Testing the multiverse hypothesis requires measuring whether our universe is statistically typical among the infinite variety of universes. But infinity does a number on statistics.
If modern physics is to be believed, we shouldn’t be here. The meager dose of energy infusing empty space, which at higher levels would rip the cosmos apart, is a trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion times tinier than theory predicts. And the minuscule mass of the Higgs boson, whose relative smallness allows big structures such as galaxies and humans to form, falls roughly 100 quadrillion times short of expectations. Dialing up either of these constants even a little would render the universe unlivable.
To account for our incredible luck, leading cosmologists like Alan Guth and Stephen Hawking envision our universe as one of countless bubbles in an eternally frothing sea. This infinite “multiverse” would contain universes with constants tuned to any and all possible values, including some outliers, like ours, that have just the right properties to support life. In this scenario, our good luck is inevitable: A peculiar, life-friendly bubble is all we could expect to observe.
Many physicists loathe the multiverse hypothesis, deeming it a cop-out of infinite proportions. But as attempts to paint our universe as an inevitable, self-contained structure falter, the multiverse camp is growing.
The problem remains how to test the hypothesis. Proponents of the multiverse idea must show that, among the rare universes that support life, ours is statistically typical. The exact dose of vacuum energy, the precise mass of our underweight Higgs boson, and other anomalies must have high odds within the subset of habitable universes. If the properties of this universe still seem atypical even in the habitable subset, then the multiverse explanation fails.
But infinity sabotages statistical analysis. In an eternally inflating multiverse, where any bubble that can form does so infinitely many times, how do you measure “typical”?
Guth, a professor of physics at the Massachusetts Institute of Technology, resorts to freaks of nature to pose this “measure problem.” “In a single universe, cows born with two heads are rarer than cows born with one head,” he said. But in an infinitely branching multiverse, “there are an infinite number of one-headed cows and an infinite number of two-headed cows. What happens to the ratio?”
For years, the inability to calculate ratios of infinite quantities has prevented the multiverse hypothesis from making testable predictions about the properties of this universe. For the hypothesis to mature into a full-fledged theory of physics, the two-headed-cow question demands an answer.
As a junior researcher trying to explain the smoothness and flatness of the universe, Guth proposed in 1980 that a split second of exponential growth may have occurred at the start of the Big Bang. This would have ironed out any spatial variations as if they were wrinkles on the surface of an inflating balloon. The inflation hypothesis, though it is still being tested, gels with all available astrophysical data and is widely accepted by physicists.
In the years that followed, Andrei Linde, now of Stanford University, Guth and other cosmologists reasoned that inflation would almost inevitably beget an infinite number of universes. “Once inflation starts, it never stops completely,” Guth explained. In a region where it does stop — through a kind of decay that settles it into a stable state — space and time gently swell into a universe like ours. Everywhere else, space-time continues to expand exponentially, bubbling forever.
Each disconnected space-time bubble grows under the influence of different initial conditions tied to decays of varying amounts of energy. Some bubbles expand and then contract, while others spawn endless streams of daughter universes. The scientists presumed that the eternally inflating multiverse would everywhere obey the conservation of energy, the speed of light, thermodynamics, general relativity and quantum mechanics. But the values of the constants coordinated by these laws were likely to vary randomly from bubble to bubble.
Paul Steinhardt, a theoretical physicist at Princeton University and one of the early contributors to the theory of eternal inflation, saw the multiverse as a “fatal flaw” in the reasoning he had helped advance, and he remains stridently anti-multiverse today. “Our universe has a simple, natural structure,” he said in September. “The multiverse idea is baroque, unnatural, untestable and, in the end, dangerous to science and society.”
Steinhardt and other critics believe the multiverse hypothesis leads science away from uniquely explaining the properties of nature. When deep questions about matter, space and time have been elegantly answered over the past century through ever more powerful theories, deeming the universe’s remaining unexplained properties “random” feels, to them, like giving up. On the other hand, randomness has sometimes been the answer to scientific questions, as when early astronomers searched in vain for order in the solar system’s haphazard planetary orbits. As inflationary cosmology gains acceptance, more physicists are conceding that a multiverse of random universes might exist, just as there is a cosmos full of star systems arranged by chance and chaos.
“When I heard about eternal inflation in 1986, it made me sick to my stomach,” said John Donoghue, a physicist at the University of Massachusetts, Amherst. “But when I thought about it more, it made sense.”
