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Adapted from Crush: Close Encounters with Gravity by James Riordon. Published by MIT Press. Copyright © 2025. All rights reserved.

They say the Goatman prowls the woods at night near my home in Maryland. He was once a biologist named Stephen Fletcher at the Beltsville Agricultural Research Center. That was before the accident with goat DNA transformed him into a half-­human, half-­goat monster who devours victims that he slays with an axe. It’s been decades since I first heard of the Goatman.

Honestly, I’m fairly certain that the carnivorous goat–­human hybrid isn’t real. It’s hard to prove something doesn’t exist, though. There are things you don’t believe in because they’re at odds with what you’re confident is true. Perpetual motion machines and alchemists’ stones that turn lead into gold don’t exist. If they did, lots of established science would need to be wrong.

Then, there are other things you might not believe in, mostly because of a lack of evidence. Bigfoot is one. There’s no reason I can think of that a large, hairy humanoid couldn’t exist. Still, I’m pretty sure it doesn’t. I also don’t believe in the Loch Ness monster or that there’s a Goatman stalking people in the woods out back. All three are cryptids —­ legendary, probably mythical, creatures that are among the subjects of cryptozoology. If one of them turned up, though, it wouldn’t upset science. We’d just add them to the biology books.

Einstein’s theory of relativity comes with its own set of elusive cryptids. At least one former specimen in the cryptozoo is now an established entry in astronomy and physics textbooks: the black hole. Science has long allowed black holes to be viable denizens of the cosmos, in theory. For decades after Einstein developed general relativity, though, few people believed that gravitational monstrosities actually existed any more than they believed in giant, hairy hominids rumored to be prowling the Pacific Northwest. Now we know they’re quite common (black holes, not Bigfoot).

There are plenty of other bizarre exhibits in Einstein’s cryptozoo. Without a doubt, some are destined to break free to join the strange-­but-true menagerie of general relativity. Here are a few gravitational cryptids to keep an eye out for in the coming decades.

White holes

According to general relativity, white holes exist, along with their sibling black holes. While black holes offer one-­way passage into their event horizons for anything that ventures too close, white holes are black holes running in reverse. They allow nothing in and spew matter and energy out. There’s plenty of evidence of black holes. If there are any white holes within view of our telescopes, they should be pretty obvious to the casual observer.

The white hole you can make out of water in your kitchen sink follows the same math and rules that govern the astronomical white holes. The liquid version shows that the math works. But like the Goatman, white holes haven’t been spotted anywhere in the Universe yet. Considering that the current crop of telescopes can see stars 13 billion years old, it seems likely that there aren’t any white holes in existence. Or, at least, they’re much, much rarer than the quintillions of black holes scattered through the Universe.

If white holes ever do turn up, astronomers will be stunned, but a lot of general relativity experts will get to say, “I told you so.”

Wormholes

Wormholes are tunnels through spacetime. They’re potentially shortcuts linking distant parts of the Universe, or passageways to other universes entirely. Einstein and his assistant Nathan Rosen discovered wormholes tucked in the theory of general relativity in 1935. The pair weren’t the first to catch a mathematical glimpse of them. Still, the simplest type of wormhole is known today as an Einstein–Rosen bridge. The entrance to one looks just like a black hole.

A wormhole could connect two black holes together, but there’d be no way to get out once something has gone in. So, the exit had better look like a white hole. Considering that we haven’t found any white holes, Einstein–­Rosen bridges that might be disguised as black holes must offer exclusive one-­way trips out of our Universe.

One thing relativity can say with certainty about wormholes is they are fragile things. If you tried to take one as a cosmic short-cut, it would rapidly collapse. Unless you were zipping through at near light speed, you’d be ripped to shreds along the way. If you did make it through, you might have just enough time to send back a selfie from your exotic new locale, but the wormhole would snap shut before you could ever come home.

Theoretically, you could prop open an Einstein–­Rosen bridge with ghost matter. If it exists, ghost matter repels everything instead of attracting like regular matter does. No one has run across ghost matter yet or figured out a way to make it. A super-­advanced alien might be able to whip up some without violating any rules of physics that we know of, but the recipe is beyond us for now.

Nevertheless, people have made wormholes in the lab, much as you can make a white hole in your kitchen sink. Lab-­based wormholes are tiny —­ the one that California Institute of Technology physicist Maria Spiropulu and colleagues built in 2022 would need to be millions of trillions of times longer just to cross from one side of a proton to the other. You won’t be traveling far by wormhole any time soon.

The Caltech wormhole requires the machinery of quantum computers. Instead of spacetime that’s warped with the intense gravity of concentrated matter, the wormhole-­on-­a-­chip runs in quantum components called qubits. It’s a real wormhole in much the same sense that a kitchen sink white hole is real —­ in the lab or sink, wormholes and white holes follow the rules of general relativity. But there’s no indication whether either one can exist in the wild.

Lab-­grown wormholes may prove handy. They could become pieces in super-­powerful quantum computers. Perhaps more importantly, they potentially offer insight into the conflict between gravity and quantum mechanics that tormented Einstein and still troubles many physicists today.

After a century of scientists on the hunt for the great white whale of physics, no theoretical harpoons have stuck in the elusive theory of everything.

Dark stars

Despite photos and videos, the Loch Ness monster and the Goatman remain dubious. Sometimes, though, the rumors and hazy reports from the field are true: There’s something out there. Giant squid probably never pulled ancient mariners’ ships below the waves, but they are no longer the stuff of legend.

