Large-scale Structure and Galaxy Formation in non-standard Cosmologies

We study the large-scale structure of alternative cosmologies, which intrinsically avoid the theoretical problems of the cosmological constant in the standard $\Lambda$ cold dark matter ($\Lambda$CDM) cosmology. We focus on early dark energy (EDE) models and $f(R)$ gravity, which are the typical examples of dark energy and modified gravity models respectively. We constrain the EDE model using measurements of cosmic microwave background and baryon acoustic oscillations, then use large-volume N-body simulations to study the structure formation and growth. EDE and $\Lambda$CDM can be distinguished through the shape of matter power spectrum on large scales, as well as the halo mass function at high redshift. We run high-resolution N-body simulations of $f(R)$ gravity in order to study in detail the properties of haloes and their environment. We find that halos less massive than $10^{13} M_\odot/h$ have a more compact inner structure in $f(R)$ gravity than in $\Lambda$CDM. These low-mass haloes grow faster and contain substantially more subhaloes in $f(R)$ gravity. We also measure the correlation between different halo environment definitions used in observations and the fifth force potential in $f(R)$ gravity. Although the different ways to define environment do not agree with one another perfectly, they can provide useful guidance about how well a dark matter halo is screened. We also find that the screening of subhaloes in dark matter haloes is primarily determined by the environment, with the subhalo mass playing a minor role. Finally, we investigate and improve the H$\alpha$ luminosity function predicted by semi-analytical galaxy formation model, GALFORM. The result is important for accessing the performance of Euclid redshift survey, which is one of the missions cosmological probe of accelerating cosmic expansion

A mixed-method approach to determining contact matrices in the Cox’s Bazar refugee settlement

Contact matrices are an important ingredient in age-structured epidemic models to inform the simulated spread of the disease between subgroups of the population. These matrices are generally derived using resource-intensive diary-based surveys and few exist in the Global South or tailored to vulnerable populations. In particular, no contact matrices exist for refugee settlements—locations under-served by epidemic models in general. In this paper, we present a novel, mixed-method approach for deriving contact matrices in populations, which combines a lightweight, rapidly deployable survey with an agent-based model of the population informed by census and behavioural data. We use this method to derive the first set of contact matrices for the Cox’s Bazar refugee settlement in Bangladesh. To validate our approach, we apply it to the UK population and compare our derived matrices with well-known contact matrices collected using traditional methods. Our findings demonstrate that our mixed-method approach successfully addresses some of the challenges faced by traditional and agent-based approaches to deriving contact matrices. It also shows potential for implementation in resource-constrained environments. This work therefore contributes to a broader aim of developing new methods and mechanisms of data collection for modelling disease spread in refugee and internally displaced person (IDP) settlements and better serving these vulnerable communities

Environmental screening of dark matter haloes in f(R) gravity

In certain theories of modified gravity, solar system constraints on deviations from general relativity (GR) are satisfied by virtue of a so-called screening mechanism, which enables the theory to revert to GR in regions where the matter density is high or the gravitational potential is deep. In the case of chameleon theories, the screening has two contributions – self-screening, which is due to the mass of an object itself, and environmental screening, which is caused by the surrounding matter – which are often entangled, with the second contribution being more crucial for less massive objects. A quantitative understanding of the effect of the environment on the screening can prove critical in observational tests of such theories using systems such as the Local Group and dwarf galaxies, for which the environment may be inferred in various ways. We use the high-resolution liminality simulation of Shi et al. (2015) to test the fidelity of different definitions of environment. We find that, although the different ways to define environment in practice do not agree with one another perfectly, they can provide useful guidance, and cross checks about how well a dark matter halo is screened. In addition, the screening of subhaloes in dark matter haloes is primarily determined by the environment, with the subhalo mass playing a minor role, which means that lower-resolution simulations where subhaloes are not well resolved can still be useful for understanding the modification of gravity inside subhaloes