One for the Multiverse
The multiverse hypothesis gained considerable traction in 1987, when the Nobel laureate Steven Weinberg used it to predict the infinitesimal amount of energy infusing the vacuum of empty space, a number known as the cosmological constant, denoted by the Greek letter Λ (lambda). Vacuum energy is gravitationally repulsive, meaning it causes space-time to stretch apart. Consequently, a universe with a positive value for Λ expands — faster and faster, in fact, as the amount of empty space grows — toward a future as a matter-free void. Universes with negative Λ eventually contract in a “big crunch.”
Physicists had not yet measured the value of Λ in our universe in 1987, but the relatively sedate rate of cosmic expansion indicated that its value was close to zero. This flew in the face of quantum mechanical calculations suggesting Λ should be enormous, implying a density of vacuum energy so large it would tear atoms apart. Somehow, it seemed our universe was greatly diluted.
Weinberg turned to a concept called anthropic selection in response to “the continued failure to find a microscopic explanation of the smallness of the cosmological constant,” as he wrote in Physical Review Letters (PRL). He posited that life forms, from which observers of universes are drawn, require the existence of galaxies. The only values of Λ that can be observed are therefore those that allow the universe to expand slowly enough for matter to clump together into galaxies. In his PRL paper, Weinberg reported the maximum possible value of Λ in a universe that has galaxies. It was a multiverse-generated prediction of the most likely density of vacuum energy to be observed, given that observers must exist to observe it.
A decade later, astronomers discovered that the expansion of the cosmos was accelerating at a rate that pegged Λ at 10−123 (in units of “Planck energy density”). A value of exactly zero might have implied an unknown symmetry in the laws of quantum mechanics — an explanation without a multiverse. But this absurdly tiny value of the cosmological constant appeared random. And it fell strikingly close to Weinberg’s prediction.
“It was a tremendous success, and very influential,” said Matthew Kleban, a multiverse theorist at New York University. The prediction seemed to show that the multiverse could have explanatory power after all.
Close on the heels of Weinberg’s success, Donoghue and colleagues used the same anthropic approach to calculate the range of possible values for the mass of the Higgs boson. The Higgs doles out mass to other elementary particles, and these interactions dial its mass up or down in a feedback effect. This feedback would be expected to yield a mass for the Higgs that is far larger than its observed value, making its mass appear to have been reduced by accidental cancellations between the effects of all the individual particles. Donoghue’s group argued that this accidentally tiny Higgs was to be expected, given anthropic selection: If the Higgs boson were just five times heavier, then complex, life-engendering elements like carbon could not arise. Thus, a universe with much heavier Higgs particles could never be observed.
Until recently, the leading explanation for the smallness of the Higgs mass was a theory called supersymmetry, but the simplest versions of the theory have failed extensive tests at the Large Hadron Collider near Geneva. Although new alternatives have been proposed, many particle physicists who considered the multiverse unscientific just a few years ago are now grudgingly opening up to the idea. “I wish it would go away,” said Nathan Seiberg, a professor of physics at the Institute for Advanced Study in Princeton, N.J., who contributed to supersymmetry in the 1980s. “But you have to face the facts.”
However, even as the impetus for a predictive multiverse theory has increased, researchers have realized that the predictions by Weinberg and others were too naive. Weinberg estimated the largest Λ compatible with the formation of galaxies, but that was before astronomers discovered mini “dwarf galaxies” that could form in universes in which Λ is 1,000 times larger. These more prevalent universes can also contain observers, making our universe seem atypical among observable universes. On the other hand, dwarf galaxies presumably contain fewer observers than full-size ones, and universes with only dwarf galaxies would therefore have lower odds of being observed.
Researchers realized it wasn’t enough to differentiate between observable and unobservable bubbles. To accurately predict the expected properties of our universe, they needed to weight the likelihood of observing certain bubbles according to the number of observers they contained. Enter the measure problem.
Measuring the Multiverse
Guth and other scientists sought a measure to gauge the odds of observing different kinds of universes. This would allow them to make predictions about the assortment of fundamental constants in this universe, all of which should have reasonably high odds of being observed. The scientists’ early attempts involved constructing mathematical models of eternal inflation and calculating the statistical distribution of observable bubbles based on how many of each type arose in a given time interval. But with time serving as the measure, the final tally of universes at the end depended on how the scientists defined time in the first place.
“People were getting wildly different answers depending on which random cutoff rule they chose,” said Raphael Bousso, a theoretical physicist at the University of California, Berkeley.
