate which cosmological information is accessible from a gravitational wave detection, with a specific interest in the late time Universe. Show less
The ultimate goal of cosmologists is to find a cosmological model able to explain the current observational data. In this sense, the Standard Cosmological model establishes that our universe is... Show moreThe ultimate goal of cosmologists is to find a cosmological model able to explain the current observational data. In this sense, the Standard Cosmological model establishes that our universe is mainly composed of two unknown components: a type of matter that is known to only interact through gravitation, Cold Dark Matter, and a substance responsible for the current accelerated expansion of the universe that can be modelled by a cosmological constant. Still, this model, though successful, fails to answer hot-burning questions in the field. For this reason, theoretical cosmologists focus on developing further modifications of the model to test them against astrophysical data and check whether alternative scenarios can provide a better explanation of the observations.This thesis is dedicated to the Bayesian statistical analyses of extensions of the Standard Cosmological model using several astronomical data sets, and to the forecast of new observables and experiments. The first part focuses on data science and inflation, and it aims to constrain inflationary models using advanced inference techniques. The second part of the thesis is dedicated to the novel concept of cross-correlations of gravitational-wave physics and large scale structure observables. The third part of this thesis is dedicated to the incoming ESA Euclid satellite, and in particular, it focuses on a crucial data science analysis software for the mission: the code “Cosmological Likelihood for Observables in Euclid”, also known as CLOE. Show less
Three major cosmology-focused missions are planned for the next decade: the Euclid space telescope, the Vera C. Rubin Observatory in Chile, and the Nancy Grace Roman Space Telescope. Their surveys... Show moreThree major cosmology-focused missions are planned for the next decade: the Euclid space telescope, the Vera C. Rubin Observatory in Chile, and the Nancy Grace Roman Space Telescope. Their surveys will detect billions of galaxies over more than a third of the celestial sphere and out to redshifts of z~2 to constrain the evolving matterdistribution using weak gravitational lensing. This thesis focusses on ways of disentangling the contribution of normal matter, baryons for cosmologists, from the measured weak lensing signal for cosmic shear analyses and galaxy cluster mass calibrations. Combining the predictions of computer-simulated universes, evolving billions of particles in time, with simplified models that reproduce the observeddistribution of hot gas inside clusters of galaxies, but that freely vary the amount of matter where no observations are available, we have quantified how strongly our ignorance of the relation between ordinary and dark matter will affect the analysis of the planned surveys. Additionally, we have studied how baryons affect cluster massdeterminations and we suggest a new analysis method for cluster abundance studies that is less sensitive to our lack of knowledge of the exact distribution of normal matter in the outskirts of galaxy clusters. Show less
At the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The... Show moreAt the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The second is dark energy, an enigmatic component responsible for the accelerated expansion of the Universe. Under these two forces, matter in the Universe organizes itself in the so-called cosmic web. The nodes of this network are large dark matter haloes, and this thesis explores how their boundaries provide information about the nature of dark energy and cosmology. Chapters 3 and 4 present robust theoretical predictions for this titular edge and discuss its simple physical interpretation. Chapters 2 and 5 corroborate these results by presenting measurements of this feature in weak-lensing data. The last scientific chapter of this thesis is a collection of studies in gravitational-wave physics. This chapter explores how these spacetime ripples observed from across the cosmos can be used to detect alternative theories of gravity. Show less
In astronomy, the interpration of observations and measurements plays a crucial role: we rely purely and fundamentally on the information that reaches us as observers. And 80% of all matter in the... Show moreIn astronomy, the interpration of observations and measurements plays a crucial role: we rely purely and fundamentally on the information that reaches us as observers. And 80% of all matter in the universe is undetectable directly.This so called 'dark matter' can be observed indirectly, by the gravitational effect on its surroundings and the curvature of spacetime predicted by the theory of relativity.In this thesis, we combine scientific research, a description of our universe, with theoretical considerations of the statistical methods used for that research, or how we can deduce that description from the 'reflections' that we see. In this work, we focus on the matter distribution in groups and clusters of galaxies, and consider the intricacies of the method of weak gravitational lensing that we use, respectively. Show less
