- Scientists suggest biggest black holes form in dense star clusters via frequent collisions
- Study analyzed 153 black hole mergers detected by LIGO, Virgo, and KAGRA gravitational-wave detectors
- Two black hole groups found: lower-mass from star collapse, higher-mass from repeated mergers
Scientists have discovered new clues about how the universe's biggest black holes may form, suggesting that these massive cosmic objects are created in dense star clusters where violent collisions happen frequently. The findings are based on the study of gravitational waves, which are ripples in spacetime produced by powerful cosmic events such as black hole mergers, reported Space.com.
Researchers studied 153 black hole merger detections recorded in version 4.0 of the LIGO-Virgo-KAGRA Gravitational-Wave Transient Catalog. The observations were made using highly sensitive gravitational wave detectors including the Laser Interferometer Gravitational-Wave Observatory (LIGO), KAGRA and Virgo.
Gravitational waves were first predicted by Albert Einstein in 1915 as part of his theory of general relativity. Scientists explained that these waves are created when massive events like black hole collisions disturb the fabric of spacetime.
The research team aimed to understand whether the heaviest black holes are formed directly from collapsing massive stars or through repeated mergers of smaller black holes inside dense stellar environments known as globular clusters.
Team leader Fabio Antonini from Cardiff University said that gravitational-wave astronomy is now doing more than simply counting black hole mergers. He said it is helping scientists understand how black holes grow, where they grow and what that reveals about the lives and deaths of massive stars. Antonini added that this information can help researchers test their understanding of how stars and clusters evolve in the universe.
The study revealed two separate populations of black holes. Scientists found a lower-mass group that likely formed after massive stars exploded in supernovas and their cores collapsed under gravity.
They also discovered a higher-mass group of black holes with rapid and randomly oriented spins, which researchers believe were formed through repeated mergers between smaller black holes in dense star clusters.
Team member Isobel Romero-Shaw said the researchers were surprised by how clearly the high-mass black holes appeared as a separate population. She explained that unlike the lower-mass systems, which generally showed slow spins, the higher-mass systems had faster spins in random directions.
According to her, this is the exact signature expected if black holes repeatedly merged in dense star clusters, making the cluster-origin theory more convincing than before.
The research also points to evidence of a long-theorised “mass gap” connected to the deaths of massive stars. Scientists believe that extremely massive stars may not collapse into black holes when they die, but instead explode completely in supernova blasts, leaving nothing behind.
According to the study, this creates a forbidden mass range for stellar-mass black holes formed from collapsing stars. Researchers believe this mass gap begins at around 45 times the mass of the sun. Black holes larger than this may instead be formed through mergers.
Antonini said the study found evidence for the long-predicted pair-instability mass gap, which is a range where stars are not expected to leave behind black holes. He explained that gravitational-wave detectors have discovered black holes that appear to exist in or near this gap at around 45 solar masses.
Antonini added that scientists are now trying to determine whether current models of stellar evolution are incorrect or whether these black holes are being formed in another way. The team also said the findings could help scientists better understand the final stages of massive stars and the behaviour of stellar bodies packed into extremely dense environments.
Antonini further said that the biggest black holes in the current sample seem to reveal more about cluster dynamics than stellar evolution alone. He explained that above about 45 solar masses, the spin patterns become difficult to explain using normal stellar binaries but can naturally be explained if the black holes had already undergone earlier mergers inside dense clusters. The findings were published on May 7 in the journal Nature Astronomy.
