Institute for Astronomy

Cosmology PhD projects

Research projects on offer in our Cosmology Group:

Cosmology with peculiar velocities of galaxies

Yan-Chuan Cai and John Peacock

This project aims to use peculiar velocities of galaxies to probe the growth of structure in cosmology.  Density fluctuations grow with time as gravity drives matter to fall towards high-density regions and move away from low-density regions, and so there must be an associated cosmic velocity field. This process depends on the amount of dark matter and also on the nature of gravity, so studying the growth of structure can tell us a great deal about fundamental cosmology.

The peculiar velocities can be measured via their impact on the temperature fluctuations of the cosmic microwave background (CMB). The most important effect arises because galaxies are associated with clumps of ionized gas, which perturb CMB photons via Compton scattering (the Sunyaev-Zeldovich effect). There is a thermal SZ effect due to internal thermal motions, but also a kinetic Sunyaev-Zeldovich effect (kSZ) that arises directly from the Doppler effect due to the bulk motion of the scattering plasma (https://arxiv.org/pdf/astro-ph/0304534.pdf).  The kSZ effect can be used to probe peculiar velocities by cross-correlating galaxies with the CMB, and this will be the major focus of the PhD project.

Some initial kSZ detections have been made by stacking pairs of galaxies and assuming that these must be moving together on average (https://arxiv.org/pdf/2101.08374.pdf), but a more sensitive detection should be possible if we have an estimate of the peculiar velocity field, so that a predicted kSZ signal can be pattern-matched with the CMB, averaging away the noise from the primary intrinsic CMB fluctuations. Such velocity predictions can be performed using galaxy number-density fluctuations in large spectroscopic surveys (https://arxiv.org/pdf/1410.7622.pdf), but the largest galaxy surveys only provide imaging. Here, galaxy colours are used to estimate photometric redshifts, yielding low-precision distances for of order 100 million galaxies (https://arxiv.org/pdf/2010.00466.pdf). A first aim for this project will be to see how well kSZ predictions can be made from such datasets, and how they compare with estimates from more precise but sparser spectroscopic data (along the lines of https://arxiv.org/pdf/1603.01608.pdf). This will be followed up by a search for the kSZ signal in the best existing CMB data, from the Planck satellite.

The next step will be to make forecasts for the kSZ signal and its cosmological implications, anticipating the next generation of galaxy surveys, such as DESI, and CMB Stage-4 observations.  New modelling techniques will be developed to exploit these catalogues, which will be released within the timescale of the PhD.  This project will involve a mixture of theory, data handling, and also the use of large numerical simulations, so it will give a broad grounding in research on cosmological structure formation.

Exploring the Universe with radio telescopes

Alkistis Pourtsidou

The analysis of the large scale structure (LSS) of the Universe aims to answer the most important questions in modern cosmology. The advent of large sky radio surveys with instruments like MeerKAT and the SKA, and the 300 petabytes of data products they will generate every year, makes this analysis a complex and data-driven endeavour. This PhD project will investigate multiple aspects of radio data analysis, exploiting new modelling and simulation techniques as well as providing a publicly available Python toolkit to analyse simulated and real data. It will also apply these tools to currently available data from MeerKAT and its cross-correlations with optical galaxy surveys to derive exquisite constraints on Hi and cosmological parameters.

Lensing and Clustering with the Rubin Observatory

Joe Zuntz

 

Combining weak lensing and galaxy clustering statistics yields one of the most powerful probes in cosmology for understanding the distribution of dark matter and the gravitational and cosmic processes that lead to that distribution. We measure and cross-correlate the two-point statistics of each and compare the results to increasingly rich theoretical models. In this project, you'll be part of the Vera Rubin Observatory, the largest upcoming ground-based photometric survey, which will build the largest dark matter maps and statistics over the course of a decade. It will measure the positions and shapes of 10 billion galaxies over the course of its mission You will work with the analysis pipeline that connects our measurements of these galaxies to our theories of physics and cosmology, becoming part of a large international team to do so. You'll help build the supercomputer analysis that will characterise and model Rubin data from core pixel modelling all the way to theoretical modelling, depending on your interests. This will be a heavily computational project, so it would be good to have a strong interest and some experience in that area.

Probing the distribution of dark matter haloes with galaxy groups

John Peacock & Shadab Alam

It is now well established that structure in our universe formed by gravitational instability, which magnifies initial density fluctuations and concentrates matter (predominantly dark matter) into nonlinear clumps called dark matter "haloes". Galaxies form within these structures, and a central task for cosmology is to understand how the resulting galaxy populations depend on the properties of the haloes that they inhabit.  Next-generation galaxy surveys aim to use the galaxy distribution as a proxy for the overall mass density, in order to study fundamental issues such as the nature of gravity and the properties of the "dark energy" that drives accelerating cosmic expansion. In this work, it is therefore essential to have a detailed knowledge of the mapping between dark and visible matter. One way of approaching this is to study galaxy groups and clusters, since these systems must correspond to individual dark matter haloes. The state of the art in galaxy groups is the catalogue created from the GAMA spectroscopic survey (https://arxiv.org/abs/1106.1994), but the opportunity now exists to improve on this in two ways: increasing the range of halo masses probed and also increasing the range of redshifts studied, so that the evolution of the group population can be measured. New spectroscopic data are now arriving from the DESI survey (https://www.desi.lbl.gov), which will increase the GAMA sample by about a factor 100. But most galaxies will still lack a spectroscopic redshift, and so it is important to use the lower precision "photometric redshifts" that can be estimated from the colours of fainter galaxies. A great resource here exists in the form of the Legacy Survey imaging (https://www.legacysurvey.org), which we have recently exploited to create a new catalogue of photometric redshifts (https://arxiv.org/abs/2010.00466). By combining this information with spectroscopy, it should be possible to construct a new group catalogue that surpasses all existing work in terms of fidelity and coverage in mass and redshift. Once this catalogue is in place, it will open the door to many follow-up projects in cosmology. The total masses of the groups can be estimated by gravitational lensing, and Edinburgh is a world leader in this work (https://arxiv.org/abs/1507.00735), both in using ground-based data, but also in the revolutionary results expected from space with the 2023 launch of the Euclid satellite (https://www.euclid-ec.org). The DESI survey will be underway until the end of 2025, and this is a project in which Edinburgh researchers will be making leading contributions to the analysis.  A particularly interesting project will be to probe the strength of gravity on cosmological scales by measuring the amplitude of infall velocities onto haloes, via the "redshift-space distortions" of the cross-correlations between groups and the general galaxy distribution (https://arxiv.org/abs/2006.05383).  There are many other possible lines of research, and this project will form an effective introduction to forefront issues in statistical cosmology, including both the theory of large-scale structure and handling of large datasets from surveys and numerical simulations.