Can Early Dark Energy Explain EDGES?

Abstract

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.