Gravitational wave detectors have spotted a cosmic collision in which a giant black hole swallowed up a mystery object seemingly too heavy to be a neutron star, but too light to be a black hole. Weighing in at 2.6 times the mass of the Sun, the object falls into a hypothetical “mass gap,” a desert between the heaviest neutron star and the lightest black hole that some theories predict—suggesting the gap doesn’t exist and that those theories need to be amended.
“People who thought there was a mass gap will have to rethink it, for sure,” says Cole Miller, an astrophysicist at the University of Maryland, College Park, who was not involved in the observation. He adds, however, “People aren’t going to be joining cults because they cannot survive this change in their worldview.”
The data come from physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors in Louisiana and Washington state, and Virgo, a similar detector in Italy. All three consist of huge, exquisitely sensitive optical instruments that can detect the fleeting stretching of space itself set off when two massive objects, such as black holes, swirl into each other. Since LIGO first sensed such gravitational waves in 2015, physicists have spotted dozens of mergers. And on 14 August 2019, the LIGO and Virgo detectors spotted a merger of objects with masses 23 and 2.6 times that of the Sun, the joint LIGO-Virgo collaboration announced yesterday.
It’s the 2.6–solar-mass object that raises eyebrows because it falls squarely in the mass gap, says Vicky Kalogera, an astrophysicist and LIGO team member from Northwestern University. “Now, for the first time, we have seen such an object,” she says. By sensing only the gravitational waves from the collision, LIGO and Virgo cannot tell for sure what the object is, she says. But nuclear physics suggests a neutron star heavier than about 2.2 solar masses cannot support its own weight, so the object is “almost certainly” a black hole, Miller says.
Yet it’s not easy to form a black hole this light, explains Feryal Özel, an astrophysicist at the University of Arizona. Prior to the advent of LIGO and Virgo, the only observational evidence for black holes came from the study of about 30 in our own galaxy, each of which orbits a companion star that feeds hot matter into it. None of these black holes weighs less than five solar masses, Ozel noted and colleagues noted in 2010. So they posited a mass gap between about 2.5 and five solar masses in which there should exist neither neutron stars nor black holes. But that notion rests on observation, Özel stresses. “Is there a fundamental physics reason black holes can’t form below five solar masses? We certainly don’t think so,” she says. “But, there must be something in the way massive stars evolve that makes it very hard.”
Subsequently, theorists explained why that may be so. Either a neutron star or a black hole can form when a massive star runs out of hydrogen fuel and its core begins to collapse. If the star is light enough, the core will collapse to a neutron star in a supernova explosion that blows away the rest of the star. If the star is too massive, however, its core will shrink to an infinitesimal point, leaving behind only its superintense gravitational field: a black hole. Theories suggest that, for these heavy stars, all but the outermost layers of the star fall in, boosting the black hole’s mass to five solar masses or more.
The new observation may put a dent in that theory—which Miller says has already met with some skepticism. “The supernova theorists who do the real modeling basically say, ‘Look, show us for sure there’s a mass gap and we’ll work on it,’” he says. And Özel, one of the original proponents of the mass gap, seems unperturbed by the find. “It’s very exciting,” she says. What’s more, LIGO and Virgo have shown that it’s possible to form a low-mass black hole in a different way. In August 2017, they spotted the merger of two neutron stars, which produced, presumably, a black hole of 2.7 solar masses.
The real puzzle may be the extreme mismatch in the masses of the black holes in the new observation, says Brian Metzger, a theoretical astrophysicist at Columbia University who was not involved in the work. Just a few weeks ago, LIGO and Virgo announced an event in which one black hole outweighed the other by a ratio of four to one. In the new event, the ratio is nine to one. “The interesting thing is the extreme mass ratio, which is hard to produce through most [models] people have focused on,” Metzger says.
Nobody knows how tightly orbiting pairs of black holes form in the first place. Most theorists have focused on two general scenarios, Metzger says: either orbiting pairs of stars that both collapse to form black holes or individual black holes that rattle around in tiny, old galaxies called globular clusters that somehow manage to pair up. Both scenarios tend to make pairs in which the black holes have similar masses. So theorists may have to dream up new hatcheries for lopsided black hole pairs, Metzger says, perhaps by starting in the dense centers of large galaxies.