The collapse of a wormhole connecting two universes may have been witnessed by the world’s three gravitational wave observatories, according to a new theory put forward that attempts to explain an extremely anomalous gravitational wave pattern recorded by the arrays in 2019. Although the likelihood of an Einstein–Rosen bridge having connected to our universe through the collision between two black holes in an alien universe is comparatively far-fetched, the study’s findings revealed that there’s more than enough uncertainty regarding the conventional explanation—two extremely large black holes in our universe going bump in the night—that it leaves plenty of room for the idea that the signal had a more exotic origin.

The gravitational wave event in question, GW190521, was recorded by both the two LIGO Laser Interferometer Gravitational-Wave Observatories (LIGO) and the Virgo interferometer array on May 21, 2019, and proved to be a remarkable event for a number of reasons: first, the two objects that collided to generate the gravitational wave were 85 and 66 times the mass of the Sun, the two largest colliding bodies recorded up to that point.

Consequently, the merger formed a black hole with the mass of 142 Suns, marking the first time an intermediate-mass black hole (IMBH), a class of previously-theoretical black holes between 100 and 100,000 solar masses, had been glimpsed. IMBHs are supposed to occupy an otherwise unobserved gap in mass between the more familiar stellar black holes (between 5 and 100 solar masses) and supermassive black holes (over 100,000 solar masses), like Sagittarius A*, the 4 million-solar mass SMBHthat resides near the center of the Milky Way.

GW190521 was also peculiar in that it was the only black hole merger that appears to have emitted a burst of electromagnetic radiation, coinciding with a flash of light that occurred 17 billion light-years away; all other recorded mergers between two black holes have been entirely dark—they are a celestial phenomenon that light cannot escape from—with only an outburst of gravitational waves marking the collisions of these colossally-massive objects.

The merger did happen in the vicinity of a third, supermassive black hole that was otherwise not connected to the event, and it was theorized that if the newly-merged black hole passed though this SMBH’s accretion disk—captured matter that is slowly spiraling into the event horizon—it could have generated a burst of EM radiation like the one recorded. However, it was expected that the flash would reoccur after 1.6 years as the merged black hole re-connected with the accretion disk as it orbits the SMBH, but such a burst has not been seen in the six years since GW190521.

But it was GW190521’s unique gravitational signature that made it truly unique: ordinarily, black holes don’t simply collide with one-another; rather, the intersecting pair fall into a binary orbit, spinning rapidly around one-another as they spiral inward closer and closer together. This gradual merging results in the generation of relatively mild gravitational waves at first, rising into a more pronounced “chirp” when the waves are converted to audible sound.

However, GW190521 lacked the quarter-second-long rise in tone characteristic of other mergers’ chirps, something that would have been quite pronounced given the combined mass of the two objects; instead, it produced something better described as a “crack” that lasted less than a tenth of a second.

This unusual lack of an inspiral buildup led researchers to conclude that the two bodies that generated GW190521 weren’t in a binary orbit, but instead were two independent bodies that had smacked into one-another head-on, a highly-improbable event given the low likelihood of two objects as seemingly rare as these two crossing paths in such a direct manner in the vastness of the cosmos. However, such a circumstance could explain the event’s peculiar signature.

That improbability—along with the other unusual aspects of the event—led a team of physicists led by University of Chinese Academy of Sciences physicist Qi Lai to hypothesize that the black hole merger that generated GW190521 may have taken place in another universe altogether, and briefly connected to our own universe by an extremely short-lived wormholethat formed as a result of the energy released in the collision.

To test the idea, Lai’s team developed two waveform models: one that simulated a gravitational signal as if it had passed through a wormhole, and another that simulated an ordinary binary black hole merger, and compared the results to the recorded data. While the outcome favored the more prosaic, in-universe explanation, it only did so by a narrow margin, leaving more than enough leeway for the wormhole hypothesis to be plausible.

“The ringdown signal after BBHs [binary black holes] merged in another universe can pass through the throat of wormhole and be detected in our universe as a short-duration echo pulse,” according to the study text, titled “Is GW190521 a gravitational wave echo of wormhole remnant from another universe?”

Channeling the signal through a wormhole in this manner would explain the lack of an inspiral phase to the resulting gravitational wave’s signature: since the wormhole would only come into existence at the moment of the merger itself, it would only be that portion of the event that would be broadcast to our universe, leaving only GW190521’s “crack” detectable to LIGO and Virgo.

However, four years later another merger of a similar magnitude occurred that, while lacking GW190521’s burst of EM radiation, also resulted in a crack-like signal instead of the usual chirp, implying that something about the merger of especially massive black holes may preclude the gradual rise in volume of the incoming gravity wave. Recorded in late November 2023, the two black holes that generated 2023’s GW231123were both already IMBH-sized—137 and 103 solar masses, respectively—resulting in a whopper of a 225-solar mass IMBH.

Needless to say, more research will be required before it can be reasonably assumed that we’ve recorded the first evidence of an honest-to-goodness Einstein–Rosen bridge. In the meantime, we can only imagine what it’s like to surf the crest of a gravity wave through a rapidly-collapsing wormhole…