  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. Before the photons decouple from
matter, hydrogen is in an ionized state and does not emit in 21cm. After
recombination and during the Dark Ages, at z ~ 30-1000, the 21cm emission is
associated with density fluctuations in the neutral hydrogen medium. After the
first stars turn on and galaxies begin to form, the 21cm emission traces
bubbles of ionized hydrogen in the sea of the neutral medium. This epoch,
spanning z ~ 6-30, is often referred to as cosmic dawn and the Epoch of
Reionization (EoR). At redshifts below z<6, the intergalactic medium is largely
ionized, but pockets of self-shielded neutral gas form in dense galactic
environments and 21cm emission traces the distribution of galaxies. The vastly
different emission mechanisms allow us to probe very different physics at
different redshifts, corresponding to different observational frequencies. The
instrumental challenges, namely building very sensitive and exquisitely
calibrated radio telescopes, however, share many commonalities across frequency
bands. The potential of the 21cm probe has been recognized by the Decadal
Survey of Astronomy & Astrophysics, whose Panel on Cosmology identified the
Dark Ages as its sole discovery area. We argue that HEP should recognize the
potential of 21cm as a probe of fundamental physics across many axes and invest
in the technology development that will enable full exploitation of this rich
technique.
