Dispersion of gravitational waves in cold spherical interstellar medium

Abstract

We investigate the propagation of locally plane, small-amplitude, monochromatic gravitational waves through cold compressible interstellar gas in order to provide a more accurate picture of expected waveforms for direct detection. The quasi-isothermal gas is concentrated in a spherical symmetric cloud held together by self-gravitation. Gravitational waves can be treated as linearized perturbations on the background inner Schwarzschild spacetime. The perturbed quantities lead to the field equations governing the gas dynamics and describe the interaction of gravitational waves with matter. The resulted field equations decouple asymptotically for slowly varying short waves to a set of three PDEs of different orders of magnitude. A second-order WKB method provides transport equations for the wave amplitudes. The influence of background curvature already appears in the first-order amplitudes, which gives rise to diffraction. We have shown that the transport equation of these amplitudes provides numerical solutions for the frequency-alteration. The energy dissipating process is responsible for decreasing frequency. The decrease is significantly smaller than the magnitude of the original frequency and exhibits a power-law relationship between original and decreased frequencies. The frequency deviation was examined particularly for the transient signal GW150914. Considering AGNs as larger background structures and high-frequency signals emitted by BNS mergers, the frequency-deviation grows large enough to be relevant in future GW-observations with increased sensitivity.