On May 21, 2019, Earth was caught in a surging swell of the cosmic sea. A gravitational wave, a ripple in the fabric of space-time, washed over the planet, pinging a trio of laser detectors on the surface of our planet. The wave was caused by a collision between two huge black holes in a deep corner of space.
Today, astronomers have announced it is officially the biggest collision ever detected, forming a black hole 150 times more massive than the sun.
Details of the event, dubbed GW190521, were published in two prominent astrophysics journals on Wednesday, and they come from the LIGO/Virgo collaboration, a huge team of scientists studying gravitational waves detected by the dual Laser Interferometer Gravitational-Wave Observatory facilities in the US and the Virgo detector in Italy.
"One of the big hopes for LIGO and Virgo is to discover brand new types of systems, and hopefully something that surprises us," says Meg Millhouse, an astrophysicist at the University of Melbourne and co-author on the study. "GW190521 was definitely one of those new surprising events."
The collision provides definitive evidence for a class of black holes long theorized to exist, but only observed indirectly in the past. It also allows astronomers to probe the nature of the stars, how they live and die and what becomes of them once they've exploded.
"GW190521 originated from the collision of two black holes that are each heavier than any LIGO/Virgo has observed before," explains Rory Smith, an astrophysicist at Monash University in Australia who works with the LIGO/Virgo collaboration. "One is around 66 times more massive than the sun, the other is 85 times more massive."
The two black holes danced with each other for eons, performing a cosmic minuet, before eventually falling into each other. Upon colliding, they formed a bigger black hole with a mass 142 times greater than the sun, converting the leftover mass into energy and releasing it as gravitational waves. It was those waves that roiled across the universe and eventually found their way to Earth, pinging our detectors.
"This signal was basically a blip, but it carried a huge amount of information," Smith notes.
The information contained in the wave provides researchers with new insight into the composition of black holes and underscores the power of gravitational wave astronomy to cut through the dark forest of our cosmos, reveal its secrets and unveil new mysteries. "This event opens more questions than it provides answers," said Alan Weinstein, a physicist at Caltech and part of the LIGO/Virgo collaboration.
"From the perspective of discovery and physics, it's a very exciting thing."
A black hole heard
Black holes are often referred to as monstrous and gargantuan; cosmic colossi that lord over the rest of space. And some are undoubtedly beastly, with masses that make our sun seem as inconsequential as a mote of dust. But black holes can range in size from small to unfathomably huge.
Some of the smallest black holes we've detected contain about 10 times the mass of the sun and are usually referred to as "stellar black holes." They form when a huge star explodes at the end of its life. Our current understanding suggests these types of black holes form if the exploding star is between around 65 and 135 times the mass of our sun.
Stars that fall into the middle range -- not too small, not too big -- become ghosts, completely eradicating themselves when they die.
Right down the other end of the spectrum are supermassive black holes. They can reach billions of masses more than the sun. These are the true monsters of the universe and usually lie in the center of a galaxy, their huge gravitational pull a sinkhole the rest of the galaxy revolves around. The Milky Way, our home galaxy, contains a supermassive black hole,, and a neighboring galaxy, Messier 87, was where .
The two black holes that caused the GW190521 event fall into the ghostly range. The larger of the two, the one with 85 solar masses, has been described as an "impossible" black hole. Smith says he wouldn't call these black holes impossible, but the sizes of the pair do present a significant puzzle for astronomers to solve.
"We don't really know yet how the two black holes that collided formed," says Smith.
One hypothesis is that black holes of this size, creatively named "intermediate mass black holes," are formed when two smaller black holes meet, dance and merge. Kind of like The Blob, one black hole eats another, allowing it to become more massive. It seems the two black holes that collided in GW190521 were themselves the result of black hole collisions between two lighter black holes.
So some time ago, when the universe was much younger, four black holes became two, then those two met, danced and became one.
For that to occur, astronomers theorize the black holes must have existed in a cosmic mosh pit, where black holes constantly bash into each other.
Saavik Ford, an astrophysicist at the American Museum of Natural History and not affiliated with the new studies, says there are "only three models" that provide plausible explanations. All of them involve turbulent, dense environments from a time when the universe felt at least a little busier. They include globular clusters, a huge collection of stars bound together by gravity, or a nuclear star cluster, a grouping of stars close to the center of a galaxy.
But Ford says the third option is the most likely environment for this mosh pit: the disks of active galactic nuclei, regions at the center of a galaxy dense with stars, gas and dust. In these regions, she says, black holes would "play nice with each other" and more mergers would be seen. Such a busy dance floor would be the perfect place for black holes to meet.
A black hole seen?
In June 2020, Caltech's Zwicky Transient Facility at the Palomar Observatoryclose to where GW190521 originated. It was a gigantic flare of light.
Black holes are invisible. They swallow light, so we can't see them directly, we can only spot their effects on the cosmos, such as the gravitational waves they send out when they smash into each other. But in June, the researchers reported they may have seen an electromagnetic signal -- light -- from the GW190521 event, the first time astronomers had done so. Saavik Ford was a co-author on the report.
At the time, GW190521 was still a "candidate" gravitational wave event. It hadn't been confirmed by the LIGO/Virgo collaboration, yet, but Ford and her team put forth a strong hypothesis that the event and their flare were related. The researchers reasoned that this event occurred in a huge gaseous debris disk surrounding an active galactic nucleus and that the flare was caused by a black hole being kicked out of the gaseous disk.
Off the back of the announcement of GW190521 on Wednesday, Ford and her colleagueswith the data from the LIGO/Virgo team. The new data reveals it's unlikely the two events are linked based on the most favored models explaining GW190521, but Ford notes there's still some work to be done before completely discounting the possibility they did see a black hole merger.
Smith is skeptical about the electromagnetic signal being related, but remains hopeful we might capture visible light from a gravitational wave event in the future. If black holes are in a mosh pit near an active galactic nucleus, it's likely only a matter of time. And even if the black hole hypothesis is discounted, Ford and her colleagues did find something really weird for an active galactic nucleus.
"If it wasn't a binary black hole merger, it was a deeply unusual flaring event around a supermassive black hole," says Ford. "So, not quite as cool, but still cool."
But what? That's difficult to say. According to Ford, the researchers are analyzing the full LIGO data set for any evidence of coincident electromagnetic signals which may help, but ultimately "more studies of AGN flaring, both observational and theoretical, would be very helpful," she says.
A black hole record
The GW190521 event adds to a growing body of data collected by the facilities run by the LIGO/Virgo collaboration and with each detection, some small mystery of the universe is revealed.
The first detection, made in 2015 and revealed in 2016, showed Einstein's theory of general relativity holds up, and since then, the facilities have made more than a dozen additional discoveries. In June, the collaboration found a "mysterious object" -- far too small to be a black hole and seemingly too large to be a neutron star.
Wednesday's discovery of the biggest recorded merger is another feather in the cap, but scientists aren't done with it yet.
"There's still lots of science that can be done with this event," says Millhouse.
LIGO and Virgo's last observation run ended in March, a month earlier than planned due to the coronavirus pandemic, but dozens of candidate events were observed and are still being investigated. The instrument's fourth observation run is set to begin in 2022 and its sensitivity will be improved. It will also benefit from a new Japanese gravitational wave detector, built underground in Japan: the Kamioka Gravitational Wave Detector.