Most massive merger of black holes ever

 Scientists have detected something impossible in deep Space   

The very concept of black holes seems improbable. Albert Einstein infamously refused to believe they could exist, even though his theory of general relativity was instrumental in predicting them. Now, scientists have witnessed evidence of something about these baffling cosmic monstrosities which further stretches the boundaries of both physics and credulity: a titanic collision of two already enormous black holes so utterly extreme that it has scientists wondering if the event they seem to have detected is even possible. Scientists have detected the most massive merger of black holes ever. This titanic collision, "heard" in ripples in space time called gravitational waves, involves black holes so massive that it could challenge current models of the universe. The merger was detected by the gravitational wave detector network LIGO-Virgo-KAGRA (LVK) on 23 Nov, 2023, during the fourth observing run of these three sensitive laser interferometers located in the US, Italy and Japan.   

As detailed in a new yet-to-be-peer-reviewed paper by a consortium of physicists, the resulting black hole, whose signal has been designated GW231123, boasts an astonishing mass about 225 times that of our Sun, easily making it the largest black hole merger ever detected. Previously, the record was held by a merger which formed a black hole of about 140 solar masses. The merger event that set space time ringing with this gravitational wave signal, involved progenitor black holes with masses of 100 and 140 times that of the sun. These two were so massive that when they merged, they created a "daughter" black hole 225 times the mass of our sun, with the missing mass converted to energy, propelling gravitational waves which rippled out from the violent event. "Black holes this massive are forbidden through standard stellar evolution models," Mark Hannam at the Laser Interferometer Gravitational-Wave Observatory (LIGO), which made the detection, said about the work. "This is the most massive black hole binary we've observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation."

Prior to GW231123, the most massive black hole created in a merger and detected in gravitational waves had a mass of 140 times that of the sun. This was detected in 2021 as the signal GW190521. "One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes." The monstrous masses of these black holes are not the only things that make GW231123 so interesting. The signal seems to indicate that prior to the merger, at least one of the progenitor black holes was spinning rapidly. Perhaps as rapidly as the laws of physics allow, in fact. Black holes can produce huge, propagating ripples in space time called gravitational waves, which were predicted by Einstein back in 1916. Nearly 100 years later, LIGO, which consists of two observatories on opposite corners of the US, made history by making the first ever detection of these cosmic shudders. The merger was first spotted in November 2023 in a gravitational wave, GW231123, that lasted just a fraction of a second. Even so, it was enough to infer the properties of the original black holes. One had a mass roughly 137 times the mass of the Sun, and the other was around 103 solar masses. During the lead up to the merger, the pair circled around each other like fighters in a ring, before finally colliding to form one.

"The black holes appear to be spinning very rapidly, near the limit allowed by Einstein's theory of general relativity," LVK member Charlie Hoy of the University of Portsmouth said. "That makes the signal difficult to model and interpret. It's an excellent case study for pushing forward the development of our theoretical tools." The Laser Interferometer Gravitational-wave Observatory (LIGO) is no stranger to making history and breaking records. In 2015, its twin detectors based in Livingston, Louisiana, and Hanford, Washington, made the first ever detection of gravitational waves. This detection came exactly a century after Einstein had first predicted the existence of gravitational waves in his 1915 theory of gravity, general relativity. The signal, which would become known as GW150914, was the result of the merger of black holes that created a daughter black hole with a mass around 62 times that of the sun. These black holes are physically problematic because it's likely that one, if not both of them, fall into an "upper mass gap" of stellar evolution. At such a size, it's predicted that the stars that formed them should have perished in an especially vicious type of explosion called a pair-instability supernova, which results in the star being completely blown apart, leaving behind no remnant, not even a black hole.

The creation of gravitational waves are from two orbiting black holes as ripples in space-time. In March 2014, astronomers announced the first detection of long-sought gravitational waves, though some critics now say the finding could be merely dust. Since 2015, LIGO has been joined by the gravitational wave detectors Virgo and the Kamioka Gravitational Wave Detector (KAGRA). This resultant collaboration has now detected over 300 black hole mergers. Over 200 of these detections have occurred in the fourth operating run of these instruments. As impressive as that is, the high-mass and rapid spin of the black holes which clashed to create GW231123 have pushed the limits of gravitational-wave detection technology and perhaps the bounds of current theoretical models, too. Some astronomers argue that the "mass gap" is really a gap in our observations and not the cause of curious physics. Nonetheless, the idea is "a hill at least some people were willing to get wounded on, if not necessarily die on," Cole Miller of the University of Maryland, who was not involved in the research, said. But perhaps the black holes weren't born from a single star. "One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes," Hannam said.

"This event pushes our instrumentation and data-analysis capabilities to the edge of what's currently possible," LVK member and California Institute of Technology (Caltech) researcher Sophie Bini said. "It's a powerful example of how much we can learn from gravitational-wave astronomy, and how much more there is to uncover." Fully unlocking the secrets of this signal and others that LVK detected up until the end of its fourth operating run in January 2024 will require the refinement of analysis and interpretation methods. Equally extreme as their weight classes are their ludicrously fast spins, with the larger spinning at 90 % of its maximum possible speed and the other at 80 %, both of which are equal to very significant fractions of the speed of light. In earthly terms, it's somewhere around 400,000 times our planet's rotation speed, according to the scientists. "The black holes appear to be spinning very rapidly, near the limit allowed by Einstein's theory of general relativity," Charlie Hoy, a member of the LIGO Scientific Collaboration at the University of Portsmouth, said. "That makes the signal difficult to model and interpret. It's an excellent case study for pushing forward the development of our theoretical tools." The researchers will present their findings at the GR-Amaldi meeting in Glasgow. "It will take years for the community to fully unravel this intricate signal pattern and all its implications," according to LIGO member Gregorio Carullo at the University of Birmingham, so, tantalizingly, we're likely only scratching the surface of this mystery." Despite the most likely explanation remaining a black hole merger, more complex scenarios could be the key to deciphering its unexpected features.