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A third detection of gravitational waves is changing our understanding of black holes

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LIGO just keeps picking up ripples in space-time

An artist rendering of two black holes merging, like the ones LIGO detected.
Image: LIGO / Caltech / MIT / Sonoma State (Aurore Simonnet)

For the third time, scientists have detected gravitational waves — the ripples in space-time created by objects moving throughout the Universe.

The discovery was made once again by LIGO, or the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration — the same team that last year made headlines for measuring gravitational waves for the first time in history. With each new detection, LIGO further solidifies that its measurements are sound, not just statistical blips. And now, researchers are starting to use these detections to learn more about distant space objects, as well as spot trends that they didn’t quite expect to see.

“There’s a famous saying: once is chance, twice is coincidence, and thrice is a pattern,” Bangalore Sathyaprakash, a physicist at Penn State and Cardiff University, as well as the lead author on a new study about the detection, tells The Verge. “We are seeing a pattern now.”

Similar to the first two detections, the latest gravitational waves LIGO measured stemmed from two dense black holes chaotically merging billions of light-years away. Black holes don’t just simply come together when they merge; they rapidly spin around each other — up to several times per second — before joining to form one single super-dense object. These rotations produce ripples in the fabric of space and time, which then move throughout the Universe at the speed of light. These latest waves, detailed in a new paper that will be published in the journal Physical Review Letters, traveled from a merger nearly 3 billion light-years away before reaching LIGO’s wave-detecting observatories on January 4th.

A simulation of the black hole merger that LIGO detected. The gravitational waves are indicated by the blue and yellow bands. Video: S. Ossokine/A. Buonanno/T. Dietrich (MPI for Gravitational Physics)/R. Haas (NCSA)/SXS project

Though all three of LIGO’s detections have been from black hole mergers, each event was unique. The first detection came from fairly massive black holes, while the second pair were much smaller. This new merger fits right in the middle: the black holes were around 19 and 31 times the mass of our Sun, which is still pretty massive, according to Sathyaprakash. Prior to LIGO’s first detection, scientists didn’t think many black holes bigger than 20 solar masses existed. Now not only we know they exist, but they may be pretty common. “There is a population of heavy black holes out there and LIGO has started seeing them,” says Sathyaprakash. “I think we will detect more and more as we go along.”

How LIGO works

Today, gravitational waves are no longer a novelty but a new tool for studying the cosmos. But up until LIGO’s first detection, gravitational waves were the last big unconfirmed part of Albert Einstein’s theory of general relativity. That theory, which came out in 1916, revolutionized our understanding of the Universe, by combining space and time together into a single concept known as space-time. Einstein theorized that objects actually leave imprints on the space-time around them — and when objects move, they create ripples in space-time, similar to how a moving object creates ripples in a pond.

Technically, every object that moves — from a person to a planet — creates these ripples. But the waves you and I produce are so small they’re basically impossible to detect. LIGO is designed to pick up waves coming from the most massive objects in the Universe moving at rapid speeds. The mergers of black holes or neutron stars — dense leftovers of stars that have collapsed — produce gargantuan waves that can be picked up from Earth. But by the time they reach our planet, they’ve diminished substantially, making them extremely difficult to measure.

Fortunately, LIGO’s two observatories in Washington and Louisiana are sensitive enough to do the trick. Each observatory is shaped like an L, with “arms” made of vacuum-sealed tubes that run 2.5 miles long. At the end of each tube is a suspended mirror, and where the arms meet, there's a laser pointed to each mirror. Whenever a gravitational wave passes, the ends of each L are warped differently, making it seem like one mirror is getting closer to the source of the laser while the other is getting farther away. LIGO can pick up this phenomenon by timing how long it takes the laser to hit each mirror. LIGO is so precise that it can pick up changes that are 1,000 times smaller than the size of a proton.

An illustration of how LIGO detects gravitational waves when they pass the observatories.
Image: American Museum of Natural History

Massive black holes

The three detections we have from LIGO are already changing scientists’ ideas about how massive black holes can get — much more massive than people thought possible.