Alex Vilenkin, director of the Institute of Cosmology at Tufts University in Medford, Mass., has proposed and discarded several multiverse measures during the last two decades, looking for one that would transcend his arbitrary assumptions. Two years ago, he and Jaume Garriga of the University of Barcelona in Spain proposed a measure in the form of an immortal “watcher” who soars through the multiverse counting events, such as the number of observers. The frequencies of events are then converted to probabilities, thus solving the measure problem. But the proposal assumes the impossible up front: The watcher miraculously survives crunching bubbles, like an avatar in a video game dying and bouncing back to life.
In 2011, Guth and Vitaly Vanchurin, now of the University of Minnesota Duluth, imagined a finite “sample space,” a randomly selected slice of space-time within the infinite multiverse. As the sample space expands, approaching but never reaching infinite size, it cuts through bubble universes encountering events, such as proton formations, star formations or intergalactic wars. The events are logged in a hypothetical databank until the sampling ends. The relative frequency of different events translates into probabilities and thus provides a predictive power. “Anything that can happen will happen, but not with equal probability,” Guth said.
Still, beyond the strangeness of immortal watchers and imaginary databanks, both of these approaches necessitate arbitrary choices about which events should serve as proxies for life, and thus for observations of universes to be counted and converted into probabilities. Protons seem necessary for life; space wars do not — but do observers require stars, or is this too limited a concept of life? With either measure, choices can be made so that the odds stack in favor of our inhabiting a universe like ours. The degree of speculation raises doubts.
The Causal Diamond
Bousso first encountered the measure problem in the 1990s as a graduate student working with Stephen Hawking, the doyen of black hole physics. Black holes prove there is no such thing as an omniscient measurer, because someone inside a black hole’s “event horizon,” beyond which no light can escape, has access to different information and events from someone outside, and vice versa. Bousso and other black hole specialists came to think such a rule “must be more general,” he said, precluding solutions to the measure problem along the lines of the immortal watcher. “Physics is universal, so we’ve got to formulate what an observer can, in principle, measure.”
This insight led Bousso to develop a multiverse measure that removes infinity from the equation altogether. Instead of looking at all of space-time, he homes in on a finite patch of the multiverse called a “causal diamond,” representing the largest swath accessible to a single observer traveling from the beginning of time to the end of time. The finite boundaries of a causal diamond are formed by the intersection of two cones of light, like the dispersing rays from a pair of flashlights pointed toward each other in the dark. One cone points outward from the moment matter was created after a Big Bang — the earliest conceivable birth of an observer — and the other aims backward from the farthest reach of our future horizon, the moment when the causal diamond becomes an empty, timeless void and the observer can no longer access information linking cause to effect.
Bousso is not interested in what goes on outside the causal diamond, where infinitely variable, endlessly recursive events are unknowable, in the same way that information about what goes on outside a black hole cannot be accessed by the poor soul trapped inside. If one accepts that the finite diamond, “being all anyone can ever measure, is also all there is,” Bousso said, “then there is indeed no longer a measure problem.”
In 2006, Bousso realized that his causal-diamond measure lent itself to an evenhanded way of predicting the expected value of the cosmological constant. Causal diamonds with smaller values of Λ would produce more entropy — a quantity related to disorder, or degradation of energy — and Bousso postulated that entropy could serve as a proxy for complexity and thus for the presence of observers. Unlike other ways of counting observers, entropy can be calculated using trusted thermodynamic equations. With this approach, Bousso said, “comparing universes is no more exotic than comparing pools of water to roomfuls of air.”
Using astrophysical data, Bousso and his collaborators Roni Harnik, Graham Kribs and Gilad Perez calculated the overall rate of entropy production in our universe, which primarily comes from light scattering off cosmic dust. The calculation predicted a statistical range of expected values of Λ. The known value, 10-123, rests just left of the median. “We honestly didn’t see it coming,” Bousso said. “It’s really nice, because the prediction is very robust.”
Bousso and his collaborators’ causal-diamond measure has now racked up a number of successes. It offers a solution to a mystery of cosmology called the “why now?” problem, which asks why we happen to live at a time when the effects of matter and vacuum energy are comparable, so that the expansion of the universe recently switched from slowing down (signifying a matter-dominated epoch) to speeding up (a vacuum energy-dominated epoch). Bousso’s theory suggests it is only natural that we find ourselves at this juncture. The most entropy is produced, and therefore the most observers exist, when universes contain equal parts vacuum energy and matter.
