Cosmological perturbations from non-equilibrium phenomena in the very early universe
File(s)
Author(s)
Ghoderao, Pulkit
Type
Thesis
Abstract
I predict the cosmological perturbations in temperature and galaxy distribution observable today if the non-equilibrium phenomena of preheating or vacuum transition occurred in the very early universe.
Preheating involves the rapid production of daughter particles after the end of inflation. I study the case where an inflaton field decays to a single light scalar spectator field during preheating. A separate universe approach combined with non-perturbative delta N formalism is required to calculate its observable effects on the non-Gaussianity of primordial curvature perturbations. By extending the non-perturbative formalism to include scale dependence of the spectator field, I find that cosmic variance plays a key role in determining the non-Gaussianity. Using lattice field theory simulations, I perform a full numerical calculation of the non-Gaussianity parameter $f_\text{NL}$. This calculation is illustrated for an observationally-viable model of preheating motivated by non-minimal coupling to gravity. Furthermore, I also study the dependence of $f_\text{NL}$ on the parameters in this model.
Vacuum or first order phase transitions are ubiquitous in theories containing multiple fields when the potential has several minima. If such a transition takes place during inflation in the presence of a light spectator field, I show that it can generate a large, potentially detectable non-Gaussianity. To do so I calculate the instanton solution that describes quantum tunnelling between vacuum states on a de Sitter background, obtain its dependence on the spectator field and thereby its modulation of the expansion of space at different locations. I apply this procedure to a toy inflationary model using the thin-wall approximation in Minkowski space, which makes analytical calculation feasible. Then I construct a realistic model based on Higgs inflation and study the parameter dependence of $f_\text{NL}$ in this scenario as well.
Preheating involves the rapid production of daughter particles after the end of inflation. I study the case where an inflaton field decays to a single light scalar spectator field during preheating. A separate universe approach combined with non-perturbative delta N formalism is required to calculate its observable effects on the non-Gaussianity of primordial curvature perturbations. By extending the non-perturbative formalism to include scale dependence of the spectator field, I find that cosmic variance plays a key role in determining the non-Gaussianity. Using lattice field theory simulations, I perform a full numerical calculation of the non-Gaussianity parameter $f_\text{NL}$. This calculation is illustrated for an observationally-viable model of preheating motivated by non-minimal coupling to gravity. Furthermore, I also study the dependence of $f_\text{NL}$ on the parameters in this model.
Vacuum or first order phase transitions are ubiquitous in theories containing multiple fields when the potential has several minima. If such a transition takes place during inflation in the presence of a light spectator field, I show that it can generate a large, potentially detectable non-Gaussianity. To do so I calculate the instanton solution that describes quantum tunnelling between vacuum states on a de Sitter background, obtain its dependence on the spectator field and thereby its modulation of the expansion of space at different locations. I apply this procedure to a toy inflationary model using the thin-wall approximation in Minkowski space, which makes analytical calculation feasible. Then I construct a realistic model based on Higgs inflation and study the parameter dependence of $f_\text{NL}$ in this scenario as well.
Version
Open Access
Date Issued
2025-03-14
Date Awarded
01/07/2025
License URL
Advisor
Rajantie, Arttu
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)