14 February 2011

How to spot a spinning black hole

Cosmos Online
Rotating black holes leave a detectable imprint on passing radiation which, according to an international team of researchers, could provide a further test of Einstein’s general theory of relativity.
rotating black hole

Black holes put a twist on light passing by, according to new technique that could help astronomers and astrophysicists detect rotating black holes.
Credit: Fabrizio Tamburini

SYDNEY: Rotating black holes leave a detectable imprint on passing radiation which, according to an international team of researchers, could provide a further test of Einstein’s general theory of relativity.

Using the most sensitive radio telescopes available to observe the rotation of black holes in active galactic nuclei – a compact region at the centre of a galaxy – could also provide valuable information about how galaxies evolve.

“Now we have a direct way of seeing if there are rotating black holes, which is something that we did not have before,” said study author, Gabriel Molina-Terriza from Macquarie University in Sydney. “We think that most of the black holes in the universe are rotating, but until now we’ve never had a direct way of measuring that.”

Adding direct measurements to theory

General relativity tells us that very massive objects such as black holes warp space-time such that the path of any passing light is bent, an effect known as gravitational lensing.

The theory also predicts that when a black hole rotates it will drag space-time around with it, creating a vortex that constrains all nearby objects, including photons, to follow that rotation.

Astronomers already have evidence that the supermassive black holes believed to lie at the core of many galaxies rotate. However, this evidence is indirect.

Taking a different approach

The rotation of the Milky Way’s black hole, for example, is suggested by the velocity distribution of stars within the galaxy, but this approach is undermined because we don’t know exactly how much matter – particularly dark matter – the galaxy contains.

Some astronomers believe that the Milky Way’s black hole is rotating very quickly while others maintain it is rotating much more slowly.

However, in this recent study, published in Nature Physics, lead author Fabrizio Tamburini of the University of Padova in Italy and colleagues instead show how to detect the rotation by measuring changes to the light from a distant star or from the disk of accreted material surrounding a black hole.

Teaming up different telescopes

The researchers point out that a wavefront travelling in a plane perpendicular to the black hole’s axis of spin will get twisted as it passes close to the black hole, since half of the wave front will be moving in the direction of advancing space-time and the other half in the direction of receding space-time. In other words, the phase of the radiation emanating from close to a rotating black hole should have a distinctive distribution in space.

Using a computer simulation to model the phase distribution resulting from the rotation of the Milky Way’s black hole, the team found that this variation ought to be visible from the ground.

They say the way to measure it is to point an array of radio telescopes at the centre of the galaxy – using different telescopes to observe different segments of the approaching wave front – and then superimpose these segments to calculate their relative phase.

Challenging experiments ahead

This procedure would be repeated, each time the telescopes pointing to a different section of the tiny patch of sky surrounding the black hole.

According to Tamburini, his group could carry out such measurements within two years using an existing array of radio telescopes, such as the Very Long Baseline Array in the U.S., or the LOIS-LOFAR in Europe, depending on the availability of funding.

“In the last few years, some clever techniques have been developed for measuring the spin of black holes indirectly, but this new result has the potential to be a much more direct, precise way of studying this phenomenon,” said Bryan Gaensler from the Sydney Institute for Astronomy in the School of Physics at the University of Sydney.

“However, the experiments needed to progress further will be challenging – it will be interesting to see if this small effect will be detectable with the latest generation of radio telescopes.”


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