In 2010 Harnik and Bousso used their idea to explain the flatness of the universe and the amount of infrared radiation emitted by cosmic dust. Last year, Bousso and his Berkeley colleague Lawrence Hall reported that observers made of protons and neutrons, like us, will live in universes where the amount of ordinary matter and dark matter are comparable, as is the case here.
“Right now the causal patch looks really good,” Bousso said. “A lot of things work out unexpectedly well, and I do not know of other measures that come anywhere close to reproducing these successes or featuring comparable successes.”
The causal-diamond measure falls short in a few ways, however. It does not gauge the probabilities of universes with negative values of the cosmological constant. And its predictions depend sensitively on assumptions about the early universe, at the inception of the future-pointing light cone. But researchers in the field recognize its promise. By sidestepping the infinities underlying the measure problem, the causal diamond “is an oasis of finitude into which we can sink our teeth,” said Andreas Albrecht, a theoretical physicist at the University of California, Davis, and one of the early architects of inflation.
Kleban, who like Bousso began his career as a black hole specialist, said the idea of a causal patch such as an entropy-producing diamond is “bound to be an ingredient of the final solution to the measure problem.” He, Guth, Vilenkin and many other physicists consider it a powerful and compelling approach, but they continue to work on their own measures of the multiverse. Few consider the problem to be solved.
Every measure involves many assumptions, beyond merely that the multiverse exists. For example, predictions of the expected range of constants like Λ and the Higgs mass always speculate that bubbles tend to have larger constants. Clearly, this is a work in progress.
“The multiverse is regarded either as an open question or off the wall,” Guth said. “But ultimately, if the multiverse does become a standard part of science, it will be on the basis that it’s the most plausible explanation of the fine-tunings that we see in nature.”
Perhaps these multiverse theorists have chosen a Sisyphean task. Perhaps they will never settle the two-headed-cow question. Some researchers are taking a different route to testing the multiverse. Rather than rifle through the infinite possibilities of the equations, they are scanning the finite sky for the ultimate Hail Mary pass — the faint tremor from an ancient bubble collision.
Our universe is a miracle which is beyond our comprehension. However much we advance through science and begin to unravel the mysteries of the world, the more we get confused and messed up in them.
No human can be said to know all the secrets of the universe, not even our most knowledgeable scientists. Science is not about facts; facts are easy to learn. Science is about exploring and questioning these pre-known facts and establishing new ones.
One such kid, Max Laughlin is definitely much smarter than the average 13-year-old or 30 year old for that matter and has been called the smartest kid on the planet earth. Before his 13th birthday, he had invented a device which was capable of giving free energy to everyone in the world (once the logistics of the production could be taken care of).
He has been discussing and debating extensively on the multi-verse theory and alternate realities for quite a while now and with the biggest brains in the business. He is one of the many physical theorists who are of the opinion that when CERN used the Hadron Collider, it leads to a permanent destruction of our universe as it existed. And now we are living in a parallel one, which was closest to our own in that space-time continuum.
Multiverse is the theory that says that our reality is not the only one which exists in our space-time continuum. In the beginning, when the universe began to take shape, right from the next instant it started spiraling outwards and kept forming parallel universes right next to each other. Down the line, through infinity, there has been an uncountable number of parallel universes. And we inhabit just one of these parallel universes.
How it happened
When CERN set off the super collider it destroyed one single electron. That immediately set off a chain reaction which annihilated our entire universe. We were shifted to the next closest universe to our own but we didn’t make the shift unscathed. Many were not able to accompany us and were left behind and forgotten. And the new universe we now inhabit, though similar to our own is not exactly the same. Here is the proof.
The Mandela effect
The Mandela effect is the phenomenon which best supports this theory. Not everyone remembers how Nelson Mandela died in the same way. There are also many pop culture references and real-world events that we swear to remember in a certain way than what is available to us through records. These little glitches are a proof that the reality we remember is different than the one we now inhabit.
Astronomers are arguing about whether they can trust this untested—and potentially untestable—idea.
e universe began as a Big Bang and almost immediately began to expand faster than the speed of light in a growth spurt called “inflation.” This sudden stretching smoothed out the cosmos, smearing matter and radiation equally across it like ketchup and mustard on a hamburger bun.
That expansion stopped after just a fraction of a second. But according to an idea called the “inflationary multiverse,” it continues—just not in our universe where we could see it. And as it does, it spawns other universes. And even when it stops in those spaces, it continues in still others. This “eternal inflation” would have created an infinite number of other universes.
