Scientists think they might have explained the origin of a bizarre ripple in spacetime that swept through Earth on May 21, 2019, and has defied easy explanation ever since.
This disturbance in the very fabric of the universe, known as a gravitational wave, may have been produced by a type of cataclysmic merger between black holes that has never been seen before, potentially shedding light on the mysterious dynamics between these exotic objects, reports a new study.
Gravitational waves are generated by extreme cosmic phenomena, such as collisions between black holes or the explosive deaths of massive stars. Since 2015, scientists have been able to capture these incredibly subtle waves using sophisticated detectors, a breakthrough that has opened an entirely new window into the universe.
Over the past several years, detectors have captured about 90 gravitational waves created by black hole mergers that occurred far away in space and time, but none have been quite like the event called GW190521, which is named after the date that it arrived at Earth. The two black holes that merged to make this wave were both several dozen times the mass of the Sun, making this the biggest black hole union ever detected.
However, the short duration and unusual signature of the wave have sparked debate about the masses, spins, and orbits of the two black holes that sent these ripples into space. In other words, it’s not exactly clear just what kind of a merger it would take to make these weird waves.
Now, scientists led by Rossella Gamba, a physicist at the Friedrich Schiller University Jena in Germany, have suggested that the strange properties of GW190521 might be explained if two nonspinning black holes passed each other in space and become gravitationally entangled in an unusual hyperbolic orbit.
If this hypothesis is true, it would make GW190521 “the first gravitational wave detection from the dynamical capture of two stellar-mass nonspinning black holes,” a finding that could help to resolve longstanding questions about how these strange systems form, according to a study published on Thursday in Nature Astronomy.
“Gravitational waves from ~90 black hole binary systems have been detected and their progenitors' properties inferred so far by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo experiments,” Gamba and her colleagues said in the study. “This has allowed the scientific community to draw conclusions on the formation channels of black holes in binaries, informing population models and at times defying our understanding of black hole astrophysics.”
“The most challenging event detected so far is the short-duration gravitational-wave transient GW190521,” the team continued. “We analyze this signal under the hypothesis that it was generated by the merger of two nonspinning black holes on hyperbolic orbits” and “find that the hyperbolic merger hypothesis is favored” compared to previous models.
The new study envisions a crowded space environment that might contain a higher than normal population of black holes, such as young star clusters or near the cores of active galaxies. In these conditions, the odds are higher that black holes on totally separate trajectories might pass close enough to each other and become entangled in what’s known as a “dynamic capture.”
These dynamic systems behave very differently from other black hole binaries that have more stable circular orbits. Gamba and her colleagues suggest that some of the weird properties of GW190521 might be explained if two black holes—neither of which is rotating—capture each other, producing two brief but intense encounters followed by a merger into an intermediate-mass black hole.
“Dynamical captures have a phenomenology radically different from quasi-circular mergers,” the researchers said. ”The close passage and capture of the two objects in hyperbolic orbits naturally accounts for the short-duration, burst-like waveform morphology of GW190521 even in the absence of spins.”
“While observational evidence for GWs from dynamical captures before our work was lacking, such events are not incompatible with the current detection rates, although these rates would require corrections to take into account the large masses of GW190521,” the team added.
The hypothesis offers a new look at a signal from space that has puzzled scientists for years. Still, it will take more wave detections, and simulations of black hole mergers, to confirm this particular origin story—or any others—for GW190521, assuming this cosmic quagmire can be solved at all.
To that end, scientists continue to develop more sensitive gravitational wave detectors, and we are likely to be awash with new discoveries in the coming years. These ripples in spacetime are allowing us to peer deep into the universe to glimpse objects that have long been invisible to us and that have backstories we are only now learning to decipher.