Abstract

The modification of gravitational waves (GWs) passing through matter is normally treated as very weak. We have re-investigated this matter using linearized perturbations within the Bondi-Sachs formalism, and find that, while the effect is normally very weak, there are special astrophysical circumstances in which it can be significant

We consider a GW source surrounded by a spherical shell of matter. We find that the shell causes the GWs to be modified both in magnitude and phase; and that if the shell is viscous, then the shear induced in the velocity field results in an energy transfer so damping the magnitude of the GWs. Both effects can be significant if the shell radius is much smaller than the GW wavelength, and there are astrophysical scenarios for which the modification to the GW signal is large enough to be measurable. These scenarios include core collapse supernovae (CCSNe), quasinormal mode emission from the remnant of a binary neutron star (BNS) merger, and primordial GWs. In particular, there are feasible values of the CCS Ne parameters for which viscous damping of the GW signal would be almost complete.

In numerical relativity, an evolution of the full Einstein and matter field equations with GW extraction far from the source will properly include all effects described above. However, in situations such as CCS Ne and BNS shear viscosity is not included in the modeling, and GW extraction is handled approximately. Results obtained in these cases would need to be corrected.

The presentation is based on: 1. N.T. Bishop, M. Naidoo, and P.J. van der Walt (2020), Gen. Rel. Grav. 52:92, https://doi.org/10.1007/s10714-020-02740-9 arXiv:1912.08289 2. M. Naidoo, N.T. Bishop, and P.J. van der Walt (2021), Gen. Rel. Grav. 53:97, https://doi.org/10.1007/s10714-021-02841-z arXiv:2102.00060 3. N.T. Bishop, P.J. van der Walt and M. Naidoo (2022), arXiv:2206.15103