On May 21, 2019, a network of observatories detected a signal from deep space that was unlike anything seen before. The signal, named GW190521, was a gravitational wave—a ripple in spacetime itself—that had traveled for seven billion years to reach Earth. Its unusual nature has led to a fascinating scientific debate, with one of the most provocative ideas being that it could be an echo from a parallel universe.
While most evidence points to a more conventional, though still extreme, cosmic event, the story of GW190521 reveals how a single unexplained signal can push the boundaries of science.
The Cosmic Anomaly: What Was GW190521?
Gravitational waves are ripples in the fabric of space. You can imagine space not as an empty void, but as a flexible, four-dimensional fabric called spacetime. Massive objects, like stars and black holes, create dips in this fabric, much like a heavy ball creates a dip on a trampoline. Huge objects moving very fast, like two black holes orbiting each other, create ripples that spread out from this dip, traveling across the universe at the speed of light. For years, scientists have detected these events as a characteristic “chirp” sound that gets higher in pitch and louder as the two black holes spiral closer and closer before finally colliding.
GW190521 was different. It wasn’t a long, rising chirp; it was a sudden “bang.” The entire signal lasted less than one-tenth of a second. In that fleeting moment, it was incredibly powerful, releasing more energy than all the stars in the observable universe combined, yet it was gone in an instant. It was less like a cosmic symphony and more like a single, sharp crack in the fabric of the cosmos.
The official analysis came from the LIGO-Virgo-KAGRA (LVK) collaboration, the global group of scientists who run the incredibly sensitive observatories that detected the signal. These facilities are technological marvels, capable of measuring a distortion in spacetime thousands of times smaller than the nucleus of an atom across a four-kilometer laser beam. Their analysis concluded that the signal came from two enormous black holes merging. The crash created an even larger object called an intermediate-mass black hole (IMBH), which weighed about 142 times as much as our sun.
This was the first time an IMBH had been seen at the exact moment it was born, confirming their existence. For a long time, these objects were a critical “missing link” for astronomers. They could see small black holes made from single stars and the supermassive giants at the center of galaxies, but they didn’t know how the giants got so big. IMBHs could be the answer—the “seeds” that grow into galactic monsters. This discovery was a huge success, but it also raised two big questions that baffled scientists.
The Scientific Puzzle: Why Was This Signal So Strange?
The official explanation, while a major breakthrough, left scientists with two puzzles that were hard to ignore, each one challenging a core scientific principle.
First, the “chirp” was missing. The long, slow spiral that comes before a black hole merger—known as the inspiral phase—wasn’t in the data. This part of the signal is like a fingerprint, providing crucial clues about the black holes’ masses, spins, and orbits before they collide. Its absence suggested this was not a typical merger where two objects circled each other for ages. It was more like arriving at the scene of a car crash and seeing only the final wreckage, with no skid marks or other evidence to explain what happened. The signal started suddenly and violently, which goes against the standard model for how black holes are expected to merge.
Second, one of the black holes was a size that shouldn’t be possible. The larger of the two was about 85 times the mass of our sun. According to our understanding of how stars evolve, stars of a certain size get so incredibly hot and unstable near the end of their lives that they trigger a runaway thermonuclear explosion. This “pair-instability supernova” is so violent it blows the star apart completely, leaving nothing behind—no neutron star, and no black hole. This creates a “mass gap,” a range of sizes where black holes formed from a single star simply aren’t expected to exist. Yet, the 85-solar-mass black hole from GW190521 is right in the middle of this forbidden zone. Finding it was as surprising to astrophysicists as a biologist finding a living dinosaur, and it challenged our fundamental understanding of how massive stars die.
