Prospects of multimessenger astronomy in the next decades

Multimessenger astronomy aims to utilize all accessible information from the universe: electromagnetic waves, particles (e.g., neutrinos), and gravitational waves. The discovery of GW170817/GRB170817A and the international follow-up observations showed the power of multimessenger astronomy in understanding cosmic phenomena. The best candidates for multimessenger astronomy are compact binary coalescences, gamma-ray bursts, or supernovae. In particular, binaries consisting of neutron stars or black holes are the most attractive candidates for standard sirens in the context of gravitational-wave astrophysics. The possibility of measuring a Hubble constant with compact binaries has drawn strong attention as an independent method from electromagnetic-wave observations. In addition to the 20–2000 Hz band available on Earth, multimessenger astronomy with gravitational-wave observations in lower frequencies below ~10 mHz would be fruitful for understanding the underlying properties and cosmological implications for Galactic binaries and massive black holes. The lowest gravitational-wave frequencies (µHz–nHz) would allow us to explore supermassive black hole mergers and the stochastic GW background from the early universe. In this review paper, we summarize recent progress of and prospects for multimessenger astronomy, focusing on compact binaries.