Together, these cosmic islands form what scientists call a “multiverse.” On each of these islands, the physical fundamentals of that universe—like the charges and masses of electrons and protons and the way space expands—could be different.
Cosmologists mostly study this inflationary version of the multiverse, but the strange scenario can takes other forms, as well. Imagine, for example, that the cosmos is infinite. Then the part of it that we can see—the visible universe—is just one of an uncountable number of other, same-sized universes that add together to make a multiverse. Another version, called the “Many Worlds Interpretation,” comes from quantum mechanics. Here, every time a physical particle, such as an electron, has multiple options, it takes all of them—each in a different, newly spawned universe.
But all of those other universes might be beyond our scientific reach. A universe contains, by definition, all of the stuff anyone inside can see, detect or probe. And because the multiverse is unreachable, physically and philosophically, astronomers may not be able to find out—for sure—if it exists at all.
Determining whether or not we live on one of many islands, though, isn’t just a quest for pure knowledge about the nature of the cosmos. If the multiverse exists, the life-hosting capability of our particular universe isn’t such a mystery: An infinite number of less hospitable universes also exist. The composition of ours, then, would just be a happy coincidence. But we won’t know that until scientists can validate the multiverse. And how they will do that, and if it even possible to do that, remains an open question.
This uncertainty presents a problem. In science, researchers try to explain how nature works using predictions that they formally call hypotheses. Colloquially, both they and the public sometimes call these ideas “theories.” Scientists especially gravitate toward this usage when their idea deals with a wide-ranging set of circumstances or explains something fundamental to how physics operates. And what could be more wide-ranging and fundamental than the multiverse?
For an idea to technically move from hypothesis to theory, though, scientists have to test their predictions and then analyze the results to see whether their initial guess is supported or disproved by the data. If the idea gains enough consistent support and describes nature accurately and reliably, it gets promoted to an official theory.
As physicists spelunk deeper into the heart of reality, their hypotheses—like the multiverse—become harder and harder, and maybe even impossible, to test. Without the ability to prove or disprove their ideas, there’s no way for scientists to know how well a theory actually represents reality. It’s like meeting a potential date on the internet: While they may look good on digital paper, you can’t know if their profile represents their actual self until you meet in person. And if you never meet in person, they could be catfishing you. And so could the multiverse.
Physicists are now debating whether that problem moves ideas like the multiverse from physics to metaphysics, from the world of science to that of philosophy.
Some theoretical physicists say their field needs more cold, hard evidence and worry about where the lack of proof leads. “It is easy to write theories,” says Carlo Rovelli of the Center for Theoretical Physics in Luminy, France. Here, Rovelli is using the word colloquially, to talk about hypothetical explanations of how the universe, fundamentally, works. “It is hard to write theories that survive the proof of reality,” he continues. “Few survive. By means of this filter, we have been able to develop modern science, a technological society, to cure illness, to feed billions. All this works thanks to a simple idea: Do not trust your fancies. Keep only the ideas that can be tested. If we stop doing so, we go back to the style of thinking of the Middle Ages.”
He and cosmologists George Ellis of the University of Cape Town and Joseph Silk of Johns Hopkins University in Baltimore worry that because no one can currently prove ideas like the multiverse right or wrong, scientists can simply continue along their intellectual paths without knowing whether their walks are anything but random. “Theoretical physics risks becoming a no-man’s-land between mathematics, physics and philosophy that does not truly meet the requirements of any,” Ellis and Silk noted in a Nature editorial in December 2014.
It’s not that physicists don’t want to test their wildest ideas. Rovelli says that many of his colleagues thought that with the exponential advance of technology—and a lot of time sitting in rooms thinking—they would be able to validate them by now. “I think that many physicists have not found a way of proving their theories, as they had hoped, and therefore they are gasping,” says Rovelli.
“Physics advances in two manners,” he says. Either physicists see something they don’t understand and develop a new hypothesis to explain it, or they expand on existing hypotheses that are in good working order. “Today many physicists are wasting time following a third way: trying to guess arbitrarily,” says Rovelli. “This has never worked in the past and is not working now.”
The multiverse might be one of those arbitrary guesses. Rovelli is not opposed to the idea itself but to its purely drawing-board existence. “I see no reason for rejecting a priori the idea that there is more in nature than the portion of spacetime we see,” says Rovelli. “But I haven’t seen any convincing evidence so far.”
