Black holes are some of the most exotic objects in the universe and new research now predicts the ‘chirp sounds’ they make during mergers

The researchers, at the Heidelberg Institute of Theoretical Studies (HITS) in Germany, predict that the chirp sound that two black holes produce when they merge preferentially occurs in two universal frequency ranges

The discovery of gravitational waves in 2015 – already postulated by Einstein a hundred years ago – led to the 2017 Nobel Prize in Physics and initiated the dawn of gravitational-wave astronomy.

When two stellar-mass black holes merge, they emit gravitational waves of increasing frequency, the so-called ‘chirp signal’, that can be ‘heard’ on Earth.

From observing this frequency evolution (the chirp), scientists can infer the so-called ‘chirp mass’, a mathematical combination of the two individual black hole masses.

So far, it has been assumed that the merging black holes can have any mass. The team’s models, however, suggest that some black holes come in standard masses that then result in universal chirps.

HITS study lead Fabian Schneider said: “The existence of universal chirp masses not only tells us how black holes form, it can also be used to infer which stars explode in supernovae.”

Apart from that it provides insights into the supernova mechanism, uncertain nuclear and stellar physics, and provides a new way for scientists to measure the accelerated cosmological expansion of the universe.




Universal black hole masses


Stellar-mass black holes with masses of approximately three to one hundred times our sun are the endpoints of massive stars that do not explode in supernovae but collapse into black holes.

The progenitors of black holes that lead to mergers are originally born in binary star systems and experience several episodes of mass exchange between the components: in particular, both black holes are from stars that have been stripped of their envelopes.

Second author Philipp Podsiadlowski, from Oxford University and currently Klaus Tschira Guest Professor at HITS, said: “The envelope stripping has severe consequences for the final fates of stars.

“For example, it makes it easier for stars to explode in a supernova and it also leads to universal black hole masses as now predicted by our simulations.”

The ‘stellar graveyard’ – a collection of all known masses of the neutron-star and black-hole remains of massive stars – is quickly growing thanks to the ever-increasing sensitivity of the gravitational-wave detectors and ongoing searches for such objects.

In particular, there seems to be a gap in the distribution of the chirp masses of merging binary black holes, and evidence emerges for the existence of peaks at roughly 8 and 14 solar masses.

These features correspond to the universal chirps predicted by the HITS team.

Third author Eva Laplace said: “Any features in the distributions of black-hole and chirp masses can tell us a great deal about how these objects have formed.”


Absence of chirp masses


Ever since the first discovery of merging black holes, it became evident that there are black holes with much larger masses than the ones found in our Milky Way.

This is a direct consequence of these black holes originating from stars born with a chemical composition different from that in our Milky Way galaxy.

The HITS team could now show that – regardless of the chemical composition – stars that become envelope-stripped in close binaries form black holes of <9 and >16 solar masses but almost none in between.

In merging black holes, the universal black-hole masses of approximately 9 and 16 solar masses logically imply universal chirp masses, ie universal sounds.

Schneider said: “When updating my lecture on gravitational-wave astronomy, I realised that the gravitational-wave observatories had found [the] first hints of an absence of chirp masses and an overabundance at exactly the universal masses predicted by our models.

“Because the number of observed black-hole mergers is still rather low, it is not clear yet whether this signal in the data is just a statistical fluke or not.”

Whatever the outcome of future gravitational-wave observations, the results will be exciting and help scientists understand better where the singing black holes in this ocean of voices come from.

Image 1: Ripples in the spacetime around a merging binary black-hole system from a numerical relativity simulation. Credit: Deborah Ferguson, Karan Jani, Deirdre Shoemaker, Pablo Laguna, Georgia Tech, MAYA Collaboration. Public Domain.

Image 2: Distribution of the chirp masses of all binary black-hole mergers observed today. The top panel shows the raw data and probability distributions of the chirp masses of each individual event while the bottom panel shows a model inferred from the combined observations. The gap in chirp masses at 10–12 solar masses and the so-far identified features at about 8, 14, 27 and 45 solar masses are indicated. Figure reproduced from Abbott et al 2021. Cred: Abbott et al, 2021. Public Domain.

Research Aether / Space Uncovered