Interacting Vacuum Cosmology

Abstract

The current standard model of cosmology is known as the ⇤CDM model. The energy density in the ⇤CDM Universe contains 5% ordinary matter,26% dark matter, and 69% a vacuum component, with equation of state1 that behaves like a cosmological constant. In this thesis we generalise the study of the dark sector in cosmology to allow interactions between the dark matter and the vacuum energy.
We first present a phase-space analysis of the qualitative dynamics of cosmologies for a simple linear interaction model where the energy transfer per Hubble time, Q/H, is a general linear function of the matter density, ⇢m, and vacuum energy, V allowing for the presence of non-interacting radiation, ⇢r. We find fixed points corresponding to power-law solutions where the different components remain a constant fraction of the total energy density and give an existence condition for any fixed points with non-vanishing energy transfer. We illustrate the phase-plane behaviour, determining the equation of state and stability of the fixed points, and give approximate solutions for matter- or vacuum-dominated solutions in the case of small interaction parameters.
We then go on to explore the bounds on the dark-sector interactions with the simple linear interaction model using state-of-the-art cosmological observations, considering linear perturbations about the homogeneous background cosmology. We explain the parameter degeneracies found when fitting cosmic microwave background anisotropies alone, and show how these are broken by the addition of supernovae data, baryon acoustic oscillations and redshift-space distortions. In the general linear interaction model we show that, while it is possible to relax both the Hubble and weak-lensing tensions simultaneously. The reduction in these tensions is modest, reduced to less slightly than 4cr and 2cr respectively.
Finally, we study the behaviour of non-linear inhomogeneities in the context of the Newtonian fluid equations for an interacting vacuum plus dark matter cosmology.

Date of Award	29 May 2023
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Hooshyar Assadullahi (Supervisor) & David Wands (Supervisor)