The Universe in Explosive Expansion

Sanchez Lopez, Samuel and Dimopoulos, Konstantinos (2024) The Universe in Explosive Expansion. PhD thesis, Lancaster University.

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Abstract

Cosmic inflation, a phase of accelerated expansion of the early Universe, not only solves the horizon and flatness problems of the Hot Big Bang but also provides the initial conditions for the density perturbations that source all structure in the Universe. 9 billion years later, the Universe started engaging in another bout of accelerated expansion, observed today, 13.8 billion years after the Big Bang. This thesis is mainly concerned with quintessential inflation, a framework that suggests that the same substance responsible for the period of primordial inflation, the inflaton field, is also responsible for the current accelerated expansion. By considering a simple and theoretically motivated setup in modified gravity, we manage to bring back to life two of the most popular inflationary models, chaotic and power-law inflation, hitherto discarded by the Planck data. We also achieve late-time inflation for fairly natural parameter values, with significantly less fine-tuning than in $\Lambda$CDM. Furthermore, we explore one specific limit of the modified gravity setup, characterised by a period of quartic kinetic domination of the inflaton, and its effects on the production of primordial gravitational waves by inflation. We find that during this period, which we call hyperkination, the peak in the density spectrum of gravitational waves corresponding to kination is truncated, thereby safely evading Big Bang Nucleosynthesis constraints. This allows us to bring the gravitational wave spectrum down to observable frequencies. If detected by future gravitational wave interferometers, it would provide valuable insight into the underlying theory. Lastly, mirroring the minimalist philosophy of quintessential inflation, we propose a toy model of unified early dark energy and quintessence, which raises the value of the Hubble constant inferred from the Planck data to values compatible with local measurements. It simultaneously explains the current accelerated expansion of the Universe, without significant additional fine-tuning than in $\Lambda$CDM.