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Third detection of gravitational waves

by Juanita Bawagan Jun 28 / 17
Artist’s conception shows two merging black holes similar to those detected by LIGO.  (Courtesy Caltech/MIT/LIGO Laboratory)

The first detection of gravitational waves has been compared to opening a new window on the universe. With the third detection, scientists have opened this window wider and stared deeper into the universe to understand black holes and gravity.

Gravitational waves are the ripples in space and time caused by massive accelerating objects. To date, all detections have been generated by two black holes colliding and merging into a larger one. The latest detection comes from a binary black hole of about 49 solar masses located three billion light years away.

Cosmology & Gravity Fellow Harald Pfeiffer is a member of the 1,500-person Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration behind these breakthrough detections. Pfeiffer’s team of seven University of Toronto astrophysicists contributed to the latest LIGO detection described in Physical Review Letters.

This marks LIGO’s third gravitational wave detection in 18 months and Pfeiffer expects to see more.

“One binary black hole collision happens somewhere in the universe every 15 minutes. So as the detectors run longer and become more sensitive, we’re going to see a larger fraction of all these merging black holes.  Besides learning about their properties and the origin of the merging black holes, they will also be used to test if Einstein’s is the right theory of gravity,” he says.

This animation shows the inspiral and merger of two black holes with masses and spins consistent with the GW170104 observation. (SXS Collaboration/CITA)

Theorists in the Cosmology & Gravity program have been particularly interested in how gravitational waves could test General Relativity. The third detection – named GW170104 and made on Jan. 4, 2017 – was a prime candidate. Because GW170104 was so far away, researchers were able to better see if gravitational waves move at the speed of light independent of their frequency as Einstein predicted.  Hypothetically, if high frequency waves move faster, they could overtake the low-frequency waves that occur before black holes collide. No deviation in speed was detected in the waves detected on January 4, 2017, and all observations have been consistent with Einstein’s prediction.

LIGO has also uncovered black holes in a range of sizes that have never been observed before.

“All of these systems have different masses so it drives home the point that there is a wide variety of stellar-mass black holes,” Pfeiffer notes.

Previously, all known black holes were either less massive or in the centres of galaxies with millions and billions of solar masses. The LIGO-detected black holes, which measure 21 to 62 solar masses, are in the middle range that had not been seen before.

Pfeiffer says this raises many questions: “How did these black holes come into existence and what other environments and circumstances were near the stars that made these black holes?” These questions have driven his research for the last 20 years and he hopes that LIGO may one day answer them.

GW170104 offers some new clues. Details of the rotation-states of the black holes in a binary give hints into how a binary black hole formed. If the binary black hole formed from two orbiting stars that each become a supernova and then a black hole, then their rotation axes should be parallel. In contrast, if the black holes form individually and then find each other, then the rotation should be random. This system slightly hints toward black hole spins being randomly oriented. More data is needed, however, before one can be certain.

Further gravitational wave detections and technological improvements will open LIGO’s window on the universe even wider. Pfeiffer hopes to see more black holes but is excited about what else LIGO might see.

“Most importantly, everyone is hoping to detect collisions’ from two neutron stars or a black hole and a neutron star … or even very different things because all of these sources can teach us about very compact objects with strong gravity,” he says.