Galaxies form and live inside dark matter haloes. As a consequence, they are exposed to the tidal fields generated by the surrounding matter distribution: this imprints a preferential direction to... Show moreGalaxies form and live inside dark matter haloes. As a consequence, they are exposed to the tidal fields generated by the surrounding matter distribution: this imprints a preferential direction to the galaxy shapes, which leads to a coherent alignment on physically close galaxies, called intrinsic alignment. Intrinsic alignment is an important contaminant to weak lensing, which instead uses the correlation of galaxy shapes caused by the lensing effect of the matter distribution along the line of sight to infer the amount and the distribution of matter in the Universe.This dissertation studies the dependence of intrinsic alignment on galaxy properties such as luminosity, redshift and halo mass, using different techniques to measure it. It presents a model to account for the scale and sample dependence of the intrinsic alignment signal when modelling it in weak lensing studies. It also investigates the amount of biasing that incorrect modelling of intrinsic alignment would induce in the inferred cosmological parameters for ongoing and future surveys. The potential of weak lensing magnification is also explored to help constrain the cosmological parameters in upcoming surveys. Show less
This thesis is dedicated to the exploration of the primordial dark ages: unknown physics during the earliest stages of the Universe’s expansion that have not yet been directly probed by... Show moreThis thesis is dedicated to the exploration of the primordial dark ages: unknown physics during the earliest stages of the Universe’s expansion that have not yet been directly probed by observations. Cosmic inflation is a burst of exponential expansion of space after the “Big Bang”. The energy that drives inflation must be transferred to elementary particles and radiation. This process is called reheating. The unknown expansion history of the universe during the reheating era connects the cosmic microwave background (CMB) observations to inflationary physics. CMB is a relic radiation that provides us a snapshot of the primordial universe. Both the inflationary and reheating eras generate signatures to be seen via upcoming gravitational waves and CMB polarization experiments. In this thesis we show analytically a scaling behaviour that allows for an easy estimate of the reheating efficiency for one broad family of multi-field models of inflation that is called α-attractors. We show the influence of the asymmetry around the minimum of potential on the reheating efficiency. Moreover, we study the predictions for chiral gravitational waves production by a spectator gauge field sector in scalar single-field inflation. Finally, we present a new class of inflationary models that is called “shift-symmetric orbital inflation”. Show less
The civilizations of ancient China and Mesoamerica, although not historically related and obviously different in many specific aspects (such as language and writing), show many interesting... Show moreThe civilizations of ancient China and Mesoamerica, although not historically related and obviously different in many specific aspects (such as language and writing), show many interesting commonalities and similar tendencies in their cultural manifestations. The aim of this thesis is to examine the validity of contemporary cosmological interpretations of architecture and related visual art in both regions.In comparing the cosmological characteristics and the performance in the architecture of ancient China and Mesoamerica this thesis has a multidisciplinary focus (considering architecture, archeology, astronomy, history and anthropology). Through comparison, it is found that the two have many similarities in their cosmological theory: they have similar azimuth characteristics (associated with colors, directional gods, seasons, life and death, etc.), their buildings are rich in space orientation; they both have Three Realms (Heaven, Earth, Underworld) and a similar life and death concept, especially they all worship their ancestors; furthermore, they use a similar Calendar Round in their respective calendars. In the performance of architecture, they use their own methods to integrate the concept of cosmology into architectural creation, which makes their architecture establish connections among the cosmos, gods, and time. Show less
The theoretical explanation of cosmic acceleration is nowadays one of the biggest puzzles in cosmology. Within the standard cosmological model (LCDM) the expansion is sourced by the vacuum energy... Show moreThe theoretical explanation of cosmic acceleration is nowadays one of the biggest puzzles in cosmology. Within the standard cosmological model (LCDM) the expansion is sourced by the vacuum energy associatedto the Cosmological Constant L. Despite its simplicity, the Cosmological Constant presents various unresolved problems from both the theoretical and the observational side.However, even if we dismiss these puzzles, the study of theoretical alternatives to LCDM is still of primary importance. In fact, the wealth and quality of cosmological data that we are expecting for thenext decade will allow us to test gravity on cosmological scales with unprecedented accuracy. This will give us the chance to investigate many of our theoretical ideas and to assess the strength of the standard model of cosmology on the largest scales.In this thesis we present different approaches that we can adopt to study modifications of gravity by means of cosmology. Show less
