Browsing by Subject "Inflationary cosmology"

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The inflationary universe: a primordial bridge between fundamental theory and observable phenomena
(2023-12) Aragam, Vikas; Paban, Sonia; Distler, Jacques; Kilic, Can; Boylan-Kolchin, Michael
This dissertation encapsulates three connected projects that I have worked on as a graduate student. I begin with an introduction to the beautiful intersection of fundamental physics and the primordial universe that is inflationary cosmology. I then present the three main chapters of this dissertation. The first explores the possibility of realizing inflation in steep potentials via multi-field dynamics, which draws on motivations from string theory and the Swampland program. The second chapter examines multi-field inflation at a more fundamental level by rigorously constructing the types of solutions we can achieve in models with features expected in UV-complete theories. The last chapter shifts focus to the gravitational wave signatures that multi-field models of inflation exhibit, with special emphasis on the conditions for these signatures to be observable in upcoming gravitational wave observatories.
Toward understanding of the complete thermal history of the universe : probing the early universe by gravitation
(2009-12) Watanabe, Yuki; Komatsu, Eiichiro
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.