Scientists at the Heidelberg Institute for Theoretical Studies (HITS) in Germany have made a significant breakthrough in understanding black holes. Their research, published in The Astrophysical Journal Letters, suggests that the ‘chirp’ noise generated when two black holes collide and merge occurs in two universal frequency ranges.
The detection of gravitational waves in 2015, a phenomenon predicted by Albert Einstein a century earlier, marked a turning point in our understanding of the universe. It led to the 2017 Nobel Prize in Physics and opened up the field of gravitational-wave astronomy. When two stellar-mass black holes merge, they release gravitational waves with an escalating frequency known as the chirp signal. By analyzing the progression of this frequency, scientists can calculate the “chirp mass,” which represents the combined mass of the merging black holes.
In previous research, it was assumed that merging black holes could have any mass. However, the models developed by the HITS team suggest that some black holes come in standard masses, resulting in universal chirps. These universal chirp masses provide valuable insights into the formation of black holes and can even be used to infer which stars explode in supernovae. The research also sheds light on uncertain nuclear and stellar physics and offers a new method for measuring the accelerated cosmological expansion of the universe.
Stellar-mass black holes, with masses ranging from approximately 3 to 100 times that of our Sun, are formed when massive stars collapse into black holes instead of exploding in supernovae. These black holes, which eventually merge, are born in binary star systems and undergo multiple episodes of mass exchange between the components. The envelope stripping process, where stars are stripped off their outer layers, has a significant impact on the fate of these stars. It makes it easier for stars to explode in supernovae and leads to the formation of black holes with universal masses.
The ever-increasing sensitivity of gravitational-wave detectors and ongoing searches for neutron stars and black holes have contributed to an expanding “stellar graveyard” of these remnants. There appears to be a gap in the distribution of the chirp masses of merging binary black holes, as well as evidence suggesting the existence of peaks at approximately 8 and 14 solar masses. These peaks correspond to the universal chirps predicted by the HITS team.
The study also revealed that black holes with much larger masses than those found in our Milky Way Galaxy exist. These black holes originate from stars with a different chemical composition. Regardless of the chemical composition, the HITS team found that stars that become envelope-stripped in close binaries form black holes with masses below 9 and above 16 solar masses but very few in between.
While the number of observed black-hole mergers is still relatively low, the hints of an absence of chirp masses and an overabundance at the universal masses predicted by the models are intriguing. Further gravitational-wave observations will provide more clarity on these findings and yield exciting results that deepen our understanding of black holes and their origins.
The research conducted by the HITS team was funded by the H2020 European Research Council, further contributing to advancements in our knowledge of the universe’s most enigmatic entities.