COSMOLOGY DOES THE MULTIVERSE REALLY EXIST? Proof of parallel universes radically di erent from our own may still lie beyond the domain of science By George F. R. Ellis : universe we see around us is not the only one; that billions of other universes are out there, too. There is not one universe—there is a multiverse. In Scientifi c American articles and books such as Brian Greene’s latest, The Hidden Reality, leading scientists have spoken of a super-Copernican revolution. In this view, not only is our planet one among many, but even Iour entire universe is insignifi cant on the cosmic scale of things. It is just one of countless uni- verses, each doing its own thing. The word “multiverse” has di erent meanings. Astronomers are able to see out to a distance of about 42 billion light-years, our cosmic visual horizon. We have no reason to suspect the uni- verse stops there. Beyond it could be many—even infi nitely many—domains much like the one we see. Each has a di erent initial distribution of matter, but the same laws of physics operate in all. Nearly all cosmologists today (including me) accept this type of multiverse, which Max Tegmark calls “level 1.” Yet some go further. They suggest completely di erent kinds of univers- es, with di erent physics, di erent histories, maybe di erent numbers of spatial dimensions. Most will be sterile, although some will be teeming with life. A chief proponent of this “level 2” 38 Scientifi c American, August 2011 Photograph by Levi Brown sad0811Elli4p.indd 38 6/21/11 4:34 PM George F. R. Ellis is a cosmologist and emeritus mathematics pro- fessor at the University of Cape Town in South Africa. He is one of the world’s leading experts on Einstein’s general theory of relativity and co-author, with Stephen Hawking, of the seminal book The Large Scale Structure of Space-Time (Cambridge University Press, 1975). IN BRIEF The notion of parallel universes leapt out of the pages of fi ction into scientifi c journals in the 1990s. Many scientists claim that mega- millions of other universes, each with its own laws of physics, lie out there, beyond our visual horizon. They are collectively known as the multiverse. The trouble is that no possible as- tronomical observations can ever see those other universes. The ar- guments are indirect at best. And even if the multiverse exists, it leaves the deep mysteries of na- ture unexplained. August 2011, Scientifi cAmerican.com 39 sad0811Elli4p.indd 39 6/21/11 4:34 PM THE PERILS OF EXTRAPOLATION What Lies Beyond? When astronomers peer into the universe, they see out to a distance of about 42 billion light- years‚ our cosmic horizon, which represents how far light has been able to travel since the big bang (as well as how much the universe has expanded in size since then). Assuming that space does not just stop there and may well be infi nitely big, cos- mologists make educated guesses as to what the rest of it looks like. Level 1 Multiverse: Plausible The most straightforward assumption is that our volume of space is a representative Us sample of the whole. Distant alien beings see diff erent volumes‚ but all of these look basically alike, apart from random variations in the distribution of matter. Together these regions, seen and unseen, form a basic type of multiverse. 42 billion light-years Level 2 Multiverse: Questionable Many cosmologists go further and speculate that, suffi ciently far away, things look quite diff erent from what we see. Our environs may be one of many bubbles fl oating in an otherwise empty background. The laws of physics would diff er from bubble to bubble, leading to an almost inconceivable variety of outcomes. Those other bubbles may be impossible to observe even in principle. The author and other skeptics feel dubious about this type of multiverse. Observable Universe multiverse is Alexander Vilenkin, who paints a dramatic picture Tegmark and Dennis Sciama [see “Parallel Universes,” by Max of an infi nite set of universes with an infi nite number of galax- Tegmark; S A, May 2003]. ies, an infi nite number of planets and an infi nite number of peo- Of these options, the most widely accepted is that of chaotic ple with your name who are reading this article. infl ation, and I will concentrate on it; however, most of my re- Similar claims have been made since antiquity by many cul- marks apply to all the other proposals as well. The idea is that tures. What is new is the assertion that the multiverse is a scien- space at large is an eternally expanding void, within which tifi c theory, with all that implies about being mathematically quantum e ects continually spawn new universes like a child rigorous