- Large collisions in space, like those between black holes or neutron stars, create ripples in space-time called gravitational waves. The phenomenon was first predicted by Albert Einstein.
- The Laser Interferometer Gravitational-Wave Observatory(LIGO) recently sensed gravitational waves from a collision of two neutron stars – the second such event ever detected.
- But these neutron stars were more massive than scientists thought possible, and nobody picked up light or radiation from the aftermath of their collision.
- That could indicate a new a new type of collision in deep space, in which neutron stars meet and immediately collapse into a black hole.
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Scientists may have stumbled upon a previously unknown class of massive collision in the universe.
On Monday, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had yet again detected ripples in space-time. They think these particular disturbances in the fabric of the universe – which were observed in April 2019 – came from the collision of two neutron stars, the super-dense remnants of dead stars.
That would make this the second neutron-star collision ever detected, but it was quite different from the earlier one.
After the first collision was detected in 2017, telescopes turned to its location in the sky. Scientists studied the collision’s aftermath in visible light, radio waves, X-rays, and gamma rays. But this time, they didn’t find any signs of the collision other than its ripples in space-time.
That has led astrophysicists to wonder whether they have discovered an entirely new class of cataclysmic event in deep space – one in which neutron stars meet and immediately collapse into a black hole, leaving no trace behind.
Einstein’s predictions led scientists to violent new events in space
Ripples in space-time are called gravitational waves. They usually come from distant collisions between massive objects, like black holes and neutron stars. Albert Einstein first predicted the phenomenon, but he didn’t think gravitational waves would ever be detected. They seemed too weak to pick up on Earth amid all the noise and vibrations.
For 100 years, it seemed Einstein was right.
But in the late 1990s, LIGO’s machines in Washington and Louisiana were built in an attempt to pick up the signals Einstein thought we’d never detect.
Finally, in September 2015, after 13 years of silence, LIGO detected its first gravitational waves: signals from the merger of two black holes some 1.3 billion light-years away. The discovery opened a new field of astronomy and earned a Nobel prize in physics for three researchers who helped conceive of LIGO.
Since then, LIGO and its Italian companion Virgo have identified two other types of collisions. The observatories registered gravitational waves from two neutron stars merging for the first time in October 2017. In August 2019, LIGO and Virgo detected what scientists believe was a black hole swallowing a neutron star.
The waves LIGO picked up in April most likely came from another collision of two neutron stars, about 520 million light-years away. That’s about four times more distant than the 2017 event.
Virgo did not detect the latest gravitational waves (probably because of the distance); only the more sensitive LIGO detector in Louisiana picked them up. Its companion machine in Washington was temporarily offline for maintenance.
As a result, scientists didn’t get much data beyond the neutron stars’ distance and mass. They calculated that the neutron stars were 3.4 times the mass of the sun – far more massive than scientists thought possible.
Something else was weird, too: Scientists didn’t see any gamma rays, the radiation signals that are supposed to arrive just seconds after gravitational waves from a neutron-star merger.
A new class of massive space collision?
Scientists expect neutron stars to collide in high-energy explosions, causing heavy elements to form and emitting bright light and gamma-ray radiation. This explosion is known as “kilonova” because it’s about 1,000 times brighter than a nova (the bright flash emitted by newborn stars).
But the gamma rays were mysteriously missing following the collision, and no telescopes spotted a bright light, either.
“We’re in that uncertain position of not knowing if we didn’t observe a kilonova because there really wasn’t one, or if we simply didn’t observe it because it was too far away,” Martin Hendry, a LIGO scientist and astrophysicist at the University of Glasgow, told Business Insider.
Scientists think a possible explanation could be that the colliding neutron stars were so massive that they collapsed immediately into a black hole, as astrophysicist Ethan Siegel proposed in a Forbes article.
“Perhaps, above a certain mass threshold, higher-mass neutron-star mergers simply interact and go directly to a black hole, swallowing up all of the matter associated with both stars, producing no heavy elements and emitting no further observable signal at all,” Siegel wrote.
If true, that could mean collisions between low-mass neutron stars – which produce kilonovae – have dramatically different results than collisions between massive neutron stars, which might not explode at all.
In that case, the newly observed neutron-star merger would represent an entirely new class of catastrophic collision.
“For a long, long time we had a sample size of zero, and now we’ve got a sample size of two,” Hendry said. “So there’s the tantalising question as to whether they really are two distinct classes or whether it’s just that, as we observe more and more, we’ll kind of fill in a continuum.”
Hundreds of Earths’ worth of gold and platinum
When the 2017 neutron-star collision was confirmed, the discovery solved a long-standing mystery about the origins of most of the universe’s heavy elements.
Scientists discovered that the explosion had forged roughly 50 Earth masses’ worth of silver, 100 Earth masses of gold, and 500 Earth masses of platinum. The gold alone was worth about $US100 octillion at 2017 market price, or $US100,000,000,000,000,000,000,000,000,000 written out.
A bit of nuclear alchemy called the rapid process, or r-process, is responsible for creating these metals.
It goes like this: As neutron stars move toward each other, a tiny bit of their material gets shot into space at incredible speeds. Those neutrons are very hot and crowded, so they smash together while moving outward, forming giant atomic cores. Very big atoms are highly unstable, so they almost immediately break apart and decay into smaller atoms – stuff like platinum, gold, silver, and even iodine.
However, if the neutron stars in the most recent collision collapsed directly into a black hole as some astronomers suspect, there would have been no r-process, and the crash would not have forged gold or platinum, Hendry said.
Astronomers could detect new collisions every day by the mid-2020s
To detect more gravitational-wave events with more precision, researchers are fitting the LIGO and Virgo detectors with a series of technical upgrades. In December 2019, they installed a device that squeezes light, allowing LIGO to make 50% more detections.
“We’re really looking, by the middle of this decade, at having an event every day or so,” Hendry said.
Most of the gravitational waves observed so far have come from collisions between two black holes. But the ability to make more detections could mean that scientists wind up finding dozens of neutron-star mergers each year.
An upcoming Japanese observatory, the Kamioka Gravitational-wave Detector (KAGRA), will also help scientists narrow down the source of gravitational waves to smaller points in the sky.
KAGRA was built underground and will use cryogenically cooled mirrors to help protect sensitive instruments from picking up false signals. The detector is slated to come online by April 30, but scientists will then need to fine-tune its instruments before it can fully join the gravitational-wave hunt later this year.
Another observatory, LIGO India, is expected to join the global network in 2025.
Together, these new and upgraded detection tools could help scientists determine whether there are indeed various classes of catastrophic neutron-star collisions in space.
“What we need to do is observe a whole bunch of neutron-star events,” Hendry said. “It’s too soon to say with only our second neutron-star merger, but once we have a few dozen or so, we can start really looking at the statistics.”
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