Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gravitational Waves from Primordial Magnetic Fields via Photon-Graviton
  Conversion   We explore a novel process in the early Universe in which thermalized photons
are converted into gravitons in the presence of strong primordial magnetic
fields. It is found that the frequency of generated gravitational waves (GWs)
is typically of order of GHz and their amplitude can be up to $
\Omega_\mathrm{GW}h^2 \sim 10^{-10}$. If detected with future developments of
the technology to explore this frequency region, the produced stochastic GW
background enables us to know when and how strong the primordial magnetic
fields are generated. From the peak frequency of the GWs, we can also probe the
number of relativistic degrees of freedom at that time.

7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
13
The categories of this paper are: quant-ph
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph
26
The categories of this paper are: bayes-an
17
The categories of this paper are: q-fin
27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
9
The categories of this paper are: nlin
12
The categories of this paper are: nucl-th