This thesis contributes to studying primordial cosmology theories and their detectability in future observations. The first part of the thesis studies a class of inflation models with curved field... Show moreThis thesis contributes to studying primordial cosmology theories and their detectability in future observations. The first part of the thesis studies a class of inflation models with curved field spaces, which are typically motivated in high energy physics theories. The second part of the thesis focuses on one particularly important cosmological observable -- primordial non-Gaussianity, whose phenomenology may reveal new physics effects in the very early Universe. Show less
The focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In... Show moreThe focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In the first chapter I have given an overview of modern cosmology. Particularly, an introduction to cosmological perturbation theory, and a general overview of the cosmological standard model, as well as an overview to beyond-standard-model scenarios is presented. Chapter two discusses cosmological models based on the so-called alpha-attractor framework. Chapter three is dedicated to the study of constraints imposed by gravitational wave observations on the so-called doubly-coupled massive bimetric gravity theory. Chapter four studies the cosmological stability of the so-called massive mimetic gravity theory. Finally, Chapter five studies the effect of the Symmetron gravity on the so-called splashback radius of dark matter halos. Show less
In the early universe, the dynamics of the Higgs field can give rise to many interesting phenomena. In the first part of this thesis, we study the behavior of the Higgs field during the reheating... Show moreIn the early universe, the dynamics of the Higgs field can give rise to many interesting phenomena. In the first part of this thesis, we study the behavior of the Higgs field during the reheating phase after cosmological inflation. We investigate the stability of the electroweak vacuum when the Higgs field is nonminimally coupled to gravity. We also study reheating in the scenario where the Higgs field itself drives inflation. In the second part, we study the possibility of generating the asymmetry between matter and antimatter during the electroweak phase transition -- the moment when the Higgs field starts to give mass to the other particles of the Standard Model. We show that it is not possible to constrain this process in a model-independent way. We also demonstrate that the predicted value of the baryon asymmetry is enhanced when leptons are included in the computation. Show less
This thesis is about cosmological inflation and its relation to observations. In part I we study the observational consequences of an additional scalar field besides the inflaton field. In... Show moreThis thesis is about cosmological inflation and its relation to observations. In part I we study the observational consequences of an additional scalar field besides the inflaton field. In particular, we focus on several different regimes where we vary both the coupling between the fields and the mass of the second field. In part II we perform a statistical analysis to understand whether we can extract some of the traces of new physics present during inflation from the three-dimensional map of galaxies in our universe. Show less
Of all the mass in our Universe, 80% is thought to consist of a hypothetical and invisible substance called dark matter (DM). So far, all observations of DM are based on its gravitational... Show moreOf all the mass in our Universe, 80% is thought to consist of a hypothetical and invisible substance called dark matter (DM). So far, all observations of DM are based on its gravitational interaction, either through the dynamics of normal (baryonic) matter or through the deflection of light. The latter approach, called ‘gravitational lensing’, is a unique way to probe the distribution of DM without making any assumptions on its dynamical state, and on scales larger than the extent of baryons. Using weak gravitational lensing with the Kilo-Degree Survey (KiDS), we first study the relation between galaxies and their dark matter halos on the scale of individual galaxies and galaxy groups. We then attempt to measure the effect of the local and large scale (cosmic web) density distribution on galaxies and halos, and we measure the interplay between galactic and DM structures at the scale of the cosmic web. Finally, we perform the first test of Verlinde’s theory of Emergent Gravity, all with the ultimate goal of gleaning some insight into the possible nature of the elusive ‘missing mass’. Show less
