Browsing by Author "Gamonal San Martín, Mauricio "

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    A nontrivial footprint of standard cosmology in the future observations of low-frequency gravitational waves
    (2020) Alfaro, Jorge ; Gamonal San Martín, Mauricio
    Recent research shows that the cosmological components of the Universe should influence on the propagation of Gravitational Waves (GWs) and even it has been proposed a new way to measure the cosmological constant using Pulsar Timing Arrays (PTAs). However, these results have considered very particular cases (e.g. a de Sitter Universe or a mixing with non-relativistic matter). In this work we propose an extension of these results, using the Hubble constant as the natural parameter that includes all the cosmological information and studying its effect on the propagation of GWs. Using linearized gravity we considered a mixture of perfect fluids permeating the spacetime and studied the propagation of GWs within the context of the Lambda CDM model. We found from numerical simulations that the timing residual of local pulsars should present a distinguishable peak depending on the local value of the Hubble constant. As a consequence, when assuming the standard Lambda CDM model, our result predicts that the region of maximum timing residual is determined by the redshift of the source. This framework represents an alternative test for the standard cosmological model, and it can be used to facilitate the measurements of gravitational waves by ongoing PTAs projects.
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    A nontrivial footprint of standard cosmology in the future observations of low–frequency gravitational waves
    (2019) Alfaro, J.; Gamonal San Martín, Mauricio
    Recent research show that the cosmological components of the Universe should influence on the propagation of Gravitational Waves (GWs) and even it has been proposed a new way to measure the cosmological constant using Pulsar Timing Arrays (PTAs). However, these results have considered very particular cases (e.g. a de Sitter Universe or a mixing with non-relativistic matter). In this work we propose an extension of these results, using the Hubble constant as the natural parameter that includes all the cosmological information and studying its effect on the propagation of GWs. Using linearized gravity we considered a mixture of perfect fluids permeating the spacetime and studied the propagation of GWs within the context of the ΛCDM model. We found from numerical simulations that the timing residual of local pulsars should present a distinguishable peak depending on the local value of the Hubble constant. As a consequence, when assuming the standard ΛCDM model, our result predicts that the region of maximum timing residual is determined by the redshift of the source. This framework represents an alternative test for the standard cosmological model, and it can be used to facilitate the measurements of gravitational wave by ongoing PTAs projects. Copyright © 2019, The Authors. All rights reserved.
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    Cosmic inflation in modified models of gravity and an analysis on gravitational waves
    (2021) Gamonal San Martín, Mauricio; Alfaro Solís, Jorge Luis; Pontificia Universidad Católica de Chile. Facultad de Física
    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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    Slow-roll inflation in f(R,T) gravity and a modified Starobinsky-like inflationary model
    (2021) Gamonal San Martín, Mauricio
    In this work, we studied the slow-roll approximation of cosmic inflation within the context of f (R,T) gravity, where R is the scalar curvature, and T is the trace of the energy-momentum tensor. By choosing a minimal coupling between matter and gravity, we obtained the modified slow-roll parameters, the scalar spectral index (n(s)), the tensor spectral index (n(T)), and the tensor-to-scalar ratio (r). We computed these quantities for a general power-law potential, Natural & Quartic Hilltop inflation, and the Starobinsky model, plotting the trajectories on the (n(s), r) plane. We found that one of the parameters of Natural/Hilltop models is non-trivially modified. Besides, if the coupling is in the interval -0.5 < alpha < 5.54, we concluded that the Starobinsky-like model predictions are in good agreement with the last Planck measurement, but with the advantage of allowing a wide range of admissible values for r and nT.