# Perturbations in Lemaitre-Tolman-Bondi and Assisted Coupled Quintessence Cosmologies

## TL;DR

This thesis investigates linear perturbations in LTB and ACQ cosmologies, deriving gauge-invariant quantities, developing numerical tools, and comparing models to observational data, highlighting deviations from common approximations relevant for future surveys.

## Contribution

It introduces gauge-invariant perturbation analysis in LTB and ACQ models, develops the PYESSENCE code, and evaluates the accuracy of the small scale approximation for upcoming cosmological experiments.

## Key findings

- Identified a conserved quantity in LTB perturbations.
- Developed the PYESSENCE numerical code for ACQ models.
- Found significant deviations from the small scale approximation for future surveys.

## Abstract

In this thesis we present research into linear perturbations in Lemaitre-Tolman-Bondi (LTB) and Assisted Coupled Quintessence (ACQ) Cosmologies. First we give a brief overview of the standard model of cosmology. We then introduce Cosmological Perturbation Theory (CPT) at linear order for a flat Friedmann-Robertson-Walker (FRW) cosmology. Next we study linear perturbations to a Lemaitre-Tolman-Bondi (LTB) background spacetime. Studying the transformation behaviour of the perturbations under gauge transformations, we construct gauge invariant quantities in LTB. We show, using the perturbed energy conservation equation, that there is a conserved quantity in LTB which is conserved on all scales. We then briefly extend our discussion to the Lemaitre spacetime, and construct gauge-invariant perturbations in this extension of LTB spacetime. We also study the behaviour of linear perturbations in assisted coupled quintessence models in a FRW background. We provide the full set of governing equations for this class of models, and solve the system numerically. The code written for this purpose is then used to evolve growth functions for various models and parameter values, and we compare these both to the standard $\Lambda$CDM model and to current and future observational bounds. We also examine the applicability of the "small scale approximation", often used to calculate growth functions in quintessence models, in light of upcoming experiments such as SKA and Euclid. We find the results of the full equations deviates from the approximation by more than the experimental uncertainty for these future surveys. The construction of the numerical code, PYESSENCE, written in Python to solve the system of background and perturbed evolution equations for assisted coupled quintessence, is also discussed.