The Redshifted 21 cm Signal in the EDGES Low-band Spectrum

Abstract

The EDGES collaboration reported an unexpectedly deep absorption in the radio background at 78 MHz and interpreted the dip as the first detection of a redshifted 21 cm signal from cosmic dawn (CD). We attempt an alternate analysis by adopting a maximally smooth function approach to model the foreground. A joint fit to the spectrum using such a function together with a flattened absorption profile yields a best-fit absorption amplitude of 921 ± 35 mK. The depth of the 21 cm absorption inferred by the EDGES analysis required invoking nonstandard cosmology, or new physics, or new sources at CD, and this tension with accepted models is compounded by our analysis that suggests absorption of greater depth. Alternatively, the measured spectrum may be equally well fit assuming there exists a residual unmodeled systematic sinusoidal feature, and we explore this possibility further by examining for additional 21 cm signal. The data then favor an absorption with Gaussian model parameters of amplitude 133 ± 60 mK, best width at half-power 9 ± 3 MHz, and center frequency 72.5 ± 0.8 MHz. We also examine the consistency of the measured spectrum with plausible redshifted 21 cm models: a set of 3 of the 265 profiles in the global 21 cm atlas of Cohen et al. are favored by the spectrum. We conclude that the EDGES data may be consistent with standard cosmology and astrophysics, without invoking excess radio backgrounds or baryon–dark matter interactions.