First Detected Collision Between Two Black Holes Of Wildly Different Masses

Artist impression of the two black holes before collision. N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project.

Scientists with the LIGO and EGO/Virgo gravitational wave observatories have detected a black hole collision quite different from any we have observed over the last four years. Since the first-ever gravitational-wave event was detected back in 2015, most mergers have been between black holes of roughly the same size, but this newly detected collision is the first with clear evidence of unequal mass components.

The event, named GW190412, was detected on April 12 last year only weeks into LIGO's third observing run, having been on a maintenance break since August 2017. Gravitational waves generated by collisions usually emanate at a single frequency, however, the researchers detected waves at multiple frequencies, suggesting an imbalance in the mass of the two objects colliding. A black hole eight times the mass of our Sun smacked into another black hole 30 times the mass of the Sun, 3.6 times heavier than the first. Their findings were presented at this month's online American Physical Society meeting, and the yet-to-be-peer-reviewed paper is available to read here.

"Every observing run of Advanced LIGO and Virgo has so far given us new insights into our universe and the third observing run is no exception,” Frank Ohme, LIGO scientist and Research Group Leader at the Max Planck Institute for Gravitational Physics, said in a statement. “We had detected several binary black hole mergers before, but never one where the bigger black hole is nearly four times more massive than its companion.”

The difference in mass is interesting, not just because this is a first-seen event. It also allowed researchers to perform some new tests of Einstein's theory of general relativity. In line with Einstein's theory, it is expected that collisions between objects of a different mass would produce “High Order Modes” in the gravitational radiation, overtones in the signal. The researchers did indeed observed this.

"The unequal masses of this source caused overtones of the main signal to be visible for the very first time. This provided us with an exciting new opportunity to test an important prediction of Einstein’s theory about what happens when black holes of unequal size collide," explained Anuradha Samajdar, a postdoc fellow at the Dutch National Institute for Subatomic Physics and member of the Virgo Collaboration, in a statement.

The team released the “sound” or chirp that translates the gravitational waves – tiny vibrations in the fabric of space-time – into something we can hear. What is even more exciting is because of the properties of this unequal system, the two black holes played a perfect fifth. As Science Magazine explains, if the main frequency of the waves was a C on a piano, the overtone would be the next higher G – a perfect fifth, and incidentally, the jump in the opening two notes of Elvis Presley singing Can't Help Falling In Love With You.

The disparity between the masses has also allowed for more accurate estimations of the distance of the now single black hole. It is most likely 700 Megaparsecs or roughly 2.3 billion light-years from us.

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