A Guide to Different Kinds of Parallel Universes

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 These days it often seems that if a theory has loose ends, its dangling threads are surreptitiously tied together out of view within the hidden fabric of a parallel universe. While some researchers recoil from introducing unseen aspects to a theory, others find that the invisible knots create an irresistibly pretty package.
 

 Depending on one’s taste, there are so many types of parallel universes to choose from—alternative cosmos galore. If extra dimensions are not your thing, maybe bifurcating timelines would work. If an endless array of gigantic bubble universes seems intimidating, then perhaps a nursery of baby universes is more endearing. While there is not yet a GPS device or app to navigate through the cartography of scientifically sanctioned parallel possibilities, perhaps this guide to all things alternative will help.
 

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 Let’s start with the oldest, most basic idea and work our way toward newer, more complex models:
 


 What if?
 
 Here is the simplest way to transport yourself to a parallel universe: Just imagine all the ways in which our universe might have turned out differently. Each of these might-have-been realities represents a parallel universe. The mathematician Gottfried Leibniz posited that we live in the “best of all possible worlds” (famously satirized by Voltaire in “Candide”) and that all these other, unrealized, possibilities for creation would have been less desirable. His perspective has persisted for three centuries as a way of explaining why the cosmos is the way it is. Contemporary physicists who make use of the so-called Anthropic Principle argue that if the universe’s conditions were slightly different, it couldn’t have supported intelligent life, and we wouldn’t be here today to speculate about it. For example, if the inflationary era, a fleeting period of ultra-rapid growth in the very early universe, had continued for a long enough time, the stable structures we see in the cosmos today, such as stars and galaxies, couldn’t have formed. The super-quick expansion would have ripped them apart.
 


 Alternative realities made possible by time travel:
 
 Science fiction writers relish the intricate plots woven by introducing time travellers into a story. Einstein’s general theory of relativity does not distinguish between space and time and hence hypothetically permits travels to the past, though the mechanics of such a journey are still largely beyond us. In recent decades, backward time travel ideas have been explored in serious articles published in reputable physics journals. If journeying back in time is possible, what would happen if someone changed history? Would they launch a new timeline, and hence a new universe, in which the chain of events was different? The answer won’t be known until backward time travel is either developed or ruled out.
 


 Sum over histories
 
 : Physicist Richard Feynman had a practical, no-nonsense approach to physics, supporting notions that are potentially testable. Yet his approach to quantum field theory introduced the startling concept of reality as a weighted sum of alternative histories. For example, according to Feynman’s formulation, if two electrons approach each other, deflect and scatter, their overall behavior from start to finish must take into account every possible intermediate path—weighted according to each path’s likelihood. It is like assessing how tired someone will be after taking a walk in the woods by assuming that they somehow split up and took every possible route from entrance to exit—assigning more weight to the shortest (and therefore likeliest) paths, but still taking all of them into account.
 


 Many-worlds interpretation of quantum mechanics
 
 : While Feynman did not assert that the ghostly alternative histories he described represented actual parallel universes, a young graduate student, Hugh Everett III (who shared the same research advisor as Feynman, John Wheeler), made the case that they are. Everett proposed a fundamental reinterpretation of quantum mechanics in which each time that particles interact, reality bifurcates into a set of parallel streams, each representing a different possible outcome. Researchers observing the outcome of such quantum experiments would similarly split up into multiple selves—each thinking that he or she is the only one. For example, suppose a physicist named Eve wants to measure the position of an electron and there are three possible outcomes. Upon taking the measurement, she would instantly divide into three distinct selves, each recording a different result. Each version of Eve would be convinced that she was the real one—wholly unaware of her near-doppelgangers.
 


 Copycat regions of the universe
 
 : We now turn from the exceedingly small to the incomprehensibly large. If the universe is infinite, as many cosmologists surmise, then if you travel far enough you will eventually reach regions nearly identical to ours. That’s because if you take a finite number of elements and mix them into an infinite number of combinations, eventually chance will reproduce one of the previous arrangements. It is like playing tic-tac-toe—play enough times and you are bound to repeat yourself. Hence somewhere, by pure chance, there could be a near-parallel Earth where a nearly-identical version of you is reading this article on a parchment scroll illuminated by a glowworm.
 


 Bubble Universes and Baby Universes
 
 : In general relativity, an energy field of the right variety can trigger space to grow explosively. Researchers use this phenomenon to explain how the universe expanded so rapidly during the inflationary era. However, they’ve come to realize that if explosive expansion took place in one part of space, it probably happened elsewhere, too. Hence, myriad bubble universes could have emerged from the primordial cosmic sea of energy. We would never have access to other bubble universes, though, because they would have since moved away from us well beyond the limits of observation. Baby universes represent a related idea, in which universes would be seeded in the extreme conditions of black holes. The embryonic regions of space would then grow into successor universes in their own right.
 


