Third gravitational wave detection puts new spin on black holes

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And we're not just talking about the heated disc of material swirling around it - the actual mass has angular momentum, meaning the black hole itself rotates on its axis.

The new data not only supports the existence of heavy stellar black holes-black holes more than 20 times the mass of the sun-they also provide clues about how binary black hole systems form.

A huge worldwide team of scientists have just announced the discovery of yet another gravitational wave signal sent out by the violent collision of two black holes circling each other 3 billion light-years away and felt on Earth on January 4.

"It is remarkable that humans can put together a story, and test it, for such odd and extreme events that took place billions of years ago and billions of light-years distant from us", Shoemaker said in a statement. "Indian scientists played a leading role in deriving this result", said Sanjit Mitra from the Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA), researchers of which have participated in the LIGO discoveries. A second observing run began on 30 November 2016, and the third signal showed up just a month later.

"Normally we don't think of space as having any properties at all, so it's counterintuitive", says Michael Landry, director of LIGO's Hanford observatory. This marked the third time in about a year that physicists, thanks to the Laser Interferometer Gravitational-wave Observatory, or LIGO, discovered gravitational waves from the violent death spirals of merging black holes. It confirmed the second wave in December 2015 at a distance of 1.4 billion light years.

The observatories, which use laser interferometers to sense the presence of gravitational waves, are operated by Caltech and MIT with funding from the National Science Foundation (NSF). These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time.

The third area of significance is that the most recent detection appears to be the farthest yet, with the two black holes located about 3 billion light-years away. It revealed the spins that are hard to measure may not be aligned with the mutual orbit of the black holes.

The readings for GW170104 indicate that at least one of the black holes might have been spinning in a direction that was different from the two objects' orbital motion around each other.

"The black holes are not necessarily lined up", said Professor Scott, of the Australian National University, one of several Australian universities involved in the research.

Gravitational waves are caused by cosmic events like colliding black holes or neutron stars, explosive supernovas - even the birth of the universe.


With a star's mass packed into a radius of around 10 kilometres, and hundreds of rotations a second, even a bump just 10 centimetres high could be enough to throw detectable gravitational waves out into the Universe. That would imply that the black holes were not born together as stars.

According to Bangalore Sathyaprakash of the University of Cardiff in the United Kingdom, the relative orientations of the spin and orbital angular momenta of a binary black hole provide important information about how the system formed.

The new finding is also important because it was possible to calculate whether the colliding black holes were spinning in the same direction as they were circling each other in their orbit before the collision, or in a different direction.

In the other model, the black holes come together later in life within crowded stellar clusters. Because LIGO sees some evidence that the GW170104 black holes are non-aligned, the data slightly favor this dense stellar cluster theory.

Gravitational waves are shaping up to be the hot new astronomical tool of the 21st century, offering glimpses into the universe's darkest corners and providing insights into the workings of the cosmos that we can't get by any other means. And because the gravitational wave arrived undiminished, it provides yet another proof of one of Einstein's theories, showing that gravity travels at light speed. According to Einstein's theory of gravity, the general theory of relativity, massive objects bend the fabric of space and create ripples when they accelerate - for example, when two objects orbit one another. Nevertheless, he says, Einstein's theory of general relativity predicts that space can expand, contract, or vibrate, thereby distorting the medium in which we all live. Soon the complementary Virgo detector is expected to come online in Italy, and in 2024 another LIGO detector is scheduled to start up in India.

These ripples in space-time can only be detected by powerful instruments.

For much of the past year, the LIGO detectors were offline to allow an upgrade.

"Two solar masses worth of energy were released from the binary system in the form of gravitational waves", said Raymond Frey, who heads the UO's team in LIGO project, about the newly detected event.

If neutron star mergers are happening at an appreciable rate, we should also be able to detect them (and, since they're not black holes, we should also see a signal of the event with photons).

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