PI: Miguel Bezares

This project explores gravitational waves (GWs) generated by the mergers of binary systems containing neutron stars (NS), including binary black holes (BBH), binary neutron star (BNS) and black hole–neutron star (BH–NS) mergers. These extreme events are crucial for testing both general relativity and alternative theories of gravity. The work involves fully general-relativistic magneto-hydrodynamics (GRMHD) simulations using large-eddy techniques to model post-merger disks and magnetic field amplification. It also extends numerical relativity simulations to scalar-tensor theories by investigating how screening mechanisms (e.g., K-essence) influence scalar radiation, focusing on dipole and quadrupole emission. A key highlight is the 3+1 numerical relativity study [https://arxiv.org/abs/2410.16367] showing that in K-essence theories, dipole emission is strongly suppressed, while quadrupole emission is only mildly reduced. Notably, this was the first simulation of mixed binary systems under these screening conditions. These findings suggest that mixed binaries could be significant sources of scalar quadrupole radiation, providing an opportunity for next-generation gravitational wave detectors to test K-essence and other scalar-tensor theories. Another significant result comes from BH–NS merger simulations using the MHDuet code [https://arxiv.org/abs/2403.09770], which reveal that tidal disruption leads to a robust accretion disk and rapid magnetic field amplification (from about 10^11 G to over 10^14 G in roughly 20 ms). Crucially, this was also the first GRMHD simulation of BH–NS mergers to employ large-eddy simulation techniques. However, higher resolutions will be required to confirm the full convergence of magnetic amplification.

Figure 1: Snapshots showing the evolution of the magnetic field magnitude and density contours during disk formation. The magnetic field is strongly amplified by the Kelvin-Helmholtz instability in the shear layers and spiral patterns of field growth can be seen migrating outward to form an accretion disk. Taken from