Research
I am a generalist who likes to solve key problems in observational cosmology with whatever methods are called for by the big problems.
My research draws together theory, data-analysis, and instrumentation to realize a powerful new cosmological probe -- 21cm emission from Hydrogen gas at high redshifts (1 <~ z <~ 150). Observing this this emission will open up revolutionary windows into key unobserved chapters of our Universe's history; the Cosmic Dawn, when the first stars, galaxies and black holes formed, and the Cosmic Dark Ages -- before there were any stars at all. We can also use 21cm intensity mapping to unveil the nature of dark energy by mapping large-scale structure just as dark-energy driven acceleration was starting to kick off several billion years ago.
While hydrogen cosmology promises transformative science, it has not been realized due to a formidable systematics challenge. Radio emission from our own galaxy and the sea of galaxies between us and the faint cosmological signal dwarf it by four-to-five orders of magnitude. To recover this signal, we must deal with large radio data sets that are heavily contaminated by noise and foregrounds. In addition, radio instrumentation effects that come in at the one-part-in-one-thousand level including antenna-antenna mutual coupling and interactions between the antennas and their environment must be addressed. Thus we must reimagine much of radio astronomy from the ground up; from the design considerations of radio antennas and observatories to the statistical methods that we use to calibrate, reduce, and infer cosmological parameters from our data. I pioneer techniques in these areas.
I anchor my techniques in data from multiple radio observatories which I helped design targeting both the Cosmic Dawn and the Epoch of Acceleration; the Hydrogen Epoch of Reionization Array (HERA), and the Hydrogen Intensity mapping and Realtime Analysis eXperiment (HIRAX). I also continue to develop methods on the Murchison Widefield Array (MWA).
A plot of discrete prolate spheroidal sequences -- a maximally efficient basis for modeling band limited signals. I use these and related functions to efficiently mitigate foreground systematics. My DPSS foreground filter has enabled the first detections of cosmological 21cm with an interferometer.
Cutting Edge Data Analysis Tools
I leverage technological advances at the heart of the machine-learning revolution include GPUs and auto-differentiation to build analysis and calibration tools that redefine what is possible in hydrogen cosmology. Another key innovation in my approaches is my identification and use of maximally efficient sets of features to model systematics. I would best describe my approach as using flexible and efficient feature sets derived from physical principals to enable fast and thorough systematics mitigation. My analysis tools include a calibration platform that is able to robustly solve for instrumental gains without many unrealistic assumptions used in other state-of-the-art approaches and a simple and powerful foreground filter. You can find the code for these tools and the HERA software stack, which I am also a core contributor to, at my github
Left: A model for foreground contamination of a HERA visibility in the two-dimensional Fourier transform of the cross-correlation between two antennas in the array. Center: predictions from a model that my graduate student constructed for antenna-antenna reflections predicts and "X" shaped feature in this space, potentially contaminated a large range of cosmological scales. Right: We observe the same feature in HERA data. The "X" contamination is efficiently removed with a time-domain filter that only preserves time-domain Fourier modes that fall within sources transiting the main beam of the instrument (red dashed lines).
Theory
Hydrogen Cosmology is an emerging field and the daunting systematics challenges require that instrumental design, data analysis techniques, and theoretical models must all be considered hand-in-hand. I have therefor undertaken numerous theoretical projects delving into the impact of calibration strategies and antenna-antenna mutual coupling on 21cm experiments and have proposed models to explain a recent unusual detection of 21cm in absorption at z~17 by the EDGEs experiment using radio emission from violently accreting super massive black-hole progenitors.
Top: The Hydrogen Epoch of Reionization Array (HERA) under construction in South Africa. Bottom: A close up of one of HERA's broad-band feeds that will allow it to observe 21cm emission across the redshifts of 6 to 25. -- Image Credit https://reionization.org/
The Cosmic Dawn - HERA and the MWA
HERA is a new radio interferometry that my collaborators and I are commissioning in South Africa which is designed to detect large-scale Hydrogen fluctuations during our Universe's Cosmic Dawn and learn about the properties of the first stars, galaxies, and black-holes. My work in setting the first limits on fluctuations due to heating by high-mass X-ray binaries at z~20 on the Murchison Widefield Array unveiled some of the major systematic challenges in interferometer design. I brought these lesson into ensuring that HERA's dishes keep spectral structures, which are a leading systematic, below acceptable levels using electromagnetic simulations and field measurements. I also established the potential for HERA to constrain the properties of high-mass X-ray binaries in heating Hydrogen before reionization leading to the HERA collaborations design and deployment of a broad band Vivaldi Feed to extend HERA's bandwidth down to 50 MHz.
I am currently leading the HERA effort to observe these pre-reionization 21cm fluctuations.
The HIRAX prototype telescope array at HartRAO in 2017 -- Image Credit Ben Saliwanchik
Constraining Dark Energy with HIRAX
At the end of the Cosmic Dawn, the Hydrogen in the Intergalactic Medium was ionized. The remaining Hydrogen inside of galaxies traces the density structure on large scales, allowing us to use it as a tracer of cosmological expansion using Baryon Acoustic Oscillations. I am currently leading the electromagnetic modeling group in designing the antenna element of the Hydrogen Intensity and Realtime Analysis eXperiment to ensure that spectral structure, mutual coupling, and polarization leakage are all at acceptable levels to ensure that HIRAX can obtain clean measurements of the BAO Hydrogen signal. HIRAX is currently funded by the South African Research Foundation to deploy 256 elements at the SKA site in South Africa.