Newly discovered gravitational waves hint that 'cities' of black holes may lurk in space

• Scientists have announced their third detection of gravitational waves.
• These ripples in space-time are likely caused by colliding black holes.
• Albert Einstein predicted the phenomenon more than 100 years ago, but he didn't expect we'd ever record it.
• An increasing number of detections opening up a strange new form of astronomy that can "listen" to black holes, neutron stars, and more.

For the third time in less than two years, physicists have detected billion-year-old ripples in the fabric of space-time.

The new recording, like the two others before it, was made using the Laser Interferometer Gravitational-Wave Observatory (LIGO).

More than 1,000 scientists were involved in the LIGO collaboration, and published a study in the journal Physical Review Letters on June 1. Their analysis suggests the gravitational waves were almost certainly created by the collision of two black holes that smashed together and merged into a larger black hole.

Binary black mergers like this are so catastrophic that they can unleash the energy locked in several suns' worth of matter in an instant; in this case, two black holes of about 20 t0 25 solar masses formed a black hole of 49 solar masses.

LIGO's third detection is striking, but researchers say its importance lies in the story it's helping to tell about black holes - one of the most powerful yet enigmatic forces of nature.

"[T]hese are objects we didn't know existed before LIGO detected them," David Shoemaker, a physicist at MIT and LIGO collaborator, said in a press release by Caltech. "It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us."

Timothy Brandt, an astrophysicist at the Institute for Advanced Study who isn't involved with LIGO or the new study, told Business Insider that the finding is a "lovely, wonderful discovery" and that it "fills in a gap perfectly" between LIGO's first and second detections, which formed black holes of about 21 and 62 solar masses, respectively.

"You'd be shocked if you didn't detect something of this size," Brandt said.

Astrophysicist and LIGO data analyst Vicky Kalogera says this increasingly common population of black holes, which range in size from about 10 to 30 times the mass of the sun, may suggest that binary black hole systems form on crowded "dance floors" or in dense "cities" that lurk within galaxies.

"Nobody has proven anything," Kalogera told Business Insider, "but there's hints here, and hints there."

Black-holes 'cities'

Astrophysicists aren't sure where these systems of black holes are made, or when, but they've narrowed it down to roughly three options.

One is that they all formed moments after the universe came into existence, some 13.77 billion years ago, as primordial black holes. Conveniently, some scientists believe these dense yet practically invisible objects might explain dark matter: the mysterious, missing 80% of the universe's mass.

Yet Brandt says this option has grown increasingly unlikely over the years, even though the black holes LIGO has detected fit the bill for size.

"Primordial black holes should be doing a variety of other things that we should observe, but don't," Brandt said.

Another option is that black holes form normally in galaxies. In this scenario, Kalogera says, massive stars run out of fuel and collapse under their own weight, forming black holes, then - as the galaxy rotates and moves through space - they randomly pair up and eventually collide.

But this may take too long to produce black hole mergers at a rate LIGO seems to be detecting.

So a third option - the one Kalogera says she is betting on - is that most black holes are born and collide in dense groups of stars called globular clusters.

Most large galaxies contain globular clusters, which are hotbeds for the formation of stars. Some are hefty enough to form black holes.

Kalogera says the newest detection revealed that each of the two black holes were spinning before they merged together. But those spins were not aligned, she says, which suggests they were zooming around each other in a busy region of space - not aligned with the plane of a flat galaxy, as the second scenario would lead to.

Black holes that form inside globular clusters would, through known physics, naturally drift and crowd toward the center of such systems.

"Since it's such a dense environment, with lots of flybys, black holes can more easily get partners, get into orbit around one another. They get captured in sort of these binary dances," Kalogera says.

"We could think of them as very active dance floors" for black holes, she says - or 3D cities that are crowded with them. "Galaxies don't have consistent densities. Like human populations, stars cluster in 'rural' and 'urban' areas."

Kalogera says there's not yet enough evidence to back up this idea, since the detection of three black hole mergers (and the six smaller black holes that created them) is a very small sample size.

However, LIGO is just warming up.

A new form of astronomy gets more powerful

The third detection of gravitational waves may not seem as monumental as the first, but astronomers would beg to differ: It proves that an entirely new field of astronomy - one in which scientists can "listen" to the music of the universe, and perhaps even peer into its origins - is not only possible, but productive.

Scans of the sky using visible light, X-rays, and electromagnetic waves only provide circumstantial evidence of how black holes warp the light and space near them. Gravitational waves, on the other hand, are what black holes themselves emit: It's the "natural language" of their existence.

Astrophysicist and LIGO collaborator Imre Bartos previously told Business Insider that gravitational waves give scientists a "peek into the heart" of how black holes form, coalesce, and evolve.

"For the first time, we've been able to understand [black holes'] language and understand what they're telling us," Kalogera said.

Albert Einstein first predicted the existence of gravitational waves more than 100 years ago. However, he doubted they'd ever be detected.

"We are starting to get a glimpse of the kind of new astrophysical information that can only come from gravitational wave detectors," physicist David Shoemaker, who led the construction of LIGO,  said in a 2016 press release.

Scientists are now hoping to boost their newly minted field of gravitational wave astronomy with more gear.

An upgraded version of the Virgo interferometer in Europe - a similarly giant L-shaped gravitational wave detector - went online in 2016. Working in concert with LIGO's two detectors, Virgo should help give astronomers a third source of data to observe black hole behavior and, by extension, the inner workings of the universe.

As both instruments advance in the coming years, Kalogera suspects the Virgo-LIGO partnership could detect roughly 100 events per year, or one every three days.

Some of those could be collisions of incredibly compact neutron stars, which may be detected within 5 years "if they're lucky," says Brandt. (Unlike black holes, however, such collisions may be visible to traditional telescopes.)

LIGO may also eventually aid in listening to the parts of the universe that were created shortly after the Big Bang, though Kalogera says the ability to pick out those signals is still a long way off. She likens the early universe to a "distant symphony that all comes together as one sound," while colliding black holes "dominate anything the early universe will have produced."

But there's no doubt that an entirely new era of astronomy has begun - one in which we can encounter events Einstein could only dream of observing.

"LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe," deputy director Albert Lazzarini said in a press release.

Sarah Kramer contributed reporting to this post.