Although the path of history may sometimes appear to be linear, that of progress—particularly in science—is certainly anything but.
In the case of multimessenger astronomy, roughly a dozen monumental advances spanning nearly two centuries and three continents coalesced in one serendipitous moment in 1987; and then everything went dark again for almost thirty years, until a globe-spanning collaboration of more than 1,000 scientists confirmed an almost infinitesimally tiny ripple in the fabric of space-time—a mere fraction of the width of a single proton—opening the door to a rapid succession of discoveries that would forever change our understanding of the cosmos.
Named for the so-called cosmic messengers—photons, cosmic rays, neutrinos, and gravitational waves—multimessenger astronomy is based in the coordinated and correlated observation of these messenger particles produced by cosmic phenomena such as black holes and active galactic nuclei, neutron star mergers, and supernovae. Multimessenger astronomy has already brought a sea change to the world of science, and it promises to even further advance our understanding of the universe and our place in it.
To even begin to fully appreciate the magnitude of this new astronomical age, we must start by revisiting the very beginning of our scientific understanding of light beyond the visible spectrum, in the year 1800. In that year, William Herschel discovered infrared radiation; and in 1801, Johann Wilhelm Ritter discovered ultraviolet radiation. But it would be another more than 60 years before James Clerk Maxwell theorized the concept of electromagnetic radiation. In 1867, Maxwell predicted the existence of radio waves, but more than 20 years would pass before they were discovered in the lab of Heinrich Hertz, in 1887. Less than a decade later, in 1895, Wilhelm Röntgen discovered X-rays. And in the first year of the 20th century, Paul Villard made the final fundamental discovery in electromagnetism—gamma radiation, which would later be named gamma rays by Ernest Rutherford, in 1903. In the span of a century, electromagnetic radiation was brought entirely from the realm of the unknown into the light of scientific knowledge—a succession of truly monumental achievements that would change our understanding of what exists in the space beyond terra firma.
In little more than a half century following, scientists would discover even more strange things seemingly raining down from beyond the sky. In 1912, Victor Hess discovered cosmic rays, for which he would receive the Nobel Prize in Physics in 1936. Then, in 1915, Albert Einstein made a revolutionary prediction—of the existence of gravitational waves, part of his General Theory of Relativity—but it would be another 100 years before their existence would be confirmed. In 1956, Clyde Cowan and Frederick Reines would make the first discovery of neutrinos, for which they would receive the Nobel Prize in Physics in 1995. Twelve years after Cowan’s and Reines’s discovery, deep in an abandoned mineshaft at the Homestake Mine in Lead, South Dakota, Raymond Davis Jr. would make the first discovery of solar neutrinos, in 1968—another milestone in our understanding of our own solar system and its relation to the universe beyond, and an achievement that would earn Davis a portion of the Nobel Prize in Physics in 2002. These discoveries of cosmic rays and neutrinos, and the eventual discovery of gravitational waves, would expand our knowledge of space in extraordinary ways—especially when coupled with observations in the electromagnetic spectrum. But that achievement would be another 20-plus years in the making.
Meanwhile, evidence of the one cosmic messenger yet unconfirmed—the gravitational wave—was continuing to make its way to Earth. In 1974, the discovery of the Hulse-Taylor binary provided indirect evidence of the existence of gravitational waves. But still, a definitive confirmation of gravitational waves’ existence continued to elude the scientific community.
A New Era
Then, in February 1987, astronomers observed a bright flash in the sky, originating in the Large Magellanic Cloud. Supernova 1987A, as it would later be named, also produced a burst of neutrinos, detected by the Kamiokande II experiment in Japan, led by Masatoshi Koshiba. Koshiba and his team correlated the burst of neutrinos with the visible flash and the stellar explosion, and multimessenger astronomy was born. For this unprecedented achievement, Koshiba would be awarded a portion of the Nobel Prize in Physics, along with Raymond Davis Jr. and Riccardo Giacconi, in 2002.
