Scientists have "heard" the symphony of two newborn black holes—each created when its respective parent black holes crashed together and merged. One of these collision events, in particular, was the first of its kind ever observed.
The detection of these baby black holes, along with information about the four parent black holes that forged them, came courtesy of gravitational waves—ripples in spacetime caused by the violent cosmic events. These waves were registered by the LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and KAGRA (Kamioka Gravitational Wave Detector) gravitational wave detectors, collectively known as the LIGO-Virgo-KAGRA collaboration.
The Two Groundbreaking Mergers
The two events, designated GW241011 and GW241110, are described as "among the most novel events among the several hundred" that the network has observed.
GW241011: The Rapid Rotator
The first merger, GW241011, was detected on October 11, 2024.
Progenitors: A black hole with around 17 times the mass of the sun crashed into a partner black hole with a mass around seven times that of our star.
Distance: The event is calculated to have happened around 700 million light-years from Earth.
Unique Feature: Decoding the signal revealed that the larger black hole is one of the most rapidly spinning black holes ever observed.
This rapid rotation allows scientists to probe the limits of Albert Einstein's 1915 theory of general relativity. The deformation caused by the rotation leaves a unique impression in the gravitational waves, which the team used to confirm physicist Roy Kerr's solution for rotating black holes, verifying Einstein's magnum opus theory in extreme circumstances. This event also confirmed for the third time the "hum" of a higher harmonic (overtone) within a gravitational wave signal.
GW241110: The Backward Spin
Less than a month after the first detection, on November 11, 2024, the instruments "heard" another newborn black hole screaming after the collision of its progenitors.
Progenitors: Black holes with 16 and eight times the mass of the sun.
Distance: About 2.4 billion light-years away.
Unique Feature: This signal revealed that the larger black hole was spinning in the opposite direction of its orbit around the smaller black hole. This characteristic had never been seen before for merging binary black holes, making this birth truly unlike anything previously observed .
Evidence for "Second-Generation" Black Holes
Both events provide tantalizing evidence for the existence of second-generation black holes.
The idea is that the black holes detected were themselves born from earlier black hole mergers. This conclusion is supported by two key factors:
Mass Difference: In both mergers, the larger black holes are significantly more massive than their companions (almost double in mass).
Opposite Spin (GW241110): The orbit-opposing spin of the larger black hole in GW241110 is evidence of a prior, chaotic merger having produced that dominant black hole.
This process of growth by repeated collision is known as hierarchical merger and is believed to occur in densely populated regions like star clusters, where black holes are more likely to meet and coalesce.
Implications for Fundamental Physics
Beyond validating general relativity, these events have the potential to reveal more about an unrelated scientific field: particle physics.
Scientists can use the rapidly rotating black holes to test for the hypothesized existence of ultralight bosons—particles that exist beyond the Standard Model of particle physics. Should they exist, these particles would draw rotational energy from spinning black holes, slowing them down. The fact that the progenitor black hole of GW241011 is still rotating at a rapid rate after millions or billions of years rules out a specific range of ultralight boson masses.
"Each new detection provides important insights about the universe," said Carl-Johan Haster, an assistant professor of astrophysics. "Binaries like these had been predicted given earlier observations, but this is the first direct evidence for their existence."
The team's research was published in the Astrophysical Journal Letters.