Damping of long wavelength gravitational waves by the intergalactic medium

Abstract

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.