Cosmology

Supervisor: Pedro Ferreira

We are able to place reasonably tight constraints on the amplitude of fluctuations of the Universe using a wide range of observations: from the cosmic microwave background to the clustering and weak lensing of galaxies. It is intriguing that there are inconsistencies between the constraints that are obtained with different methods - this is often known as the S8 problem. These inconsistencies are not glaring enough for one to question the standard model of cosmology but they do deserve to be studied more thoroughly. One way of doing that is to, given the existing or up-coming data, find more efficient methods for extracting a reliable estimate of the amplitude. We will try to do so by going beyond the standard approach of using 2-point statistics (such as the correlation function or power spectrum) and add additional information from the data which may contain additional properties about the distribution of matter in the Universe. The particular approach we will take is to constuct higher moment maps of the data which will be sensitive to such objects as the bispectrum, trispectrum and higher order spectra. The hope is that it will be possible to repurpose existing analysis methods to include these new aspects.

The project will involve a healthy combination of mathematical methods (the statistics of random fields), coding and data analysis. We have an extensive repository of data sets that can be used for this analysis but are also excited about using very new data which will come from the Rubin telescope and Euclid.

For background on cosmology you can look at the Scott Dodelson's book on Cosmology, an interesting talk on the S8 tension is here https://pirsa.org/23050129. This is a fairly old but nice snapshot of higher order statistics https://arxiv.org/pdf/astro-ph/0505391.pdf. For current examples of people trying to tackle the S8 problem with additional information, you can check out Figure 3 in https://arxiv.org/pdf/2304.04785.pdf and Fig 5 in https://arxiv.org/pdf/2204.13717.pdf

Supervisors : Maria Vicenzi and Stephen Smartt

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will start in January 2025 and is expected to completely revolutionise transient astronomy and, in particular, cosmological measurements using Type Ia supernovae. 

Rubin and LSST involve a wide network of international scientists, and our team is leading the preparation for the beginning of the survey and will be heavily involved in the analysis of the first transient data. We work on a wide range of projects (both observations oriented and simulations oriented) that will have a high impact on LSST science. These projects revolve around the development and testing of transient discovery and classification pipelines, design and optimisation of LSST spectroscopic follow-up programs, and the design of new strategies to use and compile cosmological samples using LSST supernova data.

We are also leading various analyses of recently concluded supernova surveys like the Dark Energy Survey supernova sample (the largest and deepest supernova survey from a single telescope in the pre-LSST era) and current/upcoming SN surveys with the space telescopes Euclid (an ESA led facility, now working) and Nancy Grace Roman (a future NASA mission). These surveys will provide tremendous datasets to tackle some of the most pressing questions in supernova cosmology and astrophysics. The DPhil project can focus on either the astrophysics of the explosions or the application to cosmology. On the astrophysics side, we aim to answer the question of how do white dwarfs explode to give type Ia supernovae and how many possible stellar evolutionary channels lead to this end point. How they are affected by dust extinction and their host galaxy properties critically affect their use as cosmological probes.

Supervisors: Mike Jones, Angela Taylor

A major goal in observational cosmology is to try to detect the 'B-mode' of the polarization of the cosmic microwave background, which is linked to the fundamental physics of inflation and the Big Bang. There is a huge experimental effort around the world both to build the telescopes that will detect the B-mode, and to understand the instrumental and confusing systematic effects that could either mask the polarization signal or give rise to false detections. In Oxford we have been concentrating on one of the key potential contaminants to the B-mode signal, polarized emission from synchrotron radiation in our own Galaxy. We have a unique data set, from the C-Band All-Sky Survey (C-BASS), and close collaborations with other groups in Europe with similar experiments. In this project the student will use these data and other observations to improve our understanding of synchrotron contamination of the CMB, apply this to upcoming observations from the Simons Observatory, and predict the impact of foreground contamination on CMB experiments currently being developed, including the satellite experiment LiteBIRD due for launch in 2027. 

