Gravitational waves are truly transparent to matter in the Universe and carry the information of the very early epoch. We show that the energy density spectrum of the primordial gravitational waves has characteristic features due to the successive changes in the relativistic degrees of freedom during the radiation era. Our calculations are solely based on the standard model of cosmology and particle physics, and therefore these features must exist. Our calculations significantly improve the previous ones which ignored these effects and predicted a smooth, featureless spectrum. Going back in time to the beginning of the radiation era, reheating of the Universe must have taken place after inflation for primordial nucleosynthesis to begin. We show that reheating occurs spontaneously in a broad class of inflation models with [scientific symbols] gravity (Ø is inflaton). The model does not require explicit couplings between Ø and bosonic or fermionic matter fields. The couplings arise spontaneously when Ø settles in the vacuum expectation value (vev) and oscillates. This mechanism allows inflaton quanta to decay into any fields which are not conformally invariant in [scientific symbols] gravity theories. Applying the above method, we study implications of the large-N species solution to the hierarchy problem, proposed by G. Dvali, for reheating after inflation. We show that, in this scenario, the decay rates of inflaton fields through gravitational decay channels are enhanced by a factor of N, and thus they decay into N species of the quantum fields very efficiently. Without violating energy conservation, cosmological consideration places non-trivial constraints on Dvali's solution to the hierarchy problem. Going back in time still further, we study the period just before the beginning of reheating, the era of coherent oscillation of scalar fields. We show that non-Gaussian primordial curvature perturbations appear temporarily in the coherent oscillation phase after multi-field inflation. We directly solve the evolution equation of non-Gaussianity on super-horizon scales caused by the non-linear influence of entropy perturbations on the curvature perturbations during this phase. We show that our approach precisely matches with the so-called "separate universe approach" or "δN formalism" by studying a simple quadratic two-field potential. / text

Inflationary cosmology is the leading candidate for explaining the homogeneity, isotropy and spatial flatness of the universe whilst also providing the mechanism for the seeding of large scale structure. The central theme of inflationary dynamics involves the evolution of a scalar field, called the inflaton, such that its potential drives an accelerated expansion. Warm inflation is the dynamical realization in which interactions between the inflaton and other fields can lead to dissipation of inflaton energy to other dynamical degrees of freedom. Heavy fields coupled to the inflaton mediate the transfer of inflaton energy to light degrees of freedom which thermalize and heat the universe. This damps the inflaton’s motion and allows for the potential formation of a thermal bath during the inflationary period. Hybrid inflation models are a natural way in which warm inflation can be realized, with dissipation of inflaton energy mediated by the waterfall fields to fields in the light sector. In this thesis I outline the dynamics and observational predictions of supersymmetric hybrid inflation driven by radiative corrections in the warm regime. As in the standard cold inflationary scenario inflation ends when the effective mass squared of the waterfall field becomes negative, with the tachyonic instability driving the system to a global minimum in a process called the waterfall transition. I present the effect of including thermal mass corrections to the waterfall fields, and SUSY mass splittings on the quantum effective potential and the resulting dissipation coefficient. I show that including dissipative effects can significantly prolong the inflationary period to produce 50-60 e-folds of inflation with an observationally consistent primordial spectrum. Inflation still requires a microphysical description within a fundamental theory of quantum gravity. This has prompted the search for inflaton candidates within the superabundance of scalar fields present in string theory compactifications, with brane-antibrane inflation in particular emerging as a concrete implementation of SUSY hybrid inflation in a UV complete particle physics model. Inflation proceeds in a brane-antibrane system through the movement of a stack of branes towards a stack of antibranes, with the inflaton field being the interbrane distance. Warm inflation can be implemented in a brane-antibrane system with dissipation of inflaton energy mediated by fields corresponding to strings stretched between the brane and antibrane stacks. It has been shown that this dissipation of inflaton energy in warm inflation can greatly alleviate the η-problem in brane-antibrane scenarios. Whilst these strings mediating dissipation have end points fixed on to both the D3 and D3 stacks, the compact nature of the geometry within which the system is constructed allows these strings to have different winding modes. We investigated how strings with increasing winding number can provide an enhancement to the dissipation coefficient, allowing a significant reduction in the number of branes and antibranes in the warm inflation system, whilst also modifying the inflationary dynamics by reducing the speed at which the system evolves. This may go some way to alleviating the η-problem associated with some constructions of brane-antibrane inflation whilst also potentially providing the best way to motivate the large field multiplicities associated with warm inflation models.

