Black holes outside our galaxy may be fundamentally different from those found in the Milky Way, scientists have found.

In our galaxy, scientists have been able to detect black holes from their electromagnetic emissions. Outside our galaxy, gravitational waves carry information about black holes that can tell us about their nature and origins.

Gravitational waves are incredibly subtle perturbations in spacetime. It takes remarkably sensitive instruments to be able to detect them, as the 'ripples' caused by massive objects such as black holes are absolutely tiny – thousands of times smaller than an atomic nucleus. They have only been detected a handful of times by the LIGO gravitational wave detector.

These waves have revealed that black holes outside our galaxy may be spinning more slowly, or go through a tumbling process as they form, according to a study in the journal Nature. This tumbling would mean that their direction of spin is no longer aligned with their orbit.

If the latter option is true, it suggests the black holes form in a particularly dense environment, probably within star clusters. This would make their formation considerably more dramatic and dynamic, the researchers say.

"By presenting these two explanations for the observed behaviour, and ruling out other scenarios, we are providing those who study and try to explain the formation of black holes a target to hit. In our field, knowing the question to ask is almost as important as getting the answer itself," study author Will Far of the University of Birmingham said in a statement.

A third option is that the black holes are both spinning more slowly and also in a tumbled orientation. Further studies will be necessary to determine which explanation is correct.

Ilya Mandel, also a study author from Birmingham, added: "We will know which explanation is right within the next few years. This is something that has only been made possible by the LIGO detections of gravitational waves in the last couple of years. This field is in its infancy; I'm confident that in the near future we will look back on these first few detections and rudimentary models with nostalgia and a much better understanding of how these exotic binary systems form."

gravitational waves
Gravitational waves are distortions, or 'ripples', in the fabric of space-time. Nasa