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Register a new userProbing Compensated Isocurvature with the 21-cm Signal during Cosmic Dawn
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Upcoming measurements of the 21-cm line of neutral hydrogen will open a new observational window into the early stages of structure growth, providing a unique opportunity for probing large-scale cosmological signatures using the small-scale signals from the first stars. In this paper we evaluate the detection significance of compensated isocurvature perturbations (CIPs) from observations of the 21-cm hydrogen-line during the cosmic-dawn era. CIPs are modulations of the relative baryon and dark-matter density that leave the total matter density unchanged. We find that, under different assumptions for feedback and foregrounds, the ongoing HERA and upcoming SKA1-low experiments will provide constraints on uncorrelated CIPs at the level of $sigma(A_{rm CIP})= 10^{-3}-10^{-4}$, comparable to the sensitivity of upcoming CMB experiments, and potentially exceeding the constraints from cosmic-variance limited BAO surveys.
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With the advent of the first luminous sources at Cosmic Dawn (CD), the redshifted 21-cm signal, from the neutral hydrogen in the Inter-Galactic Medium (IGM), is predicted to undergo a transition from absorption to emission against the CMB. Using simulations, we show that the redshift evolution of the sign and the magnitude of the 21-cm bispectrum can disentangle the contributions from Ly$alpha$ coupling and X-ray heating of the IGM, the two most dominant processes which drive this transition. This opens a new avenue to probe the first luminous sources and the IGM physics at CD.
A compensated isocurvature perturbation consists of an overdensity (or underdensity) in the cold dark matter which is completely cancelled out by a corresponding underdensity (or overdensity) in the baryons. Such a configuration may be generated by a curvaton model of inflation if the cold dark matter is created before curvaton decay and the baryon number is created by the curvaton decay (or vice-versa). Compensated isocurvature perturbations, at the level producible by the curvaton model, have no observable effect on cosmic microwave background anisotropies or on galaxy surveys. They can be detected through their effect on the distribution of neutral hydrogen between redshifts 30 to 300 using 21 cm absorption observations. However, to obtain a good signal to noise ratio, very large observing arrays are needed. We estimate that a fast Fourier transform telescope would need a total collecting area of about 20 square kilometers to detect a curvaton generated compensated isocurvature perturbation at more than 5 sigma significance.
We study prospects of constraining the primordial magnetic field (PMF) and its evolution during the dark ages and cosmic dawn in light of EDGES 21-cm signal. Our analysis has been carried out on a `colder IGM background which is one of the promising avenues to interpret the EDGES signal. We consider the dark matter-baryon interactions for the excess cooling. We find that the colder IGM suppresses both the residual free electron fraction and the coupling coefficient between the ionised and neutral components. The Compton heating also gets affected in colder IGM background. Consequently, the IGM heating rate due to the PMF enhances compared to the standard scenario. Thus, a significant fraction of the magnetic energy, for $B_0 lesssim 0.5 , {rm nG}$, gets transferred to the IGM and the magnetic field decays at a much faster rate compared to the simple $(1+z)^2$ scaling during the dark ages and cosmic dawn. This low PMF is an unlikely candidate for explaining the rise of the EDGES absorption signal at lower redshift. We also see that the PMF and DM-baryon interaction together introduces a plateau-like feature in the redshift evolution of the IGM temperature. We find that the upper limit on the PMF depends on the underlying DM-baryon interaction. Higher PMF can be allowed when the interaction cross-section is higher and/or the DM particle mass is lower. Our study shows that the PMF with $B_0$ up to $sim 0.4 , {rm nG}$, which is ruled out in the standard model, can be allowed if DM-baryon interaction with suitable cross-section and DM mass is considered.
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 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 smoking gun of the velocity-dependent scattering and could be used by interferometers to verify the dark matter explanation of the EDGES detection.
Detection of individual luminous sources during the reionization epoch and cosmic dawn through their signatures in the HI 21-cm signal is one of the direct approaches to probe the epoch. Here, we summarize our previous works on this and present preliminary results on the prospects of detecting such sources using the SKA1-low experiment. We first discuss the expected HI 21-cm signal around luminous sources at different stages of reionization and cosmic dawn. We then introduce two visibility based estimators for detecting such signal: one based on the matched filtering technique and the other relies on simply combing the visibility signal from different baselines and frequency channels. We find that that the SKA1-low should be able to detect ionized bubbles of radius $R_b gtrsim 10$ Mpc with $sim 100$ hr of observations at redshift $z sim 8$ provided that the mean outside neutral Hydrogen fraction $ x_{rm HI} gtrsim 0.5$. We also investigate the possibility of detecting HII regions around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock et al. 2011. We find that a $5 sigma$ detection is possible with $600$ hr of SKA1-low observations if the QSO age and the outside $ x_{rm HI} $ are at least $sim 2 times 10^7$ Myr and $sim 0.2$ respectively. Finally, we investigate the possibility of detecting the very first X-ray and Ly-$alpha$ sources during the cosmic dawn. We consider mini-QSOs like sources which emits in X-ray frequency band. We find that with a total $sim 1000$ hr of observations, SKA1-low should be able to detect those sources individually with a $sim 9 sigma$ significance at redshift $z=15$. We summarize how the SNR changes with various parameters related to the source properties.
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