21cm was initially misleading; that's the origination of the hydrogen line at 1420MHz; by the time it reaches earth it is doppler shifted down (redshift) to 78M

21cm was initially misleading; that's the origination of the hydrogen line at 1420MHz; by the time it reaches earth it is doppler shifted down (redshift) to 78MHz! I worked as an antenna engineer on the VLA's low-band Epoch of Reionization expansion project in 2014[1], which focused on 74MHz signals. They've been looking at this frequency since 1999 [2] but who knew they were 4MHz low! (I doubt that they just didn't look there, likely some other effects or local RFI problems, as Australia is a lot more remote than New Mexico.) [1] http://lwa.phys.unm.edu/users14/Ellingson_MJP.pdf http://lwa.phys.unm.edu/users14/Ellingson_MJP.pdf [2] https://www.nrao.edu/pr/1999/74mhz/ https://www.nrao.edu/pr/1999/74mhz/