Snowmass2021 Cosmic Frontier White Paper: 21cm Radiation as a Probe of Physics Across Cosmic Ages

Understand

The 21cm line refers to a forbidden transition in neutral hydrogen associated with alignment of spins of the proton and electron.

• It is a very low energy transition that is emitted whenever there is neutral hydrogen in the Universe.
• Since baryons are mostly (~75%) hydrogen, one can in principle detect this emission throughout much of the history of the Universe.
• The dominant emission mechanism is different across cosmic ages.