A global team of thousands of scientists has tracked a burst of gravitational waves to its source for the first time.
After detecting these tiny ripples in space-time for only the fifth time in history, they used telescopes to pinpoint and directly observe the source: a fiery collision between two dead stars.
Ending weeks of speculation, researchers confirmed that on August 17 they detected gravitational waves from the explosive merger of two neutron stars, each about half the mass of the Sun, 130 million light years from Earth.
The unprecedented discovery was made by the 1,500-strong Advanced LIGO and Virgo teams in the US and Italy, and followed up by astronomers using space and land-based telescopes all over the world, including Australia.
It has been hailed as the dawn of a new era in astronomy.
LIGO executive director David Reitze called the phenomenon detected "the most spectacular fireworks in the universe".
"We've moved from the era of 'silent movies' to 'talking movies'," he said.
"In this case, the 'audio soundtrack' comes from the chirp of the neutron stars as they are inspiralling — as they are orbiting together and colliding — and the 'video' is basically the light that we see after the collision."
France Cordova, director of the US National Science Foundation, said the discovery "realises a long-standing goal many of us have had — that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories".
Professor Matthew Bailes, director of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) said "multi-messenger astronomy" had finally arrived, thanks to global cooperation.
"Not only are we seeing the universe like we normally do, but we're feeling it with gravitational waves," he said.
"This is a mega-science project that needs a global community in order to make it happen."
The discovery and its implications are outlined in a barrage of scientific papers, published simultaneously in various journals including Physical Review Letters, Nature and Astrophysical Journal Letters.
Among the ramifications are a confirmation of Einstein's theories about the speed of light, a more accurate estimate of the size of the universe, an identification of the source of mysterious gamma ray bursts, and the first recorded glimpse of the power of these stellar explosions to produce rare, heavy elements like gold.
Searching for a needle in a cosmic haystack
In 2015, the first detection of gravitational waves was made by the Advanced LIGO team, using their two detectors 3,000 kilometres apart in the US. Those ripples came from the collision of two black holes, each one about 30 times the mass of our Sun.
Since then, three more detections have emerged from the death spiral of black hole pairs — the latest in August this year with the addition of the Advanced Virgo observatory in Italy.
Picking up the ripples from merging neutron stars has been much more difficult, because these events are much smaller.
Neutron stars are the incredibly dense, collapsed cores of giant stars that have exploded in supernovas; they can be heavier than the Sun and just 20 kilometres across.
On 17 August this year, the LIGO detectors picked up the first tell-tale signal or "chirp" from a gravitational wave, which lasted longer than the signals received in the previous detections.
Nearly two seconds later the FERMI space telescope detected a short gamma-ray burst — an explosive high-energy flash.
Then the race was on. More than 70 telescopes scanned the broad patch of sky identified by the gravitational wave detectors, searching for light and radio waves to pinpoint and confirm the source.
Light from the stellar collision was first detected 11 hours later by the Swope telescope in Chile.
Over subsequent hours and days, astronomers around the world watched the fireball fade. They detected signals right across the electromagnetic spectrum — visible light, infrared, X-ray and radio waves — confirming that the source was a neutron star collision.
They also traced the event to the outskirts of a galaxy called NGC 4993 in the constellation of Hydra.
Neutron stars, the universe and everything
Scientists from several Australian universities were involved in the discovery, either as part of the LIGO project or in teams that scrambled to spot the collision using telescopes.
Professor Susan Scott from the Australian National University said the discovery revealed new insights into the mysterious nature of neutron stars.
"The thing about neutron stars is that they are not very well understood as yet," she said.
"The combination of observing this event with gravitational waves, plus other electromagnetic signals that we've got on this event, has enabled us to probe the nature of neutron stars much, much further."
For example, Professor Scott explained, most of the precious metals in the universe such as gold, platinum and uranium are thought to be produced by the explosion that happens when these stars rip each other apart — and analysis of light from this explosion indicates this elemental production is at work.
Professor David Blair from the University of Western Australia said the discovery would change our understanding of the universe.
"It's amazing that this faint sound, this incredibly faint sound, then a momentary burst of gamma rays, and then a faint decaying light from this explosion has told us so much," he said.
According to Professor Blair, the observations show that gravitational waves, which are ripples in the fabric of space, and electromagnetic waves, which are ripples of energy travelling through space, travel at the same speed.
"This is the most amazing vindication of all of Einstein's theories, which had behind them this idea that the speed of light is universal," he said.
Professor Blair said the discovery also proved that neutron star collisions were the source of mysterious gamma ray bursts.
"It was just a theory. Now we know for sure what makes those gamma ray bursts.
"There's an amazing quantity of astrophysics, just from these three teeny weeny little measurements."
Scrambling for a signal
When the LIGO message went out on August 17, it was about midnight eastern Australian time.
Dr Christian Wolf from the Australian National University was among the astronomers suddenly racing to see if they could spot the source of the waves using traditional telescopes.
He ran a team which observed the fireball using the Skymapper telescope at Siding Springs near Coonabarabran — one of three Australian telescopes which spotted the event and monitored its afterglow as it faded.
"Seeing it on my own screen was a special moment," said Dr Wolf.
The colour of the light indicated the fireball was about 6,000 degrees Celsius.
"When I saw the colours I realised this was unlike anything we'd seen before."
The fading fireball was also detected at the Zadko Telescope just outside Perth, by a team led by Associate Professor David Coward from the University of Western Australia.
"We knew on day one, when the event happened, that this was something big," he said.
But the team could not see anything when the first images came in, because light from the surrounding galaxy was drowning the fireball.
"Our colleagues did some very clever image analysis, removed the galaxy and then we had this beautiful afterglow.
"Initially we were in devastation that we didn't see anything, and then absolute ecstasy when we knew that this was a one-off. This is like gold for scientists."
Meanwhile, Associate Professor Tara Murphy and her team at Sydney University were the first in the world to confirm radio waves coming from the same location, using CSIRO's Australian Compact Array Telescope near Narrabri.
"We're still monitoring this source so we can understand the physics going on in this merger," she said.
"This is probably unprecedented in how much new science has been done in such a short period of time over the past couple of months."