NEWS BRIEFING: Gravitational waves unveil cosmic cataclysm that sparks astronomical gold rush

This was the first time that any cosmic event was observed through both light it emitted and the gravitational ripples it caused in the fabric of space-time.

Never before have we seen where in the universe gravitational waves came from; the subsequent avalanche of science was virtually unparalleled in modern astrophysics.

The gravitational wave observations of black-hole collisions by the LIGO and Virgo detectors has been an incredible feat in technology and has accelerated our knowledge of the extreme and dynamic universe. However, on August 17th “colliding neutron stars invited themselves to the gravitational wave party and announced their presence with authority”.

We had not expected to detect such an event so close; as a result it was very loud. To be accompanied by a gamma-ray burst was incredible. The association between this type of short duration gamma-ray burst and neutron star collisions had been predicted for around 30 years. It is now confirmed. Thousands of gamma-ray bursts have been recorded, but this was the closest of this type ever observed. Much is unknown about these bursts and given previous understanding, this event simply should not have happened. 

The beamed fireball of a gamma ray burst can be observed at later times by x-ray, optical and radio telescopes. Astronomers from OzGrav teamed with other astronomers all over the world to work in unison to observe this amazing event – such coordination towards a common goal is a fantastic achievement and heartwarming. Over the next few years such events will become commonplace. Looking ahead, what we can learn about the universe in this new era of coordinated observations of gravitational wave sources is incredibly exciting.

I started working on the first high sensitivity gravitational wave detectors in  the USA in 1973. I expected to spend a year or two detecting Einstein’s waves and then move on to something else.

We pinned our hopes on gravitational waves from neutron stars. This was our holy grail but it eluded us even when gravity waves from black holes had been detected. Forty four years later we have found the holy grail!

It is astonishing that a single very faint signal lasting a minute combined with an even briefer burst of gamma rays and a fading glow of light, can reveal so much: the scale of the universe, the speed of gravitational waves, the mechanism of gamma ray bursts, and the origin of gold.

Neutron stars are the densest stars in the universe. The astronomers have found many of them, alone but also in pairs orbiting each other. As they circle each other they radiate gravitational waves and their orbit shrinks. We knew that eventually many of them must smash together in violent collisions but we had never seen it happen. The astronomers simply did not know where to point their telescopes at the right time.
 
The LIGO and Virgo gravitational wave observatories can’t be pointed to a place on the sky, they just sit there and wait for something 'big' to happen in the Universe. And on 17 August this year something really big happened - a gravitational wave swept into our detectors which was the hallmark signal of two neutron stars colliding. It was our closest source and our strongest signal of our five detections announced so far.
 
We quickly sent details of a patch of sky to astronomy partners all over the world. What followed was an unprecedented avalanche of telescopes and satellites scrambling to scan this patch to pinpoint the source and image it with light, x-rays, radio and gamma rays. The age of multi-messenger astronomy had truly begun.

It is fair to say that this is one of the biggest astronomical discoveries of the century so far. In 2015 we made the first detection of gravitational waves which came from two massive black holes. This meant that only the gravitational waves were detected, but traditional telescopes saw nothing.

This time we managed to catch the collision of two neutron stars — dead stellar remnants that weigh more than our Sun but are just 10 kilometers across. When these neutron stars merged they created a huge explosion which was seen using gamma-ray telescopes two seconds after the collision as seen by the gravitational-wave detectors.

Over the subsequent hours, days and weeks we saw this event across all different types of light, including x-rays, ultraviolet, optical and radio. The amount of physics that is being learned from this one collision is truly immense. This is a watershed moment in astrophysics, and brings us into the era of ‘multimessenger’ astronomy.

On August 17, the LIGO and Virgo gravitational wave detectors recorded two neutron stars, for the first time, merging about 130 million light years away (one light year is about 10 trillion kilometres).  However, this distance is 'very close' in astronomical terms and is essentially in our ‘back yard’. As such, it gave us a great view of the event. Although an event like this was predicted to be detected (eventually) by the very sensitive LIGO/Virgo detectors, no one expected it to occur so soon. Mainly because it's a very difficult detection and would have to occur very close, which would be very rare.  

Gravitational wave detectors can detect black holes and neutron stars merging, but cannot locate them with great accuracy. As such, we don’t know exactly where in the vast universe they occur or, specifically, in which galaxy they occur. Until now, only black hole mergers have been detected and they do not produce light, so we have no way to locate them precisely or study them with our telescopes. Neutron star mergers do produce light and, as such, we can pinpoint where they are and study them in great detail. This was the first time astronomers can accurately locate a gravitational wave event and observe it in detail with telescopes.

