On May 21, 2019, scientists using a set of ultra-sensitive detectors heard something strange. For years, gravitational waves from colliding black holes had been detected, which usually sound like a rising “chirp.” But this signal was different. It was a sharp, violent “crack” in the fabric of the universe that lasted for less than a tenth of a second. This bizarre event, named GW190521, was so baffling that it challenged the current understanding of the cosmos and led some scientists to consider a mind-bending possibility: that a signal from a parallel universe may have been received.

A Mysterious “Crack” in Spacetime

The universe can be imagined as a giant, stretchy trampoline, a concept physicists call spacetime. When massive objects like black holes move and crash into each other, they create ripples in this fabric, much like a bowling ball rolling across a trampoline. These ripples are gravitational waves. For a century, they were just a theory Albert Einstein conceived, but thanks to incredible detectors called LIGO and Virgo, it is now possible to “hear” them. These L-shaped detectors use powerful lasers bounced between mirrors down miles-long vacuum tunnels. They are so sensitive they can measure a wobble in spacetime smaller than a fraction of an atom—a disturbance a thousand times smaller than the nucleus of an atom.

Normally, when two black holes are about to merge, they perform a final, frantic dance, spiraling around each other faster and faster. This cosmic waltz creates a very specific signal that rises in pitch, like a bird’s chirp that gets higher and louder right at the end. This “chirp” is the gold standard, the expected sound of a black hole collision because it tells the complete story of the event. But GW190521 was missing this buildup entirely.

The experience was akin to walking into a movie theater and only seeing the final explosion, with no clue about the characters or the plot that led to it. The signal was just a sudden, powerful burst, as if two massive objects appeared from nowhere, crashed, and then vanished. It was an immediate and profound mystery that left astronomers scratching their heads.

A Cosmic Rule-Breaker

Trying to make sense of the signal, the main team of scientists concluded that it came from two huge black holes merging—one about 85 times the mass of our sun, and the other about 66 times. The crash was an event of almost unimaginable violence, releasing more energy in an instant than all the stars in the universe combined. The collision created a new black hole 142 times the sun’s mass, the first of its kind ever seen. This was a huge discovery on its own, as it confirmed the existence of a new class of “intermediate-mass” black holes, a long-sought missing link between the smaller ones born from stars and the supermassive giants that live at the heart of galaxies.

But this explanation, while accounting for the signal’s power, created an even bigger problem. The 85-solar-mass black hole shouldn’t exist. According to the “rules” of how stars live and die, stars of a certain medium-large size are supposed to blow themselves to smithereens in a massive explosion, leaving nothing behind.

Think of it like a star getting too big and hot for its own good. Its core becomes so intensely hot that the energy it produces starts turning into pairs of matter and antimatter, a process that makes the star incredibly unstable. It becomes so unstable that it completely self-destructs in a runaway thermonuclear explosion. This creates a “mass gap”—a range of sizes, from about 65 to 120 times the sun’s mass, where black holes just shouldn’t be found.

Yet, GW190521 came from a black hole sitting right in the middle of this forbidden zone. It was a cosmic rule-breaker, and no one knew how it got there.

A Message from Another Universe?

Faced with a signal with a missing beginning and a black hole that shouldn’t exist, a team of researchers proposed a wild idea. What if the event didn’t happen in our universe at all? They suggested the signal was an “echo” that traveled to us from a parallel universe through a wormhole. A wormhole, also called an Einstein-Rosen bridge, is a theoretical shortcut through spacetime, like a tunnel connecting two distant places.

The theory goes that a normal black hole merger happened in another universe, but instead of forming a stable black hole, the crash briefly tore open a tunnel in spacetime—a wormhole—that connected their universe to ours.

A tiny fragment of the signal, the final burst of waves, slipped through this tunnel. Because wormholes are thought to be incredibly unstable, it would have slammed shut almost instantly. This would explain why we only heard the end of the event, not the beginning. It would also neatly explain the rule-breaking black hole, as it could have been formed under a different set of physical laws in its home universe.

The scientists who proposed this are careful to say it’s a long shot, but based on the strange data, it’s a possibility they can’t completely ignore.

More Down-to-Earth Ideas

As fascinating as the wormhole theory is, it relies on a lot of unproven ideas that sound like they’re straight out of science fiction. Most scientists, following a principle called Occam’s Razor (which states the simplest explanation is often the best), are looking for answers closer to home.

One of the leading “normal” explanations is called Hierarchical Mergers. This idea suggests the rule-breaking black hole wasn’t born from a single star. Instead, it was a “second-generation” black hole, created when two smaller, perfectly “legal” black holes merged in an earlier collision. This could happen in a very crowded part of the universe, like a dense star cluster, which acts like a giant cosmic mosh pit with stars and black holes packed so tightly together that collisions become much more likely.

Another possibility is that the black holes were on a highly Eccentric Orbit. Rather than circling each other for ages in a predictable dance, they may have had a near-miss, “fly-by” encounter that resulted in a quick, violent merger. This would naturally explain the sudden “crack” signal. It would be like two comets whipping around the sun and crashing, rather than two planets in a stable, circular orbit.

What This Mystery Teaches Us

An event like GW190521 is a powerful reminder of how much is still to be learned about the universe. A true mystery isn’t a dead end for science; it’s the opposite. It is an exciting doorway to new discoveries that forces humanity to question what is thought to be known and to come up with creative new ideas. This isn’t the first time a puzzling message has been received from space. In 1977, a radio telescope picked up a powerful, 72-second burst of energy that was so unusual the astronomer who found it wrote “Wow!” on the data printout. The “Wow! signal” has never been heard again and remains unexplained, a testament to the fact that the cosmos can still surprise.

From a different perspective, a cosmic puzzle like this invites a sense of wonder and humility. It shows that the universe is far stranger and more magnificent than can be imagined. The scientific debate itself—weighing the possibility of a wormhole against a rare type of collision in our own universe—reflects a deeper question humanity faces when encountering the unknown. Should answers be sought in other worlds, or can they be found by looking more closely at the world right in front of us? GW190521 is a signpost in the cosmos, pointing not necessarily to another universe, but to the vast, undiscovered frontiers that are still waiting to be explored. It is a call to keep listening, to keep questioning, and to remain open to possibilities beyond the wildest dreams.

Source:

1. 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

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