For thirty years, Supernova 1987A remained the only confirmed multimessenger source outside our solar system. No observations would be made correlating cosmic rays with an electromagnetic signal, and the existence of gravitational waves had not yet been confirmed. But then, in September 2015—a century after Einstein’s prediction—the LIGO Scientific Collaboration made the first-ever detection of gravitational waves, produced by the merger of two black holes. This discovery of the last of the cosmic messengers would earn the three founding members of the LIGO collaboration—Barry Barish, Kip Thorne, and Rainer Weiss—the Nobel Prize in Physics in 2017.
Less than two years later—in August 2017—just two months before the announcement of the Nobel Prize, LIGO made another gravitational-wave detection. This time, the wave was produced by the merger of two neutron stars, and its detection was correlated to observations across the electromagnetic spectrum. Finally, after thirty years, a second multimessenger source had been discovered.
And then, less than a month after the second LIGO detection, in September 2017, the IceCube Observatory, based near the South Pole, detected a high-energy neutrino whose trajectory they traced back to a flaring supermassive black hole called a blazar. Follow-up observations by observatories worldwide correlated the blazar’s electromagnetic emissions with the neutrino event. A third multimessenger source had been confirmed—the second in less than a year.
Those two multimessenger discoveries by LIGO and IceCube, however, may not have happened were it not for groundwork laid at Penn State and in the Eberly College of Science. In fact, Penn State scientists were absolutely integral in both breakthroughs. As far back as 1993, a group of Penn State scientists from a range of disciplines came together in the Institute for Gravitational Physics and Geometry (now the Institute for Gravitation and the Cosmos), led by Abhay Ashtekar, who foresaw the importance of the multimessenger approach to next wave of astronomical discoveries. Two decades of work led to the March 2013 launch of Penn State's Astrophysical Multimessenger Observatory Network (AMON), a first-of-its-kind worldwide system based at University Park and led by Miguel Mostafá, providing real-time data analysis, detection of so-called subthreshold events—statistically indistinguishable from background or noise processes—and rapid notification of observatories around the world and in orbit for coordinated observation of events of interest, such as the August 2017 LIGO binary neutron star merger and the September 2017 IceCube neutrino event.
Prior to the 2017 LIGO detection, B.S. Sathyaprakash spent several decades developing data-analysis methods that would be foundational to LIGO’s discoveries, and a Penn State team led by Chad Hanna and including his graduate student Cody Messick developed the code that allowed LIGO to discern the 2017 gravitational-wave signal despite multiple concurrent technical issues. Messick was also the first to see the alert from LIGO and the one to send to the broader collaboration the notification that triggered the multimessenger observations around the world. Following LIGO’s detection, a notification sent to NASA’s Neil Gehrels Swift Observatory via AMON enabled rapid observation of the coincident gamma-ray burst and a number of other spectral emissions from the binary neutron star merger. And following the September 2017 IceCube neutrino event, the notification via AMON was crucial in enabling Swift to identify and observe the blazar source of the neutrino while also providing other observatories worldwide a target for their own observations across the electromagnetic spectrum. Among the IceCube scientists credited with the 2017 blazar discovery are Doug Cowen and Derek Fox. And Swift—with its science operations and mission control based at University Park, led respectively by Jamie Kennea and John Nousek, and two of its three instruments led by Mike Siegel and David Burrows—is likewise an outstanding credit to Penn State science.
As for what discoveries await farther down this winding road of scientific progress, who can truly say? AMON is entering a new and exciting phase of development, its cyberinfrastructure soon to be enhanced by deep learning and able to simultaneously analyze and correlate data on all four cosmic messengers in real time—opening yet another frontier in our understanding of the cosmos. Whatever the future holds for multimessenger astronomy, chances are that Penn State scientists will continue to figure prominently in it.
Hero image (at top): This Hubble Space Telescope image shows Supernova 1987A—the first confirmed multimessenger source outside our solar system—within the Large Magellanic Cloud, a neighboring galaxy to our Milky Way. Credit: NASA/STScI