“Proof” needs to evolve
Other scientists say that the definitions of “evidence” and “proof” need an upgrade. Richard Dawid of the Munich Center for Mathematical Philosophy believes scientists could support their hypotheses, like the multiverse—without actually finding physical support. He laid out his ideas in a book called String Theory and the Scientific Method. Inside is a kind of rubric, called “Non-Empirical Theory Assessment,” that is like a science-fair judging sheet for professional physicists. If a theory fulfills three criteria, it is probably true.
First, if scientists have tried, and failed, to come up with an alternative theory that explains a phenomenon well, that counts as evidence in favor of the original theory. Second, if a theory keeps seeming like a better idea the more you study it, that’s another plus-one. And if a line of thought produced a theory that evidence later supported, chances are it will again.
Radin Dardashti, also of the Munich Center for Mathematical Philosophy, thinks Dawid is straddling the right track. “The most basic idea undergirding all of this is that if we have a theory that seems like it works, and we have come up with nothing that works better, chances are our idea is right,” he says.
But, historically, that undergirding has often collapsed, and scientists haven’t been able to see the obvious alternatives to dogmatic ideas. For example, the Sun, in its rising and setting, seems to go around Earth. People, therefore, long thought that our star orbited the Earth.
Dardashti cautions that scientists shouldn’t go around applying Dawid’s idea willy-nilly, and that it needs more development. But it may be the best idea out there for “testing” the multiverse and other ideas that are too hard, if not impossible, to test. He notes, though, that physicists’ precious time would be better spent dreaming up ways to find real evidence.
Not everyone is so sanguine, though. Sabine Hossenfelder of the Nordic Institute for Theoretical Physics in Stockholm, thinks “post-empirical” and “science” can never live together. “Physics is not about finding Real Truth. Physics is about describing the world,” she wrote on her blog Backreaction in response to an interview in which Dawid expounded on his ideas. And if an idea (which she also colloquially calls a theory) has no empirical, physical backing, it doesn’t belong. “Without making contact to observation, a theory isn’t useful to describe the natural world, not part of the natural sciences, and not physics,” she concluded.
The truth is out there
Some supporters of the multiverse claim they have found real physical evidence for the multiverse.Joseph Polchinski of the University of California, Santa Barbara, and Andrei Linde of Stanford University—some of the theoretical physicists who dreamed up the current model of inflation and how it leads to island universes—say the proof is encoded in our cosmos.
This cosmos is huge, smooth and flat, just like inflation says it should be. “It took some time before we got used to the idea that the large size, flatness, isotropy and uniformity of the universe should not be dismissed as trivial facts of life,” Linde wrote in a paper that appeared on arXiv.org in December. “Instead of that, they should be considered as experimental data requiring an explanation, which was provided with the invention of inflation.”
Similarly, our universe seems fine tuned to be favorable to life, with its Goldilocks expansion rate that’s not too fast or too slow, an electron that’s not too big, a proton that has the exact opposite charge but the same mass as a neutron and a four-dimensional space in which we can live. If the electron or proton were, for example, one percent larger, beings could not be. What are the chances that all those properties would align to create a nice piece of real estate for biology to form and evolve?
In a universe that is, in fact, the only universe, the chances are vanishingly small. But in an eternally inflating multiverse, it is certain that one of the universes should turn out like ours. Each island universe can have different physical laws and fundamentals. Given infinite mutations, a universe on which humans can be born will be born. The multiverse actually explains why we’re here. And our existence, therefore, helps explain why the multiverse is plausible.
These indirect pieces of evidence, statistically combined, have led Polchinski to say he’s 94 percent certain the multiverse exists. But he knows that’s 5.999999 percent short of the 99.999999 percent sureness scientists need to call something a done deal.
Eventually, scientists may be able to discover more direct evidence of the multiverse. They are hunting for the stretch marks that inflation would have left on the cosmic microwave background, the light left over from the Big Bang. These imprints could tell scientists whether inflation happened, and help them find out whether it’s still happening far from our view. And if our universe has bumped into others in the past, that fender-bender would also have left imprints in the cosmic microwave background. Scientists would be able to recognize that two-car accident. And if two cars exist, so must many more.
Or, in 50 years, physicists may sheepishly present evidence that the early 21st-century’s pet cosmological theory was wrong.
“We are working on a problem that is very hard, and so we should think about this on a very long time scale,” Polchinski has advised other physicists. That’s not unusual in physics. A hundred years ago, Einstein’s theory of general relativity, for example, predicted the existence of gravitational waves. But scientists could only verify them recently with a billion-dollar instrument called LIGO, the Laser Interferometer Gravitational-Wave Observatory.
