Three billion years ago, in a third of a second, two black holes crashed into each other and merged into a single entity, converting two solar masses into energy that shook the fabric of spacetime, sending gravitational ripples across the universe that were detected on Earth last January, researchers announced Thursday.

It was the third confirmed detection of coalescing black holes detected so far by the U.S.-led Laser Interferometer Gravitational-Wave Observatory, or LIGO, a project made up of two observing stations, one near Hanford, Washington, and the other 1,800 miles away near Livingston, Louisiana, and hundreds of scientists around the world. The observations confirmed a prediction first made by Albert Einstein a century earlier.

As the gravitational waves passed by, they caused space to lengthen in one direction and compress in the other, squeezing and stretching the LIGO detectors ever so slightly and causing laser beams to cover slightly different distances as they bounced back and forth between massive mirrors.

Exhaustive tests and analyses confirmed the reality of the signal in another milestone for the growing field of gravitational wave astronomy.

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"We have observed, on the fourth of January, 2017, another massive black hole-to-black hole binary coalescence, the merging of black holes roughly 20 and 30 times the mass of our sun," David Shoemaker, the spokesperson for the LIGO Scientific Collaboration, told reporters.

"The key thing to take away from this third event is we're really moving from novelty to new observational science, a new astronomy of gravitational waves."

The discovery was detailed in a paper accepted by the journal Physical Review Letters.

The ripples detected by LIGO indicate the single black hole formed by the merger has a mass of about 49 times that of the sun, midway between the black holes detected by LIGO in September and December 2015. Two times the mass of Earth's sun was converted directly into energy in a fraction of a second.

Black holes are among the most bizarre objects in the known universe. They are believed to form when massive stars run out of nuclear fuel at the end of their lives. Without the outward pressure generated by nuclear fusion to offset the inward pull of gravity, the core suddenly collapses as the star is blown apart.

LIGO has discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple). The three confirmed detections by LIGO (GW150914, GW151226, GW170104), and one lower-confidence detection (LVT151012), point to a population of stellar-mass binary black holes that, once merged, are larger than 20 solar masses. LIGO/Caltech/Sonoma State (Aurore Simonnet)

For stars similar to the sun, core collapse stops due to quantum mechanical effects and a white dwarf remains, a compact remnant that slowly radiates its residual heat away into space. The cores of more massive stars can collapse even further, crushed to the point where protons merge with electrons. The result is a city-size ball of neutrons with the density of an atomic nucleus.

The cores of even more massive stars can collapse past the neutron star state, disappearing from the observable universe. Their gravity is so strong not even light can escape.

A major question mark is how binary black hole systems like those observed by LIGO form.

One school of thought holds the binary black holes form when two already paired stars explode and collapse to the ultimate state, spiraling into each other in a cataclysmic crash. The spins of each pre-merger black hole likely would be aligned with respect to their orbital motion.

A second theory holds that black holes form separately and later became gravitationally bound. In that case, the spins would be more randomly oriented.

The LIGO observatory near Hanford, Wash., one of two built to detect gravitational waves. The second observatory is in Louisiana. LIGO/Caltech/MIT

LIGO's latest discovery "likely favors the theory that these two black holes formed separately in a dense stellar cluster, sank to the core of the cluster and then paired up rather than being formed together from the collapse of two already paired stars," said Laura Cadonati, a LIGO researcher at the Georgia Institute of Technology.

"This is an important clue in understanding how black holes form," she said. "We have found a new tile to put in the puzzle of understanding the formation mechanism."

Gravitational waves were predicted in 1916 by Einstein's general theory of relativity. The equations showed that massive bodies under acceleration, like binary black holes or the collapsing cores of huge stars in supernova explosions, would radiate gravitational energy in the form of waves distorting the fabric of space.

The waves would spread out in all directions, traveling at or near the speed of light. But detecting them is a major challenge. By the time a wave from an event many light years away reaches Earth, its effects are vastly reduced, becoming hard-to-detect ripples rather than powerful waves.

To detect those ripples, the LIGO observatories were designed to measure changes in distance that are vastly smaller than the width of an atomic nucleus.

"Gravitational waves are distortions in the metric of space, in the medium that we live in," said Michael Landry, director of the LIGO observatory near Hanford. "Normally, we don't think of the nothing of space as having any properties at all, so it's quite counter intuitive that it could expand or contract or vibrate.

"But that's what Einstein's relativity tells us. When a gravitational wave passes, the medium that we live in is distorted, and that causes what looks to us like length changes."

By way of analogy, Landry likened spacetime to the canvas of a painting.

"If I stretch the medium of a painting, I can see the painting get distorted," he said. "It's the medium that's vibrating, that's really what a gravitational wave is, and so we register the passage of those gravitational waves by comparing the length of the two long arms of our L-shaped detector."

How LIGO observatories work

Each LIGO observatory features a pair of 2.5-mile-long vacuum tubes arranged in an L shape in which precisely tuned laser beams flash back and forth between multiple mirrors that effectively increase the distance each beam travels to nearly 1,000 miles. The laser beams then are recombined and directed into a sensor.

If the laser beam in each vacuum tube travels exactly the same distance before it is recombined, the LIGO detectors do not "see" anything. But if gravitational waves pass through, that distance would change very slightly in a very predictable way, affecting the path of the laser beams.

The resulting interference patterns allow scientists to compute the masses involved and, in some cases, how the initial black holes were spinning with respect to their orbital motion.

The LIGO system features two widely separated observing stations to make sure a local vibration is not misinterpreted. A confirmed gravitational wave must be seen by both stations at roughly the same time.

And that's precisely what the LIGO researchers found in the three confirmed cases to date. The first two events happened 1.3 and 1.4 billion light years away respectively. The collision that generated the waves detected in January occurred some 3 billion light years away.

"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," Shoemaker said in a statement.

LIGO's current observing campaign runs through the summer. After that, upgrades are planned to increase the sensitivity of the detectors, possibly bringing less powerful events like neutron star mergers into view. And there's always a chance a nearby supernova or merger might occur, one that would give space a major shake.

"If one of this size were to actually coalesce in the Milky Way, it would make a marvelous signal for us, it would be enormously strong," said Shoemaker. "But the likelihood there's one in our Milky Way that's about to coalesce is very, very low, so that's not something that we're betting on."