Exploring Signatures of New Physics in Cosmology

Abstract

Cosmology is the study of the origin and evolution of our Universe as a whole. Even if the theoretical framework of Cosmology was developed a century ago, with the formulation of General Relativity by Albert Einstein, it was only during the last decades that we have seen an improvement in experimental capabilities so relevant to transform Cosmology from a “data-scarce” to a “data-driven” science. This thesis is divided in five parts. The first one is constituted by an introductory chapter which describes the standard model which we use today in Cosmology, the ΛCDM model. The main ingredients of this model are a theory of gravity that describes as the Universe evolves, in this case General Relativity; the different components existing in our Universe, namely photons, neutrinos, baryons, cold dark matter (CDM) and dark energy, described by a cosmological constant Λ; and a theory explaining the initial conditions of the Universe, which we assume to be Inflation. Even if we can describe many of these aspects in great detail, we still have several open problems in the three topics (theory of gravity, components of the Universe and initial conditions theory). The second part is focused on the possible degeneracy between the effects that neutrinos y modified gravity theory have on cosmological observables. In the paper “Hiding neutrino masses in modified gravity cosmology” we investigated how Horndeski theory has enough freedom to reproduce a cosmological expansion as in ΛCDM and, at the same time, to boost the growth of structures at large scales. In fact this growth could hide the effects that massive neutrinos have in the matter power spectrum. In the third part we discuss about one of the dark matter candidate, primordial black holes. In “Primordial black holes as dark matter: converting constraints from monochromatic to extended mass distributions” we show how it is possible to obtain upper limits on the abundance of primordial black holes with an extended mass distribution starting from upper limits obtained assuming a monochromatic mass distribution. We also prove that for lognormal and power-law distributions the constraints on primordial black holes abundance in the 10 solar masses window are tighter with respect to the monochromatic case. In “GW×LSS: chasing the progenitors of merging binary black holes” we explain how, correlating gravitational waves maps and galaxies maps, we can understand if the origin of the black holes that form the detected binaries is stellar or primordial. The fourth part describes how we can obtain new probes of the first fractions of seconds of our Universe. In “Measuring the energy scale of inflation using large scale structure” we show how we can measure the energy scale of inflation through the measurement of a specific primordial non-Gaussianity signal, called “graviton exchange”. In particular we show that this primordial signal is of the same order of magnitude of the three-point function at large scale, opening the possibility of a detection. In “From primordial black holes abundance to primordial curvature power spectrum (and back)” we use the primordial black holes to put constraints on the maximum amplitude of the primordial curvature power spectrum. Specifically, we developed a procedure which connects numerical simulations of primordial black holes formation to a correct cosmological interpretation of these simulations, to a calculation of the abundance of these objects using peak theory. The fifth part includes the summary of the results found and a discussion of those very results. Moreover we discuss about future perspectives and about how to extend these works in future scientific projects.

La Cosmología es el estudio de las orígenes y evolución de nuestro Universo como un todo. Aunque bajo el lado teórico la Cosmología nació hace un siglo, se estableció como una ciencia “guiada por los datos” unicamente en las ultimas décadas. La primera de la tesis está constituida por un capítulo introductorio que describe el modelo estándar que se utiliza hoy en Cosmología, el modelo ΛCDM. La segunda parte se centra en la degeneración entre efectos de neutrinos y teorías de gravedad modificada. En “Hiding neutrino masses in modified gravity cosmology” se investigó como la teoría de Horndeski tenga suficiente libertad para reproducir una expansión como la que predice el ΛCDM y pueda incrementar el crecimiento de las estructura, escondiendo el efecto de neutrinos masivos. En la tercera parte se debate de agujeros negros primordiales. En “Primordial black holes as dark matter: converting constraints from monochromatic to extended mass distributions” se enseña como, desde los límites existentes para agujeros negros primordiales con distribución de masa monocromática, sea posible obtener límites en la abundancia de agujeros negros primordiales que tengan una distribución de masa extendida. En “GW×LSS: chasing the progenitors of merging binary black holes” se explica cómo, relacionando mapas de galaxias y mapas de ondas gravitatorias, se puede llegar a entender cuál es el origen, estelar o primordial, de los agujeros negros que forman las binarias detectadas. La cuarta parte describe como se pueda obtener nuevas pruebas sobre el Universo primigenio. En “Measuring the energy scale of inflation using large scale structure” enseñamos como se podría medir la escala energética de la Inflación mediante la medida de una señal específica de no-Gaussianidad primordial, llamada “graviton exchange”. En “From primordial black holes abundance to primordial curvature power spectrum (and back)” se utilizan los agujeros negros primordiales para poner límites en la amplitud máxima que pueda tener el espectro de potencia de la curvatura primordial. La quinta parte incluye el resumen de los resultados encontrados, una discusión de esos resultados y se debate sobre cuáles son las perspectivas futuras de estos trabajos.