“…The spin temperature in turn is determined by coupling with atoms, electrons and Lyman-α line radiation (Wouthuysen, 1952b,a;Purcell & Field, 1956;Furlanetto, Oh, & Briggs, 2006). At intermediate redshifts, techniques such as co-adding (Chengalur, Braun, & Wieringa, 2001;Zwaan, 2000;Lah et al, 2007;Rhee et al, 2013;Delhaize et al, 2013;Rhee et al, 2016;Kanekar, Sethi, & Dwarakanath, 2016) and cross-correlation (Chang et al, 2010;Masui et al, 2013) have been used to detect neutral Hydrogen. As we get to even higher redshifts, statistical detection is the only viable approach for observing the large scale distribution of HI.…”