When hydrogen atoms first formed, they absorbed and then emitted ambient 21-centimeter radiation at equal rates, which made the clouds of hydrogen that filled the primordial universe effectively invisible.
Then came cosmic dawn. Ultraviolet radiation from the first stars excited atomic transitions that enabled hydrogen atoms to absorb more 21-centimeter waves than they emitted. Viewed from Earth, this excess absorption would appear as a drop in brightness at a specific radio wavelength marking the moment the stars turned on.
In time, the first stars collapsed into black holes. The hot gases swirling around these black holes generated x-rays that heated hydrogen clouds throughout the universe, increasing the rate of 21-centimeter emissions. We would observe this as an uptick in brightness at a slightly shorter radio wavelength than that of the older light. The net result would be a dip…
