More than seven billion years ago, two black holes, so dense with gravity that nothing could escape their grasp, smashed into each other, releasing eight suns’ worth of mass as powerful ripples in the space-time fabric—gravitational waves that would race across the Universe at the speed of light. These black holes were engaged in a binary system, slowly rotating and pulling each other closer, until they collided and merged into a more massive black hole. This merger, dubbed GW190521, was hidden to humans until last year when its gravitational waves reached Earth and the twin detectors of LIGO (Laser Interferometer Gravitational-wave Observatory) in the United States and the smaller Virgo observatory in Italy recorded the event. Analyses of the event were published in scientific journals in early September.

Although dozens of these events have been recorded by LIGO and Virgo in the past five years, GW190521 proved to be an exceptional discovery. It is the largest and most distant merger ever recorded, with the two original black holes weighing around 65 and 85 times the mass of the sun. The resulting black hole is predicted to be currently 5.3 gigaparsecs away (around 17 billion light-years) and 142 times the mass of the sun. GW190521 provides the first direct observation of an intermediate-mass black hole (IMBH), a class of black holes (from 100 to 100,000 solar masses) that had been seemingly absent from any detection. This class is considered to be a “missing link” that serves as the building block for supermassive black holes.

GW190521 has also brought with it questions that astronomers cannot definitively answer yet. To understand them, an explanation of some astrophysics is required. A stellar-mass black hole, formed when a star runs of nuclear fuel and then collapses on itself, creating a violent supernova, can be present in a wide variety of masses except for a specific range. This is due to pair-instability (PI), a theoretical phenomenon that occurs once a star reaches a mass of greater than 65 solar masses. In stars smaller than this, photon pressure supports the nuclear fusion present in the star’s core by counteracting its gravitational pull. Once a star reaches this limit, however, photon support weakens, and the star cannot push back enough against its gravity. Eventually, the outer layers of the star will collapse inwards, the nuclear reaction will accelerate uncontrollably, and the star will go into supernova, with no black hole left behind. After about 135 solar masses, however, a star will once again collapse into a black hole. However, the two progenitor black holes from GW190521 (at 65 and 85 solar masses) fit neatly into the pair-instability gap.

Before GW190521, none of the black holes detected by LIGO and Virgo were in this pair-instability gap. Thus the 142 mass remnant has proved that IMBHs exist, and the 85 mass progenitor has shown that it is also possible for a black hole to exist within the PI gap. To explain this, the joint LIGO-Virgo team provided multiple scenarios, before eventually determining the two most probable. In the first scenario, one or both of the progenitors could have been second-generation black holes, which can have masses within the PI gap. This means that the final black hole would be a third-generation black hole and the result of multiple mergers. After two black holes combine, however, the resulting black hole is often flung away by the merger’s gravitational recoil; this scenario would have high chances of happening only if the progenitor black holes were formed inside a dense stellar environment, where the nearby gravity would be strong enough to prevent them from escaping. There the progenitor black hole could form a new binary system with another nearby progenitor and thus produce the 142 mass remnant.

Instead of the progenitors being formed from multiple black holes, it is possible that one (or both) of them was formed from multiple stars. This means that the 85 mass progenitor could have been formed from a star directly in the forbidden PI gap. The LIGO-Virgo team speculated that the star in the PI gap could produce such a black hole under specific conditions. The star would have had to collapse before its helium core could enter the PI range; to accomplish this, the star itself had to be the result of “one or more mergers between a helium-core giant and a main-sequence companion.” Once again, if the 85 mass progenitor was present inside a dense stellar environment, it would have the chance to capture a companion. If this is the case, the models for nuclear reactions in stellar cores would have to be revised. 

Thirty-five days after the detection of GW190521, the Zwicky Transient Facility in California identified a flare emerging from the same region. It is possible that gas near the newly formed black hole was heated by the merger’s shock waves, thus providing a possible electromagnetic counterpart to the merger. If the flare and merger are related, then GW190521 occurred in the disk of an active galactic nucleus, an environment that would support the formation of second and third-generation black holes. Thus the presence of a black hole in the PI gap would not be surprising.

For now, unfortunately, it is impossible to definitively determine how the black holes were formed. GW190521 most likely occurred due to one of the above explanations, but other scenarios still exist: the black holes could have been primordial, around since the beginning of the universe, or perhaps the ripples came from cosmic strings, left behind by the Big Bang. LIGO and Virgo are being upgraded and will hopefully restart observations in 2022. With increased sensitivity, they will be able to observe three times as much of the universe. Gravitational-wave astronomy is in its infancy, and the mysteries of the cosmos are just beginning to be revealed. In the future, the detection of new mergers and improved statistics may settle questions that cannot be answered right now and bring to light new ones to consider.