Cosmic inflation in modified models of gravity and an analysis on gravitational waves
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2021
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Abstract
This work comprises three different lines of research related to the study of cosmological inflation and the propagation of gravitational waves. In the first issue, we developed -for the first time in a systematic way- the slow-roll approximation for a single field inflaton within the framework of f(R,T) gravity, a modified model of gravity such that the Lagrangian is a function of the scalar curvature and the trace of the energy-momentum tensor. We obtained the modified slow-roll parameters and the spectral indices by choosing a minimal coupling between matter and gravity. We computed these quantities for several models and contrasted the predictions with the constraints of the Planck data, obtaining corrections to the Starobinsky model.
In the second line of research, we studied how a free Lorentz-valued bosonic 0-form coupled to Einstein-Cartan gravity's action can be considered the inflaton field. In this model, the interacting terms of the fields come directly from the torsionful contributions of the action. Hence, the inflationary dynamics can be extended so that we can define an effective potential that describes the evolution of the background fields. We found that for a particular combination of initial conditions, the model could adequately guarantee the slow-roll conditions over more than 55 e-folds. However, a more detailed analysis is required to confirm the viability of this inflationary scenario.
Finally, we addressed a different problem related to the propagation of low-frequency gravitational waves coming from sources located at cosmological distances. Within the linearized regime of gravity, we performed a coordinate system transformation between a frame which origin is the source of gravitational waves and the comoving frame of the FLRW metric. Then, we studied the observational consequences in Pulsar Timing Arrays experiments, finding a non-trivial modification to the timing residual of pulsars that depends on the value of the Hubble constant.
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Tesis (Master in Physics)--Pontificia Universidad Católica de Chile, 2021