About two seconds after the gravitational wave detection, a telescope in space (Fermi) detected a burst of gamma-rays. Astronomers suspected that this burst and the gravitational wave event were connected. For about 50 years, it had been theorised that highly energetic gamma-rays of light would occur when two neutron stars merge. The later confirmation of this connection solved a 50 year-old mystery.
 
Because the light was seen at the same time as the gravitational waves, we can confirm that gravitational waves travel at the speed of light through the universe.
 
Alerts of the gravitational wave event and the gamma-ray burst were sent out to teams of astronomers worldwide that had been preparing years for this. Astronomers worldwide dropped what they were doing to turn their telescopes to this event.  We discovered a bright explosion called a kilonova (named so, because they are about 1000 (kilo) times brighter than a nova). Such explosions have been theorised to accompany neutron star mergers but have never been observed. With these observations, we witnessed a kilonova for the first time, confirming the theory, and we now understand how they work.  
 
Astronomers stayed with the event, because it was expected to fade away forever, several days later.  The event was observed with every type of telescope at every wavelength of light. It was identified and located in a galaxy named NGC 4993 that is located near our Milky Way galaxy and in the Southern Hemisphere of our night sky. We observed an explosion, a kilonova, unfold in front of our eyes, more beautifully than ever imagined. Kilonovae produce the heaviest elements around us today. The event enabled us to see where, and how, exotic elements such as gold, platinum, and uranium are formed.
 
Knowing the distance and the strength of the event, we now have another means to measure the expansion rate of the universe and its age.
 
Finally, the location of the event was a bit troublesome. It was in a direction in the sky near the Sun. This made things difficult for some telescopes to observe it, as they could only get snippets of time (about an hour or less) each night to catch it before it set with the Sun. As a result, a coordinated combination of telescopes worldwide and in space was needed to observe the event in a series of short segments as we raced the sunset around the world as the Earth turned.

Before this first-ever detection of a binary neutron-star merger, electromagnetic telescopes could see the gamma rays emitted by a merger but didn't know what caused it.

When the LIGO-Virgo collaboration made the landmark detection of gravitational waves, we could identify the source of the gravitational waves but only knew approximately the location. Now we know both what happened and where it happened – multi-messenger gravitational astronomy has been born.

We’ve now seen the first event using multi-messenger gravitational astronomy but with improved sensitivity we can observe many more of these cosmic events.

With more observations, we will be able to build a clear picture of the evolution of our stars and galaxies and the birth and development of the universe. Here at the University of Adelaide we working with LIGO and OzGrav colleagues to improve the sensitivity of the current detectors and developing the technology for the next generation of detectors.

Here at CSIRO we are excited to be part of this discovery where we have turned some of the world’s best radio telescopes and joined them together with the gravity wave detectors and other telescopes around the world to try and understand the nature of gravity waves coming from across the Universe.
 
The Australia Telescope Compact Array, operated by CSIRO, just had its 25th birthday but we are keeping it up to date so that it is ready for these kinds of discoveries – we can turn it with just a few hours to the most exciting things happening in the Universe.


 
We have a program where astronomers like Tara Murphy, at the University of Sydney, can come in with just a couple of hours’ notice and completely reschedule the telescope to follow up the latest exciting scientific discoveries while they are going on.
 
The current program with the Australia Telescope Compact Array has been going now for 40 hours since the gravity waves were first discovered. It’s an ongoing program in collaboration with telescopes all around the world.
 
Congratulations to Tara Murphy and the University of Sydney team for leading this Australian effort to follow up gravity waves, and understand what they are coming from, using radio telescopes here in Australia as part of an international effort.

[After seeing the report from LIGO] We immediately rang our team in Australia and told them to get onto the CSIRO telescope as soon as possible, then started planning our observations. We were lucky in a sense in that it was perfect timing, but you have to be at the top of your game to play in this space. It is intense, time-critical science.

This international discovery, with Sydney playing an integral role, demonstrates that the best science and modern innovation is intrinsically a collaborative effort.

What a terrific way to confirm that Einstein’s theory of relativity was correct, gain insights into massive bodies like black holes and, with this knowledge, start to re-think our understanding of the universe.

This is a landmark discovery for astrophysics. It uncovers a whole new way of measuring the universe.  Even just this one event where we see the explosion that accompanied these gravitational waves, already confirms many predictions — such as how the heavy elements were created, what happens when neutron stars collide, and how fast is the universe expanding.   

This is just the beginning, it’s as though we’ve discovered a new sense. We can feel the universe now, as well as seeing it. The biggest discoveries are still to come.