The 21 cm HI transition offers a window onto the reionization history of the universe and the formation of the first generation of stars and quasars, when the universe was still metal poor. The global 21 cm signal consists of the non-Galactic distortion of the CMB blackbody radiation through absorption or emission by HI (atomic hydrogen) depending on whether the HI spin temperature is less or greater than the CMB temperature. Recently the EDGES experiment has detected a large signal that appears incompatible with existing theoretical models.

The primary objective of cosmological research in the coming decade is to understand the accelerated expansion of the Universe, attributed to either a dark energy component or a modification to gravity on cosmic scales.  This thesis will focus on evaluating the Euclid galaxy cluster selection function, an essential element of using the mission’s cluster catalog as a probe of dark energy and modified gravity.