Improvements of weak gravitational lensing shape measurements are presented and some used for data analysis. Accurate estimates of masses are calculated for clusters of galaxies and member... Show moreImprovements of weak gravitational lensing shape measurements are presented and some used for data analysis. Accurate estimates of masses are calculated for clusters of galaxies and member galaxies. Show less
In this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions... Show moreIn this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions between ions and molecules have reached a steady state. However, this assumption may not be valid if the physical conditions of the gas are evolving rapidly. Therefore, we developed a chemical model to follow the non-equilibrium evolution of ions and molecules. We then incorporated this model into hydrodynamic simulations of galaxies. We ran simulations with different metallicities (i.e. different proportions of heavy elements) and UV radiation fields, first using our full non-equilibrium chemical model and then assuming chemical equilibrium. We found that the total star formation rate is higher at higher metallicity and for weaker radiation fields. In contrast, non-equilibrium chemistry does not strongly influence the total star formation rate or outflow properties of the galaxy. However, it does affect the abundances of individual chemical species, for example in molecular outflows. Finally, we explored the properties of molecular clouds in our simulations. At low metallicity, the molecular fraction of young clouds tends to be below equilibrium, as the molecules are still forming. This also affects the observable CO emission from young clouds. Show less
Ten billion years ago the Universe was at the peak of its star formation activity, which has been declining since then. This thesis investigates, with novel spectroscopic data from Hubble Space... Show moreTen billion years ago the Universe was at the peak of its star formation activity, which has been declining since then. This thesis investigates, with novel spectroscopic data from Hubble Space Telescope, the evolution of the galaxy population from that particular period, the so-called "Cosmic Noon", to the present epoch. The main topics addressed are the contribution of emission lines to the optical light of galaxies through cosmic time, the star formation rates of actively star-forming galaxies and quenched galaxies, and the evolution of the stellar ages of galaxies from 10 billion years ago to the current time. Show less
The question of how the first stars formed and assembled into galaxies lies at the frontier of modern astrophysics. The study of these first sources of cosmic illumination was transformed by the... Show moreThe question of how the first stars formed and assembled into galaxies lies at the frontier of modern astrophysics. The study of these first sources of cosmic illumination was transformed by the installation of new instrumentation aboard the Hubble Space Telescope during one of the final Space Shuttle missions in 2009. Hubble has since unveiled a population of ultra-faint galaxies seen just a few hundred million years after the Big Bang, an epoch often termed the Cosmic Dawn. This thesis presents pioneering observational studies of the first generations of galaxies, enabling an examination of their properties and the physics that governed the illumination of the early cosmos. Show less
Galaxies have changed drastically over the past 10 billion years. This thesis deals with these changes, focusing on evolution in the structure of very massive galaxies with a range of stellar... Show moreGalaxies have changed drastically over the past 10 billion years. This thesis deals with these changes, focusing on evolution in the structure of very massive galaxies with a range of stellar population properties. The main subjects addressed are the rapid changes in the sizes of old galaxies, the gradients in stellar population content within galaxies, and the predictions from theoretical models regarding these properties. Show less
In this PhD thesis, I describe work done in the CosmoGrid and AMUSE projects. CosmoGrid is a _CDM N-body simulation containing 8.5 billion dark matter particles within a volume of only (30 Mpc)^3,... Show moreIn this PhD thesis, I describe work done in the CosmoGrid and AMUSE projects. CosmoGrid is a _CDM N-body simulation containing 8.5 billion dark matter particles within a volume of only (30 Mpc)^3, resulting in very high mass resolution. In order to run such a large simulation, we investigate a method of combining multiple supercomputers via a fast network; combining them into one large machine for the final calculation. We compare the results of CosmoGrid to previous _CDM simulations, and use the finalised CosmoGrid data to investigate how an aligned group of galaxies in a void region might have formed. Combining CosmoGrid with the Astrophysical Multipurpose Software Environment (AMUSE), we investigate a method to embed simulated star clusters in a large-scale environment such as a dark matter halo. Finally, we use this method to run self-consistent simulations of open star clusters embedded in a forming Milky Way-like galaxy. Show less
