IN THE YEARS following the debut of our multiverse theory, research on the multiverse has gone from the cosmological fringes to being a very active, popular field. My theory about the multiverse, once an outlier, is now far from the only one.
Interestingly, one of these alternative multiverse theories comes from Roger Penrose, the physicist who inspired my quest all those years ago and who we first met in this book as one of the strongest proponents of a singular universe explained by the theory of everything.
In 2007, three years after I proposed my theory of the multiverse, I invited Roger to UNC Chapel Hill. A giant void had just been observed, and I was excited by this dramatic development. I wanted to talk with Roger and get his reaction, especially because he has also been a world leader of the efforts toward a theory of everything. So, while I respected his opinion, I also expected resistance.
I collected Roger from the airport and drove him to his hotel on campus. He told me he would like to have dinner because he had been traveling all day. While I drove and listened to his travel stories, I was panicking inside, knowing full well that our chances of finding a late dinner in a small town like Chapel Hill were not high. The kitchen at his hotel was already closed, and the chef and staff had all gone home. Roger and I walked along the main road, Franklin Street, trying to find an open restaurant. After a mile or so, we spotted a Turkish restaurant called Talulla’s. Its owner was just placing the Closed sign on the front door. I rushed over and pleaded with him to stay open, explaining that I had a very important guest and we had to find him something to eat. The restaurant owner’s kindness and hospitality saved the day. Without hesitation, he welcomed us inside and apologized that he had only cold mezes; the stoves had been shut off. Roger and I took a table, and the owner brought over an array of dishes, then sat at the bar with a beer, patiently waiting for us to finish our dinner.
One of the most enjoyable aspects of discussing a hard problem with Roger, even when we disagree, is that his incredible passion for physics comes through; he is like a five-year-old in a toy store who is told he can have anything he wants. We discussed the recent landscape crisis, and I shared my opinion on the multiverse, then Roger excitedly told me about his own idea of a sequential universe.
In Roger’s model, the beginning and the end of the universe are connected in a sequence. Roger envisions that in the distant future and thanks to dark energy, our universe will empty out completely; all observers will suffer the cosmic heat death, and our universe will keep a constant entropy forever. By the laws of physics, time will stop; we cannot build clocks in a universe where nothing changes. The universe is so unchanging—so big, uniform, smooth, and empty—that we can rescale its size from big to small without losing any information. (This rescaling of the size of the universe and everything in it is, in the language of math, “conformal transformation.”) When we rescale, what we have in our hands is a small and smooth bit of space with lots of energy that we now know will again bang into inflated growth, go through the typical history of a universe, and then, in the far future, empty out, freeze, and start over.
In Roger’s proposal, since the inflaton at the beginning and the dark energy at the end of the life of a universe are identified as one source, the universe keeps repeating these cycles, producing an infinite number of sequential universes or, as Roger calls them, aeons. Each aeon is a universe; therefore, a collection of universes spread out in time is also a multiverse. Interestingly, since clocks freeze at the end of each cycle, the second law of thermodynamics is not violated by Roger’s theory because the arrow of time resets in each aeon. But it comes at the price of having a sequential multiverse, one where universes are produced one after another.
As we talked excitedly and wrote pictures and equations on paper napkins snatched from nearby empty tables, I kept an eye on the owner, who was still sitting at the bar, drinking his third beer, and talking on his phone in Turkish to his family while politely trying not to look at us as a signal to hurry up. But unhappily for him, we still had more to discuss.
Near the end of our dinner, I realized that, as much as Roger was motivated to find a unified theory of a single universe, his model inadvertently produced a multiverse; indeed, an infinite number of sequential universes. So it, too, produced a multiverse, but his collection of universes existed in time rather than space. I made this point and tried to convince Roger that all our attempts to solve the mystery of the origin of our universe would always end up with a multiverse—his model included. We debated this for hours and finally left the restaurant around one in the morning—but not before thanking the owner profusely.
My discussion with Roger that night at Talulla’s remains one of the most intellectually memorable I have ever had. Roger and I have met and publicly debated the origin of the universe on numerous occasions since. In one of our more recent discussions, he announced that he had revised his “ridiculous number” concerning the likelihood of our universe coming into existence—he increased it from 10 to the 123rd power to 10 to the 124th!
Roger’s multiverse evasion of the second law of thermodynamics is wholly original, but there are others that have rivaled it for sheer novelty. On a somewhat similar trajectory to Roger, Andrei Linde and Alan Guth, the founding fathers of cosmic inflation, have tried to solve the problem of our universe’s origin from the day they realized their model of cosmic inflation produced the insurmountable problem of its origin, as Roger had demonstrated. In fact, Linde was the first one to propose a vision of an eternally reproducing universe, another type of sequential multiverse. Linde argued that if cosmic inflation can spontaneously happen once, then it may spontaneously do so over and over again. A single universe may multiply to produce new bubble universes that then branch out and similarly keep reproducing, creating yet more offspring. (Our four-dimensional universe is flat, as I’ve mentioned, but in its three spatial dimensions, it looks like a sphere. Thus the term bubble universe, which refers to our universe’s three-dimensional shape rather than its four-dimensional flat geometry.) If we have eternity at our disposal, then we have all the time in the world to wait for more episodes of inflation to occur and produce new bubble universes. Each bubble universe bangs spontaneously from a patch of the previous one, which itself had inflated and branched out from a patch of its predecessor, and so on.
Linde’s eternal inflation theory is an appealingly organic view of the multiverse. The closest comparison, indeed, is a natural one—just like a very old tree keeps growing new branches and leaves every year, this inflationary universe will keep endlessly reproducing new bubble universes.
