The cosmic microwave background is the afterglow of the Big Bang, which we see at millimeter wavelengths. But it also has a long-wavelength tail extending into the radio range. At wavelengths longer than 21 centimeters, this light begins to be absorbed by neutral hydrogen scattered across distant space. Hydrogen acts like a frosted glass: some photons get stuck in it, making the sky map at these frequencies slightly smoother than expected.

The effect depends on two things: how hot the gas is (its spin temperature) and how many neutral atoms are there, as opposed to ionized ones. The colder the hydrogen, the stronger the absorption. The absorption peak occurs at frequencies around 100 megahertz: this corresponds to the cosmic "dark ages," before the first stars ignited. The fraction of absorbed light can reach a few percent.

Directly detecting such a faint radio signal is a daunting challenge. But the authors propose a workaround: compare radio maps with the already precise millimeter-wave maps of the cosmic microwave background. If the radio map shows a slight smoothing that the millimeter map does not, then hydrogen is indeed absorbing. This cross-analysis could be easier than catching hydrogen's own glow.

This approach offers a new way to measure the ratio of neutral hydrogen fraction to its spin temperature at different epochs. This is important for understanding how gas heated up in the early universe and when the first stars ignited. In essence, we get a thermometer and an atom counter for times when the universe was dark and cold.