Analysis of three gravitational wave signals and of a fourth possible gravitational wave candidate, reported in this week’s Nature, adds to our understanding of how black hole pairs form and evolve.
Studying gravitational waves from merging pairs of black holes can provide valuable clues to the formation of such systems. There are considered to be at least two different scenarios for binary black hole formation and looking at the combination of the spins of the black holes (the angular momentum of the material from which the black holes formed) could provide a way to test these competing theories. If the black holes came from pairs of stars that were born together, the spins would be expected to be aligned, whereas if the system formed through dynamical interactions among already-collapsed stars, the spins would probably be misaligned.
Will Farr and colleagues examined the spin properties of four black hole merger events, three detected in 2015 (GW150914, LVT151012, GW151226) and one in 2017 (GW170104). If these black holes were spinning as fast as those observed in our own galaxy, then the findings indicate that they probably have a random alignment, suggesting that the mergers took place after the stars had already become black holes. The authors suggest that with as few as ten additional detections, it may be possible to conclude with confidence the origin of black hole binaries; it was thought previously that many more detections would be needed to achieve this level of confidence.
In an accompanying News & Views article, Steinn Sigurdsson writes that the results 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.”