Detectability and Model Discriminability of the Dark Ages 21 cm Global Signal

TL;DR

This work evaluates the detectability and model-discriminating power of the Dark Ages 21 cm global signal using a Bayesian evidence framework with physically motivated foregrounds and optimistic noise assumptions across multiple observing strategies. By comparing nine signal models, including a NO21 baseline, the study finds that wide-band observations from 1–50 MHz generally yield strong detections and enable discrimination among models, whereas restricting to 15–50 MHz eliminates detectable signals for most cases. The analysis shows that a few well-chosen frequency channels can suffice to distinguish models if the per-channel noise is sufficiently small, but wide frequency coverage is essential to break foreground degeneracies, particularly for smooth-spectrum models peaking at low frequencies. The findings underscore the importance of low-frequency, space-based measurements to access the full Dark Ages 21 cm signal and inform instrument design and observing strategies.

Abstract

The 21 cm signal from neutral hydrogen atom is almost the only way to directly probe the Dark Ages. The Dark Ages 21 cm signal, observed at frequencies below 50 MHz, can serve as a powerful probe of cosmology, as the standard cosmological model predicts a well-defined 21 cm spectral shape. In this work, we assess the detectability and model-selection power of 21 cm observations assuming physically motivated foregrounds, optimistic error levels, and several observing strategies for the signals predicted in various cosmological models. Using a Bayesian evidence-based comparison, we find that wide-band observations covering 1-50 MHz can identify the evidence of non-zero 21 cm signals from models considered in this paper except the one with a smooth spectrum that peaks at lower frequencies. In particular, observations below 15 MHz are essential to avoid degeneracies with the foreground. Furthermore, even with observations measured at 5 MHz intervals over the frequency range 1-50 MHz, the 21 cm signal can be identified if the errors are sufficiently small. This indicates that the intrinsic 21 cm spectral shape can be captured without foreground degeneracy even with a limited number of frequency channels.

Figures (8)

• Figure 1: The 21 cm signal spectra used in this work for model comparison. There are nine models: eight cosmological models ($i=1$--8) and one zero-signal model ($i=0$). The thin dashed line shows the thermal-noise level assuming 10,000 hours of integration time.
• Figure 2: The solid line shows the foreground model used to generate the mock data. The red dots show the observed values from 1979MNRAS.189..465C.
• Figure 3: Dependence of $\Delta \ln Z_{i,0}$ on the assumed minimum noise level in each 1 MHz channel. The three horizontal lines indicate the interpretation thresholds for $\Delta \ln Z_{i,0}$.
• Figure 4: Residuals after foreground subtraction using the maximum-likelihood parameter sample for the case $j=0$. The two blue lines indicate the noise level for reference.
• Figure 5: $\ln Z$ values for multiple combinations of input and subtraction models assuming a wide-band observation from 1 MHz to 50 MHz. Here, $i$ is the input model and $j$ is the subtraction model.