Astrofísica, Cosmologia e Gravitação

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http://repositorio.ufes.br/handle/10/17488
Programa de Pós-Graduação em Astrofísica, Cosmologia e Gravitação

Centro: CCE
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URL do programa: https://cosmologia.ufes.br/pt-br/pos-graduacao/PPGCosmo

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    Quantum effects in cosmology
    (Universidade Federal do Espírito Santo, 2020-08-28) Frion, Emmanuel; Pinto Neto, Nelson; https://orcid.org/0000-0001-6713-5290; http://lattes.cnpq.br/6196081550581346; https://orcid.org/0000000312800315; Fabris, Julio Cesar; https://orcid.org/000000018880107X; http://lattes.cnpq.br/5193649615872035; Rodrigues, Davi Cabral; https://orcid.org/0000000316835443; http://lattes.cnpq.br/5465449494182034; Bergliaffa, Santiago Esteban Perez; Alcaniz, Jailson Souza de; Peter, Patrick; Wands, David Graham
    Even though predictions from inflationary models fit observations with great accuracy, inflation is not a theory, and it is therefore important to look at alternative mechanisms whose phenomenology can be constrained by cosmological data. It is possible to explain the origin of large-scale structures through bouncing models, taking into account the evolution of quantum perturbations in both a contracting phase and an expansion phase. This thesis is motivated by the effects of quantum perturbations on cosmological models. We first focus on a semi-classical description of quantum perturbations in the form of stochastic noise in a collapsing universe. The growth of perturbations is a fundamental issue in bouncing cosmologies that any acceptable model must treat accurately. To this end, we quantified how quantum perturbations may overcome the classical energy density in the simple case of a massless field with exponential potential. This is generally not the case within this configuration, although there is an important growth of quantum diffusion in the case of a matter-dominated model, which could possibly drive the system away from the classical evolution. This stochastic collapse is the first step towards a complete stochastic bouncing model. Numerous non-singular bouncing models resolve the initial singularity issue thanks to quantum effects. They constitute a broad class of relevant cosmological models, since they solve many problems of standard inflation. A subclass is obtained by considering the canonical quantisation of general relativity using the de Broglie-Bohm interpretation of quantum mechanics. In the second part of the thesis, we show that the generation of magnetic fields in such models compatible with observations on cosmological scales can be obtained with a simple coupling between gravity and electromagnetism. Interestingly, bouncing magnetogenesis have intrinsically less issues than inflationary magnetogenesis. The model presented here shows that acceptable magnetic fields can be obtained, depending on the energy scale of the coupling and the time when the bounce occurs. To close the thesis, we finish with a bouncing model obtained from the affine quantisation of the Brans-Dicke theory. Contrary to canonical quantisation, the affine procedure needs less assumptions and kinetic energy terms possess a quantum potential regularising the dynamics, resulting in a smooth bounce. Another advantage of this method lies in the choice of mathematical models one can use to tackle the same physical problem. We employ this asset to deal with the quantum equivalence of Jordan and Einstein frames, an issue of modified gravity models. The results point toward an unitary equivalence of the frames. We conclude with a summary of achievements.