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Register a new userAstro 2020 Science White Paper: Fundamental Cosmology in the Dark Ages with 21-cm Line Fluctuations
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The Dark Ages are the period between the last scattering of the cosmic microwave background and the appearance of the first luminous sources, spanning approximately 1100 < z < 30. The only known way to measure fluctuations in this era is through the 21-cm line of neutral hydrogen. Such observations have enormous potential for cosmology, because they span a large volume while the fluctuations remain linear even on small scales. Observations of 21-cm fluctuations during this era can therefore constrain fundamental aspects of our Universe, including inflation and any exotic physics of dark matter. While the observational challenges to these low-frequency 21-cm observations are enormous, especially from the terrestrial environment, they represent an important goal for cosmology.
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The epoch of reionization, when photons from early galaxies ionized the intergalactic medium about a billion years after the Big Bang, is the last major phase transition in the Universes history. Measuring the characteristics of the transition is important for understanding early galaxies and the cosmic web and for modeling dwarf galaxies in the later Universe. But such measurements require probes of the intergalactic medium itself. Here we describe how the 21-cm line of neutral hydrogen provides a powerful probe of the reionization process and therefore important constraints on both the galaxies and intergalactic absorbers at that time. While existing experiments will make precise statistical measurements over the next decade, we argue that improved 21-cm analysis techniques - allowing imaging of the neutral gas itself - as well as improved theoretical models, are crucial for testing our understanding of this important era.
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The origin of dark matter is a driving question of modern physics. Low-energy antideuterons provide a smoking gun signature of dark matter annihilation or decay, essentially free of astrophysical background. Low-energy antiprotons are a vital partner for this analysis, and low-energy antihelium could provide further discovery space for new physics. In the coming decade, AMS-02 will continue accumulating the large statistics and systematic understanding necessary for it to probe rare antinuclei signatures, and GAPS, which is the first experiment optimized specifically for low-energy cosmic antinuclei, will begin several Antarctic balloon campaigns. The connection of cosmic-ray antinuclei and dark matter is reviewed and the outlook in light of experimental progress is presented.
Measurement of the spatial distribution of neutral hydrogen via the redshifted 21 cm line promises to revolutionize our knowledge of the epoch of reionization and the first galaxies, and may provide a powerful new tool for observational cosmology from redshifts 1<z<4 . In this review we discuss recent advances in our theoretical understanding of the epoch of reionization (EoR), the application of 21 cm tomography to cosmology and measurements of the dark energy equation of state after reionization, and the instrumentation and observational techniques shared by 21 cm EoR and post reionization cosmology machines. We place particular emphasis on the expected signal and observational capabilities of first generation 21 cm fluctuation instruments.
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 states 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 Moons 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 >5$sigma$}) 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.
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