It all has to do with where these objects came from. The black holes that LIGO has been observing are thought to be the leftover remnants of dead stars that have used up all their fuel and collapsed. And the masses of these black holes are largely dictated by the stars they came from, which depends a lot on where and when the stars formed. For instance, the first stars that emerged just after the Big Bang — about 14 billion years ago — differ a lot from those that formed much later.

The earliest stars were made up of only the lightest elements, hydrogen and helium — the first elements to form. But as the stars evolved, they eventually created heavier elements inside their cores through nuclear fusion reactions. At some point, those heavier elements were expelled throughout the Universe when the stars exploded and became the initial ingredients for the next generation of stars. That means the stars that formed more recently are richer in these heavier elements, like carbon and nitrogen.

The amount of heavy elements a star has is thought to affect how that star evolves over time. Heavier elements absorb a lot more radiation, according to Sathyaprakash. That energizes the elements, giving them enough momentum to be thrown off the star. That means if a star is made up of a lot of heavier elements, it is more likely to lose mass than a star that is made up of pristine hydrogen and helium.

The three confirmed detections by LIGO (GW150914, GW151226, GW170104), and one lower-confidence detection (LVT151012). Many of these black holes are larger than what people expected.
Image: LIGO / Caltech / Sonoma State (Aurore Simonnet)

Most people assumed that the black holes LIGO saw would come from stars rich in heavy elements, meaning they probably would never exceed 20 times the mass of our Sun. But now that LIGO has detected at least three black holes more massive than that, Sathyaprakash says it’s possible there are a lot more stars out there made of lighter elements. Perhaps the observatories are spying much older stars that were formed closer to the Big Bang. Either way, people are rethinking their models. “There are some people who did think heavier black holes could form, but they were in a minority,” says Sathyaprakash. “So the past detection of black holes by LIGO was a turning point in astrophysics.”

Out of alignment

The LIGO collaboration is using this latest detection to figure out more details about the two black holes that created the latest waves. For instance, the signals hold clues as to how the holes were spinning up until the point they merged. And that, in turn, paints a picture of how the merger occurred in the first place.

When black holes merge, there are three types of rotations involved: the two holes each rotate individually and they revolve around each other. If both the black holes rotate in the same general direction as how they’re revolving around each other, then they’re considered aligned, according to Laura Cadonati, a LIGO collaborator and professor of physics at Georgia Institute of Technology. However, if one black hole is spinning in the opposite direction of the other, then they’re not aligned.

A rendering of the ripples created by black holes as they merge.
Image: NASA

The LIGO researchers can try to determine the black holes’ alignment by studying the gravitational wave signal a bit more closely. If aligned, the black holes take slightly longer to merge than expected. That’s because the aligned orbits would create more rotational energy, making it harder for the holes to come together. “It takes a little bit longer,” says Cadonati. “They do another couple dances around each other.” In the case of the latest detection, the wave signal indicates that these black holes were not in alignment when they merged.

And that tells scientists a bit about how the black holes might have come together. If the black holes stemmed from a two-star system, where the stars were already orbiting around each other, then chances are the orbits would be aligned. However, this signal seems to indicate that these black holes came from stars that formed separately, but within the same stellar cluster. They then slowly came together over time. “It’s just a hint [that this happened]; we cannot say with certainty,” says Cadonati. “But if this becomes the pattern — if we see more of this — that will be a more definitive statement.”

LIGO spotted these most recent gravitational waves during an observational run that started on November 30th, 2016 and will continue through the end of the summer. So it’s possible we’ll hear about even more detections soon enough, as LIGO continues to look through the data. Once the current observational run ends, though, the next one doesn’t start up again until 2018. During that down time, the team will be doing technical upgrades to the observatories to make them even more sensitive. That means even more detections could be on their way, allowing LIGO to learn more about how massive objects merge billions of light-years away.

“It’s only three detections so far, so it’s only the beginning,” says Sathyaprakash. “But with this detection we are sending a message to the world that we are here to do gravitational wave astronomy.”