Understanding our universe through gravitational wave astronomy

It was a big day for physics: 14 September 2015. Scientists made the first confirmed detection of a gravitational wave – a ripple in space-time theorised by Albert Einstein a century ago, but until that moment unproven.

At the same time, they made the first observation of the collision and merger of a pair of black holes, regions in space with gravitational forces so strong not even light can escape. It was this collision that caused the gravity wave.

The blip of data lasting just a tenth of a second verified the last prediction of Einstein’s theory of general relativity. It creates the ability to use gravity to look further back in time than any current technology allows, opening a new field of science – gravitational astronomy. It offers  the same potential for discovery as did optical and radio astronomy.

The detection came from the first observing run of the Advanced Laser Interferometer Gravitational-Wave Observatory – Advanced LIGO, for short – a pair of highly specialised observatories in Livingston, Louisiana, and Hanford, Washington, in the US. The observatories bounce laser beams from  mirrors  at the end of two intersecting, four-kilometre-long vacuum tunnels. Perceived changes occur in the length of the tunnel cavities when gravity waves disturb the lasers.

Advanced LIGO is now being upgraded to increase its sensitivity. Earlier this year, the team confirmed it had detected a second wave, on December 26 2015.

Dr Eric Thrane and the Monash University team are part of the LIGO Scientific Collaboration (LSC), an international collaboration of more than 900 scientists. Dr Thrane co-chairs one of the four LSC teams that analyse data produced by Advanced LIGO for gravitational wave signals.

How long have you been studying gravitational waves, and what led you to this field of study?

Since 2008. It was a bit of luck and love that led me into gravitational waves.

I had done my PhD at the University of Washington in Seattle in particle astrophysics. My wife was starting her PhD at the University of Minnesota, and when I visited there

I met Vuk Mandic, who had just been brought on to build a gravitational wave group. He convinced me to join his group.

The LSC involves more than 900 scientists. What’s been the professional and scientific impact of working with such a large collaboration?

Because there are so many people involved in the LSC and they’re spread all over the world, it affords you opportunities to work with brilliant people with all kinds of expertise.

Can you explain the ‘false data’ insertion you worked on in the detection of the gravitational waves, and the critical role this played?

One of the ways we can be confident in the detections is that my team at Monash injects fake gravitational wave signals into our detectors, and picks them up as if they were real. Then we compare what we measured with what we put in, and check if the two match.

So when you’ve been working on this for so many years and seen nothing but noise, and all of a sudden you see this beautiful, obvious binary black hole signal, you think it has to be a false injection.

This placed Monash at the centre of some of the first communications on this event, because we had to step up and say, “No, this was not us, this was not a test”.

The next step for Advanced LIGO is increasing its sensitivity. How will Monash researchers be involved?

We’re sending one of our best honours students, Chris Whittle, to the LIGO Hanford observatory. Chris will be embedded with the commissioning team, which is the group of scientists and engineers who are trying to make the detector more sensitive.

Monash is also working on the detector – improving sensitivity, trying to make the instruments give us even more astrophysics and more science during the second observing run.

As the detector improves, we expect that eventually we may be looking at hundreds of gravity wave detections every year. I think that will happen in 2019 or 2020.

THE DETECTION

The gravity wave detected in September 2015 was generated by the collision and merger of two black holes.

• Frequency of signal: 35–250Hz, an audible sound rising from lowest A up to middle C
• Duration of signal: 0.2 seconds
• Distance from Earth: 1.3 billion light years
• Mass of colliding black holes:29 and 36 times that of our sun

What knowledge pathways might last September’s detection open for future research and understanding of universal laws?

So many. Before, there were a lot of things you could work on in gravitational wave astronomy, but they were all conjectural. A lot of people thought binary neutron stars would be LIGO’s first detection, but instead we saw these black holes. So my research and the research of our team at Monash has shifted to think more about big black holes.

One of our projects is looking at the effect of gravitational wave memory. When really big black holes emit gravitational waves, they create a significant stretching of space-time that lingers after the gravitational wave passes.

And actually seeing real gravitational wave events is very different to the hypotheticals that science had to work with prior to September 2015. It generates new and real ideas for astrophysics. It really is inspiring.

What excites you most about your research?

I’m confident we will now detect many more gravitational wave events. With the accumulation of many more events, we can really start to do some interesting science on black holes.

The most exciting thing, of course, would be if we detect something that no-one is even expecting, that no-one has even predicted. I think there is reason to be optimistic that when we look at the sky with a new form of radiation – gravitational waves – we will see something that no-one has even anticipated. This is the most exciting possibility of gravitational wave astronomy.