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 7 orders of magnitude. Future advances in 21 cm astronomy may conceivably push these bounds below the sensitivity of cosmological constraints on the total energy density of gravitational waves.

Figure 1

The Gertsenshtein effect.

Figure 2

Left: Parameter space for cosmic magnetic fields today. Gray shaded areas show the exclusion discussed in the text. The solid (dashed) colored curves indicate contour lines for the rescaled conversion probability, $(T_0/{\omega }_0{)}^2\mathcal{P}$. See Eqs. (6) and (7). Right: Upper bounds on the stochastic GW background derived from ARCADE2 and EDGES (this work), compared to existing laboratory bounds from (a) superconducting parametric converter [3], (b) waveguide [4], (c) 0.75 m interferometer [5], (d) magnon detector [6], and (e) magnetic conversion detector [7]. The solid lines indicate the allowed parameter space for cosmic magnetic fields, as given in the left panel. The dashed lines mark the $N_{\mathrm{eff}}$ constraint for broad GW spectra and for a peaked spectrum with $\mathrm{\Delta }\omega /\omega ={10}^{-3}$. For reference, the dotted lines indicate ${\rho }_g={\rho }_c$.