Observational features of the early and late-time universe

The first half of this Thesis investigates the observational features of dark energy dominated cosmologies. Using semi-analytic expressions for the Cold Dark IVlatter (CDM) halo mass function , we make predictions for halo abundance in models where t.he dark energy scalar field is coupled to CDM. "'e find that the mass function is a sensitive probe of coupled dark energy models, and that for certain forms of the dark energy potential, these models can offer a viable alternative to the standard ACDlVI paradigm. A generic prediction of string theory is the existence of multiple axion fields - giving rise to an axiverse. VVe couple such an axion to its corresponding modulus, extending the original axiverse scenario. A dynamical systems analysis is presented, which reveals how the axion and modulus fields leave observable signatures as both dark matter and dark energy. Among these signatures include multiple periods of accelerated expansion in the matter era, the presence of dark energy at early times, and future cosmological collapse. "'e use such phenomena to place constraints on the model parameter space, leaving open a small region that is observationally acceptable. Rather than appealing to fundamental theory to describe dark energy, we can reconstruct its properties in a model independent way. VIle introduce a parametrisation of the dark energy equation of state which uses the dark energy density as a time variable. Using our parametrisation, we place constraints on the time evolution of dark energy which are considerably tighter than those obtained using existing parametrisations. The second half of this Thesis examines the sensitivity of multi- field inflationary observables to perturbative reheating. Vve include in our analysis the local- type non- linearity parameter fi-?Lal , the spectral index of the primordial curvature perturbations nc;, and the tensor-to- scalar ratio TT. Vle demonstrate with concrete examples that if the curvature perturbation is still evolving after inflation has ended, the primordial observables can acquire substantial reheating dependent corrections. By way of a sudden decay calculation, which we verify with numerical simulations, we show that if both fields participate in the reheating phase, the corrections to the primordial observables can be within the sensitivity of CMB experiments such as Planck. Our analytic results are very general, encompassing the curvaton scenario, the inflating curvaton scenario, and any applicable multi- field model between these two regimes. Our results imply that the statistics of (, especially fi-fLal , can only be reliahly used as a probe of multi- field inflation if the physics of perturbative reheating is accounted for.EThOS - Electronic Theses Online ServiceGBUnited Kingdo