Proof of black hole ‘plunging regions’

The black hole pulls material from a companion star towards it, forming a disc that rotates around the black hole before falling into it. Credit: NASA/CXC/M. Weiss
The black hole pulls material from a companion star towards it, forming a disc that rotates around the black hole before falling into it. Credit: NASA/CXC/M. Weiss
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Einstein has been proved correct with a key prediction about black holes by an international team led by researchers from the Department of Physics at Oxford. Using X-ray data to test a key prediction of Einstein’s theory of gravity, their study, published [in May] in the Monthly Notices of the Royal Astronomical Society, gives the first observational proof that a ‘plunging-region’ around black holes not only exists, but exerts some of the strongest gravitational forces yet identified in the galaxy.

The new findings are part of wide-ranging investigations into outstanding mysteries around black holes by astrophysicists at Oxford’s Department of Physics. The study, Continuum emission from within the plunging region of black hole discs, focuses on smaller black holes relatively close to Earth, using X-ray data gathered from NASA’s space-based NuSTAR and NICER telescopes. Later this year, a second Oxford team hopes to move closer to filming first movies of larger, more distant black holes as part of multi-million European initiative.

Unlike in Newton’s theory in gravity, Einstein’s theory states that sufficiently close to a black hole it is impossible for particles to safely follow circular orbits, instead they rapidly plunge toward the black hole at close to the speed of light – giving the plunging region its name. The Oxford study focused on this region in depth for the first-time, using X-ray data to gain better understanding of the force generated by black holes.

‘This is the first look at how plasma, peeled from the outer edge of a star, undergoes its final fall into the centre of a black hole, a process happening in a system around 10,000 light years away,’ said Dr Andrew Mummery, from the Department of Physics, who led the study. ‘What is really exciting is that there are many black holes in the galaxy, and we now have a powerful new technique for using them to study the strongest known gravitational fields.

‘Einstein’s theory predicted that this final plunge would exist, but this is the first time we have been able to demonstrate it happening. Think of it like a river turning into a waterfall – hitherto, we have been looking at the river. This is our first sight of the waterfall. We believe this represents an exciting new development in the study of black holes, allowing us to investigate this final area around them. Only then can we fully understand the gravitational force. This final plunge of plasma happens at the very edge of a black hole and shows matter responding to gravity in its strongest possible form.’

Astrophysicists have for some time been trying to understand what happens close to the black hole’s surface and do this by studying discs of material orbiting around them. There is a final region of spacetime, known as the plunging region, where it is impossible to stop a final descent into the black hole and the surrounding fluid is effectively doomed.

Debate between astrophysicists has been underway for many decades as to whether the so-called plunging region would be detectable. The Oxford team has spent the last couple of years developing models for it and, in the study just published, demonstrate its first confirmed detection found using X-ray telescopes and data from the international space station.

While this study focuses on small black holes closer to Earth, a second study team from Oxford’s Department of Physics is part of a European initiative to build a new telescope, the Africa Millimetre Telescope, which would hugely enhance our ability to make direct images of black holes.  More than 10 million Euros funding has already been secured, part of which will support several first PhDs in astrophysics for the University of Namibia, working closely with Oxford’s Department of Physics. The new telescope is expected to enable observation, and filming, for the first time of large black holes at the centre of our own galaxy, as well as far beyond.

This article was first published here.

Continuum emission from within the plunging region of black hole discs, A Mummery et al, Monthly Notices of the Royal Astronomical Society June 2024

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