Restrictions in the darksector of the universe andmodified gravity with largescale structure andgravitational waves

Resumen

The current paradigm of Cosmology is founded on the¿Cold Dark Matter (LCDM) prescription, thanks to the enormous amount of experiments in agreement with it. The building process of the LCDM paradigm has three pillars, namely: General Relativity, the Cosmological Principle and the presence of a puzzling Dark Sector. However, LCDM model is not able to answer all questions. For example, about the intrinsic nature of the Dark Sector, certain issues on the formation of structures in the Universe and current tensions between datasets. Consequently, the last word has yet to be spoken regarding the description of the Cosmos.Because of that, this PhD dissertation intends to be a step forward in the improvement of our cosmological knowledge.After two chapters devoted to the current concordance model and its main problems respectively, as an introduction, the main topic of this PhD thesis is presented in the third chapter: the so-called pure momentum transfer models. They are an alternative description of the Cosmos which add to the concordance scenario the presence of a momentum exchange between certain components of the Universe involving the Dark Sector. That kind of interactions preserve the background cosmology while modifying the perturbation regime, where precisely certain tensions have emerged in recent times. Moreover, we can also understand them as the macroscopic description of a microscopic force acting in the Dark Sector due to a yet invisible charge. The main implication of this pure momentum exchange is the freezing of the density perturbations of the pressureless fluid, leading to a late-time mechanism eraser of structure. In this thesis, we first present three different momentum trans-fer models to subsequently thoroughly study, both analytically and numerically,their background cosmology and their linear perturbation regime. We then explore all the different effects on several observables to later use the most recent data to constrain the model parameters along with the cosmological parameters. After that,we devote a chapter to the analyses of how future planned surveys will be capable or not of disentangling the presence of the pure momentum transfer interactions.In the final chapter, we focus on the very small scales where the non-linear physics takes place, as the next natural step once the background and linear cosmology has been studied along with the performance of future surveys. We analyse how these models affect the formation of structures in our Cosmos, using the first one presented before as a proxy. Finally, we summarise the work done and we present our conclusions together with the possible future prospects and open questions.