Singing binaries: listening to the chirps of black holes

A couple of weeks before he was due to deliver his Inaugural Lecture, Professor Ilya Mandel received an email from the organisers expressing slight concern that so few people had signed up for the event. They needn’t have worried: the lecture theatre was packed to the rafters, with extra seating needed and some of the audience having to sit on the aisle steps.

‘I hope the audience had fun,’ Ilya, Professor of Theoretical Astrophysics, says modestly. ‘There was a wide range of people there, from members of the public to friends from other fields of science. My mother – who’s an economist – and my six-year-old son came along and both of them enjoyed it.’

The reasons the event drew such a crowd – about 150 people – were both Ilya’s engaging manner and the subject matter itself. Entitled ‘Singing binaries: listening to the chirps of black holes’, the November 9 lecture covered the latest major discovery to shed new light on the origins of the Universe – the detection of gravitational waves.

‘The discovery of gravitational waves is still new, so it’s big news in people’s minds. And I imagine some of the more senior people who contributed to this revolutionary discovery will be the next winners of the Nobel Prize in Physics. I’m hoping they will include my PhD supervisor, (renowned American theoretical physicist) Kip Thorne.’

Ilya’s address was part of a series of Inaugural Lectures run by the College of Engineering and Physical Sciences to showcase its leading scientists who are pushing the boundaries in their disciplines. These events, which are open to the public and free of charge, mark an academic’s promotion to Professor (Ilya was appointed earlier this year), but also provide a rare opportunity to hear first-hand about their research.

And Ilya’s research is as cutting-edge and hot-off-the-press as it gets. It is only a matter of months since news broke of the detection, by instruments of the Laser Interferometer Gravitational-wave Observatory (LIGO), of a tiny fluctuation in space-time emanating from a collision of two black holes more than one-and-a-half billion light years from Earth. As Ilya describes it: ‘This tiny signal was the echo of a very loud song, sung by a pair of black holes merging more than a billion years ago during the last fraction of a second of their lives.’

The discovery heralded the conclusion of a decades-long search for one of the most difficult-to-test predictions of general relativity, Einstein’s theory of gravity.

‘At the same time, this black-hole song was the first note of a beautiful symphony to reach us through a newly opened window on the Universe,’ describes Ilya, who since coming to Birmingham in 2011 has established himself as one of the leaders in the emerging field of gravitational-wave astrophysics. ‘Through this window of gravitational-wave astronomy, we are already beginning to probe the secrets of strong-field gravity. In the next few years, we anticipate hearing many more songs coming from the mergers of compact remnants of massive stars: neutron stars and black holes.’

Ilya compares this work to that of palaeontologists who examine the skeletons of dinosaurs to piece together what the creatures looked like.

‘The question is: can we solve the inverse problem and discover from the skeletal remnants how those stars lived when they were still active stars in a binary system? This is obviously challenging because we’re seeing only the end process. The answer is that we can’t evolve individual systems backwards; but if you have a whole population of binary merger events, then you can get some information, at least. If you combine that with other kinds of information and observations – gamma ray bursts, supernova, galactic radio pulsars and so on – then you get as full a picture as possible.’

In order to do this, Ilya and his team have launched a project called COMPAS (Compact Object Mergers: Population Astrophysics and Statistics) to make computational-analysis models of what they think populations of merging black holes might look like.

‘The goal is to build different models and test them against sets of assumptions. The idea is that although we can’t solve the problem backwards, we can model it forwards. The models are expensive, which means we can’t build one for every set of assumptions, so we will have to build an emulator.’

There are many challenges, such as the difficulty of making meaningful comparisons with limited data. Nevertheless, Ilya is optimistic.

‘We’ve already had our first COMPAS paper accepted by Nature Communications, so I think gradual progress is being made towards getting a true model of binary evolution,’ says Ilya. ‘It’s a long process, requiring lots of collaborations. There are also lots of competitors – we’re not the only people trying to do this, although I think we have some tools and advantages they don’t have.’

One advantage is that Ilya has, as he puts it, ‘the scientist’s version of attention deficit disorder! Every few years I get a bit bored with what I’m doing and try something quite different. The advantage for a process like COMPAS is that it’s a synthesis of lots of different approaches and tools – pulling together stellar evolution, general relativity, astrostatistics and gravitational waves. Very few people have worked on all these areas.’

Born in Russia, Ilya moved with his parents to the US when he was 13. He gained his first two degrees, a BSc in Physics and an MSc in Computer Science, from Stanford, before moving to the California Institute of Technology to study for a PhD in Physics, with a specialisation in gravitational-wave astronomy.

‘I knew I wanted to do some kind of physics, but it was a series of lucky steps that led me into this field,’ he recalls. ‘At graduate school I worked with Kip Thorne, mostly on modern general relativity, but it was fairly abstract, technical stuff. Then I went to a summer school where I heard a series of lectures given by theoretical astrophysicist Prof Cole Miller, who demonstrated the “back of the envelope” approach (an approach to solving physics problems that focuses on the key scaling relations rather than precise number-crunching) – and I was fascinated. So I looked at how I could combine general relativity with astrophysics.’

Ilya’s interest extends to all aspects of theoretical astrophysics, but with a particular focus on strong gravitational fields, compact objects and gravitational-wave astronomy. Already his work has helped to widen the new window that LIGO has opened on to the Universe: it seems highly likely his research will make that window bigger still in the years to come.
‘Of course, as we begin to understand new things, we discover there are even more things we don’t know,’ he concedes. ‘However, we’re certainly having a lot of fun doing this. I enjoy my life as a scientist – I really do.’

• The University’s Astrophysics and Space Research Group holds regular, free Astronomy in the City events. These include stargazing with portable telescopes, Observatory visits to use its half-metre telescope and talks from University experts on cutting-edge research. The next events are on January 25 and March 8, 2017. Admission is by ticket only; these can be booked via Eventbrite.
• Prof Cole Miller will be a Distinguished Visiting Fellow at the University in January 2017. He will give a public lecture on ‘The Mysteries of Black Holes’ on January 12, jointly hosted by the AstroSoc, the Institute of Advanced Studies, and the Institute of Gravitational Wave Astronomy. See Eventbrite for details and to reserve a seat.