A wormhole from another universe? Scientists revisit the puzzling black hole GW190521

In May 2019, astronomers picked up something strange in the fabric of spacetime. The LIGO and Virgo detectors recorded a gravitational wave that lasted just one-tenth of a second. The signal, known as GW190521, was unusual straight away because it didn’t look like the normal “chirps” usually produced when two black holes orbit one another before colliding. Instead, it sounded like a cosmic crack.

The official explanation was meritorious enough in itself. Researchers with the LIGO-Virgo collaboration worked out that the signal was caused by the merger of two enormous black holes—one about 85 times more massive than the Sun and one about 66 times more massive. They merged to form a new black hole with 142 solar masses. That made GW190521 the first observed detection of so-called intermediate-mass black holes.

But that explanation was contentious. According to the laws of astrophysics, scientists explain, stars large enough to form black holes in that mass range shouldn’t exist. It’s referred to as the “forbidden gap.” To some, that had made GW190521 one of the most unusual gravitational wave finds to date.

Not everyone thinks it was just a record-shattering collision between black holes. A group of Chinese physicists has even proposed something far more revolutionary: what if the signal was not caused by a merger of black holes in our universe, but a reflection from a collision in a different universe?

Their idea revolves around wormhole physics. Wormholes, or “cosmic tunnels,” were initially thought of in theoretical models in the 20th century. They are considered to link distant parts of space or even entirely different universes. While no wormhole has ever been observed, they have been fascinating scientists and science fiction writers for decades.

In the new research by the researchers, GW190521 may have been an “echo of a wormhole.” Two black holes had merged in a separate universe. Spacetime vibration waves created as a result, called ringdowns, had travelled through a wormhole’s throat and overflowed into our universe. LIGO and Virgo detected no complete signal with a spiral phase but one early echo pulse—the only piece that was left.

To attempt this possibility, researchers used a classic model of wormhole theory called the Morris-Thorne model. This type of wormhole has a wide throat through which waves can travel, as compared to a black hole’s event horizon that captures everything.

When two black holes merge in this arrangement, the ringdown waves so created would oscillate back and forth along photon spheres, much like sound reverberating between walls. Most of the energy would be confined, but some small portion may leak through, arriving as flashes in another universe.

The researchers then built a template for what such an echo would look like. They modeled it as a “sine-Gaussian” signal, in the shape of a short bell curve with a center frequency. When they aligned this template with real GW190521 data using Bayesian statistics, the fit was surprisingly good. The frequency was about 56.9 hertz, and it lasted for around 0.02 seconds, which matched the unusual, fleeting quality of the observed signal.

Naturally, any new model has to compete with the reigning explanation: a black hole merger. In doing so, the researchers used the same cautious waveform model used by the LIGO-Virgo collaboration in their original analysis, one that includes unequal masses and spin of the black holes.

The consistency of the wormhole model with data was almost as good as that of the black hole model. Signal-to-noise ratios—the quality of how well the event shows up against background noise—were the same in all three detectors. The total score was 14.45 for the wormhole theory and 15.59 for the black hole theory.

Bayesian analysis gave the black hole explanation a small statistical boost but nothing more. The researchers determined that the merger hypothesis is the winner, but the wormhole explanation has not been ruled out. That is, the entrance to more exotic physics remains open.

If the wormhole explanation is correct, the consequences would be astonishing. Wormholes are inseparable from quantum gravity theory and the enigma of whether information is lost in black holes. The discovery of one would change how scientists think about space, time, and even the possibility of other universes.

But there are huge hurdles. The Morris-Thorne wormhole will only remain open if it is fueled “negative energy matter.” This kind of material has never been observed to exist in any practical form. Without it, wormholes will collapse almost instantly, with nothing remaining.

That only leaves one other enigma: If wormholes produce a series of echoes, why did LIGO and Virgo only see one? The scientists offer two suggestions. The wormhole may have quickly collapsed after its formation, closing off subsequent signals, or the subsequent echoes may have been too faint for equipment to detect.

The mystery does not end with GW190521. In November 2023, LIGO and Virgo reported another short gravitational wave event, known as GW231123, that also lacked a clear inspiral phase. That one was a merger creating a black hole 225 times more massive than the Sun, the largest detected. Its burst-like nature sparked new controversy regarding whether such wormhole echoes could be hiding in the data.

Physicist Qi Lai, from the University of Chinese Academy of Sciences, is convinced that only systematically made comparisons of large numbers of weird events will reveal whether wormholes are at work. If the same weird signature turns up in multiple detections, the case for exotic causes will be mounting.

For the moment, the merger of black holes is the more likely explanation for GW190521, yet the wormhole theory won’t disappear. Improving on waveform templates and sensitizing detectors even further may help future research distinguish between the two. An additional new gravitational wave event contributes another piece of information to the puzzle.

Whether or not GW190521 was the product of an “impossible” black hole or the transitory sigh of another universe, it highlights just how much remains to be discovered about the universe. Gravitational wave astronomy is only in its infancy, and enigmas such as this one are probably going to be multiplied when the discipline makes further progress.

If wormhole echoes do exist, they might open a new window into observing not just black holes, but the fabric of spacetime itself. It would imply that humans glimpsed evidence from other universes and have an entirely new tool for probing physics beyond Einstein.

Even if wormholes themselves are too bizarre, more accurate models of gravitational waves will upgrade detectors, allowing astronomers to observe stranger and farther-out cosmic events. Either way, the research pushes science toward greater inquiry into the universe—and possibly beyond.”

Research findings are available online in the journal arXiv.