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Space-time is constantly being strummed by all the events that are happening in the universe. The vibrations, which we call gravitational waves, resonate across the cosmos. Since the first direct detection of gravitational waves in 2015 – and the following discoveries of several collisions between black holes, neutron stars, and each other since then – we have been able to listen to the “sounds” of these events.
Now, for the first time, researchers have detected two tones from a newly formed black hole. It had previously been assumed that just one tone could be detected, and any others would be too faint for our current technology to pick up.
The gravitational event GW150914 that kicked it all off in 2015 saw a collision between two black holes, one weighing 36 times our Sun and the other one 29, sending quivers through space. The merge created a single black hole releasing a mindboggling amount of energy as gravitational waves.
"A new black hole forms out of a violent astrophysical process and thus is in an agitated state," lead author of the first paper, Maximiliano Isi, from MIT, said in a statement. "However, it quickly sheds this surplus energy in the form of gravitational waves."
Most of the energy was in the “chirp”, the emission from the instance of the merger but some of it came from the "ringdown", the overtones produced by the black hole settling in, which researchers managed to detect and detail in two new papers: One, discussing finding them in the signal from GW150914 (published in Physical Review Letters) and the other detailing the technique used to hunt for them (published in Physical Review X).
"This was a very surprising result. The conventional wisdom was that by the time the remnant black hole had settled down so that any tones could be detected, the overtones would have decayed away almost completely," said Saul Teukolsky, a professor of physics at Cornell University. "Instead, it turns out that the overtones are detectable before the main tone becomes visible."
The overtones were assumed too weak to be detected within the current gravitational wave data, but the model developed for the ringdown by the team allowed them to spot the signal amidst the noise. The chance that the detection is a fluke is about 1 in 5,000, which is quite good but not yet the gold-standard level expected in particles physics.
The data and model are good enough to estimate the properties of the newly formed black hole, which were in agreement with what was discovered from the rest of the gravitational wave detection. The ringdown was also used to test the no-hair theorem, the idea that whatever falls into the black hole is permanently inaccessible. This is the expectation from general relativity but doesn’t sit well with quantum mechanics. The team admits that better accuracy in future observations might completely overhaul this idea.
"Einstein's theory could break down if there are quantum effects at play," lead author of the second paper Matthew Giesler, from Caltech, explained. "Newton's theory of gravity passes many tests where gravity is weak, but completely fails when it comes to describing gravity at its most extreme, like when it comes to trying to describe merging black holes. Similarly, as we eventually probe the signal from black holes with increasing accuracy, it is possible that even general relativity might someday fail the test."
The approach detailed in these papers is yet another way to understand the universe through gravitational waves, and we are still very much in the overture of this exciting field.
