Modified Gravity and Large Scale Structure Cosmology: a linear and non-linear treatment
Name: GUILHERME BRANDO DE OLIVEIRA
Type: PhD thesis
Publication date: 10/06/2022
Advisor:
Name |
Role |
|---|---|
| FELIPE TOVAR FALCIANO | Advisor * |
Examining board:
| Name |
Role |
|---|---|
| FELIPE TOVAR FALCIANO | Advisor * |
| JÚLIO CÉSAR FABRIS | Internal Examiner * |
| MIGUEL BOAVISTA QUARTIN | External Examiner * |
Summary: This thesis consists of a broad study on cosmologies beyond ΛCDM, in particular the possible consequences of scalar-tensor theories of gravity on the large-scale structure of the Universe were studied. Within the Standard Model of Cosmology, General Relativity is taken
as the theory that describes gravity at all scales, a fact that is supported in Astrophysical and Solar System tests. However, on cosmological scales, gravitational tests still do not have the same binding power. In this way, the study of alternative theories of gravity, in addition to the usual motivation to explain conceptual problems of the Cosmological Constant, is still extremely necessary.
To increase the power of precision in gravity tests on cosmological scales, I developed numerical tools based on cosmological perturbation theory, in linear and non-linear order, as well as a non-perturbative treatment using quasi N-body simulations.
I also expose different ways to test the great freedom we have in the parameter space introduced by modified gravity theories. In fact, when working with such models, the introduction of extra parameters, in addition to the six cosmological parameters of the ΛCDM model, is inevitable.
The main results of this thesis were published in four articles, cited throughout the text, and their contents were condensed in Chapters 3, 4 and 6. In Chapter 3, the impact that alternative gravity theories have on cosmological observables is discussed. In particular, I present a detailed, linear-level analysis of the No Slip gravity model, and how this theory impacts structure formation and light propagation. Furthermore, in the same chapter, I briefly discuss how primitive gravity theories modify the power spectrum at large and small scales.
In Chapter 4, I start by analyzing the power spectrum of matter at linear scales, specifically how it is defined in the ΛCDM model, and how massive neutrinos introduce a scale dependence on the growth function. Afterwards, I introduce the formulation of the N-body gauge, a specific coordinate system that facilitates the interpretation of Newtonian simulations in a relativistic context. This is possible through the introduction, in a manner consistent with the Newtonian treatment, of relativistic species, such as photons, neutrinos and dark energy. How stage-IV cosmological surveys survey the Universe on ever-larger scales; it is imperative that we include such species in our analyses, as their effect on large scales can go beyond the 1% threshold when compared to a Universe with only dark matter. At the end of the chapter I present how we can combine modified gravitation with Newtonian simulations.
In Chapter 5, I expose all the mathematical tools at the nonlinear level that I studied and developed throughout this thesis. And, in Chapter 6, I present the results of how we can build computationally fast tools using all the tooling I developed in modified gravity, going from linear to nonlinear scales. Chapter 7 closes the present test with some conclusions, and three possible lines of future research that I aim to develop in the next few years.
