Gravitational waves shine a light on colliding black holes

The detection of gravitational waves has launched “an entirely new subfield of astronomy” that will eventually explain how black holes join up into pairs.

That’s the prediction of Steinn Sigurðsson from Penn State University in the US, commenting in the journal Nature on research examining three gravitational waves emanating from binary black hole systems.

The research in question, published in the same edition of the journal, was conducted by a team led by Will Farr from the University of Birmingham in the UK.

Farr and his colleagues focussed on four observed black hole pairs – three discovered in 2015 and one found this year – comparing modelled gravitational waves against the actual data recorded.

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The object of the exercise was to gain insight into a compelling astronomical question: how do binary black hole systems form?

There are two primary theories to explain their existence. The first is that they are created when two previously existing black holes fall into each other’s gravitational field. 

The second is that they arise from binary star systems: paired stars that remain in each other’s orbit in death as in life.

One key piece of evidence that will eventually solve the mystery is the angular distribution of each black hole’s spin in relation to its orbit.

Farr’s team build on work by other astronomers – including Sigurðsson, as early as 1993 – that the angle of spin should be determined by the way the black hole pair formed.

If the black holes had previously existed independently before merging, the theory suggests, then the distribution of the measured spin should be “isotropic”. That is, the spins of each black should be aligned at random, with no connection to the direction of their orbit around each other.

If, on the other hand, the black holes arose from the death of an already paired star system, then the spin should be preferentially aligned with the orbit.

Farr and colleagues report that on the analysis of data gleaned from gravitational wave detections associated with the black hole detections dubbed GW150914, LVT151012, GW151226 and GW170104, the odds are very slightly in favour of isotropic results – indicating that pairs are created when individual black holes effectively collide with each other.

Available data, however, is not sufficient to draw firm conclusions. However, the study authors and Sigurðsson agree that real benefit of the research lies in the precision of the analysis and the consequent reduction of the amount of new evidence required before a definitive conclusion can be made.

Farr’s team estimate that only another 10 gravitational wave detections associated with binary systems will be needed. At that point, the authors say, “the existing preference for either an isotropic spin distribution or low spin magnitudes for the observed systems will be confirmed (or overturned) confidently in the near future”.

Sigurðsson adds that the findings are “important because they tell us how many data are needed to test the main formation theories, and show that the number of required observations is likely to be achieved in the near future.”

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