 Higher Dimensions
 
 : For this type of parallel universe, we move beyond the three dimensions of space itself and consider the possibility of a higher, unseen dimension. While such a scenario sounds a bit like “The Twilight Zone,” higher dimensions are a vital part of string theory and other attempts at unifying the natural forces. If a higher dimension exists beyond space and time, why can’t we travel through it? Theorists hypothesize that the particles of matter and light cling to our three-dimensional space, preventing us from entering or even observing the extra dimension.
 

 While our bodies have remained in our own universe, our minds have completed an excursion through a weird assortment of parallel universe possibilities. Do any of these types of parallel universes exist? If so, how are they connected? Suggestions for testing these various hypotheses are too numerous to recount in this post. I refer the reader to several interesting proposals:
 

<a href="http://arxiv.org/abs/0809.4422">
 Testing Many-Worlds Quantum Theory By Measuring Pattern Convergence Rates
 </a>


<a href="http://arxiv.org/abs/1101.0003">
 Testing for Large Extra Dimensions with Neutrino Oscillations
 </a>


<a href="http://www.sciencedaily.com/releases/2011/08/110803102844.htm">
 Is Our Universe Inside a Bubble? First Observational Test of the ‘Multiverse’
 </a>



 Go Deeper
 
 

 Editor’s picks for further reading
 


 FQXi:
 <a href="http://www.fqxi.org/community/articles/download/__details/Philosophy_of_the_Multiverse__RC.pdf">
 Philosophy of the Multiverse
 </a>
 
 In this essay, discover why many theorists are drawn to the idea that our universe is just one among many.
 

 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/manyworlds/">
 Parallel Worlds, Parallel Lives
 
</a>
 Discover web resources associated with NOVA’s “Parallel Worlds, Parallel Lives,” a film about the life and work of Hugh Everett III.
 

 Scientific American:
 <a href="http://www.scientificamerican.com/article.cfm?id=parallel-universes">
 Parallel Universes
 </a>
 
 In this article, physicist Max Tegmark explores four “levels” of multiverses.
 
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The Phantom Energy Menace:  Is the Cosmos Doomed to a Big Rip?

