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Most massive merger of two black holes results in a powerful ripple through space-time

The signal traveled seven billion years to reach us, but only lasted one tenth of a second.

Four short wiggles that lasted about one tenth of a second. Detected on 21 May last year by scientists searching for signs of powerful ripples in space-time, this was a brief signal, and an unusual one.

As short as it was, this blip was a gravitational wave — the result of some of the most violent processes taking place in the universe. In a new study, scientists explain how this particular one stands out: It was caused by the most massive collision of two black holes ever detected, creating an intermediate mass black hole, the first of its kind ever seen.

The groundbreaking detection was made using the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO).

It is detailed in a study published Wednesday in the Astrophysical Journal Letters.

The finding has the potential to fundamentally alter our understanding of the universe. That is due to the nature of gravitational waves, which are ripples in the fabric of space-time, and offer a completely different way to 'see' the universe than via the light that different cosmic bodies give out.

An artist's impression of the black hole merger in action, and the powerful energy released.

Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), and a member of the team who detected the gravitational wave, tells Inverse he could sense there was something unusual about this signal when he first heard it.

"It was quickly apparent that it was different because it was so short in time and so low frequency," Christensen tells Inverse.

"It was really like what we call a burst, but we could see it in the detectors so we knew it was real."

The scientists labeled the signal GW190521. Further analysis revealed this ripple in the cosmic fabric was likely created by the merger of two black holes, both with unusual properties.

This gif shows the out-of-kilter spin of the black holes.Raul Rubio/Valencia Group/Virgo Collaboration

These two black holes appear to have had masses of about 85 and 66 times the mass of the Sun respectively, and were not only spinning around each other, but appear to have also been spinning around their own axes at angles out of alignment with the axis of their orbit, according to the study.

This out of alignment spinning caused the plane of the black holes' orbit to rotate.

"That changed the signal pretty noticeably and it affected our ability to analyze it," Christensen says.

The merger released a huge amount of energy (about 8 Suns-worth), which then spread across the universe in the form of gravitational waves.

These waves traveled around 7 billion light-years to reach Earth. This means the merger took place when the universe was only half of its current age. Scientists believe that the universe was created around 13.8 billion years ago.

As the two black holes merged, they created an even larger black hole, of about 142 solar masses. The newborn black hole marks the first time scientists have observed the birth of an intermediate mass black hole. These cosmic behemoths weigh-in between 100 to 1,000 solar masses.

As the black holes merged, they gave birth to an intermediate mass black hole — the first of its kind ever detected.

In the past, scientists had predicted intermediate mass black holes, but they had never observed them before.

"We’ve seen lower mass black holes and we know there’s supermassive black holes, but what’s in between, and how do you make these black holes?" Christensen says.

So far, all other black holes ever observed are either stellar-mass black holes, which measure from a few solar masses up to tens of solar masses, or supermassive black holes, which measure anywhere between hundreds of thousands, to billions of times that the mass of our Sun. The black hole at the center of our galaxy is a supermassive black hole, about four million times the mass of the Sun.

Hearing the universe

Gravitational waves are caused by the accelerated masses of cosmic beings, which send out waves at the speed of light.

"With electromagnetic radiation, light, we can see the universe but with gravitational waves, we can hear the universe," Christensen says. "It’s a completely different way to explore the universe."

Christensen compares it to a doctor seeing a patient and noting some physical symptoms that appear on the outside, but then placing a stethoscope and listening to their chest to get a better understanding of what may be wrong.

"They complement each other and provide different information, but it’s all about describing how the universe works," he adds.

Gravitational waves are also a way for astronomers to explore the 'dark side' of the universe, or the invisible matter that would otherwise not show up using light-based observation techniques.

"There would be no way to see two black holes spinning around each other and colliding if not for gravitational waves," Christensen says. "It's a different sense."

Scientists detected gravitational waves for the first time in September, 2015, using LIGO. Since then, the gravitational-wave detector has listened in to more of these ripples in space-time.

"Each time we turn on the detectors, we see more interesting things," Christensen says. "The universe is providing us with all kinds of surprises, and that’s really wonderful."

Abstract: The gravitational-wave signal GW190521 is consistent with a binary black hole merger source at redshift 0.8 with unusually high component masses, 85+21−14 M and 66+17 −18 M, compared to previously reported events, and shows mild evidence for spin-induced orbital precession. The primary falls in the mass gap predicted by (pulsational) pair-instability supernova theory, in the approximate range 65−120 M. The probability that at least one of the black holes in GW190521 is in that range is 99.0%. The final mass of the merger (142+28−16 M) classifies it as an intermediate-mass black hole. Under the assumption of a quasi-circular binary black hole coalescence, we detail the physical properties of GW190521’s source binary and its post-merger remnant, including component masses and spin vectors. Three different waveform models, as well as direct comparison to numerical solutions of general relativity, yield consistent estimates of these properties. Tests of strong-field general relativity targeting the merger-ringdown stages of the coalescence indicate consistency of the observed signal with theoretical predictions. We estimate the merger rate of similar systems to be 0.13+0.30−0.11 Gpc−3 yr−1. We discuss the astrophysical implications of GW190521 for stellar collapse, and for the possible formation ofblack holes in the pair-instability mass gap through various channels: via (multiple) stellar coalescences, or via hierarchical mergers of lower-mass black holes in star clusters or in active galactic nuclei. We find it to be unlikely that GW190521 is a strongly lensed signal of a lower-mass black hole binary merger. We also discuss more exotic possible sources for GW190521, including a highly eccentric black hole binary, or a primordial black hole binary.
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