Axion-photon-dark photon oscillation and its implication for 21 cm
  observation
  We examine the resonant conversion of axion-like particle (ALP) or dark
photon to the electromagnetic photon in the early Universe, which takes place
due to the ALP-photon-dark photon oscillations in background dark photon gauge
fields. It is noted that the corresponding conversion probability can have an
unusual spectral feature which allows strong conversion at low frequency
domain, but has negligible conversion at high frequencies above certain
critical frequency which is determined by the ALP coupling to dark photon and
the strength of background dark photon gauge field. We apply this scheme to
heat up the 21 cm photons without affecting the Cosmic Microwave Background,
which can explain the tentative absorption signal of 21 cm photons detected
recently by the EDGES experiment.