and experimentally testable. I am skeptical about this blowing bubbles. The concept of infl ation goes back to the claim. I do not believe the existence of those other universes has 1980s, and physicists have elaborated on it based on their most been proved—or ever could be. Proponents of the multiverse, as comprehensive theory of nature: string theory. String theory al- well as greatly enlarging our conception of physical reality, are lows bubbles to look very di erent from one another. In e ect, implicitly redefi ning what is meant by “science.” each begins life not only with a random distribution of matter but also with random types of matter. Our universe contains OVER THE HORIZON particles such as electrons and quarks interacting through forc- to a broad conception of the multiverse es such as electromagnetism; other universes may have very dif- have various proposals as to how such a proliferation of univers- ferent types of particles and forces—which is to say, di erent lo- es might arise and where they would all reside. They might be cal laws of physics. The full set of allowed local laws is known as sitting in regions of space far beyond our own, as envisaged by the landscape. In some interpretations of string theory, the land- the chaotic infl ation model of Alan H. Guth, Andrei Linde and scape is immense, ensuring a tremendous diversity of universes. others [see “The Self-Reproducing Infl ationary Universe,” by Many physicists who talk about the multiverse, especially ad- Andrei Linde; S A, November 1994]. They might vocates of the string landscape, do not care much about parallel exist at di erent epochs of time, as proposed in the cyclic uni- universes per se. For them, objections to the multiverse as a con- verse model of Paul J. Steinhardt and Neil Turok [see “The Myth cept are unimportant. Their theories live or die based on internal of the Beginning of Time,” by Gabriele Veneziano; S consistency and, one hopes, eventual laboratory testing. They as- A, May 2004]. They might exist in the same space we do sume a multiverse context for their theories without worrying but in a di erent branch of the quantum wave function, as advo- about how it comes to be—which is what concerns cosmologists. cated by David Deutsch [see “The Quantum Physics of Time For a cosmologist, the basic problem with all multiverse pro- Travel,” by David Deutsch and Michael Lockwood; S posals is the presence of a cosmic visual horizon. The horizon is A, March 1994]. They might not have a location, being the limit to how far away we can see, because signals traveling completely disconnected from our spacetime, as suggested by toward us at the speed of light (which is fi nite) have not had time 40 Scientifi c American, August 2011 Illustrations by Chad Hagen sad0811Elli3p.indd 40 6/20/11 2:08 PM Us Us Level 1 Multiverse Level 2 Multiverse since the beginning of the universe to reach us from farther out. known, is that no one can prove you wrong. How can scientists All the parallel universes lie outside our horizon and remain be- decide whether their picture of an unobservable region of space- yond our capacity to see, now or ever, no matter how technology time is a reasonable or an unreasonable extrapolation of what we evolves. In fact, they are too far away to have had any infl uence see? Might other universes have di erent initial distributions of on our universe whatsoever. That is why none of the claims made matter, or might they also have di erent values of fundamental by multiverse enthusiasts can be directly substantiated. physical constants, such as those that set the strength of nuclear The proponents are telling us we can state in broad terms forces? You could get either, depending on what you assume. what happens 1,000 times as far as our cosmic horizon, 10100 Known physics predicts other domains. Proposed unifi ed times, 101,000,000 times, an infi nity—all from data we obtain with- theories predict entities such as scalar fi elds, a hypothesized rel- in the horizon. It is an extrapolation of an extraordinary kind. ative of other space-fi lling fi elds such as the magnetic fi eld. Such Maybe the universe closes up on a very large scale, and there is fi elds should drive cosmic infl ation and create universes ad infi - no infi nity out there. Maybe all the matter in the universe ends nitum. These theories are well grounded theoretically, but the somewhere, and there is empty space forever after. Maybe nature of the hypothesized fi elds is unknown, and experimental- space and time come to an end at a singularity that bounds the ists have yet to demonstrate their existence, let alone measure universe.