So far no one has definitively seen back to the cosmic dark ages, although one experiment claimed to detect the 21-cm hydrogen line from 180 million years after the big bang.

The behavior of the 21-cm signal is related to the characteristics of the first stars, which reionized the hydrogen and let there be light.

Malhotra has been studying galaxies that emit light in a certain wavelength that is characteristic of the recombination of hydrogen—the moment when neutral atoms formed.

And the entire universe was shrouded in neutral hydrogen gas. Then, perhaps 100 million years or so in, everything started to ...

Conditions around cosmic dawn would affect the 21-cm light. (21-cm light is emitted when a neutral hydrogen atom, with one proton and one electron, transitions from a state with both particles ...

Earth, between sweltering Venus and frozen Mars, formed and orbits within a band where water could persist in solid, liquid, and gaseous phases. Mars had surface water, but not as much as Earth, and ...

The 21-cm emission isn’t particularly bright, but because there is so much hydrogen out there, the signal is easy to detect. And wherever there is matter, so too is the hydrogen line.

Bosma, A. (1981) 21-cm Line Studies of Spiral Galaxies. II. the Distribution and Kinematics of Neutral Hydrogen in Spiral Galaxies of Various Morphological Types. The Astronomical Journal, 86, ...

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