So far, all of science has relied on testability. It has been what makes science science and not daydreaming. Its strict rules of proof moved humans out of dank, dark castles and into space. But those tests take time, and most theoreticians want to wait it out. They are not ready to shelve an idea as fundamental as the multiverse—which could actually be the answer to life, the universe and everything—until and unless they can prove to themselves it doesn’t exist. And that day may never come.
Perimeter Associate Faculty member Matthew Johnson and his colleagues are working to bring the multiverse hypothesis, which to some sounds like a fanciful tale, firmly into the realm of testable science.
Never mind the big bang; in the beginning was the vacuum. The vacuum simmered with energy (variously called dark energy, vacuum energy, the inflation field, or the Higgs field). Like water in a pot, this high energy began to evaporate – bubbles formed.
Each bubble contained another vacuum, whose energy was lower, but still not nothing. This energy drove the bubbles to expand. Inevitably, some bubbles bumped into each other. It’s possible some produced secondary bubbles. Maybe the bubbles were rare and far apart; maybe they were packed close as foam.
But here’s the thing: each of these bubbles was a universe. In this picture, our universe is one bubble in a frothy sea of bubble universes.
That’s the multiverse hypothesis in a bubbly nutshell.
It’s not a bad story. It is, as scientists say, physically motivated – not just made up, but rather arising from what we think we know about cosmic inflation.
Cosmic inflation isn’t universally accepted – most cyclical models of the universe reject the idea. Nevertheless, inflation is a leading theory of the universe’s very early development, and there is some observational evidence to support it.
Inflation holds that in the instant after the big bang, the universe expanded rapidly – so rapidly that an area of space once a nanometer square ended up more than a quarter-billion light years across in just a trillionth of a trillionth of a trillionth of a second. It’s an amazing idea, but it would explain some otherwise puzzling astrophysical observations.
Inflation is thought to have been driven by an inflation field – which is vacuum energyby another name. Once you postulate that the inflation field exists, it’s hard to avoid an “in the beginning was the vacuum” kind of story. This is where the theory of inflation becomes controversial – when it starts to postulate multiple universes.
Proponents of the multiverse theory argue that it’s the next logical step in the inflation story. Detractors argue that it is not physics, but metaphysics – that it is not science because it cannot be tested. After all, physics lives or dies by data that can be gathered and predictions that can be checked.
That’s where Perimeter Associate Faculty member Matthew Johnson comes in. Working with a small team that also includes Perimeter Faculty member Luis Lehner, Johnson is working to bring the multiverse hypothesis firmly into the realm of testable science.
“That’s what this research program is all about,” he says. “We’re trying to find out what the testable predictions of this picture would be, and then going out and looking for them.”
Specifically, Johnson has been considering the rare cases in which our bubble universe might collide with another bubble universe. He lays out the steps: “We simulate the whole universe. We start with a multiverse that has two bubbles in it, we collide the bubbles on a computer to figure out what happens, and then we stick a virtual observer in various places and ask what that observer would see from there.”
Simulating the whole universe – or more than one – seems like a tall order, but apparently that’s not so.
“Simulating the universe is easy,” says Johnson. Simulations, he explains, are not accounting for every atom, every star, or every galaxy – in fact, they account for none of them.
“We’re simulating things only on the largest scales,” he says. “All I need is gravity and the stuff that makes these bubbles up. We’re now at the point where if you have a favourite model of the multiverse, I can stick it on a computer and tell you what you should see.”
That’s a small step for a computer simulation program, but a giant leap for the field of multiverse cosmology. By producing testable predictions, the multiverse model has crossed the line between appealing story and real science.
In fact, Johnson says, the program has reached the point where it can rule out certain models of the multiverse: “We’re now able to say that some models predict something that we should be able to see, and since we don’t in fact see it, we can rule those models out.”
For instance, collisions of one bubble universe with another would leave what Johnson calls “a disk on the sky” – a circular bruise in the cosmic microwave background. That the search for such a disk has so far come up empty makes certain collision-filled models less likely.
Meanwhile, the team is at work figuring out what other kinds of evidence a bubble collision might leave behind. It’s the first time, the team writes in their paper, that anyone has produced a direct quantitative set of predictions for the observable signatures of bubble collisions. And though none of those signatures has so far been found, some of them are possible to look for.
