Will the observed behavior of redshifted neutral hydrogen redefine the standard cosmological model?
After the Cosmic Microwave Background photons decoupled from baryons, the Dark Ages epoch began: density fluctuations imprinted from earlier times grew under the influence of gravity, eventually collapsing into the first stars and galaxies during the subsequent Cosmic Dawn. Exploring these early unobserved epochs, a key NASA science goal, was demonstrated to be achievable by recent observations with the Experiment to Detect the Global Epoch of Reionization (EoR) Signature (EDGES).
In the Dark Ages, most of the baryonic matter was in the form of neutral hydrogen (HI), detectable via its ground state’s “spin-flip” transition. This line’s rest frame frequency (wavelength) of 1420 MHz (21-cm) arrives today highly redshifted to low radio frequencies ≲100 MHz due to cosmic expansion. 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 EoR, when ionization of HI extinguished the signal. EDGES recently reported an absorption trough at 78 MHz (redshift z~17), similar in frequency to expectations for Cosmic Dawn, but ~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 an 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 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). DAPPER will observe this pristine epoch (17-38 MHz; z~83-36), and will measure the amplitude of the 21-cm spectrum to the level required to distinguish (at >5 sigma) the standard cosmological model from that of additional cooling derived from current EDGES results. In addition to dark matter properties such as annihilation, decay, temperature, and interactions, the low-frequency background radiation level can significantly modify this trough. Hence, this observation constitutes a powerful, clean probe of exotic physics in the Dark Ages. A second objective for DAPPER will be to verify the recent EDGES results for Cosmic Dawn, in the uncontaminated environment above the lunar farside, with sparse frequency sampling from 55-107 MHz (z~25-12). The combination of both troughs will robustly augment the constraining power of DAPPER on early-Universe astrophysics and cosmology.
DAPPER's specific science objectives are:
- Determine the level of (dis)agreement with the standard cosmological model caused by dark matter in the Dark Ages.
- a) Determine the level of excess cooling above the adiabatic limit for Cosmic Dawn. b) Determine when the first stars and black holes formed.
- DAPPER Science Diagram 1
- DAPPER will use the 21-cm all-sky signal to observe redshifts z=83-12, associated with the Dark Ages and Cosmic Dawn.
- The 21-cm spectrum has broad absorption troughs corresponding to the Dark Ages and Cosmic Dawn.
- Objective 1 is shown in the Dark Ages
- Objective 2 is shown in the Cosmic Dawn
- The legend also shows the Dark Ages Dapper measurement, Cosmic Dawn DAPPER measurements, Edges results, Potential Model with low cooling, Potential Model with medium cooling, Potential Model with high cooling, and the standard model.
- DAPPER Science Diagram 2
- DAPPER separates Galaxy foreground from 21-cm signal using differences in spectral spahes, spatial structure, and polarization.
- The foreground's spatial structure is irregular.
- The foregrounds' spectral structure is smooth in the galactic foreground line.
- The foreground is polarized in the 21-cm signal, neutral hydrogen is not.
- The science timeline asks the question, "Will the observed behavior of redshifted neutral hydrogen redefine the standard cosmological model?"
- Redshift, EM Spectrum and time in billions of years are shown with the corresponding epochs of the formation of the universe with the Big Bang, Cosmic Dark Ages, Cosmic Dawn, and Reionization.
- Missions features are the Hubble Ultra-Deep Field, JWST/WFIRST, DAPPER, HERA, and EDGES missions are shown along with their corresponding area of research.