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
An important unsolved problem that affects practically all attempts to connect string theory to cosmology and phenomenology is how to distinguish effective field theories belonging to the string... Show moreAn important unsolved problem that affects practically all attempts to connect string theory to cosmology and phenomenology is how to distinguish effective field theories belonging to the string landscape from those that are not consistent with a quantum theory of gravity at high energies (the ``string swampland{''}). It was recently proposed that potentials of the string landscape must satisfy at least two conditions, the ``swampland criteria{''}, that severely restrict the types of cosmological dynamics they can sustain. The first criterion states that the (multi-field) effective field theory description is only valid over a field displacement Delta phi <= Delta similar to O (1) (in units where the Planck mass is 1), measured as a distance in the target space geometry. A second, more recent, criterion asserts that, whenever the potential V is positive, its slope must be bounded from below, and suggests vertical bar del V vertical bar/V >= c similar to O (1). A recent analysis concluded that these two conditions taken together practically rule out slow-roll models of inflation. In this note we show that the two conditions rule out inflationary backgrounds that follow geodesic trajectories in field space, but not those following curved, non-geodesic, trajectories (which are parametrized by a non-vanishing bending rate Omega of the multi-field trajectory). We derive a universal lower bound on Omega (relative to the Hubble parameter H) as a function of Delta,c and the number of efolds N-e, assumed to be at least of order 60. If later studies confirm c and Delta to be strictly O (1), the bound implies strong turns with Omega/H >= 3 N-e similar to 180. Slow-roll inflation in the landscape is not ruled out, but it is strongly multi-field.}} 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
In this thesis we study some aspects of the very early Universe. We focus on the period of inflation, and show how the presence of many fields during this period can affect its low energy... Show moreIn this thesis we study some aspects of the very early Universe. We focus on the period of inflation, and show how the presence of many fields during this period can affect its low energy description. We focus on the predictions for the two and three point correlation functions of the curvature perturbations, in the case in which apart from the inflaton field there is a very heavy isocurvature field participating in the dynamics. We also treat briefly the case in which the isocurvature field is light, in particular applied to the case of so-called natural inflation. Show less
This thesis consists of two different parts, separating research projects carried out in two different groups. In the first and longest part of this thesis, we attempt to fit the signal for a... Show moreThis thesis consists of two different parts, separating research projects carried out in two different groups. In the first and longest part of this thesis, we attempt to fit the signal for a reduction in the speed of sound of the inflaton. In chapter 1, we shortly introduce the topics discussed in this thesis, namely _CDM cosmology, transient reductions in the speed of sound during inflation, and Bayesian statistical inference. Afterwards, we attempt to fit a particular hypothesis for the speed of sound reduction using Cosmic Microwave Background data (chapter 2) and later adding Large Scale Structure data to the search (chapter 3). Finally, in chapter 4 we present two pieces of code that were elaborated for the research in this thesis, and later released to the community. In the second part, consisting solely of chapter 5, we present a classification of all possible 6-dimensional symmetric toroidal orbifolds over which Heterotic String Theory leads to a supersymmetric model. To do that, we made use of standard crystallographic tools. Show less
Cosmological inflation is the most successful theory that explains the homogeneity and flatness of the early universe. It also provides a quantum origin for the primordial perturbations that we... Show moreCosmological inflation is the most successful theory that explains the homogeneity and flatness of the early universe. It also provides a quantum origin for the primordial perturbations that we observe in the Cosmic Microwave Background Radiation (CMB). The simplest models make use of a single scalar field, which produces the exponential expansion of the early universe. In this thesis the effects of additional heavy fields is studied from several points of view. On the one hand, possible signatures due to the presence of such fields are searched for in the current CMB data, and a new formalism is developed, allowing the analytical study of features arising from the presence of these fields. On the other hand, in the context of supergravity theories, the presence of additional heavy fields is ubiquitous, and their stability is needed in order to not spoil the basic predictions of single-field inflation. The viability of inflation in the presence of an additional heavy supersymmetric sector is studied, and several stability constraints are derived. The conclusion is that CMB data might be already sensitive enough to the presence of additional heavy fields, and that the landscape of supergravity scenarios which stabilize these fields is very constrained Show less
The theory describing physics at the highest energy scales likely contains extra dimensions, whose internal degrees of freedom result in many massive field and particles. At accelerator experiments... Show moreThe theory describing physics at the highest energy scales likely contains extra dimensions, whose internal degrees of freedom result in many massive field and particles. At accelerator experiments these fields and particles generally decouple from the low energy physics. However, in cosmology gravity couples everything, thereby invalidating the decoupling assumption. In this thesis we have shown that massive particles and field that do not decouple during cosmological inflation will generate corrections, which lead to possibly observable features in the Cosmic Microwave Background. Furthermore, in specific setups where the massive particles and fields do decouple their stability can still be affected by the low energy physics energy scale Show less
