Black holes, enigmatic and captivating, remain one of the most extraordinary entities in the cosmos. Utilizing gravitational-wave detectors, scientists have successfully identified around 70 “chirps,” the distinctive sounds produced when two black holes merge.
At the Heidelberg Institute for Theoretical Studies (HITS), a team of researchers has made a fascinating prediction. Chirps appear to have a preference for occurring within two specific frequency ranges, creating an “ocean of voices.” The study, published in The Astrophysical Journal Letters, sheds new light on black hole behavior.
The groundbreaking discovery of gravitational waves in 2015, as theorized by Einstein a century earlier, earned the 2017 Nobel Prize in Physics. This milestone marked the beginning of gravitational-wave astronomy. When two black holes with stellar mass merge, they emit gravitational waves with increasing frequency, forming the characteristic chirp signal, which can be detected on Earth. By analyzing this frequency evolution, scientists can deduce the “chirp mass,” a mathematical representation of the combined black hole masses.
Previously, it was assumed that merging black holes could have any mass. However, the team’s models indicate that some black holes possess standard masses, leading to universal chirp patterns.
According to Fabian Schneider, who led the study at HITS, the existence of universal chirp masses offers valuable insights into black hole formation and can help identify which stars undergo supernovae. Furthermore, it provides a means to study supernova mechanisms, uncertain nuclear and stellar physics, and contributes to measuring the accelerated cosmological expansion of the universe. This newfound knowledge opens up exciting possibilities for scientific exploration and understanding.
‘Severe consequences for the final fates of stars’
Stellar-mass black holes, ranging from 3 to 100 times the mass of our Sun, are the end result of massive stars that don’t undergo supernova explosions but instead collapse into black holes. These black holes, which eventually merge, originate from binary star systems and go through multiple episodes of mass exchange, resulting in both black holes being stripped of their envelopes.
According to Philipp Podsiadlowski from Oxford University, the stripping of envelopes has significant consequences for the stars’ final fate. It makes supernova explosions more likely and leads to the prediction of universal black hole masses, as revealed by their simulations.
Thanks to the growing sensitivity of gravitational-wave detectors and ongoing searches, the “stellar graveyard” is expanding, encompassing neutron-star and black-hole remnants of massive stars. Notably, there appears to be a gap in the distribution of chirp masses for merging binary black holes, with evidence pointing to peaks around eight and 14 solar masses, which align with the universal chirps predicted by the HITS team.
Eva Laplace, the study’s third author, highlights that any features observed in the distributions of black-hole and chirp masses provide valuable insights into the formation of these enigmatic objects.
Not in our galaxy: Black holes with much larger masses
Since the initial discovery of merging black holes, it has been evident that some black holes have significantly larger masses than those found in our Milky Way. This is because these black holes originate from stars with different chemical compositions than those in our galaxy. The HITS team’s research shows that regardless of the chemical composition, stars that undergo envelope stripping in close binary systems form black holes with masses less than nine and greater than 16 solar masses, but very few in between.
The universal masses of approximately nine and 16 solar masses observed in merging black holes logically imply the presence of universal chirp masses, representing universal sounds.
Fabian Schneider, reflecting on the data from gravitational-wave observatories while updating his lecture on gravitational-wave astronomy, noticed hints of an absence of chirp masses and an overabundance at precisely the universal masses predicted by their models. However, due to the relatively low number of observed black-hole mergers, it remains uncertain whether this signal is merely a statistical fluke or not.
Regardless of the outcome of future gravitational-wave observations, the results will undoubtedly be intriguing and contribute to a deeper understanding of the origins of the captivating singing black holes in this vast ocean of voices.