Role of Auxiliary Fields during Cosmological Inflation and Reheating

Alam, Khursid (2025) Role of Auxiliary Fields during Cosmological Inflation and Reheating. PhD thesis, Indian Institute of Science Education & Research Kolkata.

Text (PhD thesis of Khursid Alam (20RS114)) 20RS114.pdf - Submitted Version Restricted to Repository staff only Download (9MB)

Abstract

Cosmological inflation, a well-established framework for explaining the cosmic structure and fluctuations in the cosmic microwave background (CMB), is often modeled as a single slowrolling scalar field. However, inflation likely occurred in a high-energy regime involving physics beyond the Standard Model, with contributions from multiple fields, such as scalars and gauge fields. This thesis explores how secondary fields influence inflationary dynamics during and after inflation, focusing on three aspects: (A) CMB constraints on natural inflation with gauge field production, (B) Effects of reheating on moduli stabilization, and (C) Non-thermal moduli production during preheating in ω-attractor inflation models. The natural inflation model, characterized by a periodic potential, encounters challenges from recent cosmological observations, including Planck 2018, BICEP Keck, and BAO measurements. Here, the inflaton is a pseudo-scalar coupled to a gauge field via a Chern-Simons term α/f ɸFF̃. This coupling significantly modifies both the inflationary background and perturbation dynamics due to gauge field production. With increasing ω, gauge fields notably influence CMB-relevant scales. Our numerical analyses confirm the model’s consistency with observations for specific range of ω, with scalar spectral index constraints beingmore stringent than those from non-Gaussianities or spectral running. We further investigated moduli stabilization during perturbative reheating. The moduli potential minimum may disappear due to external factors, such as inflaton energy density or radiation baths. To maintain stability, we impose upper limits on the inflationary energy scale and the maximum temperature of the thermal bath, referred to as the critical temperature. Our analysis, which is considering moduli dynamics, shows stabilization depends on the initial field range, even when the thermal bath’s temperature exceeds the critical limit. Furthermore, we find that the allowed initial field range increases for higher thermal bath temperatures, particularly when the e!ective potential exhibits a runaway behavior. For low-mass moduli (<̰ 30TeV), this range may still lead to cosmological moduli problems by violating Big Bang Nucleosynthesis (BBN) constraints unless the initial abundance is suppressed. Low-mass moduli fields (<̰ 30TeV) can significantly impact Big Bang Nucleosynthesis (BBN) cosmology, particularly by a!ecting the abundance of light elements such as helium and lithium. Since low-mass moduli decay during or after the production of these light elements, their decay products can disrupt the observed light element abundances, which are tightly constrained by BBN observations. To protect the primordial light element abundances, the moduli field’s initial abundance must be su”ciently suppressed, especially if it is produced during the early universe, such as during the preheating phase following inflation. During preheating, moduli fields can be generated non-thermally. In this thesis, we investigate how the abundance of the moduli field can be suppressed in the context of the ω-attractor inflationary model. Our analysis shows that the moduli abundance is reduced for small values of the inflaton potential parameter, ω. Specifically, we calculate the moduli abundance using two approaches: a fully numerical method implemented with the LATTICEEASY code and a semi-analytical method. Our results demonstrate that for O(1) inflaton-moduli coupling, the condition α <̰ 10⁻⁸M²pl satisfies the BBN constraints, thereby imposing an upper limit on the inflationary energy scale and the reheating temperature.

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