Now, there are some indistinct photos that may have captured a tribe of gravity’s cryptids in their native environment. Like an out-of-focus Bigfoot lurching through the background of a vacation camping photo, dark stars seem to be lurking in the far distance in James Webb Space Telescope images of some of the oldest objects in the Universe.

Just as Bigfoot is a larger, furrier version of a human, dark stars are larger, fluffier versions of regular stars. If they exist, dark stars are mostly made out of the same stuff as other early stars — hydrogen and helium from the Big Bang. Only 0.1% of a dark star is dark matter, but it makes all the difference.

Dark matter probably comes in antimatter particles as well as normal dark matter particles. If so, when they encounter each other, a dark matter particle and its antimatter version will annihilate and release energy. It doesn’t happen often in most of the Universe because dark matter is spread out so thinly as a rule.

When the Universe was very young, though, stars began to form as hydrogen and helium were gravitationally attracted to dark matter–­rich regions. That, in turn, drew in more dark matter, which increased the chances of matter and antimatter versions of dark matter running across each other and annihilating.

As the story goes, the annihilations released energy, heating the gas clouds and puffing them up, preventing the high pressures that lead to the nuclear fusion that powers most stars. Although they were dark matter–­driven, dark stars would have been 10 billion times brighter than our Sun and tens of millions of times more massive.

Once the dark matter/antimatter supply runs out, after millions to billions of years, the enormous clouds shrink. If they existed, the dark stars then turned into enormous fusion-­powered stars. Those would quickly burn out and end up as supermassive black holes like the one at the center of our galaxy.

As of this writing, the James Webb Space Telescope has spotted what could be three dark stars. Much like photos touted as evidence of Bigfoot, the dark star images aren’t clear enough to tell if they’re really the gravitational cryptids they appear to be. The fuzzy images could be ancient galaxies masquerading as dark stars in Webb images, like the hoaxsters in furry costumes who are usually the subjects of Bigfoot snapshots. High-­resolution photos taken with longer exposure times will probably reveal the truth, one way or the other, in the coming years.

If confirmed, dark stars could soon follow the path of black holes from membership in the gravitational cryptozoo to the catalog of confirmed astronomical entities.

Black holes used to be part of Einstein’s cryptozoo. But throughout the 20th century, the evidence for their existence continued to grow, such as these images from NASA’s Nuclear Spectroscopic Telescope Array that reveal the activity of supermassive black holes at the heart of two colliding galaxies. Today, black holes are commonplace in our understanding of the Universe. (Credit: NASA)

Other universes

Perhaps we have already met a gravitational cryptid, and it is us. Our Universe could be one of a multitude of universes. There are certainly plenty of scientists among us pondering the possibility. There may be creatures in other universes wondering if they will ever find a way to know if our Universe exists.

One reason to think our Universe isn’t alone or unique stems from the fact that, time and time again, science has shown that we are not special. You’re one of over eight billion humans, including at least a few who are undoubtedly much like you. There’s nothing much special about where you are on this spherical planet —­ you could hop up and move to another point at any time. The Earth is one of countless similar planets throughout the cosmos, probably including many others encrusted with life, and we orbit an unremarkable star in an unremarkable solar system at an unremarkable place in the galaxy. Our galaxy doesn’t stand out among the trillions of others in existence, of which not a single one is at a particularly special place in the Universe. Considering all that, why should our Universe be special?

Being the only one would certainly make our Universe unusual. Based on the progression of science that brought us here, you might guess that ours is one of many, probably infinitely many, universes. There are a few ways that could happen. We might live in one of a long line of universes that expanded in a big bang, then collapsed in a big crunch, only to bang and expand again in an eternally inflating and deflating spacetime. Or perhaps there are innumerable universes right next to us in dimensions we can’t visit.

The unifying problem so far with various ideas about other universes is there has been no way to check to see if they’re out there. That may not always be the case. Certainly, when it came to all the other ways we thought we were unusual in the past, advances in science ultimately freed us of the delusion. From oceangoing ships that showed the Earth is a globe to telescopes that revealed hosts of planets around distant stars, we have managed to burst our bubble of uniqueness at every other juncture where you could have argued that something about us or the place where we live is special.

Maybe we’ll uncover the gravitational cryptids that are other universes. Or maybe someone from outside of ours will find us as we slink about in the multiverse like a cosmic Bigfoot.

Theory of everything

The Moby Dick of gravity is out there. The theory of everything is one of gravity’s cryptids that most physicists believe in their heart of hearts is real.

Scientists have been after a theory of everything for at least 200 years. You could potentially argue that the pursuit goes back millennia or more. Einstein went to his grave chasing it, with no more success than Ahab in his pursuit of the white whale.

There are some who say that there is no theory of everything. Maybe reality isn’t so accommodating. If we can’t see inside black holes, or look back to the beginning of the Universe, does it really matter anyway? There’s nowhere else that the quantum world and relativity collide.

Physicist Roger Penrose has proposed that there may be a cosmic censor that ensures all singularities are hidden from us behind event horizons. Like triple-­X ratings on films, the censorship prevents us from seeing things we supposedly can’t handle. Apparently, the scene cut from the cosmic cinema reveals the horror of the breakdown of physics as we know it. If so, we have no need for a theory of everything, whether one exists or not.

After a century of scientists on the hunt for the great white whale of physics, no theoretical harpoons have stuck in the elusive theory of everything. Will physicists manage better than Ahab in the end? I believe so.

I only hope I’m alive to see it.

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