Multi-messenger astronomy

Multi-messenger astronomy is the practice of studying an astrophysical event using more than one of the four cosmic 'messengers': electromagnetic radiation (photons across the spectrum, from gamma rays to radio), gravitational waves, neutrinos, and cosmic rays. Each messenger carries different information and is produced by different physics, so combining them yields a far richer picture than any single channel can. Light tells you about surfaces and radiating gas; gravitational waves report the bulk motion of mass; neutrinos escape from deep inside dense, opaque interiors.

The founding event was GW170817 on August 17, 2017. LIGO and Virgo detected the gravitational-wave chirp of two inspiraling neutron stars; 1.7 seconds after the merger, gamma-ray satellites caught the short gamma-ray burst GRB 170817A from the same sky region; within hours optical telescopes localized the source to the galaxy NGC 4993 and watched a kilonova — the radioactive afterglow of freshly synthesized heavy elements — rise and fade. Some 70 observatories across every electromagnetic band observed the single merger.

The scientific payoff was immediate and broad: confirmation that neutron-star mergers forge heavy elements such as gold and platinum; a measurement of the speed of gravity equal to the speed of light to one part in 10¹⁵; and an independent, distance-ladder-free measurement of the Hubble constant using the gravitational wave as a standard siren. Multi-messenger astronomy now also includes high-energy neutrino events, such as the 2017 association of a neutrino with the flaring blazar TXS 0506+056.

The term gained currency in the 2010s as gravitational-wave detection matured. GW170817 (2017) is universally regarded as its inaugural event, the first time a source was observed in both gravitational waves and light.