The cosmic collision that blew our minds
Scientists around the world came together to make what might be the discovery of the year — mysterious cosmic ripples from the spectacular collision of two neutron stars, 130 million years ago. Science reporter Jamie Morton explains.
t began as a cosmic collision, sprinkling gold across the heavens, but culminated in a chirp in an underground lab. It was 12.40am on August 18 in New Zealand when an array of sophisticated detectors on the other side of the planet picked up something weird.
For roughly 100 seconds, gravitational signal GW170817 registered at the United States- based Laser Interferometer Gravitational- Wave Observatory ( Ligo).
At the same time, Nasa’s Fermi space telescope picked up an odd burst of gamma rays, a form of radiation capable of travelling across vast distances of the universe.
When Italy’s gravitational wave observatory, Virgo, also detected a small signal, scientists were able to put the pieces together to narrow down where in the sky these cosmic cues had whizzed in from.
Armed with the co- ordinates, a handful of observatories joined the hunt.
When optical telescopes first spotted the source, it appeared as a fresh point of light amid the gloom.
It represented something much more amazing — the collision of a pair of the smallest, densest stars known to science.
Neutron stars are formed when massive stars explode in supernovas. Though they measure only 20km in diameter, just a teaspoon of their material packs a mass of about a billion tonnes.
About 130 million years ago, when dinosaurs roamed our Earth, the ill- fated stars were in the final twirls of a space waltz, gathering speed as the mere 300km between them grew ever more narrow.
Their death spiral stretched and distorted the surrounding space- time, giving off energy in the form of powerful gravitational waves. At the moment of collision, the bulk of the two neutron stars merged into one ultra- dense object, erupting in a fireball of gamma rays.
The first telescope to report the fireball’s colour was Australia’s 1.35m, wide- angle Skymapper, indicating the inferno raged at about 6000C, roughly the surface temperature of the sun. Other forms of light — X- ray, ultraviolet, optical, infrared, and radio waves — trickled in over the next few weeks.
Ultimately, about 70 observatories on the ground and in space witnessed the stunning merger at their representative wavelengths.
Perhaps the most dazzling observation wasn’t the fireball, but what came in its wake — a kilonova.
Only theorised until that point, this phenomenon involved material left over from a neutron star collision being blown out far out into space.
Remarkably, the smash- up created heavy elements such as lead and gold, and littered them throughout the universe.
It was even likely hundreds of thousands of Earth- equivalent masses of gold and other elements were instantly produced in the fireworks display, University of Auckland astrophysicist Dr J. J. Eldridge said.
“If the rate of neutron stars mergers is as high as we now think, these dying stars are now the source of most of these elements in the universe,” he said. “We’re all made of stardust, but gold, silver and platinum are made of neutron stardust.”
In all, the climax unfolded in mere Earth seconds, confirming century- old theories, ushering in a new field of science, and blowing our minds.
“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe,” said Frances Cordova, director of Ligo’s funder, the US National Science Foundation.
“This discovery realises a long- standing goal many of us have had, to simultaneously observe rare cosmic events using traditional as well as gravitational- wave observatories.”
Shared with the world this week, the event was a shining example of how gravitational waves could help scientists unlock some of the greatest secrets of our universe.
The waves are essentially ripples, comparable to sound, that travel through space at the speed of light.
Since the speed of light is about 300,000 km per second, a light year i s about 10 trillion kilometres — and it took 130 million of these years before we were able to see the collision.
the year Anzac troops were fighting at Gallipoli, Albert Einstein theorised that space- time, which makes up our universe, is a four- dimensional fabric that can be pushed or pulled as objects move through it.
Consider dropping a bowling ball on a trampoline and watching its contortions, or the waves that spread through the water as you run your hand through a still pool.
Though we had long assumed gravitational waves were out there, it wasn’t until Ligo detectors recently recorded them that we could truly say Einstein was right.
The feat predictably earned this year’s Nobel Prize for Physics.
Having captured these waves, we can now see how the universe works in an entirely new way, just as we can observe the surface of a pond stirred by ripples.
Intercepting them hadn’t been an easy task.
Ligo and Virgo’s vast facilities consist of two long tunnels in an L shape, at the joint of which a laser beam is split in two. When laser light is sent down the length of each tunnel, it’s split and reflected back in the direction it came from by a suspended mirror.
Normally, the laser light in each tunnel should return to the location where the beams were split at precisely the same time.
But, in the rare instance that gravitational waves zoom through, other things happen.
These alter each laser beam’s arrival time, creating an almost imperceptible change in the observatory’s output signal.
The first waves detected by Ligo, on September 14, 2015, were born from the violent merging of two black holes in deep space.
In that case, the resulting chirp that popped up in Ligo’s real- time data analysis software lasted a fraction of a second.
The longer length of August’s chirp, registering at about the same range as common musical instruments, indicated the source as objects that were much less massive than the black holes seen to date. Instead, what triggered them were estimated to be in a range of around 1.1 to 1.6 times the mass of the sun — in the mass range of neutron stars.
“It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see — and promising the world we would see,” Ligo spokesman David Shoemaker said.
Professor Joerg Frauendiener, chair of applied mathematics at the University of Otago, said the discovery was remarkable on several levels.
It was a clear indication gravitational wave astronomy was on track to become one of the most important sources of information about our universe. It had also confirmed long- held suspicions that gamma- ray bursts were linked to neutron star collisions.
Theorists had predicted that when these stars collided, they gave off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum.
The gamma- ray burst detected by Fermi, and soon after confirmed by the European Space Agency’s gamma- ray observatory, Integral, clocked what’s called a short gammaray burst.
The new observations confirmed that at least some short gamma- ray bursts were generated by the merging of neutron stars.
“It also enforces the expectation of many astrophysicists that seeing such events in different channels will help them to understand the inner structure of neutron stars which, so far, remains largely unknown,” Frauendiener said.
The collision ultimately provided the loudest, closest and most precisely located gravitational wave signal yet received by humans.
Background analysis had showed that an event of that strength happened less than once in 80,000 years by random coincidence, so scientists recognised it immediately as a confident detection and a remarkably nearby source.
Professor Matt Visser, of the Victoria University’s School of Mathematics and Statistics, said the event had given us an independent way of checking and verifying the accelerated expansion of the universe.
“There are also many other implications — from the cooking of heavy elements in the subsequent kilonova to direct precision tests on the speed of gravitational waves.”
We were now venturing into a new era of discovery.
“In 2016, the first direct detection of gravitational waves was announced, first predicted over 100 years ago by Einstein, opening a new window to the universe: the gravitational wave window,” said Dr Simone Scaringi, of the University of Canterbury’s Department of Physics and Astronomy.
“Today, not only have we detected the ripples in space- time caused by the merging of compact objects, but we have been able to precisely pinpoint their location in the sky, and, as a consequence, study the associated counterpart throughout the electromagnetic spectrum.”
It pointed us down a new path to understanding some of the most enduring questions in astrophysics.
“For example, where do heavy elements such as gold and silver come from?
“And, under what conditions do massive stars form in the universe, in what quantities, and how many of these exist in binaries?
“We really are at a turning point in astrophysics, and we happen to be lucky enough to be witnessing the discoveries that will revolutionise our understanding of the universe as they happen.”
Yet, though some mysteries appears to be solved, new ones have emerged.
The observed short gamma- ray burst was one of the closest to Earth seen so far, yet it was surprisingly weak for its distance.
Scientists are now beginning to propose models for why this might be — promising beautiful new insights in years to come.
Simone Scaringi University of Canterbury