The emission is the so-called ‘forbidden’ hyperfine transition of the spin state of Hydrogen’s one electron. It’s flips from spin up to spin down, and the energ

The emission is the so-called ‘forbidden’ hyperfine transition of the spin state of Hydrogen’s one electron. It’s flips from spin up to spin down, and the energy difference is a photon at about 1420 MHz. It’s called forbidden because it’s extremely unlikely to occur - so unlikely that I don’t believe it’s been observed in the lab. But in aggregate, with vast vast clouds of hydrogen it becomes likely enough and strong enough to detect. We observe this emission from hydrogen in our own galaxy and through the local universe. And it’s quite strong to easily detect. In this experiment, they’re looking for that same 1420 MHz signal, redshifted down to something like 180 MHz (where exactly it is isn’t known for sure).