The early star-forming Universe is still poorly constrained, with the properties of high-redshift stars, the first heating sources, and reionization highly uncertain. This leaves observers planning 21-cm experiments with little theoretical guidance. In this work we explore the possible range of high-redshift parameters including the star formation efficiency and the minimal mass of star-forming halos; the efficiency, spectral energy distribution, and redshift evolution of the first X-ray sources; and the history of reionization. These parameters are only weakly constrained by available observations, mainly the optical depth to the cosmic microwave background. We use realistic seminumerical simulations to produce the global 21-cm signal over the redshift range z = 6 − 40 for each of 193 different combinations of the astrophysical parameters spanning the allowed range. We show that the expected signal fills a large parameter space, but with a fixed general shape for the global 21-cm curve. Even with our wide selection of models we still find clear correlations between the key features of the global 21-cm signal and underlying astrophysical properties of the high redshift Universe, namely the Lyα intensity, the X-ray heating rate, and the production rate of ionizing photons. These correlations can be used to directly link future measurements of the global 21cm signal to astrophysical quantities in a mostly model-independent way. We identify additional correlations that can be used as consistency checks.
The recent detection of an anomalously strong 21-cm signal of neutral hydrogen from cosmic dawn by the EDGES low-band radio experiment can be explained if cold dark matter particles scattered off the baryons draining excess energy from the gas. In this Letter we explore the expanded range of the 21-cm signal that is opened up by this interaction, varying the astrophysical parameters as well as the properties of dark matter particles in the widest possible range. We identify models consistent with current data by comparing to both the detection in the low-band region and the upper limits from the EDGES high-band antenna. We find that consistent models predict a 21-cm fluctuation during cosmic dawn that is between 3 and 30 times larger than the largest previously expected without dark matter scattering. The expected power spectrum exhibits strong baryon acoustic oscillations imprinted by the velocity-dependent cross section. The latter signature is a conclusive evidence of the velocity-dependent scattering and could be used by interferometers to verify the dark matter explanation of the EDGES detection.
Long wavelength spectral distortions in the Cosmic Microwave Background arising from the 21-cm transition in neutral Hydrogen are a key probe of Cosmic Dawn and the Epoch of Reionization. These features may reveal the nature of the first stars and ultra-faint galaxies that transformed the spin temperature and ionization state of the primordial gas. SARAS 2 is a spectral radiometer purposely designed for precision measurement of these monopole or all-sky global 21-cm spectral distortions. We use 63 hr night time observing of the radio background in the frequency band 110-200 MHz with the radiometer deployed at the Timbaktu Collective in Southern India to derive likelihoods for plausible redshifted 21-cm signals predicted by theoretical models. First light with SARAS 2 disfavors the class of models that feature weak X-ray heating (with f X ≤ 0.1) and rapid reionization (with peak dT b dz ≥ 120 mK per unit redshift interval ).
Figure 2: The small-scale PTOLEMY prototype installed at the Princeton Plasma Physics Laboratory (February 2013). Two horizontal bore NMR magnets are positioned on either side of a MAC-E filter vacuum tank. The tritium target plate is placed in the left magnet in a 3.35T field, and the RF tracking system is placed in a high uniformity 1.9T field in the bore of the right magnet with a windowless APD detector and in-vacuum readout electronics. Contents Contents ii 7 Trigger and Data-Acquisition 8 Time-of-flight 9 Muon veto 10 e-Gun Calibration 11 Vacuum system 12 Cooling systems
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