For centuries, astronomers studying the stars were limited to information from visible light. Telescopes, photographic plates, and digital detectors were all developed to collect, magnify, and capture that signal.
But visible light isn’t the only message the cosmos is sending us.
“All the big, interesting events — like supernovae, gamma-ray bursts, and mergers — are disruptive,” says Péter Mészáros, a theoretical astrophysicist at Penn State. “They send out accelerated particles, photons, and waves in space-time.”
Astronomers call these messengers, and there are four types: photons, neutrinos, cosmic rays, and gravitational waves. Gradually, scientists have unlocked the ability to detect them all. Today, astronomers stand on the brink of a new era of multi-messenger astronomy — one they have been awaiting for decades and that is already providing new insights. Thanks to big instruments, big collaborations, and big data, they finally have the practical tools to interpret the cosmic detritus constantly raining down upon the Earth.
A fuller picture
The roots of multi-messenger astronomy date to the 1960s, when the U.S. government launched satellites carrying detectors for gamma rays — the most powerful photons — to track Russian nuclear tests. They found plenty of gamma-ray sources, but to their surprise, they weren’t coming from Earth, but from all around the sky.
“People thought the Russians were making nuclear explosions out in space,” says Mészáros. “But soon it was obvious that they were coming from very far away, cosmological distances.” Around the same time, astronomers detected neutrinos, which are subatomic particles with no charge and very little mass, emerging from the Sun. Cosmic rays — atomic nuclei that have been accelerated to near the speed of light — were first seen in balloon experiments about 100 years ago. And although Albert Einstein predicted gravitational waves — the ripples in space-time that occur when massive objects collide — in 1916, they were not observed until 2015.
Multi-messenger astronomy is the practice of synthesizing these various messengers from violent astronomical events. For instance, astronomers have directly observed gravitational waves and used these observations to understand what happens when neutron stars or black holes collide. Astronomers have also uncovered new mysteries, including the discovery that rare ultrahigh-energy cosmic rays originate from outside our galaxy, and that high-energy neutrinos form a cosmic background that pervades the universe. Exciting theories abound as to what kinds of exotic objects are sending out these cosmic messengers: superstrings, dark matter, and even “defects” in the structure of the universe have all been suggested.
“The ultimate goal is to witness an event with all the messengers,” says Kate Scholberg, a neutrino physicist at Duke University. “We have come close in the past few years. When it does happen — and it should soon — then hopefully we can get a full picture of the emission source.”
The power of these combined messengers comes from the fact that each one is generated by one of the four forces of nature: photons by the electromagnetic force, gravitational waves by gravity, cosmic rays by the strong nuclear force, and neutrinos by the weak nuclear force. The different messengers are the product of their particular origins, and so their presence (or absence) and their characteristics — such as composition, energy level, and direction — teach us about the object they came from.
The trick is that each of the aforementioned messengers requires vastly different detectors and an unprecedented level of cooperation across disciplines. Instantaneous communication is needed to coordinate observations of fleeting events, and processing the data requires bespoke skills in statistical analysis and data mining. It has been said that the only limit to success is of imagination, but in this case, the challenges are purely practical.