School of Physics and Astronomy

Unveiling Cosmic Dawn and the Epoch of Reionisation with HERA

Research Group:Cosmology and Relativity Group (Astronomy Unit)

Number of Students:1-2

Full-time Project: yes

Funding:

Funding through STFC or College+DISCnet (ask for details).

Project Description:

While our knowledge of very early and very late cosmic times has come on leaps and bounds over the past couple of decades, we still know comparatively little about the intervening period, when the first stars and galaxies formed and began to light up the Universe. This era can be broadly broken up into the Dark Ages, when neutral gas filled the cosmos; Cosmic Dawn, when the first stars began to shine; and the Epoch of Reionisation, when radiation from stars and galaxies ionised the neutral gas, leaving the Universe in much the same ionisation state as it is today.

Closeup of the HERA radio array in South Africa. Courtesy: Kathryn Rosie (2018).

The first stars and galaxies were rare objects, making them hard to find at such immense distances, while other forms of emission that might be easier to see were not yet in operation. As such, our understanding of this period remains very patchy, as it is hard to probe observationally. Fortunately, the neutral hydrogen gas that pervaded the Universe at this time gives off a faint glow at radio wavelengths, called 21cm emission. While this is hard to detect -- it is hidden behind the very bright radio emission of our own galaxy -- a number of experiments are currently in operation that hope to find and characterise the 21cm signal in order to finally understanding this most mysterious period in the Universe's history.

The Hydrogen Epoch of Reionisation Array (HERA), operating from the Karoo desert in South Africa, is one of the most powerful such experiments. Built as a large array of many close-packed radio dishes, HERA is already taking data, and in principle already has the sensitivity to put the best limits on fluctuations in the neutral hydrogen distribution during Cosmic Dawn and the EoR to date. Extracting this signal from the bright galactic emission and correcting for the imperfect response of the instrument to the signal remains extremely challenging however.

Illustration of reionization and cosmic dawn. Courtesy: Avi Loeb.

The aim of this project is to contribute to the development and application of advanced statistical methods to the analysis of existing and future HERA data. Key problems include:

  • Efficiently and precisely estimating the power spectrum of 21cm fluctuations, in the presence of bright foreground emission.
  • Correcting for "leakage" due to cuts in the data, imposed by excising radio frequency interference.
  • Analysing the stability of the results to various analysis assumptions, by rigorously applying null tests and feeding simulated data through the analysis pipeline.
  • Accurately estimating the covariance matrix of the data, which is needed to optimally extract the faint 21cm signal.
  • Modelling/simulating the physical EoR and Cosmic Dawn signal in a computationally-efficient way.

This work will be carried out in collaboration with other members of the HERA Collaboration based in Berkeley, McGill, U. Western Cape, JPL, U. Penn, and U. Washington. You will gain skills in some or all of the following: analysis and interpretation of radio astronomical data, the application of high-dimensional inference techniques to large volumes of data, the application of structured and unsupervised machine learning techniques to detect spurious features in data, and the development of fast parallel data analysis codes to be run on high-performance computers. You will also gain experience in project management, transferable data science skills, and working in a mid-sized international collaboration.

Figures courtesy: Kathryn Rosie and Avi Loeb.

SPA Academics: Phil Bull