An Extraordinary Claim: The Parallel Universe Hypothesis
To explain these puzzles, especially the missing chirp, a team of researchers proposed a wild idea in a paper that has not yet been peer-reviewed. Led by Dr. Qi Lai, they suggested GW190521 wasn’t a signal from an event in our universe at all. Instead, they theorized it was an “echo” from a black hole merger that happened in a parallel universe. This surprising claim is based on fascinating but unproven ideas from theoretical physics, the branch of science that uses mathematics to explore the very limits of what could be possible.
According to their hypothesis, this is what happened:
- A normal black hole merger, with its full chirp and crash, happens in another universe.
- The huge energy from the event briefly creates a “wormhole”—a theoretical tunnel through space connecting that universe to ours. This idea comes from Einstein’s theories, but scientists believe these tunnels would be incredibly unstable and collapse instantly.
- A piece of the gravitational waves from only the final “bang” of the merger travels through this short-lived wormhole.
- The wormhole then closes, cutting off the signal and stopping the earlier “chirp” from ever reaching us.
This elegantly explains why the chirp was missing, as that part of the event stayed in the other universe. However, this idea relies on a chain of unproven concepts like wormholes and parallel universes, for which we have no physical proof.
Reality Check: What Do Most Scientists Think?
While the wormhole idea is interesting, most scientists prefer a simpler explanation, following a principle called Occam’s Razor, which says the simplest answer is often the right one. They look for answers that don’t require inventing new kinds of physics. The leading theory suggests the merger happened in an Active Galactic Nucleus (AGN). An AGN is the incredibly dense, bright, and chaotic center of a galaxy, where a supermassive black hole is actively swallowing enormous amounts of gas and dust. This “cosmic mosh pit” is one of the most extreme environments in the universe and a perfect place for bizarre events to unfold.
This environment could solve both of GW190521’s puzzles:
- Solving the Mass Gap: In the thick, swirling disk of gas within an AGN, a black hole can grow quickly by “feeding” on the material around it. This process, called accretion, could easily allow a black hole to gain enough mass to grow large enough to enter the “forbidden” gap.
- Explaining the Missing Chirp: The crowded space in an AGN makes head-on collisions between black holes much more likely. Instead of a long, predictable spiral between two long-orbiting partners, two unrelated black holes could have a chance encounter and crash directly into each other. Such a collision would be brief and violent, producing the short, bang-like signal that was detected, with no preceding inspiral.
A key piece of evidence supports this theory. About a month after the gravitational wave was detected, astronomers saw a huge flare of light, named ZTF19abanrhr, coming from the same area of the sky. A merger of two black holes in empty space should be completely dark. However, this flare is exactly what you would expect if a newly formed, massive black hole was sent flying through the thick gas of an AGN after the merger. The kick from the collision would create a massive shockwave, heating the gas and causing it to glow brightly. The fact that the flare happened in the right place and at the right time is strong evidence for the AGN theory. Seeing an event with both gravitational waves (“sound”) and light (“sight”) is a powerful confirmation tool for astronomers.
A Puzzle That Pushes Boundaries
In the end, the debate over GW190521 will be settled with more data. Finding more of these short, powerful signals will help scientists figure out if they fit the AGN model or point to something else entirely. Future, more powerful observatories will surely find more of these strange events and provide a clearer answer.
While the parallel universe idea is probably not correct, its value is in how it sparks curiosity and pushes the boundaries of science. Thinking about these wild ideas, even if they turn out to be wrong, is a crucial part of the scientific process. It forces scientists to build better tools and more precise theories to test the claims. Every attempt to prove or disprove a radical idea sharpens our understanding of the universe. The legacy of GW190521 is a powerful reminder that the universe is full of deep mysteries. The ongoing mission to solve them, by testing every idea with solid proof, is what drives our understanding of the cosmos forward.m, by testing all ideas against hard evidence, is what continues to drive our understanding forward.
Source:
- Lai, Q., Lan, Q., Liu, H., Wang, Y., & Piao, Y. (2025, September 9). Is GW190521 a gravitational wave echo of wormhole remnant from another universe? arXiv.org. https://arxiv.org/abs/2509.07831
