Abstract
The standard model of cosmology, namely the CDM model, is based on Einstein's theory of General Relativity (GR) with a Cold Dark Matter (CDM) content and a positive cosmological constant , in addition to ordinary matter and radiation components. While it provides a paradigm in very good agreement with many observations, from Big Bang Nucleosynthesis (BBN) to Cosmic Microwave Background (CMB), several questions remain open and various theoretical extensions seem necessary in order to address them.An extensively studied ingredient of the CDM model is the inflationary scenario, which solves some of the issues associated with the initial conditions that the original hot Big Bang model cannot address, such as the homogeneity and flatness problems. Furthermore, it ts very well with current data, in particular, the spectrum of temperature anisotropies in the CMB. As we recall in Chapter 2, in some scenarii, the end of inflation may lead to the formation of Cosmic Strings (CS) or Cosmic SuperStrings (CSS), which can have a significant impact on some observables, even though they have been proven not to be the main source of CMB anisotropies. We focus on a particular phenomenological consequence of C(S)S, Gravitational Waves (GWs), which are becoming an important tool to gather new information on our universe. More specifically, energetic high frequency GW Bursts (GWB) are thought to be emitted by cusps, which are points on C(S)S temporarily reaching the speed of light. We investigate the occurrence of such phenomena in a particular setup where a light string is stretched between two heavy, almost fixed strings, as could appear in a C(S)S network. First, an analytical study allows us to draw simplifying hypotheses, such as the periodicity of the noninteracting movement of the string, and yields an effective rule to identify cuspy strings. In addition, we implement these assumptions in a numerical simulation, which settles the free parameter of this criterion. Also, the string and the network parameters are found to influence strongly the average number of cusps and thus the amount of energy released in the form of GWB. In particular, both the analytical and numerical studies demonstrate that the smaller the correlation length is (that is, the wavier the string is), the more cusps the string holds.
String/Mtheory yields a large variety of scenarii and thus a large phenomenological diversity, from inflation to Dark Matter (DM) candidates. It generally implies additional dimensions and additional ingredients, such as scalar fields (often involved with inflation) or extended objects (such as Cosmic SuperStrings). It can also provide a description of our universe, on which we focus in Chapter 3, in which all fields but the graviton live on a (3+1) brane, itself embedded in a largerdimensional bulk. We consider a model where the bulk is populated with a gas of punctual, effectively 0dimensional defects, which interact with our brane universe. Their collisions with open strings attached to the brane generate a recoil velocity of such D0branes, later called Dparticles. This additional vector eld acts as a new content of the universe, which from the low energy point of view behaves as a Dark Matter/Dark Energy (DE) mixture. The modifications of the graviton equations of motion are related to its squared field strength, which under certain circumstances condensate and plays the r^ole of an extra scalar field. This model, called the Dmaterial universe, can not only give a mechanism for the growth of large scale structure but, as we show here, can also lead to a successful inflationary scenario, the condensate appearing as the slowly rolling inflaton. Moreover, it provides an effective DM fluid which fits restricted  by our model's hypotheses  lensing data, thus diminishing the need of conventional DM without overclosing the universe. Finally, this supplementary ingredient alters the graviton propagation as it brings in an effective mass term and affects the refractive index experienced by radiations. This study, which spans several cosmological eras and covers several length scales, leads to constraints on the free parameters of the model including the number density of Dparticles and the string scale.
Such analyses of models beyond the CDM model may provide important information  alternative exploration routes as well as additional possible bounds on the parameters  that would help us understand the dynamics of our universe.
Date of Award  2017 

Original language  English 
Awarding Institution 

Supervisor  Eugene Lim (Supervisor) & Mairi Sakellariadou (Supervisor) 