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.

中文翻译：

---

射电望远镜作为高频重力波探测器的潜力

在磁场的存在下，重力波被转换成光子，反之亦然。我们证明了这种转换会导致宇宙微波背景（CMB）的失真，该背景可用作电离之前活跃的MHz至GHz重力波源的检测器。射电望远镜EDGES的测量值可以作为引力波振幅的界限，$H_C<{10}^{-21}（{10}^{-12}）$在78 MHz时，可获得当前天体物理和宇宙学约束所允许的最强（最弱）宇宙磁场。同样，ARCADE 2的结果暗示$H_C<{10}^{-24}（{10}^{-14}）$在3–30 GHz。对于最强的磁场，这些限制比当前的实验室限制超出了大约7个数量级。可以想象，21厘米天文学的未来发展可能会将这些界限推到低于宇宙学约束对引力波总能量密度的敏感性范围之内。