Around 400,000 years after the Big Bang the Universe cooled sufficiently for matter and radiation to decouple, allowing the first neutral atoms to form. This epoch of recombination can be observed through the Cosmic Microwave Background (CMB), at a redshift of z~1,100. Much later, at a redshift of z~10-15, the first stars and galaxies formed. Upcoming optical and infrared observatories such as the James Webb Space Telescope are expected to be able to observe these early generations of luminous objects. The period in between these two regimes, however, remains largely unobserved. This observational gap can be addressed by utilizing the 21-cm spectrum of neutral Hydrogen (HI), which was abundant in the early Universe after recombination.
The 21-cm line, also known as the hyperfine or "spin-flip" transition, arises from the interaction between the magnetic moments of the electron and proton in Hydrogen atoms. The observed brightness temperature of the 21-cm transition is determined by both the temperature and neutral fraction of the Hydrogen gas. Measurements of the highly redshifted 21-cm spectrum are therefore able to trace the history of the HI gas starting from the Dark Ages, after recombination when no luminous objects existed, through Cosmic Dawn, when gas clouds collapsed to form the first stars and galaxies, to the Epoch of Reionization (EoR), at which point the InterGalactic Medium (IGM) became fully ionized rendering the 21-cm line unobservable. 21-cm observations thus provide a method for studying the evolution of the early Universe during a time that is otherwise inaccessible.
The Radio Quiet Environment Above the Lunar Farside and its Application to 21-cm Experiments
NASA Exploration Science Forum, NASA Ames Research Center, Moffett Field, CA, 24 July 2019
The Dark Ages, probed by the redshifted 21-cm signal, is the ideal epoch for a new rigorous test of the standard LCDM cosmological model. Divergences from that model would indicate new physics, such as dark matter decay (heating) or baryonic cooling beyond that expected from adiabatic expansion of the Universe. In the early Universe, most of the baryonic matter was in the form of neutral hydrogen (HI), detectable via its ground state's spin-flip transition. A measurement of the redshifted 21-cm spectrum maps the history of the HI gas through the Dark Ages and Cosmic Dawn and up to the Epoch of Reionization (EoR). The Experiment to Detect the Global EoR Signature (EDGES) recently reported an absorption trough at 78 MHz (redshift z of 17), similar in frequency to expectations for Cosmic Dawn, but about 3 times deeper than was thought possible from standard cosmology and adiabatic cooling of HI. Interactions between baryons and slightly-charged dark matter particles with electron-like mass provide a potential explanation of this difference but other cooling mechanisms are also being investigated to explain these results. The Cosmic Dawn trough is affected by cosmology and the complex astrophysical history of the first luminous objects. Another trough is expected during the Dark Ages, prior to the formation of the first stars and thus determined entirely by cosmological phenomena (including dark matter). Observations on or in orbit above the Moon's farside can investigate this pristine epoch (15-40 MHz; z=100-35), which is inaccessible from Earth. A single cross-dipole antenna or compact array can measure the amplitude of the 21-cm spectrum to the level required to distinguish (at >5σ) the standard cosmological model from that of additional cooling derived from current EDGES results. This observation constitutes a powerful, clean probe of exotic physics in the Dark Ages.