Parallel universes seem like something you’d find in a sci-fi novel. But they could be more real than you think, say scientists.

The idea of multiple, parallel universes existing beyond the scope of our own is enormously exciting. Imagine the prospect of infinite other versions of yourself, all with wildly different lives. One of them might be an international sports star; another might be a Nobel Prize winner. It’s fun to think about, but it’s still just an idea. Now, though, scientists think they may have discovered proof of a parallel universe. And the evidence suggests that it might have once crashed into our own.

There are swathes of modern-day science that sound like they’d fit better into science fiction. Indeed, some concepts feel like they’ve come straight out of the movies. For example, the tractor beam that pulled the Millennium Falcon towards the Death Star in Star Wars could become real. And it could be sooner than you think, according to NASA engineer Paul Stysley.

Nevertheless, few sci-fi concepts sound as implausible in reality as the idea of parallel universes. Yet they’ve been a popular crutch for pop culture stories for decades. For instance, The Wizard of Oz distinguished between two separate, parallel worlds – one magical, one mundane – by filming one in color.

Comic-book fans, meanwhile, will know all too well how parallel universes can be deployed in the service of storytelling. DC Comics introduced the Phantom Zone in 1961, as a separate dimension to where Superman banishes his enemies. More recently, films including Donnie Darko and The Cloverfield Paradox have incorporated the idea of other universes into their narratives.

Plenty of TV shows have used the idea of parallel universes as the basis for an ongoing story, too. Star Trek spin-offs Deep Space Nine, Enterprise and Discovery all took the original series’ “mirror” universe and developed multiple plotlines from it, for instance. And animated hit Rick and Morty relies heavily on the idea for its wacky stories.

All this is not to say, however, that parallel universes are the sole preserve of fiction. In fact, some scientists think they may actually be completely real. You might have heard this referred to as multiverse theory. At the moment, though, there’s no real way to prove beyond doubt that multiple universes exist. Hence why it’s a theory.

According to multiverse theory, our universe is in its own “bubble” – and there are many more bubbles out there. These universes could have been born out of cosmic inflation, the period of quick expansion that occurred immediately after the big bang. As far as our particular cosmic vicinity is concerned, inflation came to a stop some 13.7 billion years ago.

The idea of cosmic inflation is broadly accepted in scientific circles, giving multiverse theory some basis in reality. However, while that inflation is generally said to have occurred in the instant after the big bang, multiverse theory posits that it’s still going. “I would say most versions of inflation in fact lead to eternal inflation, producing a number of pocket universes,” Alan Guth, a proponent of inflation theory, told New Scientist in 2015.

According to Guth and other proponents of multiverse theory, this eternal inflation is what keeps new universes or “bubbles” constantly forming, driven by energy that occupies empty space. Indeed, they even suggest the potential that separate universes could form within those bubbles – from which they derive the term “pocket universes.”

To fully understand the drive behind multiverse theory, it’s also worth considering string theory. This is what underpins everything we know about the constants of nature: Newton’s gravitational constant, or the masses of elementary particles, for instance. But it also posits that those constants could be entirely different within different bubbles.

What this tells us is that within these different bubbles, the laws of physics would be different to those that we experience in our own universe. Not in all of them – some would be nearly identical. But that’s the beauty of the multiverse. Thanks to eternal inflation, every possibility you can dream up apparently has happened, at least within the confines of the theory.

To that end, multiverse theory can also go some way to explaining life within our own universe. After all, if it’s true, then it only stands to reason that the conditions to create our existence would occur because every possible outcome also occurs. So the exact laws of physics needed to bring us into being would have ended up aligning in at least one universe.

If you take it at face value, multiverse theory can also help to explain why we’ve never encountered alien life. If every possible combination of particles exists at least once within the multiverse’s different branches, then that makes it entirely possible for one of those branches to support life – ours.

According to data scientist Jeremie Harris, though, we shouldn’t expect to see life elsewhere in our universe, if the multiverse theory is correct. “Just like you wouldn’t expect a lottery winner to win twice, the odds against life evolving a second time in the same universe would be incredibly low,” he wrote for Medium in 2018.

