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Register a new userForecasted 21 cm constraints on compensated isocurvature perturbations
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Christopher Gordon
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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.
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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.
We investigate the potential of the galaxy power spectrum to constrain compensated isocurvature perturbations (CIPs), primordial fluctuations in the baryon density that are compensated by fluctuations in CDM density to ensure an unperturbed total matter density. We show that CIPs contribute to the galaxy overdensity at linear order, and if they are close to scale-invariant, their effects are nearly perfectly degenerate with the local PNG parameter $f_{rm nl}$ if they correlate with the adiabatic perturbations. This degeneracy can however be broken by analyzing multiple galaxy samples with different bias parameters, or by taking CMB priors on $f_{rm nl}$ into account. Parametrizing the amplitude of the CIP power spectrum as $P_{sigmasigma} = A^2P_{mathcal{R}mathcal{R}}$ (where $P_{mathcal{R}mathcal{R}}$ is the adiabatic power spectrum) we find, for a number of fiducial galaxy samples in a simplified forecast setup, that constraints on $A$, relative to those on $f_{rm nl}$, of order $sigma_{A}/sigma_{f_{rm nl}} approx 1-2$ are achievable for CIPs correlated with adiabatic perturbations, and $sigma_{A}/sigma_{f_{rm nl}} approx 5$ for the uncorrelated case. These values are independent of survey volume, and suggest that current galaxy data are already able to improve significantly on the tightest existing constraints on CIPs from the CMB. Future galaxy surveys that aim to achieve $sigma_{f_{rm nl}} sim 1$ have the potential to place even stronger bounds on CIPs.
The recent Cosmic Microwave Background data from the Planck satellite experiment, when combined with HST determinations of the Hubble constant, are compatible with a larger, non-standard, number of relativistic degrees of freedom at recombination, parametrized by the neutrino effective number $N_{eff}$. In the curvaton scenario, a larger value for $N_{eff}$ could arise from a non-zero neutrino chemical potential connected to residual neutrino isocurvature density (NID) perturbations after the decay of the curvaton field, parametrized by the amplitude $alpha^{NID}$. Here we present new constraints on $N_{eff}$ and $alpha^{NID}$ from an analysis of recent cosmological data. We found that the Planck+WP dataset does not show any indication for a neutrino isocurvature component, severly constraining its amplitude, and that current indications for a non-standard $N_{eff}$ are further relaxed.
Compensated isocurvature perturbations (CIPs) are opposite spatial fluctuations in the baryon and dark matter density. They can be generated for example in the curvaton model in the early Universe but are difficult to observe because their gravitational imprint nearly cancels. We therefore propose a new measurement method by searching for a spatial modulation of the baryon acoustic oscillation (BAO) scale that CIPs induce. We find that for a Euclid-like survey the sensitivity is marginally better than the WMAP cosmic microwave background (CMB) constraint, which exploits the CIP-induced modulation of the CMB sound horizon. For a cosmic-variance limited BAO survey using emission-line galaxies up to $zsim7$ the sensitivity is between stage 3 and stage 4 CMB experiments. These results include using CIP-galaxy cross-correlations, which improves the sensitivity by a factor of $sim2-3$ for correlated CIPs. The method could be further improved with an optimal estimator, similarly to the CMB, and could provide a useful cross-check of other CIP probes. Finally, if CIPs exist, they can bias cosmological measurements made assuming no CIPs. In particular, they can act as a super-sample fluctuation of the baryon density and bias measurements of the BAO scale. For modern BAO surveys, the largest 2$sigma$ CIP fluctuation allowed by Plancks 95% bound could bias BAO measurements of $H(z)$ by 2.2%, partially reducing the tension with the local $H_0$ measurements from 3.1$sigma$ to 2.3$sigma$.
Hawking radiation from primordial black holes (PBH) can ionize and heat up neutral gas during the cosmic dark ages, leaving imprints on the global 21cm signal of neutral hydrogen. We use the global 21cm signal to constrain the abundance of spinning PBHs in mass range of $[2 times 10^{13}, 10^{18}]$ grams. We consider several extended PBH distribution models. Our results show that 21cm can set the most stringent PBH bounds in our mass window. Compared with constraints set by {it{Planck}} cosmic microwave background (CMB) data, 21-cm limits are more stringent by about two orders of magnitudes. PBHs with higher spin are typically more strongly constrained. Our 21cm constraints for the monochromatic mass distribution rule out spinless PBHs with initial mass below $1.4 times 10^{17} {rm{g}}$, whereas extreme Kerr PBHs with reduced initial spin of $a_0=0.999$ are excluded as the dominant dark matter component for masses below $6 times 10^{17} {rm{g}}$. We also derived limits for the log-normal, power-law and critical collapse distributions.
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