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.

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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

The extraction of the Kolmogorov (metric) entropy from an experimental time signal is discussed. Theoretically we stress the concept of generators and that the existence of an expansive constant guarantees that a finite-time series would be sufficient for the calculation of the metric entropy. On the basis of the theory we attempt to propose optimal algorithms which are tested on a number of examples. The approach is applicable to both dissipative and conservative dynamical systems.

Spectral distortions in the cosmic microwave background over the MHz band are imprinted by neutral hydrogen in the intergalactic medium prior to the end of reionization. This signal, produced in the redshift range z = 6 − 34 at the rest frame wavelength of 21 cm, has not been detected yet; and poor understanding of high redshift astrophysics results in a large uncertainty in the expected spectrum. The SARAS 2 radiometer was purposely designed to detect the sky-averaged 21-cm signal. The instrument, deployed at the Timbaktu Collective (Southern India) in April-June 2017, collected 63 hr of science data, which were examined for the presence of the cosmological 21-cm signal. In our previous work the first-light data from SARAS 2 radiometer were analyzed with Bayesian likelihoodratio tests using 264 plausible astrophysical scenarios. In this paper we re-examine the data using an improved analysis based on the frequentist approach and forward modeling. We show that SARAS 2 data rejects 27 models, out of which 25 are rejected at a significance > 5σ. All the rejected models share the scenario of inefficient heating of the primordial gas by the first population of X-ray sources along with rapid reionization.

We present new constraints on parameters of cosmic dawn and the epoch of reionization derived from the EDGES High-Band spectrum . The parameters are probed by evaluating global 21 cm signals generated with the recently developed Global21cm tool. This tool uses neural networks trained and tested on ∼ 30, 000 spectra produced with semi-numerical simulations that assume the standard thermal evolution of the cosmic microwave background and the intergalactic medium. From our analysis, we constrain at 68% (1) the minimum virial circular velocity of star-forming halos to V c < 19.3 km s −1 , (2) the X-ray heating efficiency of early sources to f X > 0.0042, and (3) the low-energy cutoff of the X-ray spectral energy distribution to ν min < 2.3 keV. We also constrain the star-formation efficiency ( f * ), the electron scattering optical depth (τ e ), and the mean-free path of ionizing photons (R mfp ). We re-compute the constraints after incorporating into the analysis four estimates for the neutral hydrogen fraction from high-z quasars and galaxies, and a prior on τ e from Planck 2018. The largest impact of the external observations is on the parameters that most directly characterize reionization. Specifically, we derive the combined 68% constraints τ e < 0.063 and R mfp > 27.5 Mpc. The external observations also have a significant effect on V c due to its degeneracy with τ e , while the constraints on f * , f X , and ν min , remain primarily determined by EDGES.