A lingering feeling that something wasn’t right sparked the latest research project led by Macquarie University physicist Professor Daniel Terno.

“I look at something and the puzzle doesn’t quite add up,” says Macquarie University physicist Professor Daniel Terno.

Professor Terno and his team, with support from the Julian Schwinger Foundation, revisited a question many would assume has long been settled: When astronomers and physicists refer to a ‘black hole’, what is really out there?

Black holes are usually described as regions of spacetime, where gravity is so intense that nothing – not even light – can escape. But Professor Terno says the real scientific question isn’t whether black holes exist, but whether human observations can tell us for certain that an object has an event horizon – the defining boundary of a black hole.

With a new wave of technology promising the sharpest, most detailed look we have ever had at the most extreme objects in the universe, it seems like a good time to take a step back and re-ask the big questions.

Rather than debating whether black holes truly form, Professor Terno’s research asks a different question: If light appears to become trapped, what does that tell us about the space surrounding these extreme objects?

Many theoretical models describe ultra-compact objects that could trap light in almost the same way as a black hole, while lacking an event horizon altogether.

“Observations don’t always tell us uniquely what these objects are,” he says. “Our research explores what physical conditions are required for an object to trap light the way a black hole does and whether nature might behave differently from our idealised mathematical models.”

In other words, does the universe make convincing black hole look-alikes?

The key problem, says Professor Terno, is that astronomy is always an imperfect kind of watching. We observe from far away, through noise, over limited time.

“If we only ever observe the Universe over finite times, could some objects look almost identical to black holes while behaving differently in ways that future telescopes might detect?” Terno posits.

So far, observations from projects such as the Event Horizon Telescope and gravitational-wave observatories including LIGO, Virgo and KAGRA are consistent with both the traditional picture of black holes and many alternative ‘black hole mimicker’ models. The hope is that the next generation of telescopes and detectors will be sensitive enough to reveal the subtle differences.

“The mathematical models suggest there could be subtle differences in how these objects behave,” he says. “The challenge is translating those differences into something astronomers can actually observe.”

That is where future telescope designers come in.

“If I had one request,” Professor Terno says, “it would be to observe for longer. We want to see the tails of the signal.”

The ‘tails’ are the faint signals that linger after the dramatic peak of an event such as two massive objects colliding. While the strongest part of the signal has been studied extensively, these weaker late-time signals could reveal subtle clues about whether the object behaves exactly like a black hole or something slightly different.

Support from the Julian Schwinger Foundation has made it possible for Terno and his team to pursue exactly that kind of curiosity-driven work. The funding, along with a visiting position at the Perimeter Institute, has allowed him to continue tackling some of the biggest unanswered questions in theoretical physics.

“The important thing is being willing to ask whether we’ve been asking the right question,” Professor Terno says. “That’s what keeps science moving forward.”