First, the good news: Despite earlier doomsday prognoses, the cosmos is not fated to implode. If you stay up late at night worrying that the entirety of creation will cave in on you in a universal Big Crunch, rest assured. Astronomical evidence suggests that the Big Bang expansion will never reverse itself; the universe will not collapse back down to a point. Rather, the universe, like ever-sprawling suburbs, will burgeon forever, its galaxies receding farther and farther away from each other.Now, the bad news: Cosmic expansion, <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html/">discovered in 1998 through supernova observations</a> conducted by Nobel Prize-winning physicists Saul Perlmutter, Brian Schmidt, and Adam Riess and their research teams, is not only continuing, it is picking up its pace. Though we don’t know what is causing the acceleration, one leading idea, called “phantom energy,” has ominous implications. If phantom energy continues to drive the universe faster and faster outward, it could literally tear space into shreds—a doomsday scenario called the “Big Rip.”Phantom energy, <a href="http://arxiv.org/abs/astro-ph/9908168">proposed by Dartmouth physicist Robert Caldwell</a> in 1999, is one possible variety of the mysterious stuff researchers call dark energy. Dark energy is the catchall phrase for the hidden agent of cosmic acceleration, and encompasses many different approaches. The simplest proposal adds an extra “antigravity” term, called a “cosmological constant,” to Einstein’s equations of general relativity. Einstein himself had proposed the term to stabilize his equations, but then hastily removed it after Edwin Hubble showed in 1929 that the universe was expanding—and not stable at all. Adding the term back into the equations, while assuming that the geometry of the universe is flat, leads to a prediction that the cosmic growth rate will gradually and steadily increase.While a cosmological constant seems to model the accelerating universe nicely, at least according to current astronomical observations, many researchers are seeking a more tangible explanation. Instead of an extra term arbitrarily added to the gravitational equations, they are looking for an actual physical substance with repulsive properties. This substance must have a bizarre property called negative pressure. Though we never encounter negative pressure in everyday life, you can picture how it works by imagining poking a soap bubble and seeing it inflate rather than pop.Gravitational researchers model the behavior of substances using an equation, called an equation of state, that relates the material’s pressure to its density. For conventional substances such as fluids and gases, both pressure and density are positive, so their ratio, usually called “ w ,” is positive as well. However, if a substance were to mimic a cosmological constant, its w ratio would be negative one. That is, if you try to squeeze it, it will get bigger, not smaller.In 1998, shortly after the discovery of cosmic acceleration, Caldwell, along with astrophysicists Rahul Dave, then at Penn, and Paul Steinhardt of Princeton, <a href="http://www.physics.princeton.edu/~steinh/Caldwell.pdf">argued for the greatest amount of flexibility</a> in trying to describe dark energy. Borrowing the ancient Greek term for the “fifth essence,” they dubbed it “quintessence” and asserted that its equation of state could vary over time and space. Rather than assuming that w is negative one, they urged that its actual value be pinned down through astronomical observation.This flexibility spurred Caldwell to think about the worst-case scenario imaginable for dark energy. He wondered what would happen if the amount of negative pressure exceeded the density—that is, if w was less than negative one. In our soap bubble analogy, larger negative pressure would mean that pressing on its exterior would cause it to puff up even faster. In the far future, he realized, gravity and all other forces would eventually lose potency, overwhelmed by the hulking menace of what he dubbed phantom energy. The reason is that unlike the steady density of the energy associated with a cosmological constant, the density of phantom energy would grow greater and greater, rising along with its negative pressure, building to a lethal crescendo—a Big Rip.In Caldwell’s end game scenario, our distant descendants (relocated to other planets, perhaps) will notice the first sign of trouble billions of years from now as other galaxies recede beyond detection and disappear from the sky. (That would happen under the cosmological constant picture too, but is exacerbated in the case of phantom energy.) Eventually our local group of galaxies, including Andromeda, will become cosmic hermits. Then, tens of millions of years before the ultimate doomsday, the local group and then the Milky Way itself will break up like a dropped box of peanut brittle.In the cosmic twilight times, all planetary systems will be torn apart and the stars and planets will explode. About thirty minutes later, all atoms will burst like fireworks. Then, in a mere fraction of a second, space itself will tear to shreds in the all-destroying Big Rip. 1If the devouring of reality by a ravenous shredder sounds revolting, you can console yourself with an <a href="http://arxiv.org/abs/astro-ph/0510771">alternative</a> developed by physicist Pedro Gonzalez-Diaz of the Institute of Mathematics and Fundamental Physics in Madrid. He conjectures that phantom energy could feed one of the many wormholes that could exist in the fabric of spacetime. Over billions of years, the wormhole would swell up faster than the cosmic expansion rate and eventually engulf the material content of whole universe—galaxies and such—sparing it from a Big Rip. Theoretically, then, everything could pass unscathed out the other end of the wormhole, emerging in another sector of reality, wherever that would be. Not exactly a joy ride, but at least it wouldn’t leave you in tatters!The jury is still out on what causes cosmic acceleration. It could well be the case that the culprit is not phantom energy, but rather a gentler form of dark energy—such as a type of quintessence with a w value greater than or equal to negative one. If so, the expansion of space would increase more gradually, leaving the local group and the Milky Way gravitationally bound together as other galaxies eventually recede from view. While our enclave of space would be lonely, it would remain intact. In short, we’d experience a “Big Stretch” rather than a “Big Rip.”1 A <a href="http://www.springerlink.com/content/6842285247657415/">recent calculation</a> by a team of researchers led by Chinese physicist XiaoDong Li uses current data to estimate that the Big Rip will take place 16.7 billion years from now. The dissolution of atoms would take place a mere 30 quintillionth of a second before the Big Rip.Go Deeper Editor’s picks for further readingNBCNews.com: <a href="http://www.msnbc.msn.com/id/3077730/ns/technology_and_science-space/t/will-universe-end-big-rip/#.UCkQKkQRYck">Will universe end in a ‘big rip’?</a> Space writer Robert Roy Britt talks with cosmologists about the possibility of a Big Rip.Open Yale Courses: <a href="http://www.youtube.com/watch?v=40OQdPbeM0c">Dark Energy, the Accelerating Universe, and the Big Rip
</a> In this video, Yale astronomer Charles Bailyn reviews the evidence for dark matter and dark energy and presents the possibility of a Big Rip.

Could the Universe Be Lopsided?

