Sign up for CNN’s Wonder Theory science newsletter to explore the universe with news on fascinating discoveries, scientific advancements, and more. A groundbreaking event has taken place in the field of astrophysics: a collision between two black holes, each more massive than a hundred suns, has been identified as the largest merger of its kind ever recorded. This significant event, dubbed GW231123, was discovered by a dedicated team of astronomers using the Laser Interferometer Gravitational-Wave Observatory (LIGO). This facility consists of two identical instruments located in Livingston, Louisiana, and Hanford, Washington, which detected the faint ripples in space-time, known as gravitational waves, produced by the collision of these colossal black holes.
Gravitational waves were first predicted by Albert Einstein in 1915 as part of his theory of relativity. Interestingly, Einstein believed these waves to be too weak to be detected by human technology. However, in 2016, LIGO made history by detecting these waves for the first time during a black hole collision, reaffirming Einstein's theories. Following this groundbreaking achievement, three scientists received accolades for their contributions to what has become known as a “black hole telescope.” Since then, LIGO and its sister instruments—Virgo in Italy and KAGRA in Japan—have detected signs of around 300 black hole mergers.
Mark Hannam, head of the Gravity Exploration Institute at Cardiff University and a member of the LIGO Scientific Collaboration, remarked, “These amazing detectors are really the most sensitive measuring instruments that human beings have ever built. So, we’re observing the most violent and extreme events in the universe through the smallest measurements we can make.”
GW231123 stands out among the 300 black hole mergers not only because of its size but also due to the unique characteristics of the individual black holes involved. According to Charlie Hoy, a research fellow at the University of Plymouth and a member of the LIGO Scientific Collaboration, "The individual black holes are special because they lie in a range of masses where we do not expect them to be produced from dying stars." Furthermore, the black holes are likely spinning at nearly the maximum speed possible, presenting significant challenges to current understandings of black hole formation.
Gravitational waves are the only means scientists have to observe collisions in binary systems of black holes. Hannam explains, “Before we could observe them with gravitational waves, there was even a question of whether black hole binaries existed at all.” Unlike regular celestial bodies, black holes do not emit light or electromagnetic radiation, making them invisible to traditional telescopes.
Einstein's theory of general relativity suggests that gravity distorts space and time, leading to the formation of ripples, or gravitational waves, as massive objects like black holes move rapidly. However, these waves are incredibly weak and come with limitations, including uncertainty regarding the distance of GW231123 from Earth, which could be as far as 12 billion light-years away. Nevertheless, the mass of the two black holes involved is estimated to be approximately 100 and 140 times that of the sun, leading to intriguing questions about their origins.
The mass of the black holes in GW231123 resides within what scientists refer to as the “mass gap,” a theoretical range of approximately 60 to 130 solar masses. This range has not been directly observed, leading to uncertainty about its boundaries. If the black holes from GW231123 indeed fall within this gap, it suggests they did not originate from traditional stellar collapse, but rather from an alternative formation process. In a recent study published on the open-access repository Arxiv, Hannam and his colleagues propose that the mass gap may be explained by previous mergers of black holes, indicating a chain reaction of mergers that could lead to increasingly massive black holes.
This theory posits that once black holes merge, they can continue to grow in mass through successive collisions. Hannam elaborated, “Since the black holes in GW231123 look like they’re at masses where you couldn’t get them by normal mechanisms, this is a strong hint that this other process is going on.” If confirmed, this hypothesis could unveil an unexpected population of black holes that lie between those formed from the death of massive stars and the supermassive black holes found in galactic centers.
Another remarkable aspect of GW231123 is the rapid spin of the two black holes involved in the merger. Charlie Hoy notes that "most black holes we have found with gravitational waves have been spinning fairly slowly," indicating that GW231123 may have formed through a different mechanism or signaling a need for updated models of black hole formation. High-speed spins are challenging to produce, yet they support the theory of previous mergers, as scientists expect merged black holes to exhibit faster spins.
Sophie Bini, a postdoctoral researcher at Caltech and a member of the LIGO-Virgo-KAGRA Collaboration, stated, “GW231123 challenges our models of gravitational wave signals, as it is complex to model such fast spins, and it stands out as an extraordinary event that is puzzling to interpret.” The previous record for the most massive black hole merger belonged to GW190521, which was only 60% the size of GW231123. Researchers anticipate the possibility of discovering even more massive mergers in the future with advances in detection technology like the proposed Cosmic Explorer in the US and the Einstein Telescope in Europe.
This latest discovery not only sheds light on the formation and growth of black holes but also illustrates the rapid maturation of gravitational wave astronomy. Imre Bartos, an associate professor at the University of Florida, remarked, “In less than a decade, we’ve moved from first detection to charting territory that challenges our best theories.” While the hypothesis of previous mergers could explain both the high mass and fast spin of the black holes, alternative explanations include repeated collisions in young star clusters or the direct collapse of unusually massive stars, though these are less likely to produce such fast-spinning black holes.
As Zoltan Haiman, a professor at the Institute of Science and Technology Austria, notes, “This idea was already raised immediately after the first ever LIGO detection of a black hole merger, but this new merger is very hard to explain in other ways.” Future detections will ultimately determine whether this heavyweight merger is an isolated event or indicative of a more extensive and complex landscape of black hole mergers yet to be explored.
