Testing the Field Content of Cosmic Inflation

Abstract

In this thesis we study cosmological inflation, a period of accelerated expansion
in the very early universe. According to the simplest models, inflation is driven
by a single scalar field that slowly rolls down its own potential. Despite the great
success of single-field inflation, usually high-energy theories beyond the standard model accommodate a richer particle content. For this reason, in this thesis we explore different ways of testing extra fields present during inflation, especially considering their effects in the gravitational wave sector.
In the first part of this thesis, we examine light spin-2 fields, described within an
effective field theory approach. We show that when the helicity-2 sound speed decreases during inflation, the spin-2 fields can induce primordial gravitational waves growing towards small scales. We explore the region of parameter space which can be probed by the upcoming LISA mission at interferometer scales. These light spin-2 fields would also mediate the tensor 3-point correlation function, and we therefore study the properties of the bispectrum signal, its amplitude in the equilateral and squeezed configurations and shape. We also discuss a possible way of indirectly testing the bispectrum on small scales, and identify the parameter space generating percent level anisotropies at scales
to be probed by SKA and LISA.
In the second part of this thesis, we consider the presence of an additional scalar field, working in the framework of cosmological α–attractors, originally formulated in terms of a radial and angular field living in a hyperbolic field space. We focus on potentials endowed with an inflection point, and compare single- and two-field models leading to enhanced scalar fluctuations on small scales. While in the single-field case ultra-slow-roll dynamics at the inflection point is responsible for the growth of the power spectrum, in the multi-field set-up we study the effect of geometrical destabilisation and non-geodesic motion in field space. We show that compatibility with CMB measurements on large scales constrains the small-scale phenomenology, with primordial black holes that can
only be produced with very light masses, M ≲108 g, and GWs induced at second-order peaked at ultra-high frequencies, f ≳10kHz.

Date of Award	13 Dec 2022
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	David Wands (Supervisor), Matteo Fasiello (Supervisor) & Hooshyar Assadullahi (Supervisor)