An orchestra conductor can easily tell a gong from a bell from their different sounds. Can astronomers do the same and tell a black hole from another dark object just by detecting their gravitational-wave signal? Writing in the prestigious Physical Review Letters, in a work highlighted by the Editors, selected for a synopsis and for PRL cover, a team of scientists from Instituto Superior Técnico in Portugal, Cagliari University and Sapienza University in Italy shows that this might not be the case.
Last February, the LIGO/Virgo Collaboration announced the first direct detection of gravitational waves. This historical discovery has been also welcomed as the first conclusive proof for the existence of black holes, the most extreme objects in the Universe. The detected signal, dubbed GW150914, corresponds to two massive objects that spiral inwards and eventually collide in a cosmic spacetime-quake. LIGO data shows that the two objects are extremely compact, and too massive to be neutron stars. But what makes this signal really unique is that the few-milliseconds-long gravitational-wave signal ends as a "ringdown", in which the end-product of the coalescence vibrates pretty much like a drum. Just like the notes of the drum depend on its properties (the shape, the size, the material), the "ringdown modes" should carry information about the very nature of the final object produced after the merger.
However, the team of scientists shows that the vibrations of very compact objects are almost identical to those of black holes. In particular, they studied whether "wormholes" (a tunnel through spacetime connecting two distant regions of the universe) could be distinguished from black holes. Black-hole oscillations are intrinsically connected to a special effect of Einstein's theory of general relativity: the "light ring", where photons and gravitational waves can remain on a circular orbit for some time (the ringdown time). This study indicates that any compact object with a light ring initially rings-down just like a black hole, but the real nature of the final object eventually shows up at very late times. Because time slows down enormously in the strong-gravity regions near compact objects, LIGO might have missed this tiny difference in the signal. The analysis of the authors shows that, just like an orchestra conductor should have a perfect pitch to tell the difference between two drums playing almost the same note, gravitational-wave detectors need to be extremely sensitive to the final ringdown if they want to tell a black hole from some other dark object.
Yet, with the improved sensitivity that gravitational-wave interferometers will reach in the next years, events like GW150914 will definitely boost our understanding about the nature of black holes.
Note: "Can One Hear the Shape of a Drum?" is the title of a famous work by Marc Kac, where he discusses an old conjecture that the shape of a drum can be inferred solely from its sound.