Damping of long wavelength gravitational waves by the intergalactic
  medium
  The problem of radiation by the charged particles of the intergalactic medium
(IGM) when a passing gravitational wave (GW) accelerate them is investigated.
The largest acceleration (taking a charge from rest to a maximum speed which
remains non-relativistic in the rest frame of the unperturbed spacetime) is
found to be limited by the curvature of a propagating spherical gravitational
wavefront. Interesting physics arises from the ensuing emission of radiation
into the warm hot IGM, which to lowest order is a fully ionized hydrogen plasma
with a frozen-in magnetic field $B$. It is found that for a vast majority of
propagation directions, the right-handed polarized radiation can penetrate the
plasma at frequencies below the plasma frequency $\om_p$, provided $\om<\om_b,$
where $\om_b=eB/m_e$ satisfies $\om_b<\om_p$ for typical IGM conditions.
Moreover, the refractive index under such a scenario is $n\gg 1,$ resulting in
an enhanced radiative dissipation of GW energy (relative to the vacuum
scenario), which is more severe for electrons if both charge species are in
thermal equilibrium and accelerated in the same way. The emission by the
electrons then prevails, and is further amplified by coherent addition of
amplitudes within the size one wavelength. The conversion of GWs of
$\lam\gtrsim 5\times 10^{13}$~cm to electromagnetic waves means such GWs can
only propagate a distance $\lesssim 1$~Gpc before being significantly damped by
an IGM B field of $\sim10^{-8}$ G. The low-frequency GWs
\textcolor{black}{targeted by pulsar-timing-arrays} will not survive unless the
IGM magnetic field is much lower than expected. The \textcolor{black}{mHz}
frequency GW inspirals targeted by future \textcolor{black}{space based}
detectors such as the Laser Interferometer Space Antenna remain intact and can
be detected.