Even if these parallel universes do exist, though, it doesn’t mean we’ll one day be able to travel to them, a la the Planet Express crew in Futurama. Indeed, thanks to eternal inflation, the space between each of these bubbles will simply keep expanding forever. And that means each universe will just get further away, forever out of reach.

Unfortunately, then, there’s seemingly no way to prove that these alternate universes are currently floating out beyond the reach of our own. However, some scientists believe that we may be able to tell whether they were ever there at all. After all, if each universe exists in its own bubble, then it’s not hard to imagine these bubbles touching at some point, before they began expanding away from each other. And if they did, they’re going to have left a mark.

To understand exactly how this phenomenon could occur, however, we need to know a little background – literally, because it’s all underpinned by the cosmic microwave background. In simple terms, the cosmic microwave background is radiation left over from the universe’s early years, which occupies all space. It was first discovered by a pair of U.S. radio astronomers in 1964, and is strong evidence that the Big Bang occurred.

In 2013 the European Space Agency (ESA) produced a new map of the cosmic wave background after a four-year mission by its Planck satellite. The colorful map measures the temperature of the background radiation across the known universe, charting where light imprinted on the cosmos around 380,000 years after the Big Bang.

Indeed, the real beauty of the cosmic microwave background is that it can give scientists an insight into the earliest moments of the universe. While it was once 3,000 Kelvin – or nearly 5,000 degrees Fahrenheit – it’s since cooled to near absolute zero. However, there are fluctuations in that temperature. And it’s these fluctuations that inflationary theorists use to support their ideas.

Back in 2007, Matthew Johnson – Assistant Professor of Physics & Astronomy at York University in Toronto – theorized that if a parallel universe ever collided with our own, the cosmic microwave background would tell us. Indeed, Johnson and his PhD adviser reckoned that these events would appear as “circular bruises” on the map.

These “bruises” would indicate a change in temperature, and they could go either way. If a collision did happen, it would affect the period of inflation in that area of the map. A longer than normal inflationary period would result in a comparatively lower density of matter, or a colder spot on the cosmic microwave background. On the other hand, a less lengthy period would show up as a warmer spot.

Image: NASA/JPL-Caltech/Univ. of Virginia

Using data from an earlier mission to chart the cosmic microwave background, created by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), Johnson and a team of researchers from University College London detected possible evidence of four such temperature fluctuations. “There’s no obvious, boring explanation for the features,” Johnson told New Scientist in 2010.

However, the scientists needed the full map of the cosmic microwave background to make sure. If collisions with parallel universes really were responsible for these fluctuations, then there would be other signs too. For instance, the orientation of photons in the map produced by the Planck satellite could be an indication that there’s something to the theory.

A few years later, the ESA finally produced its high-resolution map of the cosmic microwave background. It wasn’t long, then, before scientists began examining it closely for these “calling cards” of other universes. And by 2015 Ranga-Ram Chary, a researcher based in California at the Planck satellite’s U.S. data hub, revealed the results of his painstaking analysis.

However, Chary hadn’t just spent all that time staring at the ESA’s map. Instead, he had simply stripped it away from Planck’s model of the entire known universe. But he didn’t stop there: after removing the cosmic microwave background, he took away the planets, stars, dust and any other gases. With all that gone, the only remainder should have been noise.

Yet, just as Johnson theorized back in 2010, there were some strangely bright patches in among that noise. According to Chary, these spots could well be evidence that a parallel universe once bumped into our own. And there may even be a way to tell, thanks to the recombination era, which occurred a few hundred thousand years after the Big Bang.

Recombination marks the period when hydrogen was first created from protons and electrons, resulting in particular light signatures. And it’s these signatures that can help us distinguish between our own universe, and potential others. “This signal is one of the fingerprints of our own universe,” the University of Cambridge’s Jens Chluba told New Scientist in 2015. “Other universes should leave a different mark.”

