After years of searching for gravitational waves, it finally happened: LIGO bagged the biggest one ever. Approximately 10 billion years ago, two massive black holes — weighing in at 85 and 66 times the mass of our Sun — merged together, converting approximately 8 solar masses into pure energy in the form of gravitational radiation. After journeying through the expanding Universe, those signals arrived at the National Science Foundation’s LIGO and the European Gravitational Observatory’s Virgo detectors, where they were detectable over a timespan of just ~13 milliseconds. It was the most massive black hole merger ever detected.
It’s remarkable for a number of reasons, as it sets a slew of records, including:
the most distant black hole-black hole merger (at 17 billion light-years away, accounting for the Universe’s expansion),
the most massive progenitor black holes (at 85 and 66 solar masses),
the most massive final black hole (at 142 solar masses),
the greatest amount of mass turned into energy in a single event (8 solar masses),
and the shortest-duration definitive signal ever seen (at ~12.7 milliseconds).
But the biggest surprise of all is that we didn’t expect these black holes to exist at all. Here’s the enormous puzzle presented by this new discovery, and the leading ideas on what the solution might be.
The way gravitational wave detectors like LIGO actually “see” merging black holes is that these mergers create ripples in spacetime, where space alternately compresses and expands in two perpendicular directions, in phase, as the gravitational waves pass through them at the speed of light. By creating a detector where light travels repeatedly down-and-back along two long-baseline arms in perpendicular directions, those small and periodic distance changes can be seen, down to even a tiny fraction of a wavelength of the light used. Mirror displacements as small as ~10-19 meters can be detected.
But we can’t detect every source of gravitational waves in the Universe: only the ones that have both a sufficiently large amplitude (creating a large enough change in the relative positions of the mirrors) and that fall into a frequency range that the detectors are sensitive to (based on the physical size of the detector’s arms). Ground-based detectors like LIGO and Virgo are sensitive to mergers of collapsed objects — black holes and neutron stars — ranging from a few solar masses up to perhaps a few hundred solar masses.
This newest event, now officially known as GW190521, is the heaviest black hole-black hole merger ever seen. It’s so massive — and therefore, its event horizon is so large — that only the last couple of orbits before the merger could be seen by our terrestrial detectors. The “ringdown” phase, where the post-merger black hole settles down, could actually be detected as well, which provides a phenomenal amount of information to gravitational wave scientists about the properties of this merger. It really is this massive, this distant, and inconsistent with being anything other than two black holes merging together from nearly perfectly circularized orbits.
The post-merger black hole, at 142 solar masses, is also the very first “intermediate mass black hole” ever detected. We’ve detected stellar mass black holes before, which we loosely categorize as under 100 solar masses, which are assumed to form from massive stars that go supernova, experience a catastrophic instability, or otherwise collapse entirely. We’ve also detected supermassive black holes: of 100,000 solar masses or more, which live at the centers of massive galaxies. But for the in-between black holes, this is the first.
Based on the black hole-black hole mergers already seen by LIGO and Virgo, we’d already learned an important lesson: 99% of black holes in binary, merging systems are below 43 solar masses. This is, at least so far, the first and only black hole-black hole merger we know about where both members are above that ~43 solar mass threshold. It’s an important milestone for a vital reason: there must be some way to build up these supermassive black holes from smaller black holes, and that requires a population of these intermediate mass black holes. At last, we’ve discovered the very first one.
We know how the first one we’ve ever seen arose: from the merger of two lower-mass black holes. We don’t know if mergers, accretions, or some other mechanism (such as the direct collapse of material) is responsible for the majority of these intermediate mass black holes that must exist in the Universe, but at least we know how the first one came about. What we don’t know, however, is how we physically created at least one of the black holes — the 85 solar mass one — that led to its formation.
In theory, the lower-mass black holes are called “stellar mass” black holes because they arise as the remnants of stars, which live, die, and leave a black hole remnant behind. For all of the previous black holes seen by gravitational wave detectors, this explanation worked just fine, as the theoretical predictions for how massive stars died lined up with our observations of the black holes that existed.
But an 85 solar mass black hole? That, according to our best current understanding of stellar evolution, shouldn’t be possible.
Here’s why: if a star is massive enough to go supernova, it will form either a neutron star or a black hole, depending on its original mass. In general, the more massive a star is, the more massive the remnant it leads to. But this only works up to a point. Above a certain mass, the temperature inside the star gets so hot — above about 3 billion K — that the most energetic photons, which provide the radiation pressure that hold the star up against gravitational collapse, can spontaneously convert into matter-antimatter (electron-positron) pairs. This is a disaster for the star.