Black holes collide in universe-shaker science said should never happen

Two colossal black holes merging and creating a single, even larger black hole have given astrophysicists the biggest gravitational wave spike yet detected, a space-time shaking collision generating wave energy equal to eight Suns. The mysterious signal was captured by the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo detectors on May 21, 2019, a vanishingly brief jolt with huge implications.

GW190521, as the signal was labeled, only lasted less than a tenth of a second. However that undersells just what was involved in it reaching Earth, given scientists believe it was generated by a black hole merger roughly 5 gigaparsecs away from our planet.

That would've taken place when the universe was around half its age, the researchers calculate, and thus make GW190521 the result of one of the most distant gravitational-wave sources detected so far. They believe that it was generated when two black holes, spiraling together, and with masses around 85 and 66 times the mass of our own Sun respectively, finally collided. The resulting black hole had around 142 solar masses.

It also released a torrent of energy, equal to around 8 solar masses, which rippled as gravitational waves through the fabric of space-time. According to Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS) and one of the Virgo team, "it's the most massive signal LIGO and Virgo have seen."

What makes the black holes unusual is that they're neither as small, or as large, as we've been familiar with so far. Until now, we've known of stellar-mass black holes and supermassive black holes. The former, generated when massive stars die, typically measure in at under a few tens of solar masses. The latter are anything from hundreds, thousands, or even billions of solar masses in mass.

These colliding black holes, however, fall into the intermediate mass range, the first time such examples have been spotted. It runs counter to one of the current theories about how black holes are created, the so-called "pair instability mass gap," which had suggested that collapsing stars should not be able to generate black holes between around 65 and 120 solar masses.

Figuring out just what led to an 85 solar mass black hole will be occupying the researchers next, though there are already some possible theories. A hierarchical merger, for example, could mean the two progenitor black holes themselves were the result of two earlier, smaller black hole mergers. Or, it could be something more groundbreaking, with other sources of gravitational waves potentially including the resonance of a "cosmic string" generated just after the universe itself experienced inflation after the Big Bang.

"Since we first turned on LIGO, everything we've observed with confidence has been a collision of black holes or neutron stars," Alan Weinstein, professor of physics at Caltech and a member of the LIGO team, explains. "This is the one event where our analysis allows the possibility that this event is not such a collision. Although this event is consistent with being from an exceptionally massive binary black hole merger, and alternative explanations are disfavored, it is pushing the boundaries of our confidence. And that potentially makes it extremely exciting. Because we have all been hoping for something new, something unexpected, that could challenge what we've learned already. This event has the potential for doing that."

The next challenge is trying to identify more intermediate black holes which could indicate their existence is less rare than preexisting theories allow for. A paper released today includes further investigation into the broader astrophysical implications of the discovery.