Prospects for Multi-messenger Astronomy in the Era of 3G GW Detectors

A century after Einstein's prediction of gravitational waves (GWs), the 4-km scale ground-based Laser Interferometer Gravitational Wave Observatory (LIGO) unveiled GWs from merging black holes (BHs) in binary systems. Then, LIGO and Virgo together discovered GW170817, the first binary neutron star (NS) merger witnessed through both its GW siren and its electromagnetic (EM) glow at all wavelengths of light - a spectacular start of multi-messenger GW astronomy. With their subsequent observing runs, LIGO and Virgo have reported observations of tens of BH mergers and about one binary NS merger per year, while routinely issuing alerts to the broader astronomical community. Over the next 5 years, new GW+EM discoveries are expected to start trickling in as LIGO and Virgo continue to increase their sensitivity, KAGRA and LIGO India strengthen the second generation (2G) GW network, and new EM facilities become operational. At the same time, third generation (3G) GW observatory concepts are now emerging as the next frontier that will bring the full GW universe into reach and, when paired with more sensitive EM facilities, will transform the 2G-era trickle of GW+EM discoveries into a flood. In this context, starting from the example of GW170817, I will discuss prospects for multi-messenger astronomy in the 3G era, highlighting opportunities to clarify currently open questions in the field, while expanding multi-messenger studies beyond the realm of compact binary mergers.