Potential of radio telescopes as high-frequency gravitational wave
  detectors
  In the presence of magnetic fields, gravitational waves are converted into
photons and vice versa. We demonstrate that this conversion leads to a
distortion of the cosmic microwave background (CMB), which can serve as a
detector for MHz to GHz gravitational wave sources active before reionization.
The measurements of the radio telescope EDGES can be cast as a bound on the
gravitational wave amplitude, $h_c < 10^{-21} (10^{-12})$ at 78 MHz, for the
strongest (weakest) cosmic magnetic fields allowed by current astrophysical and
cosmological constraints. Similarly, the results of ARCADE 2 imply $h_c <
10^{-24} (10^{-14})$ at $3 - 30$ GHz. For the strongest magnetic fields, these
constraints exceed current laboratory constraints by about seven orders of
magnitude. Future advances in 21cm astronomy may conceivably push these bounds
below the sensitivity of cosmological constraints on the total energy density
of gravitational waves.
