# Cosmology PhD projects

Research projects on offer in our Cosmology Group:

## Cosmology with the Kinematics of Galaxies

Yan-Chuan Cai and Jorge Penarrubia

On the very large scales of the Universe, galaxies are co-moving with the Hubble expansion, and therefore, they are moving apart from each other. On relatively small scales, fluctuations in matter density induce peculiar velocities. The total kinematics of galaxies are driven by the competing forces of the Universe's expansion and the fluctuations in local matter densities, encoding rich cosmological information. This project aims to exploit the cosmological information derived from the velocities of galaxies at small scales. We will employ the existing modelling framework of the "perturbed Hubble flow" (https://arxiv.org/abs/1405.0306) to simultaneously model peculiar velocities and the Hubble expansion. We will push the accuracy to higher levels for measuring cosmological parameters. Cosmological simulations will be utilised in the development and testing of the model before applying it to analyse public data from peculiar velocity surveys to constrain cosmology.

References:

https://arxiv.org/abs/2209.11238

https://arxiv.org/abs/1405.0306

## New methods for weak lensing and galaxy clustering analysis in the era of Euclid

- Video: Alex Hall: New methods for weak lensing and galaxy clustering analysis in the era of Euclid
- Alex Hall's PhD project

Weak lensing is caused by the deflection of light from distant luminous sources by intervening matter and is a key technique for understanding our Universe. The effect causes alignments in the shapes of galaxies, imperceptible with the eye (hence “weak”), but strong enough to be detected statistically. The weak lensing signal can be used to infer the properties of dark matter between the source and the observer, in particular its spatial distribution (i.e., how clumped or how smooth it is) and its evolution throughout cosmic history. Weak lensing is also sensitive to the geometry of space-time and hence to the constituent components of the Universe. The dominant component is the so-called “Dark Energy”, a poorly understood source of energy-density which appears to mimic a cosmological constant term in the Einstein field equations. Gaining an understanding into the properties of Dark Energy is one of the main science drivers for cosmology surveys such as Euclid.

Alongside weak lensing, surveys like Euclid will make measurements of the positions of galaxies. Galaxies are not distributed uniformly across the sky; they tend to cluster together due to gravity. The statistics of galaxy positions is another rich source of information about cosmological models, particularly when combined with weak lensing measurements.

The project will involve developing new methods for comparing models with data, ensuring that small spatial scales (which are measured with high precision but are also where modelling is inaccurate) do not impart biases in the inferred cosmological parameters. A variety of techniques have been employed in previous studies (e.g., scale cuts, nuisance parameters, theoretical errors), each with their own advantages and disadvantages, but the connection between them is opaque and no single method has been shown to be optimal. The project will investigate the connections between these methods, with the aim of identifying a preferred strategy for Euclid. The project is a mixture of computational and theoretical work, but with a close connection to data analysis and statistical methodology. There is some freedom in the direction the work will take, tailored to the strengths and interests of the student.

Euclid, launched in July 2023, is a European Space Agency-led space telescope designed to conduct a survey of galaxy positions and shapes over almost the entire accessible extragalactic sky. The student will have the opportunity to join and participate in the Euclid Consortium (https://www.euclid-ec.org/), the team of scientists tasked with analysing data from Euclid.

References:

https://arxiv.org/abs/1110.3193

https://arxiv.org/abs/1411.0115

https://arxiv.org/abs/2212.03734

## Solving the Cosmological Constant Problem

One of the biggest mysteries in cosmology is the observed acceleration of the expansion of the Universe. Such an effect is unexpected and currently modelled using Einstein's Cosmological Constant. However, this is problematic as its 'natural' value in general relativity is huge, while quantum physics says the vacuum itself should make other huge contributions. Conventional methods to tame these large energies fail, while attempts to find a stable mechanism using new scalar fields to damp them to observed levels are blocked by Weinberg's 'No-Go' Theorem. Recently, we have found a new, and simple, dynamical mechanism using a scalar field that achieves this goal, avoiding both 'fine tuning' and Weinberg's Theorem. The goal of this project is to explore this model further and test its prediction for the expansion history and formation of structure in the Universe against observational data.

Reference: https://arxiv.org/abs/2201.09016

#### Under the Computational Astrophysics projects see also:

**Precision cosmology with the Lyman-Alpha forest -**Avery Meiksin**Building Galaxies at Cosmic Dawn -**Britton Smith