EDGES result versus CMB and low-redshift constraints on ionization histories

TL;DR

This paper analyzes EDGES 21 cm absorption results in the context of standard cosmology, finding that while the timing fits models, the observed absorption depth exceeds what standard models predict, indicating potential new physics or model limitations.

Contribution

It assesses the compatibility of EDGES absorption profile with existing reionization constraints within the b1CDM model, highlighting the discrepancy in absorption amplitude.

Findings

Timing of absorption fits standard reionization models

Absorption amplitude is too large for standard models

Simple models cannot reproduce the observed depth

Abstract

We examine the results from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), which has recently claimed the detection of a strong absorption in the 21 cm hyperfine transition line of neutral hydrogen, at redshifts demarcating the early stages of star formation. More concretely, we study the compatibility of the shape of the EDGES absorption profile, centered at a redshift of $z \sim 17.2$, with measurements of the reionization optical depth, the Gunn-Peterson optical depth, and Lyman-$\alpha$ emission from star-forming galaxies, for a variety of possible reionization models within the standard $\Lambda$CDM framework (that is, a Universe with a cosmological constant and cold dark matter). When, conservatively, we only try to accommodate the location of the absorption dip, we identify a region in the parameter space of the astrophysical parameters that successfully explains all of the aforementioned observations. However, one of the most abnormal features of the EDGES measurement is the absorption amplitude, which is roughly a factor of two larger than the maximum allowed value in the $\Lambda$CDM framework. We point out that the simple considered astrophysical models that produce the largest absorption amplitudes are unable to explain the depth of the dip and of reproducing the observed shape of the absorption profile.