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

Abstract

The 21 cm 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 21 cm. After recombination and during the Dark Ages, at z ∼ 30 − 1000, the 21 cm emission is associated with density fluctuations in the neutral hydrogen medium. After the first stars turn on and galaxies begin to form, the 21 cm 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 21 cm 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 21 cm 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 21 cm as a probe of fundamental physics across many axes and invest in the technology development that will enable full exploitation of this rich technique.

Dark Galaxies, Sun-Earth-Moon Interaction, Tunguska Event&amp;#8212;Explained by WUM

“…At redshifts below z < 6, the intergalactic medium is largely ionized, but pockets of self-shielded neutral gas form in dense galactic environments and 21 cm 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" [10].…”

Bootstrapping one-loop inflation correlators with the spectral decomposition

“…One of the most fascinating facts in modern cosmology is that we can access the physics of the primordial universe by measuring the correlation functions of large-scale inhomogeneities and anisotropies. Examples of such measurements include the temperature and the polarizations of the cosmological microwave background [1,2], the large-scale structure survey [3], and the more futuristic 21cm tomography [4,5]. Based on existing observational data, it is now widely believed that the large-scale inhomogeneities originated from a period of inflation in the primordial universe.…”

A New Constraint on the Simulation of the Intergalactic Medium through the Evolution of the Neutral Hydrogen Fraction in the Epoch of Reionization

“…At redshifts between 12 and 20, the spin temperature is equal to the kinetic temperature, and x α has a nonzero value. For other redshifts, the relation between the spin temperature, the kinetic temperature, and the color temperature is T C = (1 + 2.5T k )T S /(1 + 2.5T S ) (Liu et al 2022).…”