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Dr Phil Bull


Lecturer in Cosmology

Room Number: GO Jones room 501



I make most of the code used in my research public. You can find a full listing on my website, including links to the code, but here are a few highlights:

  • RadioFisher: Fisher forecasting code for 21cm intensity mapping
  • LSST Core Cosmology Library: General-purpose code with validated predictions for cosmological observables
  • Bubble: Background predictions for exact, spherically-symmetric inhomogeneous GR solutions
  • Commander 2: Full-sky CMB Gibbs sampler based on multi-grid solver

Undergraduate Teaching

I am not currently teaching, but I do have open PhD projects.

Postgraduate Teaching

I am not currently teaching, but I do have open PhD projects.


Research Interests:

My research covers the intersection of theoretical and observational cosmology. I am interested in what inhomogeneities can tell us about dark energy, and how novel observables and statistical tools can be used to make inferences about the cosmos on the largest scales. Research topics include:

  • Cosmology with multiple tracers, including optical and radio (21cm) surveys with LSST and SKA
  • Secondary anisotropies and spectral distortions of the CMB as cosmological probes
  • General Relativistic effects on matter inhomogeneities and light propagation
  • Bayesian inference, stochastic processes, and computational physics

PhD Supervision

I am currently offering PhD projects in theoretical and observational cosmology, in collaboration with other members of the Astronomy Unit.

This is not an exhaustive list and I would be happy to discuss other project possibilities.

Project Title

Mapping the cosmic horizon with next-generation surveys

On the distance scales probed by current galaxy surveys, General Relativistic effects are only mildly important, and most of the interpretation of the data can be done within the context of Newtonian gravity. Forthcoming surveys like LSST, SKA, and Euclid are much bigger and deeper than their predecessors though, and so will be able to probe much larger scales -- approaching the size of the "cosmic horizon" -- where GR effects are important and must be taken into account.

Excitingly, the observable signatures of these effects should contain valuable information about some of the most fundamental questions in cosmology and theoretical physics -- the nature of gravity, the cause of cosmic acceleration, and how inflation set the initial conditions of the Universe we see today. By measuring them, we can test fundamental physics on truly cosmological scales.

The aim of this project is to put together a suite of tools for making practical measurements of GR effects on the very largest observable distance scales in the Universe. Because of an effect called cosmic variance, these measurements can only be made if data from several different large-scale surveys can be combined in a precise, statistically-robust way, with excellent rejection of systematic effects that contaminate the data. Only a few preliminary studies have attempted to use this "multi-tracer" method so far, and substantial advances in modelling and statistical analysis methods are urgently needed to make it usable in future surveys.

This project will develop the most important components of this method, with a view to using it on simulations, precursor datasets, and eventually the large surveys themselves. The work will include:

  • Performing theoretical calculations to understand how the large-scale signal changes in different cosmological scenarios;
  • Building realistic simulations of the signal, incorporating models of how galaxies trace the matter distribution;
  • Simulations of systematic errors in the data caused by astrophysical contamination and the instrumental responses of optical and radio telescopes;
  • Development of new statistical analysis methods that can clean out systematic effects and recover the true signal with sufficient precision.

Through this project, you will gain skills and experience in a range of important topics across cosmology and astrophysics: performing calculations using cosmological perturbation theory; running and analysing N-body simulations; applying galaxy-halo models to simulations and data; simulating data produced by optical and radio telescopes; working with Bayesian statistical analysis methods; and writing efficient parallel computer codes. You will also gain experience working in large scientific collaborations, where you will develop project management and communication skills, and have the opportunity to attend international meetings and work with international collaborators in the US, South Africa, and Italy.

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