Gravitational waves and Fast Radio Bursts: from thought experiment to potential treasure trove

A lot of undergraduate physics is taught using thought experiments – for example, where Einstein imagines two trains coming together and somebody is shining a flashlight between them.

It’s all very theoretical and pure. I was never too great at the tensor calculus behind general relativity, but in my honours year at university I realised you could actually test general relativity by using neutron stars, and I’ve kind of been doing it ever since.

I didn’t have any trouble wrapping my head around the mathematics of two neutron stars orbiting each other in warped space-time, each with masses of half a million Earth masses, and going around each other every eight hours. This was something I felt quite passionate about.

What I now mainly do is radio astronomy in all its different guises, but I became a kind-of expert at building the supercomputing instrumentation to study neutron stars. And that led to quite a few breakthrough discoveries over the years.

Back in 2015, the gravitational wave community came to me and asked if I would lead a bid for a Centre of Excellence. At the time, there were groups working on gravitational wave detection in Perth, Canberra and Adelaide, but gravitational waves hadn’t ever been detected.

Between my agreeing to lead the bid and having the final interview for OzGrav (the ARC Centre of Excellence for Gravitational Wave Discovery), the discovery of gravitational waves was announced! So I joined the LIGO (Laser Interferometer Gravitational-wave Observatory) community and have been leading the national research efforts in that area since 2017.

Over the next decade we think we can use gravitational waves to help measure the distances to the mergers of neutron stars.

It’s the amplitude of the gravitational wave that tells you how far away the source is, so you get a very precise distance for each merger. And you can then compare that to the redshift (and hence velocity) of the source to work out the age of the universe. It’s a very unique and model-independent way that gravitational waves can be of use.

Our detectors keep getting better and better. When Advanced LIGO was first turned on, it could detect neutron star mergers out to just 60 megaparsecs (1 megaparsec = 3.26 million light years).

In the next run, we’re hoping to get out to 170 megaparsecs (Mpc), and it’s a distance cubed thing for the event rate. Over the next 10 years, we’ll be out to like 350Mpc. This is like five or six times the original run, so we expect 125 to 200 times the event rate.

So we are hoping to detect well over 1,000 gravitational wave triggers – this is an area of science that in the first 100 years detected zero.

We’ll learn about the curvature of spacetime, when black holes merge to become one big black hole. We can even determine how they spin! We’re hoping to get lots of neutron star mergers too and determine the age of the universe.

“Over the next decade we think we can use gravitational waves to help measure the distances to the mergers of neutron stars.”

This is nicely complemented by our work on fast radio bursts (FRBs). FRBs give us a very precise measurement of how many atoms there are in the universe. So we’re going to combine the big cosmological questions. What is the age of the universe, and how much (normal) matter is in it? Essentially, how big is it? How old is it? What’s it made of?

Unfortunately, we haven’t yet seen what dark matter comprises, so we can’t contribute much to that (yet). But the other big thing that’s happening is that the Square Kilometre Array (SKA) will be commissioned during the course of this next decade.

A third of the mid-frequency component of SKA already exists in South Africa and is returning fantastic data. I’m leading the radio pulsar project on that facility. We’re hoping to detect gravitational waves from supermassive black holes – we are effectively taking the pulse of the universe by looking at how stably neutron stars appear to spin. So that’s also a fun thing.

We discovered the first fast radio burst by accident with one of my former students, and they’re now a very large field of astrophysics on their own. There’s a big race on to build a facility that will isolate the most FRBs to their host galaxies. At the moment, Australia is winning the race in pinpointing the FRBs, but the Canadians are winning the race to find the largest number. So there’s an interesting competition there to nail down the electron content (and hence the amount of normal matter in the universe).

We’re also planning what the next type of gravitational wave detector will look like. There’s something called the Einstein Telescope that the Europeans have designed, which will increase the interferometer arm to 10km in length, with many detectors – that’s a sort of multi-billion-dollar project that they’re hoping to finish in about a decade. We’re talking to our long-term collaborators in America about a sort of a super LIGO, which is about 10 times the length. This is a project called Cosmic Explorer, but we have to work out whether or not you can actually fit such a thing on the Earth at a reasonable price.

The Earth actually bends on 40km lengths, so you’d kind of need a valley that exactly compensates for the Earth’s curvature that you can lay your laser beam vacuum tubes on.

“At the moment, Australia is winning the race in pinpointing the FRBs, but the Canadians are winning the race to find the largest number.”

We think that with the extra arm lengths and by also ramping up the power (think of a James Bond evil-villain-type laser), we could potentially be able to see virtually every black hole merger in the universe. And that will give us a wildly uninterrupted view of how the most massive stars merge at the end of their lives when they become black holes.

It will probably have a range between 10 and 50 times as big as the current detectors, and then you could basically see any event where black holes merged in the universe. At the moment we get about one a week when the detectors are operating, but it’s possible that these new detectors could detect tens of thousands of events a year in a very unbiased way. It could be like listening to popcorn pop, except it would be the Universe!

Normal light gets easily blocked by dust, and the atmosphere makes stars twinkle – both of these effects bias our view of the cosmos. But almost nothing stops the propagation of gravitational waves.

They’re a beautiful way to communicate information about the most relativistic things in the universe, and thanks to triumphs of engineering, theory and computation, OzGrav gets to explore Einstein’s universe in new and informative ways!