The expanding Universe began from a hot, nearly uniform mixture of ordinary matter (baryons) and dark matter. Once the temperature of the cosmic plasma dipped below 3000 K, protons and electrons combined to form hydrogen atoms, and the Universe became transparent to the blackbody radiation bath that filled it. We currently detect that radiation as the cosmic microwave background (CMB). Tiny fluctuations in the matter density at these early times grew gravitationally and eventually led to the collapse of the first gas clouds ~30 Myr after the Big Bang (Region B in the figure). These clumps seeded the first bursts of star formation, lighting up our Universe in the Cosmic Dawn (Region C in the figure). Because our current picture of this Cosmic Dawn of structure formation has not yet been investigated observationally, its direct exploration is one of the most exciting frontiers in astrophysics. DARE is unique in probing the Universe at sufficiently early times to follow the evolution of the first stars, galaxies, and black holes from z = 35 to 11.
DARE measures the impact on the hyperfine (or "spin-flip") 21-cm line of neutral hydrogen in the intergalactic medium (IGM) arising from X-ray and UV emission from the first stars, black holes, and galaxies. This line is produced by the tiny magnetic moments, or spins, of the proton and electron. If the spins are aligned, the atom lies at a slightly higher energy than if they are anti-aligned. The energy difference corresponds to a photon with a rest wavelength (frequency) of 21-cm (1420 MHz). The expansion of the Universe redshifts these photons from earlier epochs to lower observed frequencies, v=1420/(1+z) MHz (e.g., at z=30, v=45 MHz). The frequency-redshift relation enables the direct reconstruction of the history of the early Universe as a function of time from the 21-cm spectrum, a powerful tool that is independent of theoretical models.
The sky-average, or global, brightness temperature of the 21-cm signal as a function of redshift is given by
Several important astrophysical processes drive the evolution of TB with redshift (time). These include: (1) UV radiation from the first stars, which “activates” the 21-cm signal through the Wouthuysen-Field mechanism; (2) X-ray heating, likely generated by gas accretion onto the first black holes; and (3) ionizing photons from the first galaxies (which destroy neutral hydrogen). These radiation backgrounds grow at different times, so their interplay creates distinct features in the spectrum. When the first stars appear (z ~ 35-22; Region B), their UV radiation drives TS toward the cold temperatures that are characteristic of IGM gas, triggering a deep absorption trough. Shortly after, black holes likely formed as remnants of the first stars (z ~ 25-12; Region C). The energetic X-ray photons from these accreting black holes heat the IGM, transforming the 21-cm signal from absorption into emission as the gas becomes hotter than the CMB (Region D). This emission peaks as photons from these stars and black holes ionize the IGM gas (z < 12), eventually eliminating the spin-flip signal, i.e., the fraction of neutral hydrogen, xHI, descends to zero. This last phase corresponds to the Epoch of Reionization (EoR).
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A number of ground-based experiments will probe the EoR. Compelling observational evidence indicates that the bulk of reionization occurs at late times. However, this era could be very long due to an extended early phase of Pop III star formation. CMB measurements suggest that this early component, at z>15, accessible to DARE, is significant. Moreover, in many models the IGM remains cold during reionization. DARE constrains the early phases of reionization through the global signal and will complement ground-based measurements. The timing of this stage depends on the overall ionizing efficiency of galaxies, which in turn depends on uncertain physical factors. The combination of these complementary measurements will provide a clearer picture of reionization as well as an anchor point for DARE’s unique measurements at much higher redshifts.
The heating and ionization history of the IGM may carry clues about exotic physics. DARE will explore such models in which, for example, dark matter annihilation and decay can heat the IGM and modify the 21-cm signature, as can primordial black holes and cosmic strings.
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