Normally, any such signatures would be masked by the cosmic microwave background. When Chary stripped it away, however, he found areas that were a whopping 4,500 times brighter than even the theory suggested. And one possible reason for those anomalies is that another universe collided with ours, leaving behind excess protons and electrons – thereby amplifying the recombination process.

The caveat to that exciting prospect, however, is that you would need a universe with around 1,000 times more particles than our own on the other end of that collision. Fortunately, that’s entirely possible in the realm of multiverse theory. After all, everything is possible in a scenario where infinite universes can create infinite conditions.

However, that doesn’t necessarily mean that Chary’s theory is definitely what happened – that’s why it’s still a theory. And it’s worth remembering that we’ve already had “false alarms” when it comes to the existence of parallel universes. Back in 2014, a team of researchers at the South Pole detected a signal that could have expanded our understanding of inflation.

Using a powerful telescope, the team spotted light signals that may have been clear evidence to back up inflation theory. Or rather, they would have been, had they not actually originated from within our own galaxy. Indeed, it turned out the signals were actually just grains of dust. Consequently, the researchers hadn’t actually discovered anything groundbreaking at all.

Unsurprisingly, one of the people who helped debunk that 2014 finding also thinks Chary’s proposal could be the same thing. “I suspect that it would be worth looking into alternative possibilities,” David Spergel told New Scientist in 2015. “The dust properties are more complicated than we have been assuming, and I think that this is a more plausible explanation.”

Other scientists are even less optimistic about Chary’s theory. Joseph Silk, a physics and astronomy professor at Maryland’s Johns Hopkins University, told New Scientist that the idea of a parallel universe is “completely implausible.” And he shut down any suggestion of their existence from Chary’s analysis, saying that while the anomalies exist, they can’t possibly be an alternate universe.

Of course, Chary is aware that the burden of proof still lies with him. However, he has attempted to refute the easier explanations for the anomalies. For instance, it being simply carbon monoxide would be unusual, and the emission isn’t strong enough to be carbon. If the bright spots are dust, meanwhile, they’d have to be the least warm dust ever seen.

“I am certain he made every effort to ensure that the analysis is solid,” Chluba acknowledged. “It will be important to carry out an independent analysis and confirm his finding.” Indeed, just because other explanations could theoretically be ruled out, it doesn’t mean that the signals are definite proof of parallel universes.

It’s worth bearing in mind that if the multiverse theory could be proved, it wouldn’t be great news for physicists. That’s because if there is an infinite number of alternate universes, it would mean there’s really nothing truly special about our own. Indeed, the conditions to create life would simply be pure chance and bound to happen in a state of infinite possibilities.

Ironically, one of the main proponents of inflation theory now doesn’t agree with multiverse theory, despite helping create it. Paul Steinhardt, of Princeton University, told Scientific American magazine in 2014, “Our observable universe would be just one possibility out of a continuous spectrum of outcomes. So, we have not explained any feature of the universe by introducing inflation after all. We have just shifted the problem of the original Big Bang model… to the inflationary model.”

Moreover, Steinhardt has called multiverse theory “unscientific,” because there’s no way to rigorously test it. And Chary’s findings could also become further unstuck by the limitations of the data that he’s working with. Indeed, the Planck satellite was intended to map the cosmic microwave background. However, it wasn’t built to measure the specific distortions Chary spotted.

As a result, some physicists have argued that parallel universes aren’t a legitimate topic of scientific inquiry. Indeed, Joseph Silk and his fellow scientist George Ellis wrote in a 2014 paper, “As we see it, theoretical physics risks becoming a no man’s-land between mathematics, physics and philosophy that does not truly meet the requirements of any.”

At the moment, then, there’s really no way to totally prove or disprove the existence of parallel universes. Maybe the bright spots found by Chary indicate where another “bubble universe” once collided with our own. However, it’s not the only possible explanation for them. Nevertheless, it’s still fun to imagine what could be out there, just over the horizon…