Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The gravitational wave signal from primordial magnetic fields in the
  Pulsar Timing Array frequency band   The NANOGrav, Parkes, and European pulsar timing array (PTA) collaborations
have reported evidence for a common-spectrum process that can potentially
correspond to a stochastic gravitational wave background (SGWB) in the 1--100
nHz frequency range. We consider the scenario in which this signal is produced
by magnetohydrodynamic (MHD) turbulence in the early universe, induced by a
non-helical primordial magnetic field at the energy scale corresponding to the
quark confinement phase transition. We perform MHD simulations to study the
dynamical evolution of the magnetic field and compute the resulting SGWB. We
show that the SGWB output from the simulations can be very well approximated by
assuming that the magnetic anisotropic stress is constant in time, over a time
interval related to the eddy turnover time. The analytical spectrum that we
derive under this assumption features a change of slope at a frequency
corresponding to the GW source duration that we confirm with the numerical
simulations. We compare the SGWB signal with the PTA data to constrain the
temperature scale at which the SGWB is sourced, as well as the amplitude and
characteristic scale of the initial magnetic field. We find that the generation
temperature is constrained to be in the 2-200 MeV range, the magnetic field
amplitude must be $>1$\% of the radiation energy density at that time, and the
magnetic field characteristic scale is constrained to be $>10$\% of the horizon
scale. We show that the turbulent decay of this magnetic field will lead to a
field at recombination that can help to alleviate the Hubble tension and can be
tested by measurements in the voids of the Large Scale Structure with gamma-ray
telescopes like the Cherenkov Telescope Array.

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The categories of this paper are: chem-ph
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int
19
The categories of this paper are: eess
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The categories of this paper are: q-alg
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The categories of this paper are: bayes-an
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The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat
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The categories of this paper are: stat
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The categories of this paper are: q-fin
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The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
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The categories of this paper are: econ
10
The categories of this paper are: q-bio