Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

3
The categories of this paper are: physics
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
8
The categories of this paper are: hep-ex
2
The categories of this paper are: math
23
The categories of this paper are: solv-int
11
The categories of this paper are: hep-lat
12
The categories of this paper are: nucl-th
37
The categories of this paper are: supr-con
17
The categories of this paper are: q-fin
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
16
The categories of this paper are: stat
1
The categories of this paper are: cs