Black holes spinning faster and faster

Many giant black holes in the centre of galaxies are spinning faster than at any time in the history of the universe, and may have been set in motion comparatively recently, new research shows.

Dr Alejo Martinez-Sansigre of the University of Portsmouth and Professor Steve Rawlings of the University of Oxford used radio, optical and X-ray data to test their theoretical models of spinning black holes, and found the models stood up well for supermassive black holes with twin jets.

Using the radio observations, the two astronomers were able to sample the population of black holes, deducing the spread of the power of the twin jets. By estimating how the black holes acquire material, they could then work out how quickly they might be spinning.

The observations also give information on how the spins of supermassive black holes have evolved. In the distant past, say the researchers, practically all spun very slowly, whereas nowadays some have very high spins. So on average, they’re spinning faster than ever before.

It’s the first time that the evolution of the spin of the supermassive black holes has been closely described, and suggests that those that grow by swallowing matter will barely spin, while those that merge with other black holes will be left spinning rapidly.

“The spin of black holes can tell you a lot about how they formed. Our results suggest that in recent times a large fraction of the most massive black holes have somehow spun up,” says Dr Martinez-Sansigre.

“A likely explanation is that they have merged with other black holes of similar mass, which is a truly spectacular event, and the end product of this merger is a faster -spinning black hole.”

Later this decade, the team hopes to test the theory that these supermassive black holes have been set spinning relatively recently.

“With so many collisions, we expect there to be a cosmic background of gravitational waves, something that will change the timing of the pulses of radio waves that we detect from the remnants of massive stars known as pulsars,” says Professor Rawlings.

“If we are right, this timing change should be picked up by the Square Kilometre Array, the giant radio observatory due to start operating in 2019.”