Black holes are among the most exotic phenomena in the universe because they are enigmatic, fascinating, and unavoidable. The chirp sound that two black holes make when they merge can be heard using gravitational-wave detectors; about 70 such chirps have been discovered so far.
A team of researchers at the Heidelberg Institute for Theoretical Studies (HITS) now predicts that in this “ocean of voices” chirps preferentially occur in two universal frequency ranges. The study has been published in The Astrophysical Journal Letters.
In addition to earning Einstein the 2017 Nobel Prize in Physics, the discovery of gravitational waves in 2015 marked the beginning of gravitational wave astronomy.
The so-called chirp signal, or gravitational waves of increasing frequency, are produced by the merger of two stellar-mass black holes and may 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.
Black holes that are merging are currently thought to be capable of having any mass. However, according to the team’s models, some black holes arrive in normal masses and produce universal chirps.
“The existence of universal chirp masses not only tells us how black holes form,” says Fabian Schneider, who led the study at HITS, “it can also be used to infer which stars explode in supernovae.”
When updating my lecture on gravitational-wave astronomy, I realized that the gravitational-wave observatories had found 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.
Fabian Schneider
In addition, it offers new ways for researchers to evaluate the accelerated cosmic expansion of the universe as well as insights into the supernova mechanism, hazy nuclear and stellar physics, and the supernova mechanism itself.
‘Severe consequences for the final fates of stars’
Massive stars that do not explode in supernovae but instead collapse into black holes are the ends of stellar-mass black holes, which have masses of roughly 3-100 times that of the sun. 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 off their envelopes.
“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,” says Philipp Podsiadlowski from Oxford University, second author of the study and currently Klaus Tschira Guest Professor at HITS.
The “stellar graveyard” a collection of all known masses of the neutron-star and black-hole remnants of large stars is rapidly expanding as a result of continuous gravitational-wave searches and the ever-increasing sensitivity of gravitational-wave detectors.
Particularly, a gap in the distribution of the chirp masses of merging binary black holes appears, and evidence for peaks at about eight and fourteen solar masses appears. These features correspond to the universal chirps predicted by the HITS team.
“Any features in the distributions of black-hole and chirp masses can tell us a great deal about how these objects have formed,” says Eva Laplace, the study’s third author.
Not in our galaxy: Black holes with much larger masses
Since the initial finding of merging black holes, it has been clear that black holes with masses significantly greater than those found in the Milky Way exist. This is a direct result of these black holes developing from stars that were created with a different chemical composition than the ones 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 less than nine and greater than 16 solar masses but almost none in between.
In merging black holes, the universal black-hole masses of approximately nine and 16 solar masses logically imply universal chirp masses, i.e. universal sounds.
“When updating my lecture on gravitational-wave astronomy, I realized that the gravitational-wave observatories had found first hints of an absence of chirp masses and an overabundance at exactly the universal masses predicted by our models,” says Fabian Schneider. “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 outcomes of upcoming gravitational-wave observations, they will be fascinating and aid in the understanding of the origin of the singing black holes in this sea of voices.
