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