Black holes are rather featureless, defined only by their spin and mass. When two black holes merge, however, the result is the brief appearance of something that has bumps and curves. Under the right circumstances, some physicists claim, the gravitational wave produced by the merger can reveal this form to us, offering new insight into how gravity behaves at its most extreme.
In the week the Nobel Prize for physics was awarded for black hole research, Dr Juan Calderón Bustillo of the Galician Institute for High Energy Physics has published a paper exploring these holes from a new angle. His co-author Christopher Evans, a Georgia Tech graduate student, created computer simulations of black hole collisions so they could study the gravitational waves they expected to see produced, which depends not only on the masses of the merging holes but the orientation of their orbital plane to Earth.
The team found that “when the two original, 'parent' black holes are of different sizes, the final black hole initially looks like a chestnut, with a cusp on one side and a wider, smoother back on the other,” Calderón Bustillo said in a statement.
This stage lasts less than a tenth of a second, Calderón Bustillo told IFLScience, and enormous energy is released during this time as the combined hole relaxes into its final state. However, “it turns out that the black hole emits more intense gravitational waves through its most curved regions, which are those surrounding its cusp," he added.
To an observer witnessing the merger from above or below, the gravitational wave signal would appear straightforward, with a constant pitch but falling amplitude, like the note we hear when a well-made bell is struck once. However, Calderón Bustillo claims in Communications Physics that an observer in line with the equator of the hole would detect “a more complex signal, with a pitch that goes up and down a few times before it dies.” The authors call these “post-merger chirps.”
Although detections of black hole mergers are now so common only the more interesting ones get much attention, we have yet to detect Calderón Bustillo's predicted signal. In part, this is because its creation requires one of the holes to be substantially more massive than the other. However, Calderón Bustillo told IFLScience there is a more important reason: the signal is strongest in the face-on direction, where the varying signal can't be detected. Although black hole mergers are randomly distributed in their orientation towards Earth, we pick up on those where the Earth is situated over one pole, as these signals can be detected from a greater distance.
Nevertheless, if there was a merger of the same size and distance as one of the ones we have actually observed, LIGO detectors should be able to detect the chirps, provided the orientation is right. Consequently, while more sensitive future detectors will really test Calderón Bustillo's claims, with luck confirmation might be achieved even with those currently in operation.