Gravitational waves hint at black hole secrets

Why do some black holes exist as pairs? No one knows but researchers are confident the answer is not far away.

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 Sigurdsson, from Pennsylvania State University, commenting in the journal Nature on research examining three gravitational waves emanating from binary black hole systems.

The research in question, led by Will Farr from the University of Birmingham, focused on four black hole pairs – three discovered in 2015 and one found this year – and compared modelled gravitational waves against the actual data recorded.

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

There are two primary theories. 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 hole’s spin in relation to its orbit. As early as 1993 Sigurdsson suggested that spin angle was a vital clue – if only it could be detected with sufficient precision.

If the black holes existed independently before merging, the theory suggests, then the distribution of the measured spin should be “isotropic”. That is, the spins of each hole should be aligned at random, with no relation 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 data gleaned from gravitational wave detections associated with the black hole pairs dubbed GW150914, LVT151012, GW151226 and GW170104 set the odds very slightly in favour of isotropic results – indicating that pairs are created when individual black holes collide with each other.

Available information, however, is insufficient to make a definitive call. The study authors and Sigurdsson agree that the 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.

The Farr team estimates that only another 10 gravitational waves 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”.

Sigurdsson 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.”