  The Experiment to Detect the Global Epoch of Reionization Signature (EDGES)
collaboration has reported the detection of an absorption feature in the
sky-averaged spectrum at $\approx 78$ MHz. This signal has been interpreted as
the absorption of cosmic microwave background (CMB) photons at redshifts $15
\lesssim z \lesssim 20$ by the 21cm hyperfine transition of neutral hydrogen,
whose temperature is expected to be coupled to the gas temperature by the
Wouthuysen-Field effect during this epoch. Because the gas is colder than the
CMB, the 21cm signal is seen in absorption. However, the absorption depth
reported by EDGES is more than twice the maximal value expected in the standard
cosmological model, at $\approx 3.8\sigma$ significance. Here, we propose an
explanation for this depth based on "early dark energy" (EDE), a scenario in
which an additional component with equation of state $w=-1$ contributes to the
cosmological energy density at early times, before decaying rapidly at a
critical redshift, $z_c$. For $20 \lesssim z_c \lesssim 1000$, the accelerated
expansion due to the EDE can produce an earlier decoupling of the gas
temperature from the radiation temperature than that in the standard model,
giving the gas additional time to cool adiabatically before the first luminous
sources form. We show that the EDE scenario can successfully explain the large
amplitude of the EDGES signal. However, such models are strongly ruled out by
observations of the CMB temperature power spectrum. Moreover, the EDE models
needed to explain the EDGES signal exacerbate the current tension in low- and
high-redshift measurements of the Hubble constant. We conclude that
non-finely-tuned modifications of the background cosmology are unlikely to
explain the EDGES signal while remaining consistent with other cosmological
observations.
