tokens: i.e., α 0 = −0.5 and α a = 0. In Figure 3 , we show T 21 , as defined in Equation ( 2 ), for various values of α 0 and α a , with other cosmological parameters fixed at values consistent with a Planck 2015 cosmology (Planck Collaboration et al. 2016 ). As shown, the ΛCDM model ( α 0 = α a = 0), denoted by the star, is in tension with the EDGES measurement at a 99% CL illustrated by the hatched region. However, interacting vacuum models can in principle release the tension, namely, T 21 can be pushed into the hatched region by a large range of the α 0 and α a parameters. Figure 3. Illustration of the intensity of the 21 cm signal relative to the CMB temperature, T 21 (K), for various values of α 0 and α a . The color bar indicates the values of α a . The black solid curve corresponds to the case in which α does not evolve with time ( α a = 0). The intersect between the black solid curve and the vertical gray dashed line, marked by a yellow star, denotes the ΛCDM model. The hatched region illustrates the observed 21 cm signal from EDGES at a 99% CL. We then perform a Monte Carlo Markov Chain global fit for the parameters in Equation ( 12 ) using a modified version of CosmoMC 8 8 Available at https://cosmologist.info/cosmomc/ . (Lewis Bridle 2002 ) with a combined data set including, 1. the angular power spectra of temperature and polarization measurements of CMB from the Planck mission (Planck Collaboration et al. 2016 ); 2. the Joint Light-curve Analysis sample of supernovae (SNe) measurements (Betoule et al. 2014 ); 3. the Baryonic Acoustic Oscillations (BAO) distance measurements from 6dFGS (Beutler et al. 2011 ), Sloan Digital Sky Survey (SDSS) DR7 Main Galaxy Sample (Ross et al. 2015 ), Ly α forest of BOSS DR11 quasars (Font-Ribera et al. 2014 ; Delubac et al. 2015 ), and BOSS DR12 with tomographic information (Wang et al. 2017 ; Zhao et al. 2017 ); joint BAO and Redshift Space Distortions (RSD) measurements from WiggleZ (Blake et al. 2012 ) and from eBOSS DR14 (Zhao et al. 2019 ); and RSD measurements from 6dFGS (Beutler et al. 2012 ), 2dFGRS (Percival et al. 2004 ), SDSS LRG (Samushia et al. 2012 ), and VIPERS (de la Torre et al. 2013 ); 4. and the local H 0 measurement using the Cepheids, i.e., H 0 = 73.24 ± 1.74 km s −1 Mpc −1 (Riess et al. 2016 ). The results are summarized in Table 1 and in Figures 4 and 5 . To quantify the constraint on α 0 and α a from the EDEGS measurement, we show the 68%, 95%, and 99% CL contours of α 0 and α a using EDEGS alone in the lower left part of Figure 4 . 9 9 As EDEGS alone cannot constrain all of the parameters in Equation ( 12 ) simultaneously, we only vary α 0 and α a in this case while the other parameters are fixed to the values derived from the Planck 2015 measurement. As illustrated, the EDGES measurement favors an interacting vacuum model over the ΛCDM model at more than 99% CL, and this would induce a quintessence-like effective dark energy as shown in the lower left of Figure 1 . We then
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