For the first time, scientists have directly detected gravitational waves from the spectacular collision of two neutron stars, and at the same time detected visible light from the merger. The discovery, reported today by a collaboration of scientists from around the world, marks the first time that a cosmic event has been detected through both light and gravitational waves - the ripples in space-time predicted a century ago by Albert Einstein's general theory of relativity.
The result heralds a new era of multimessenger astronomy, in which different kinds of electromagnetic radiation and gravitational waves are combined to teach us more about the universe than either could by itself. Altogether, five CIFAR researchers in the Gravity & the Extreme Universe program are involved in the discovery.
"The G&EU program sits at the forefront of what is a genuine revolution in astrophysics, and indeed all of science," said R. Howard Webster Foundation Fellow and Program Director Vicky Kaspi (McGill University), who was not herself involved in the most recent result. "This new and truly astonishing gravitational wave result is yet another example of what the future holds. It is a privilege to be witnessing these events unfold."
The collision was detected Aug. 17 by the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground- and space-based observatories, including NASA's Fermi satellite, which detected a weak pulse of gamma rays.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds. When the stars collided they released a flash of light in the form of gamma rays. In the days and weeks following the smashup, other forms of electromagnetic radiation -- including X-ray, ultraviolet, optical, infrared, and radio waves -- were detected.
"This is the very first event which now rings in the era of multimessenger astronomy," said Harald Pfeiffer (University of Toronto), one of the members of CIFAR's Gravity & the Extreme Universe program involved in the LIGO-Virgo collaboration. Pfeiffer is currently working on a study to cross check the detection results for systematic effects and further refine the results.
"Messengers" such as astroparticles, electromagnetic waves and gravitational waves allow researchers to learn about an astrophysical system. "You can tell a lot more about the source because you see it in different ways," said CIFAR Associate Fellow Gabriela Gonzalez (Louisiana State University), an instrumentalist who works on improving the sensitivity of the LIGO detectors. Gonzalez led the LIGO Scientific Collaboration during the first discovery of gravitational waves in 2015.
Gravitational wave signals and electromagnetic signals contain complementary information. From the gravitational wave signals we can measure the masses of neutron stars, how far away they are and properties of the nuclear matter they are made up of. On the other hand, electromagnetic signals tell us about the composition of elements produced by the collision.
"It's an incredibly unique laboratory for fundamental physics and astrophysics," added Parameswaran Ajith, a researcher at the International Centre for Theoretical Sciences (TIFR) in India and a CIFAR Azrieli Global Scholar. Ajith and Pfeiffer also pointed out that the discovery opens up a new method for measuring the Hubble constant - a measure of how quickly the universe is expanding.
"This is the very first time we've been able to connect a gravitational wave source detected by LIGO-Virgo to the entirety of the rest of astrophysics via electromagnetic radiation," said Daryl Haggard, a professor at McGill University and a CIFAR Azrieli Global Scholar.
Haggard was part of a team using NASA's orbiting Chandra X-ray telescope to make the first X-ray detections of a gravitational wave source. Their observation, which came two weeks after the original detection, was the third by Chandra, and found persistent X-ray emissions coming from the location of the collision, where initially there had been none.
Their observations confirmed that the violent jet of hot plasma known as a gamma-rays, touched off by the neutron star collision, was off-axis rather than pointed at the earth. This is the first time a short gamma ray burst jet has been observable from the side. Analyzing it will tell us how the merger may have occurred, what the magnetic structure of the neutron stars was and how this energy impacts the interstellar medium surrounding the collision.
Mergers of neutron stars are thought to be responsible for producing most of the heavy elements in the universe. Further study of such collisions could help scientists determine the origin of these elements, which make up almost half of the periodic table. Already, follow-up observations by telescopes around the world have revealed signatures of recently synthesized material, including gold and platinum.
Several dozen scientific papers sharing findings will be published today in a variety of journals including the main detection in the journal Physical Review Letters. The information stemming from this event will also allow us to learn about how extremely high-density matter behaves.
"Neutron stars are about 1.4 times the mass of the sun and all the mass is condensed into a volume roughly the size of Toronto," said Pfeiffer "This is one of the very few places in the universe that we can actually learn about how matter behaves that is so dense."
For Ajith, who worked on measuring the speed of the gravitational waves produced by the collision, the joint detection marks the first time we have been able to compare the speed of gravitational waves to the speed of light.
"The fact that these two signals arrived at nearly the same time tell us that the speed of gravitational waves is incredibly close to the speed of light. This was predicted by Einstein but it is the first time we are making a direct measurement," said Ajith.
Vicky Kalogera (Northwestern University), a fifth CIFAR fellow involved in the detection, made pioneering contributions in predicting how often LIGO is likely to see mergers of double neutron stars or double black holes, based on previously existing radio wave observations and theoretical calculations. The new detections will tell us how nature forms these systems.
"This is a revolution in astronomy," Kalogera said. "Never before did we have so many astronomers, so many instruments studying one source and solving multiple mysteries in one shot."
The Gravity & the Extreme Universe program at CIFAR seeks to understand the origin and evolution of the universe, in part by using gravitational waves to study very strongly gravitating objects like neutron stars. Studying these astronomical phenomena has been a long-standing specialty of Canadian research. The program also seeks to bring together a wide selection of researchers from across the spectrum of astronomical research.
"To fully exploit [these findings] requires a very wide array of expertise. It requires the gravitational wave experts, it requires people who can run numerical simulations and it requires astronomers and theorists. This CIFAR program is one almost unique place where all these people come together, so it is the perfect venue to really make progress," said Pfeiffer.
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Journal
Physical Review Letters