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 .