Dark Ages Radio Explorer

The period between the formation of stars and galaxies and recombination is known as the "dark ages". During this time, the majority of matter in the universe is neutral hydrogen. This hydrogen has yet to be observed, but experiments are underway to detect the hydrogen line produced during this era. The hydrogen line is produced when an electron in a neutral hydrogen atom is excited to a state where the electron and proton have aligned spins or de-excited as the electron and proton spins transition from being aligned to anti-aligned. The energy difference between these two hyperfine states is ${\displaystyle 5.9\times 10^{-6}}$ electron volts, corresponding to a wavelength of 21 centimeters. At times when neutral hydrogen is in thermodynamic equilibrium with the photons in the cosmic microwave background (CMB), the neutral hydrogen and CMB are said to be "coupled", and the hydrogen line is not observable. The hydrogen line can only be observed when the two temperatures differ.

The Big Bang produced a hot, dense, nearly-homogeneous universe. As the universe expanded and cooled, particles, then nuclei, and finally atoms formed. At a redshift of about 1100, equivalent to about 400,000 years after the Big Bang, when the primordial plasma filling the universe cooled sufficiently for protons and electrons to combine into neutral hydrogen atoms, the universe became optically thin whereby photons from this early era no longer interacted with matter. We detect these photons today as the cosmic microwave background (CMB). The CMB shows that the universe was still smooth and uniform.[3][4][5]

After the protons and electrons combined to produce the first hydrogen atoms, the universe consisted of a nearly-uniform, almost completely-neutral, intergalactic medium (IGM) whose dominant matter component was hydrogen gas. This period is called the Dark Ages due to the lack of luminous sources. Theoretical models predict that over the next few hundred million years, gravity slowly condensed the gas into increasingly dense regions, within which the first stars appeared, marking Cosmic Dawn.[4][5]

As more stars formed and the first galaxies assembled, the universe was flooded with ultraviolet photons capable of ionizing hydrogen gas. A few hundred million years after Cosmic Dawn, the first stars produced enough ultraviolet photons to re-ionize nearly all the universe's hydrogen atoms. This era of reionization marks the transition of the IGM back to a nearly completely ionized state.[4][5]

The emergence of structural complexity in the universe has not yet been investigated observationally. In order to study the earliest structures in the universe, it is necessary to use a telescope more powerful than the Hubble Space Telescope. Theoretical models suggest that existing measurements are beginning to probe the tail end of Reionization, but the first stars and galaxies in the Dark Ages and the Cosmic Dawn are not yet observable with current tools.[4]

The proposed DARE mission would make the first measurements of the birth of the first stars and black holes and would measure the properties of the otherwise invisible stellar populations. Such observations would place existing measurements in context and contribute to understanding how the first galaxies grew from earlier generations of structures.[3][4][5]

DARE's proposed approach is to measure the spectral shape of the sky-averaged, redshifted 21-cm signal over a radio bandpass of 40–120 MHz, observing neutral hydrogen at redshifts in the range 11–35, which correlates to 420–80 million years after the Big Bang. The DARE mission's proposed timeline has it orbit the Moon for 3 years, taking data above the Lunar far side, the only location in the inner Solar System thought to be free of human-generated radio frequency interference and any significant ionosphere.

The scientific instrument would be mounted to an RF-quiet spacecraft bus and is composed of a three-element radiometer, including an electrically-short, tapered, biconical dipole antennae, a receiver, and a digital spectrometer. The smooth frequency response of the antennae and the differential spectral calibration approach used for DARE may be effective in removing the intense cosmic foregrounds so that the weak cosmic 21-cm signal can be detected.

Besides DARE, other similar projects are proposed to also study this area such as the Precision Array for Probing the Epoch of Reionization (PAPER), Low Frequency Array (LOFAR), Murchison Widefield Array (MWA), Giant Metrewave Radio Telescope (GMRT), and the Large Aperture Experiment to Detect the Dark Ages (LEDA).

• Reionization
• Wouthuysen–Field coupling

 This article incorporates public domain material from DARE paper in Advances in Space Research now in press. National Aeronautics and Space Administration.

• Furlanetto, Steven R.; Peng Oh, S.; Briggs, Frank H. (2006). "Cosmology at low frequencies: The 21cm transition and the high-redshift Universe". Physics Reports. 433 (4–6): 181–301. arXiv:astro-ph/0608032. Bibcode:2006PhR...433..181F. doi:10.1016/j.physrep.2006.08.002. S2CID 118985424.

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