HYPERION: The Case for Interferometry in 21 cm Reionization Global Signal Studies

Completed in November 2018, this thesis describes the theory behind what would have become the HYPERION global sky experiment. This unique instrument was being designed specifically to observe the global signal of reionization, i.e. to measure the average behavior of neutral hydrogen gas during the period of our universe’s history when the first stars, galaxies, and black holes were being born. By observing this behavior, we can learn a tremendous amount about the physics and behavior of the early universe. This signal is difficult to measure as it is so faint: at the redshifted frequencies at which we expect to see it, the reionization signature is dwarfed by galactic synchrotron radiation that is estimated to be 100,000 times brighter than it. This has proven to be a challenging problem for cosmologists to solve, as this also forces very careful calibration of the observational instruments to ensure that systemic biases aren’t being mistaken for reionization signatures.

HYPERION (i.e. the HYdrogen Probe for the Epoch of ReIONization) presented an innovative new technique for observing this signal, which aimed to use interferometry to avoid many of the systemic calibration issues that plague the single-dish experiments typical of the field. This is a trade-off, as an interferometer is definitionally not receptive to a global signal. However, by implementing some clever design to manipulate each receiver’s spatial receptivity, we can push the global signal into higher spatial orders and make it visible to the interferometer.

This thesis explores these concepts in detail, laying out the case for interferometric global signal studies and exploring how to best design an instrument for this purpose. It also explores how to calibrate an instrument with these unique elements, and the impacts of imperfect calibration on its ability to accurately recover the true global signal of reionization.

Abstract

We seek to examine the case for the viability of using a classical interferometer to search for the “global signal” of 21 cm emission from reionization. In particular, we examine how spatial windowing can be used to redistribute a spatial monopole into higher order modes that are accessible to the interferometer. In this study, the windowing is managed through the use of absorptive walls around the individual antennas of the array, which modify each antenna’s receptivity of the sky, thus introducing spatial variation into the monopole signal. We can characterize the parameters of these walls and their effects on our observed signal into a set of known sensitivity coefficients, which can then be used to accurately recover the reionization global signal. Preliminary results show that this method does have some sensitivity to the monopole, indicating that it could serve as an alternative experimental design to the traditional single-dish approach, as it benefits from different systematics and finer control of spatial scaling and sensitivity, which may help to mitigate non-isotropic foregrounds.