One hundred years ago, Einstein re-envisioned space and time as a rippling, twisting, flexible fabric called spacetime. His theory of general relativity showed how matter and energy change the shape of this fabric. One might expect, therefore, that the fabric of the universe, strewn with stars, galaxies, and clouds of particles, would be like a college student’s dorm room: a mess of rumpled, crumpled garments.Indeed, if you look at the universe on the scale of stars, galaxies, and even galaxy clusters, you’ll find it puckered and furrowed by the gravity of massive objects. But take the wider view—the cosmologists’ view, which encompasses the entire visible universe—and the fabric of the universe is remarkably smooth and even, no matter which direction you turn. Look up, down, left, or right and count up the galaxies you see: you’ll find it’s roughly the same from every angle. The cosmic microwave background, the cooled-down relic of radiation from the early universe, demonstrates the same remarkable evenness on the very largest scale.
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Physicists call a universe that appears roughly similar in all directions “isotropic.” Because the geometry of spacetime is shaped by the distribution of matter and energy, an isotropic universe must posses a geometric structure that looks the same in all directions as well. The only three such possibilities for three-dimensional spaces are positively curved (the surface of a hypersphere, like a beach ball but in a higher dimension), negatively curved (the surface of a hyperboloid, shaped like a saddle or potato chip), or flat. Russian physicist Alexander Friedmann, Belgian cleric and mathematician Georges Lemaître and others incorporated these three geometries into some of the first cosmological solutions of Einstein’s equations. (By solutions, we mean mathematical descriptions of how the three spatial dimensions of the universe behave over time, given the type of geometry and the distribution of matter and energy.) Supplemented by the work of American physicist Howard Robertson and British mathematician Arthur Walker, this class of isotropic solutions has become the standard for descriptions of the universe in the Big Bang theory.However, in 1921 Edward Kasner—best known for his coining of the term “Googol” for the number 1 followed by 100 zeroes—demonstrated that there was another class of solutions to Einstein’s equations: anisotropic, or “lopsided,” solutions.Known as the Kasner solutions, these cosmic models describe a universe that expands in two directions while contracting in the third. That is clearly not the case with the actual universe, which has grown over time in all three directions. But the Kasner solutions become more intriguing when you apply them to a kind of theory called a Kaluza-Klein model, in which there are unseen extra dimensions beyond space and time. Thus space could theoretically have three expanding dimensions and a fourth, hidden, contracting dimension. Physicists Alan Chodos and Steven Detweiler explored this concept in their paper “ <a href="http://journals.aps.org/prd/abstract/10.1103/PhysRevD.21.2167">Where has the fifth dimension gone?</a> ”Kasner’s is far from the only anisotropic model of the universe. In 1951, physicist Abraham Taub applied the shape-shifting mathematics of Italian mathematician Luigi Bianchi to general relativity and revealed <a href="http://www34.homepage.villanova.edu/robert.jantzen/bianchi/index.htm#roleinGR">even more baroque classes of anisotropic solutions</a> that expand, contract or pulsate differently in various directions. The most complex of these, categorized as Bianchi type-IX, turned out to have chaotic properties and was dubbed by physicist Charles Misner the “Mixmaster Universe” for its resemblance to the whirling, twirling kitchen appliance.
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Like a cake rising in a tray, while bubbling and quivering on the sides, the Mixmaster Universe expands and contracts, first in one dimension and then in another, while a third dimension just keeps expanding. Each oscillation is called a Kasner epoch. But then, after a certain number of pulses, the direction of pure expansion abruptly switches. The formerly uniformly expanding dimension starts pulsating, and one of those formerly pulsating starts uniformly expanding. It is as if the rising cake were suddenly turned on its side and another direction started rising instead, while the other directions, including the one that was previously rising, just bubbled.One of the weird things about the Mixmaster Universe is that if you tabulate the number of Kasner epochs in each era, before the behavior switches, it appears as random as a dice roll. For example, the universe might oscillate in two directions five times, switch, oscillate in two other directions 17 times, switch again, pulsate another way twice, and so forth—without a clear pattern. While the solution stems from deterministic general relativity, it seems unpredictable. This is called deterministic chaos.Could the early moments of the universe have been chaotic, and then somehow regularized over time, like a smoothed-out pudding? Misner initially thought so, until he realized that the Mixmaster Universe couldn’t smooth out on its own. However, it could have started out “lopsided,” then been stretched out during an era of ultra-rapid expansion called inflation until its irregularities were lost from sight.As cosmologists have collected data from instruments such as the Hubble Space Telescope, Planck Satellite, and WMAP satellite (now retired), the bulk of the evidence supports the idea that our universe is indeed isotropic. But a minority of researchers have used measurements of the velocities of galaxies and other observations, such as an odd line up of temperature fluctuations in the cosmic microwave background dubbed the “ <a href="https://www.newscientist.com/article/dn23301-planck-shows-almost-perfect-cosmos-plus-axis-of-evil/">Axil of Evil</a> ” to assert that the universe could be slightly irregular after all.For example, starting in 2008, Alexander Kashlinsky, a researcher at NASA’s Goddard Space Flight Center, and his colleagues have statistically analyzed cosmic microwave background data gathered by first the WMAP satellite and the Planck satellite to show that, in addition to their motion due to cosmic expansion, many galaxy clusters seem to be heading toward a particular direction on the sky. He dubbed this phenomenon “dark flow,” and suggested that it is evidence of a previously-unseen cosmic anisotropy known as a “tilt.” Although the mainstream astronomical community has disputed Kashlinsky’s conclusion, he has <a href="http://arxiv.org/abs/1411.4180">continued to gather statistical evidence</a> for dark flow and the idea of tilted universes.Whether or not the universe really is “lopsided,” it is intriguing to study the rich range of solutions of Einstein’s general theory of relativity. Even if the preponderance of evidence today points to cosmic regularity, who knows when a new discovery might call that into question, and compel cosmologists to dust off alternative ideas. Such is the extraordinary flexibility of Einstein’s masterful theory: a century after its publication, physicists are still exploring its possibilities.Go Deeper Editor’s picks for further readingArs Technica: <a href="http://arstechnica.com/science/2014/03/is-the-universe-lopsided/">Is the lopsided Universe telling us we need new theories?</a> Science writer Matthew Francis asks whether the “axis of evil” reveals anything meaningful about the fundamental laws of physics.Ask an Astronomer: <a href="http://curious.astro.cornell.edu/about-us/101-the-universe/cosmology-and-the-big-bang/general-questions/574-what-do-homogeneity-and-isotropy-mean-intermediate">What do “homogeneity” and “isotropy” mean?</a> A brief introduction to what astronomers mean when they talk about homogeneity and isotropy.New Scientist: <a href="https://www.newscientist.com/article/dn23301-planck-shows-almost-perfect-cosmos-plus-axis-of-evil/">Planck shows almost perfect cosmos – plus axis of evil</a> A summary of conclusions from the Planck mission, which created the highest resolution map of the entire cosmic microwave background radiation.
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Einstein&#x27;s theory of general relativity makes room for very different kinds of universes.