Supervisor: Gavin Dalton

WEAVE is a new spectroscopy facility at the William Herschel Telescope, led from Oxford and now in the commissioning phase. Over the next few years, WEAVE will deliver more than 12,000,000 high quality calibrated spectra for 8 diverse surveys ranging from the structure and evolution of the Milky Way to the formation of the earliest galaxies. Working with the WEAVE PI within a diverse science team spanning 6 countries, this project provides unique access to all aspects of the WEAVE surveys, with possible avenues of exploration including a WEAVE-based calibration of the Hobby-Eberly Telescope Dark Energy Experiment, analyses to search for rare objects within the Milky Way, or spectroscopic identification of gravitationally lensed galaxies.

Supervisor: David Alonso

One of the main obstacles preventing cosmology from becoming a truly robust science of discovery is our lack of understanding of the complex astrophysical systems that underly almost all cosmological observations of the large-scale structure. Fortunately, a broad range of probes of this large-scale structure are now available to us that provide complementary information about the physical properties of these systems. In this project, we will explore combinations between several tracers of the large-scale structure (galaxy clustering, weak lensing, the Sunyaev-Zel'dovich effect, the Cosmic Infrared Background, X-ray and radio data) to construct a complete, data-driven, theoretical model, describing the impact of astrophysical processes (galaxy formation, gas virialisation, AGN feedback) in a cosmological setting. This work will have direct impact on current and future cosmological constraints from a variety of experiments, including the Simons Observatory and the Legacy Survey of Space and Time (LSST). Interested candidates should be keen on both data analysis and theoretical modelling, as well as statistics, coding, and maths.

Further reading:

• https://arxiv.org/abs/2206.15394 
• https://arxiv.org/abs/1709.01489
• https://arxiv.org/abs/2309.11129
• https://arxiv.org/abs/1909.09102

Figure: reconstruction of the last 10 billion years of star formation history from a combination of Cosmic Infrared Background, galaxy clustering, and weak lensing data (Jego, Alonso et al. 2022).

Supervisors: Matt Jarvis,  Catherine Hale & Gavin Dalton

Over the past few decades it has become clear that the environment and dark matter halo mass in which a galaxy resides can strongly influence how it evolves. Thus, in order to understand galaxy evolution we need be able to relate the galaxies to their dark matter haloes and the larger-scale filamentary cosmic web. To do this work, we need accurate positions in 3-dimensional space and the new generation of spectroscopic surveys  will provide this information. The student will play a leading role in using these new deep spectroscopic surveys with WEAVE, 4MOST and MOONS to understand how radio sources impact and are impacted by their halo environment, using radio data from LOFAR, MeerKAT and ASKAP. They will have the opportunity to use the best multi-wavelength data to characterise the sources and use the spectroscopic information to perform a 3-D clustering analysis and measure the impact accreting black holes on the process of galaxy formation in dark matter haloes.  This project will provide new and important constraints on the feedback process from radio AGN that is used in hydrodynamic simulations of galaxy evolution to truncate star formation in massive galaxies.

The student will also have the opportunity to get involved in other WEAVE-LOFAR and MeerKAT projects and will join a group over 10 postdocs and students all working on galaxy evolution from multi-wavelength survey data. As such the student will become familiar with multi-wavelength survey astronomy and statistical techniques to measure the clustering of different galaxy populations across cosmic time, key skills for exploiting the next generation of surveys from Rubin:LSST, Euclid and the SKA.

Information about WEAVE: https://arxiv.org/pdf/2212.03981.pdf

Information about MOONS: https://arxiv.org/ftp/arxiv/papers/2009/2009.00644.pdf

Information about 4MOST-WAVES/ORCHIDSS: https://www.eso.org/sci/publications/messenger/archive/no.190-mar23/messenger-no190-25-27.pdf

Some background reading on galaxy clustering

https://arxiv.org/pdf/1606.08989.pdf

https://arxiv.org/pdf/1711.05201.pdf

https://arxiv.org/pdf/2009.01817.pdf