Ground-penetrating radar for precision radio cosmology

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The first stars in the universe were born a few hundred million years after the big bang. These stars were unlike any that exist today, and this period of the universe's history, known as "cosmic dawn," is largely uncharted territory. Observations of these first stars are possible via radio-frequency measurements of the sky. Hydrogen, which is the most abundant element in the universe, naturally emits light with a wavelength of 21 cm. Because the universe is expanding, this wavelength is stretched in proportion to how far away (or, equivalently, how long ago) the hydrogen emitted its light. The radio emission from hydrogen depends sensitively upon the first stars and is therefore a powerful tool for elucidating their physical properties and formation history. However, observations at these frequencies (<200 MHz) are exceptionally difficult because the signal from the sky can be easily contaminated by uncertainties in the instrument behavior. In particular, the antenna that receives incoming light is influenced in subtle ways by the electromagnetic properties of the underlying earth: in essence, the ground is an integral part of the instrument and therefore must be characterized as part of the experiment. This proposal will support a ground-penetrating radar (GPR) system that will be used to perform high-precision measurements of soil electromagnetic properties for cosmic dawn experiments. The cosmic dawn instrumentation has already been developed at McGill, and the subsystems have been extensively characterized. We will observe from the McGill Arctic Research Station and Uapishka Station in central Quebec, which our team has identified as outstanding radio-quiet sites that represent a unique Canadian geographic advantage. With novel cosmic dawn instrumentation in hand, as well as superb observing locations, the GPR will bring a new level of precision to cosmic dawn measurements and will help open a new window on the radio sky.