LIGO: Now, gravitational waves provide clues on how binary black holes form

Photo GW-Optimized
An international team of researchers has made a third detection of gravitational waves, ripples in space and time, in a discovery that provides new insights into the mysterious nature of black holes and, potentially, dark matter. – Image: LSC/OzGrav

On January 4, 2017, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves — ripples in space and time — for the third time. The first success in observing gravitational waves was in September 2015 and the second detection was in December 2015.

While the September 2015 detection became important as it was the first time that gravitation waves were observed, detection of gravitational waves three months later in December 2015 was considered important as it confirmed that merger of two black holes and release of huge energy in the form of gravitational energy was not a chance discovery. The second observation opened up a new class of objects to be studied in the universe.

But the detection of gravitational waves in January this year is important for two main reasons — it sheds light on the direction in which the two black holes are spinning and how the black holes formed a binary system. The newfound black hole was formed when two black holes more than 31 and 19 times the mass of the Sun formed into a single body of mass about 49 times that of our Sun. The balance one mass of the Sun was transformed into gravitational energy.

The newly detected merger occurred approximately 3 billion light years away, making it more than twice as distant as the first two events, which occurred 1.3 and 1.4 billion light years away, respectively. The results were published in the journal Physical Review Letters.

Black holes like ice skaters

Like a pair of ice skaters spinning individually while also orbiting spiralling around each other, the two black holes rotate on their own axes even as they orbit around each other. Like in the case of planets and other celestial objects, the direction of rotation will usually be the same as the direction in which they orbit. “What we usually expect in all binary black holes is that the direction of rotation will be the same as the direction in which they orbit around each other,” says Dr. Varun Bhalerao from the Department of Physics, Indian Institute of Technology (IIT) Bombay.

LIGO
The U.S.-based Laser Interferometer Gravitational-Wave Observatory has discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple). The three confirmed detections by LIGO (GW150914, GW151226, GW170104), and one lower-confidence detection (LVT151012), point to a population of stellar-mass binary black holes that, once merged, are larger than 20 solar masses — larger than what was known before. Image: LIGO/Caltech/Sonoma State (Aurore Simonnet)

In other words, if the black holes are rotating on their axes in an anti-clockwise direction, the orbital motion will also in the same anti-clockwise direction — what astronomers refer to as aligned spins. At times, the black holes can spin in a direction different to the direction of the orbital motion — it can rotate in an anti-clockwise direction and orbit around the other black hole in a clockwise direction.  Also, the spin of the black holes can be at an angle to the orbital plane.

Hints on how the binary black holes formed

The January observation provides the first evidence that at least one of the black holes in the binary system might have been spinning in a direction that is “not completely aligned with the orbital rotation of the binary”.  In simple terms, at least one of the black holes in the binary system might have been spinning in a direction that is different from the direction of orbital rotation. With the available data it cannot be determined if the axes of rotation of the black holes is at an angle to the orbital plane.

The difference in the direction in which at least one of the black holes is spinning on its axis as it orbits around the other black hole provides some clue on how these binary black holes might have formed.

“Two stars formed from a cloud of gas would rotate in the same direction. When the stars explode and black holes are born and form a binary the axes of rotation will remain the same,” says Dr. Bhalerao. Since at least one of the black holes might be rotating in a direction misaligned with the overall orbital motion, the black holes could have formed separately and come together later in life within crowded stellar clusters. The black holes pair up after they sink to the centre of a star cluster.

“Since the history of formation of the two black holes is different, the black holes can be spinning in any direction relative to their orbital motion,” says Dr. Bhalerao. As the latest evidence from black holes show evidence of black holes being non-aligned, the data supports the dense stellar cluster theory.

India’s contribution

“With the third definite detection of gravitational waves from a coalescing black hole binary, we have discovered a new class of astrophysical sources to test Einstein’s theory of general relativity in extreme conditions,” Dr. Bala Iyer, the Principal Investigator of the Indian team in LIGO says in a LIGO release. Dr. Iyer is a Simons visiting professor at the International Centre for Theoretical Sciences (ICTS), Bengaluru. “The ICTS group played a key role in developing and implementing an analysis that was used to test the consistency of the observed signals with general relativity,” the release says.

The nine-member ICTS team in LIGO also contributed to the estimation of the mass and spin of the remnant black hole produced by the merger. By studying the properties of black holes, future observations will tell us how exactly these binary systems are formed in nature.

“Until LIGO detected black holes of more than 20 solar masses we were not sure if black holes of such masses existed [the solar masses of the merged black holes detected by LIGO in September 2015 and December 2015 were 62 and 21 times that of our Sun respectively]. We had no idea how common binary black holes are, how they are distributed and how they form.  As LIGO is discovering many binary black holes we are beginning to understand this class of objects,” says Dr. Bhalerao.

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