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Physicists detect gravitational waves from colliding black holes

by Lindsay Jolivet
Feb 11 / 16

Physicists have detected gravitational waves left over from the collision of two black holes more than a billion years ago, and confirmed a prediction made by Albert Einstein as part of his general theory of relativity.

Researchers including CIFAR Fellow Harald Pfeiffer (University of Toronto) participated in the research that led to the discovery as part of the Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration. LIGO detected gravitational waves, which are ripples in the fabric of spacetime that happen when massive objects accelerate in the Universe.

Many decades of theory and indirect evidence supported the existence of gravitational waves, most notably through a technique called pulsar-timing used by CIFAR fellows of the program in Cosmology & Gravity, but this is the first direct observation. The researchers concluded the source of the gravitational waves was two black holes orbiting each other and pulling together until they collided and merged into one. This is also the first observational proof that black holes sometimes collide.

“It is absolutely stunning to see two ground-breaking discoveries at once,” says Pfeiffer, “Not only were gravitational waves that were passing through Earth measured for the very first time, but also, the origin of these waves are astronomical objects that have hitherto never been observed.”

The black holes in question were each close to 30 times more massive than the sun, and they collided about 1.3 billion years ago, according to the researchers. In the fraction of a second before they became one enormous, spinning black hole, part of their mass converted to energy. This energy blasted out in the form of gravitational waves that rippled through the Universe.

Traveling at the speed of light, the waves finally reached Earth’s southern hemisphere and after passing through Earth, were observed on Sept. 14, 2015 by two LIGO detectors in Livingston, Louisiana and Hanford, Washington. The measurements of the signals matched Einstein’s predictions in his theory of general relativity, confirming his hypothesis a century after he first presented it.

LIGO announced the finding on Feb. 11, 2016, and the paper describing the research has been accepted to Physical Review Letters.

The Canadian Institute for Theoretical Physics (CITA), at the University of Toronto, is affiliated with the LIGO Scientific Collaboration. Pfeiffer, the faculty member on the project, and other CITA researchers, helped detect and validate the gravitational wave findings. Pfeiffer also leads CITA’s research group in numerical relativity, which is one of the world’s leaders in simulating black hole collisions on supercomputers. From these calculations, many of which were performed at the SciNet Consortium at the University of Toronto, researchers learn the shapes of the gravitational wave signals, which LIGO successfully found, and they can measure the masses and spins of the black holes. Pfeiffer collaborates with CIFAR fellows in the Cosmology & Gravity program, many of whom approach the search for gravity waves from different perspectives and disciplines.

“I am grateful to CIFAR for providing valuable interactions with world-leading experts in all aspects of gravity research. The many perspectives represented in the Cosmology & Gravity program help to deepen one’s understanding, and to sharpen the research focus onto the truly relevant questions,” Pfeiffer says.

J. Richard Bond, the program director, says the discovery will bring new excitement and directions to research efforts in the program.

“Many years ago, we targeted the calculation of colliding black holes as an important area for CIFAR’s program in Cosmology & Gravity, and drew a few world-leading researchers to pursue this in Canada. Their theoretical work underpins this great experimental detection, and in fact led to us being a node in the LIGO network. We knew this day would come and I am so pleased that CIFAR helped LIGO in this discovery,” Bond says.

“The direct detection of gravitational waves is an amazing, landmark discovery,” says CIFAR President Dr. Alan Bernstein. “And it is strong confirmation of Einstein’s general theory of relativity.

“Perhaps most importantly in the longer run, these experiments have opened a new door in the history of astronomy. This international team of scientists, including CIFAR Fellow Dr. Harald Pfeiffer, have created a way of looking at our cosmos that even Galileo could not have imagined.”

Videos courtesy of the SXS Collaboration.