But Linde and Guth’s eternal-inflation theory suffers from a familiar problem: it closes off the origins of the universe from examination, just as Penrose and Hawking’s singularity theorem did when it posited that nothing, absolutely nothing, existed before creation. Under eternal-inflation theory, reconstructing our universe’s origin from its parent universes all the way down their genealogical tree is like trying to trace the origin of a single leaf all the way to the first shoot from the seed of an infinitely old and tall tree on which the leaf appeared. An eternally reproducing universe would take us from bubble universe to bubble universe, all branching out from each other, endlessly back into the past. If eternal inflation is correct, then our origin is pushed all the way back to the infinite past, hidden under an infinite number of origins of all the other bubble universes that came before it. In short, our beginnings become untraceable because the first moment of creation is hidden in the eternal past.
The inflationary-universe community has recently looked for ways to test eternal inflation. They do so by resorting to something even spookier than the quantum entanglement that I used as a testing mechanism in my theory. In eternal inflation, bubble universes, being continually produced, end up colliding with each other. Thus, tests of eternal inflation rely on the assumption that collisions between our universe and other bubble universes would create “fractures”—another sort of imprint on our sky. Of course, in the majority of these cases, collisions of two bubble universes would be catastrophic, and we wouldn’t be here to talk about them. But according to eternal-inflation colleagues, it was conceivable to anthropically “arrange” for the collisions of our universe with other bubbles to be so soft that they didn’t destroy our universe.
I’m not sold on those ideas. Observationally, if two spheres or two bubble universes collide softly with each other, based on their symmetry, we would expect to see the ripples they produce on their respective surfaces; they would look like concentric circles spreading out from the point of impact. These types of ripples have not yet been observed in our sky.
Still, my curiosity got the best of me. In 2013, together with Malcolm Perry, a colleague of mine at the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge, I decided to undertake my own effort to understand eternal inflation. Specifically, we wanted to investigate whether eternal inflation was indeed eternal and if it could be unified with the theory that Tomo, Rich, and I had presented about our universe originating from the quantum landscape multiverse. If so, then we could arrive at a unique image, a unified theory of the multiverse.
Eternal-inflation theory relies on the random quantum fluctuations of our old friend the inflaton particle. Sometimes these fluctuations can cause the inflaton to spontaneously jump in energy, creating energies high enough to trigger a local Big Bang and therefore produce a bubble universe. But having an inflaton jump to high energies is not sufficient to produce a bubble universe. For an inflaton to produce a universe, the high-energy fluctuations also need to find an incredibly smooth, tiny region of space, a piece of prime real estate on which to start this bubble universe.
In turn, the production of multiple bubble universes makes the background space coarser and coarser, so finding smooth prime real estate becomes harder, and it becomes more and more difficult, if not impossible, to produce additional bubble universes. A good analogy is an ice-skater trying to skate on ice that is becoming covered with dirt and particles; when the ice is no longer smooth, the skater cannot continue fluidly skating. Our skater will be forced to come to an abrupt stop due to the friction produced by the contact of the skate’s sharp blade with the gritty buildup on the ice’s surface. Similarly, the universe’s reproduction becomes harder to continue if the space-time is increasingly “choppy.” Malcolm and I found that in this scenario, eternal inflation eventually ceases. Building a multiverse, apparently, is harder than it seems.
Through frequent discussions with my late friend and colleague Stephen Hawking, I could observe how fast the scientific thinking about the multiverse was changing. Hawking, who spent most of his life working toward a theory of everything and was one of its iconic leaders, initially embraced the view of an anthropic selection of our universe from the string-theory landscape. Gradually he shifted away from the theory of everything and became open to the possibility of a multiverse by investigating eternal inflation. Yet once he had convinced himself that an eternal-inflation model was also problematic (which he did, independently of Malcolm and me and for different reasons), he started exploring a new model. In the last few years of his life, Hawking was actively working on the physics of the multiverse.
Hawking’s conversion on multiverse theory was indicative of the about-face that the field of physics was undergoing as the new millennium progressed. What had once been a fringe idea was now firmly in the mainstream.
To be fair, in the early years of working on the multiverse, I was not a complete anomaly. Another independent advocate for the multiverse paradigm was MIT’s distinguished theoretical physicist Max Tegmark; he proposed that many of the deep mysteries in physics, from dark energy to the existence of life, could be better explained if our universe was not alone. Tegmark advocated for a mathematical multiverse—a multiverse where all the possible mathematical objects, seen and unseen, from, say, a doughnut-shaped universe to a flying spaghetti monster, acquired a physical existence. It is a fascinating view based on entropy arguments with far-reaching philosophical implications, but the mathematical-multiverse theory is harder to test observationally than our quantum landscape multiverse theory.
Tegmark’s theory and those of Hawking, Guth and Linde, and Penrose are only the most popular models of the multiverse; today, there exist many others. But what all these theories have in common is that they posit something that sixty years ago was considered unthinkable at best and heresy at worst—the idea that we do not need a theory of everything for our singular universe; instead, we exist in a multiverse.
A few years ago, I was debating this shift at the annual HowTheLightGetsIn festival at Hay-on-Wye in Great Britain with a conservative opponent of the multiverse. Exasperated by the arguments in favor of the multiverse, he declared to me and the audience: “But half of the physics community does not believe in a multiverse!” He paused briefly in order to let his grave statement settle in, and to lighten the mood, I replied jokingly: “So you are agreeing that the other half of the community does believe in the existence of the multiverse?” Everyone laughed. And in my heart, I knew that we were both right.
The German philosopher Arthur Schopenhauer put it best when he said, “Truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.” Today, many scientists view as self-evident the possibility of our cosmos being vaster than a single universe. For the first time, we have what we need to look up in the sky from our little planet and see and test the far reaches of cosmic theory, beyond the horizon of our one universe. Now, from inside our finite and ephemeral universe, we can finally reach for infinity and eternity.