The Dark Ages Radio Explorer (DARE) will probe the epoch of formation of the first stars, black holes, and galaxies, never before observed, using the redshifted hyperfine 21-cm transition from neutral hydrogen. These first objects to illuminate the Universe at the end of the Dark Ages into the Cosmic Dawn (redshifts 35 to 11) will be studied via their heating and ionization of the intergalactic medium. Over its lifetime of 2 years, DARE observes at low radio astronomy frequencies (VHF), 40 120 MHz, in a 125 km altitude lunar orbit. The Moon occults both Earth and the Sun as DARE makes observations on the lunar farside, shielding it from the corrupting effects of radio interference, Earth's ionosphere, and solar emissions. Bi-conical dipole antennas, pseudo-correlation receivers used in differential mode to stabilize the radiometer, and a digital spectrometer achieve the sensitivity required to observe the cosmic signal. The unique frequency structure of the 21-cm signal and its uniformity over large angular scales are unlike the spectrally featureless, spatially varying characteristics of the Galactic foreground, allowing the signal to be separated from the foreground. With a straightforward measurement, proven technology, and radio-quiet environs, DARE will open a new window of discovery into the early Universe.
DARE realizes NASA's strategic objective in astrophysics to: "explore how (the Universe) began and evolved". It also executes small-scale mission described in Astrophysics Roadmap: "Mapping the Universe's hydrogen clouds using 21-cm radio wavelengths via lunar orbiter from the farside of the Moon". DARE will measure the sky-averaged spin temperature of neutral hydrogen at unexplored redshifts 11-35, to explore the origin and evolution of the first stars and galaxies. DARE's science objectives are:
1) Determine when the First Stars ignited and what were their characteristics.
2) Determine when the first Black Holes began accretion and what were their characteristic masses.
3) Determine when Reionization began.
4) Determine what surprises emerged from the Dark Ages.
DARE will answer two fundamental questions identified in the recent Decadal Survey, New Worlds, New Horizons in Astronomy and Astrophysics: What were the first objects to light up the Universe, and when did they do it? The birth of the first stars and black holes is one of the truly transformative events in the history of the Universe. DARE's approach is to measure the spectral shape of the sky-averaged redshifted 21-cm signal from neutral hydrogen over the redshift range 11-35 (80-420 million years after the Big Bang), corresponding to radio frequencies 40-120 MHz.
DARE opens a new discovery window in astrophysics DARE is a successor to COBE/WMAP/Planck in exploring the next cosmological frontier and will complement JWST's observations of bright galaxies during Cosmic Dawn. DARE is unique in observing the redshift interval 35>z>11 using the sky-averaged HI spectral features and in measuring the integrated radiation backgrounds from entire populations rather than individual sources. JWST will observe some of the earliest galaxies ~0.3-0.4 billion years after the Big Bang (z ~12-15), while other facilities operating between radio and X-ray wavelengths will observe these sources in complementary ways. DARE provides a diffrent approach to constraining early luminous sources than JWST or ATHENA Instead of detecting bright, individual sources, DARE measures the collective effcts of all sources from z = 11 - 35 By combining DARE and JWST data, we constrain a broad range of mass and luminosity functions for the fist stars and galaxies Ground-based radio telescopes operating mostly above the DARE band at 100-200 MHz, such as EDGES, MWA, LOFAR, and PAPER, are attempting to study the end of reionization. The HERA and SKA interferometers will offer exquisite constraints on most of that process using the angular power spectrum, LEDA and Sci-HI overlap with portions of the DARE band and are important technology/engineering precursors; however, ionosphere corruption will limit any detection of the 21-cm features. DARE is distinctive in its observations of Turning Point B (frequencies < 50 MHz), along with measurements of Turning Points C and D DARE's measurements will be complementary to other data including galaxy observations, the CMB, and Lya absorption studies DARE has high programmatic value to make unique scientific progress in understanding the fist stars/galaxies, providing synergy with NASA's next Great Observatory (JWST), and requiring a space mission to complete its goals
These observations are challenging because the 21-cm signal strength is predicted to be much fainter than various astrophysical foregrounds and interferences. However, DARE eliminates powerful sources of interference including: a) Earth's ionosphere and b) human-generated RFI human-generated radio frequency interference (RFI) by taking data while it is above the farside of the Moon. Exhaustive experimental and theoretical efforts demonstrated that DARE must be placed in the pristine environs of the lunar farside to guarantee success. The smooth frequency response of DARE's bi-conical dipole antenna and pseudo-correlation receivers used in differential mode are effective in removing the astrophysical foregrounds (i.e., the Galaxy and solar system objects).
This Mission draws on a rich intellectual and technological heritage from ground-based low frequency instruments as well as antennas, receivers, and S/C that have flown in similar space environments. They reflect the experiences, lessons-learned, and proven performance of EDGES, a ground-based pathfinder operating in Western Australia at higher frequencies as well as the Cosmic Twilight Pathfinder (CTP) initiatives at NRAO-GreenBank, WV operating at high frequency part of the DARE band. The DARE measurement strategy and science instrument design derives its intellectual heritage from EDGES. DARE is pioneering a new measurement from the lunar far-side, but it is not entering uncharted intellectual territory. The DARE strategy abandons the conventional absolute calibration requirement of a radiometer in favor of a differential spectral calibration requirement, an approach derived from EDGES (and analogous to differential radiometry used by WMAP).
DARE is led by Jack Burns, a highly-published scientist and successful large project and senior university administrator. The science team includes members who first modeled the global radio signal from the Cosmic Dawn, the PI of the ground-based DARE pathfinder (EDGES), and members with extensive flight mission experience. The team has decades of experience with centimeter and meter wavelength observations. The Project Manager at NASA Ames, the Spacecraft Project Manager at Ball Aerospace, and the Science Instrument Manager at JPL are seasoned veterans from multiple NASA space science missions.