Theory and observations of the 21cm line at large redshift

Radio astronomy is undergoing a period of rapid advances because of digital techniques that allow an improvement in sensitivity of several orders of magnitude. The universe nearby and the initial conditions of the universe have now been characterized at great precision thanks to modern galaxy surveys and precision measurements of the cosmic microwave background. But the intermediate epoch, sometimes dubbed the “Dark Ages,” remains largely unexplored. However precision measurements using the 21cm line will allow a detailed three-dimensional characterization of this intermediate epoch.


The thesis is composed a two parts: a theoretical component involving the physics of the 21cm line and an observational component as part of the HIRAX experiment, one of the precursors to the Square Kilometer Array (SKA).


One of the big questions of contemporary cosmology is the nature of the Dark Energy, the discovery of which led to the 2011 Physics Nobel Prize. Is the Dark Energy simply a cosmological constant, or a new form a energy with even more exotic properties?


Baryon acoustic oscillations (BAO) allow us to map the expansion history of the universe and thus deduce the properties of the Dark Energy. Although many experiments seek to map the BAO using observations in the visible and infrared bands, radio observations have many advantages. In particular observations using the 21cm line allow one to gather redshift information with no additional effort. By contrast, optical and IR measurements are intrinsically two-dimensional, and for the third dimension one must use spectrometers with hundred of individually placed fibers, or one must content oneself with approximate photometric redshifts. Radio observations, however, are inherently three-dimensional.


HIRAX will be placed in the southern hemisphere, at the future SKA site, which is one of the most radio quiet zones on Earth. HIRAX will map the BAO between z=0.8 and z=2.5 over the entire sky visible from the southern hemisphere. The student recruited will participate in the HIRAX collaboration and be responsible for a part of the analysis and interpretation of the data collected.



Martin Bucher






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