The real significance of this work is as a proof of principle: it shows that the multiverse can be testable. In other words, if we are living in a bubble universe, we might actually be able to tell.
The universe we live in may not be the only one out there. In fact, our universe could be just one of an infinite number of universes making up a “multiverse.”
Though the concept may stretch credulity, there’s good physics behind it. And there’s not just one way to get to a multiverse — numerous physics theories independently point to such a conclusion. In fact, some experts think the existence of hidden universes is more likely than not.
Here are the five most plausible scientific theories suggesting we live in a multiverse:
1. Infinite Universes
Scientists can’t be sure what the shape of space-time is, but most likely, it’s flat (as opposed to spherical or even donut-shape) and stretches out infinitely. But if space-time goes on forever, then it must start repeating at some point, because there are a finite number of ways particles can be arranged in space and time.
So if you look far enough, you would encounter another version of you — in fact, infinite versions of you. Some of these twins will be doing exactly what you’re doing right now, while others will have worn a different sweater this morning, and still others will have made vastly different career and life choices.
Because the observable universe extends only as far as light has had a chance to get in the 13.7 billion years since the Big Bang (that would be 13.7 billion light-years), the space-time beyond that distance can be considered to be its own separate universe. In this way, a multitude of universes exists next to each other in a giant patchwork quilt of universes. [Visualizations of Infinity: A Gallery]
Space-time may stretch out to infinity. If so, then everything in our universe is bound to repeat at some point, creating a patchwork quilt of infinite universes.
2. Bubble Universes
In addition to the multiple universes created by infinitely extending space-time, other universes could arise from a theory called “eternal inflation.” Inflation is the notion that the universe expanded rapidly after the Big Bang, in effect inflating like a balloon. Eternal inflation, first proposed by Tufts University cosmologist Alexander Vilenkin, suggests that some pockets of space stop inflating, while other regions continue to inflate, thus giving rise to many isolated “bubble universes.”
Thus, our own universe, where inflation has ended, allowing stars and galaxies to form, is but a small bubble in a vast sea of space, some of which is still inflating, that contains many other bubbles like ours. And in some of these bubble universes, the laws of physics and fundamental constants might be different than in ours, making some universes strange places indeed.
3. Parallel Universes
Another idea that arises from string theory is the notion of “braneworlds” — parallel universes that hover just out of reach of our own, proposed by Princeton University’s Paul Steinhardt and Neil Turok of the Perimeter Institute for Theoretical Physics in Ontario, Canada. The idea comes from the possibility of many more dimensions to our world than the three of space and one of time that we know. In addition to our own three-dimensional “brane” of space, other three-dimensional branes may float in a higher-dimensional space.
Out universe may live on one membrane, or “brane” that is parallel to many others containing their own universes, all floating in a higher-dimensional space.
Columbia University physicist Brian Greene describes the idea as the notion that “our universe is one of potentially numerous ‘slabs’ floating in a higher-dimensional space, much like a slice of bread within a grander cosmic loaf,” in his book “The Hidden Reality” (Vintage Books, 2011).
A further wrinkle on this theory suggests these brane universes aren’t always parallel and out of reach. Sometimes, they might slam into each other, causing repeated Big Bangs that reset the universes over and over again. [The Universe: Big Bang to Now in 10 Easy Steps ]
4. Daughter Universes
The theory of quantum mechanics, which reigns over the tiny world of subatomic particles, suggests another way multiple universes might arise. Quantum mechanics describes the world in terms of probabilities, rather than definite outcomes. And the mathematics of this theory might suggest that all possible outcomes of a situation do occur — in their own separate universes. For example, if you reach a crossroads where you can go right or left, the present universe gives rise to two daughter universes: one in which you go right, and one in which you go left.
“And in each universe, there’s a copy of you witnessing one or the other outcome, thinking — incorrectly — that your reality is the only reality,” Greene wrote in “The Hidden Reality.”
5. Mathematical Universes
Scientists have debated whether mathematics is simply a useful tool for describing the universe, or whether math itself is the fundamental reality, and our observations of the universe are just imperfect perceptions of its true mathematical nature. If the latter is the case, then perhaps the particular mathematical structure that makes up our universe isn’t the only option, and in fact all possible mathematical structures exist as their own separate universes.
“A mathematical structure is something that you can describe in a way that’s completely independent of human baggage,” said Max Tegmark of MIT, who proposed this brain-twistin gidea. “I really believe that there is this universe out there that can exist independently of me that would continue to exist even if there were no humans.”