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-10-18T19:17:48Z No. of bitstreams: 1 MARIA APARECIDA DOS SANTOS – DISSERTAÇÃO (PPGFísica) 2014.pdf: 628300 bytes, checksum: be5188c733755263bec183578258ca27 (MD5) / Made available in DSpace on 2018-10-18T19:17:48Z (GMT). No. of bitstreams: 1 MARIA APARECIDA DOS SANTOS – DISSERTAÇÃO (PPGFísica) 2014.pdf: 628300 bytes, checksum: be5188c733755263bec183578258ca27 (MD5) Previous issue date: 2014-02 / Capes / Considerando as diferentes abordagens possíveis referentes ao Universo, este trabalho está voltado para o estudo da Cosmologia Padrão e Inflacionária utilizando campos escalares para descrever a fase de expansão acelerada do Universo. Assim, através da Teoria da Gravitação proposta pela Relatividade Geral é possível determinar as equações de Friedmann e utilizando a Teoria de Campos em Cosmologia podemos obter uma equação de movimento que descreve a evolução temporal de um campo escalar chamado ínflaton, responsável pela inflação. Nesse sentido, propomos como alternativa a utilização de alguns modelos de potenciais já existentes, dentre os quais: V ( ) =12m2 2 (quadr atico), V ( ) = C cos2 (tipo cosseno), V ( ) = C sin2 (tipo seno), V ( ) = (t) 4 e o potencial constante V = V0. Buscando dessa forma descrever a evolução temporal do fator de escala a(t) e o comportamento do parâmetro de desaceleração q(t) com o objetivo de analisar a fase inflacionária, identi cando regiões de aceleração e desaceleração do Universo nos cenários dos espaços plano e curvo. / Taking into consideration the set of di erent approaches to the Universe existent today this work focuses on standard cosmology and in ationary expansion of the said using scalar elds to describe the expansion acceleration rate. Therefore, through a gravitation theory proposed by General Relativity is possible to set Friedmann`s equations and using Field Theory applied to Cosmology to obtain an equation of motion which describes the temporal evolution of a scalar eld called in action, which is responsible for the in ationary process. In this sense, we propose as alternative some models whose potentials are already established, among them: V ( ) = 12m2 2 (quadratic), V ( ) = C cos2 (cosinelike) , V ( ) = C sin2 (sinelike), V ( ) = (t) 4 and the constant potential V = V0 . We seek with this to describe the temporal evolution of the scale factor a(t) and how the decelerating parameter behaves and then analyze the in ationary faze, indentifying periods when the Universe was accelerating or decelerating given curve or plane space scenarios.

This dissertation is focused on three lines of work. In the first part, we consider aspects of holography and gauge/gravity duality in lower and higher dimensions. In particular, we study the duality for exact solutions localized on the Randal-Sundrum 2-branes. We also test if some holographic principles in general relativity can be generalized to include higher derivative theories of gravity; namely Lovelock gravity. In the second part we consider the role of pseudo Nambu-Goldstone bosons (pNGBs) in inflationary cosmology. Specifically, we construct an inflationary model using string theory axions, and use these pNGBs to produce the observed coherent magnetic field in the Universe. The third part of the thesis is devoted to the study of the phenomenology of emergent phenomena. we investigated whether one could test if diffeomorphism invariance, the sacred symmetry of general relativity, is emergent. We also construct a new minimal vectorial Standard Model, and argue that the absence of mirror particles predicted by this model can give us a hint about the fundamental nature of space.