How Many Dimensions Does the Universe Really Have?

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 An engineer, a mathematician and a physicist walk into a universe. How many dimensions do they find?
 

 The engineer whips out a protractor and straightedge. That’s easy, she says. With her instruments she demonstrates the trio of directions at right angles to each other: length, width and height. “Three,” she reports.
 

 The mathematician gets out his notepad and creates a list of regular, symmetric geometric shapes with perpendicular sides. Squares have four linear edges, he notes. Cubes have six square sides. By extrapolation, hypercubes have eight cubic sides. Continuing the pattern, he realizes that he could keep going forever. “Infinity,” he says.
 
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 Finally it is the physicist’s turn. She gazes at the stars and carefully records their behavior. She determines that they attract each other through gravity, which drops off as the square of their mutual distances—an indication, she thinks, of three dimensions. However, once she derives the equation for how their light moves through space, she finds that it is best expressed in four dimensions. Then, after much thought, she tries to think of ways to describe gravity and light in a common theory, which seems to require at least ten dimensions. “Three, four, or maybe even more,” she chimes in.
 

 Let’s see how she reached her conclusions.
 

 In 1917, Austrian physicist Paul Ehrenfest wrote a thought-provoking piece, “
 <a href="http://www.dwc.knaw.nl/DL/publications/PU00012213.pdf">
 In what way does it become manifest in the fundamental laws of physics that space has three dimensions?
 </a>
 ” (pdf). In the article he enumerated evidence that three dimensions are perfect for describing our world.
 

 He noted, for example, that the stable orbits of planets in the solar system and the stationary states of electrons in atoms require inverse-squared force laws. If gravity, for instance, dropped off with the cube instead of the square of distance from the Sun, the planets would not follow steady, elliptical paths.
 

 Let’s think of what an inverse-squared law means. Imagine a
 <a href="http://www.lightandmatter.com/html_books/0sn/ch10/ch10.html#Section10.6">
 bubble that roughly encompasses a planet’s orbit
 </a>
 . The strength of the Sun’s gravitational field at that distance is diluted over the bubble’s surface area. Surface area is proportional to radial distance squared, explaining why gravity drops off by that factor. Because a bubble, including its interior, is three-dimensional, space itself must be as well. In short, the fact that gravity tapers off with distance squared—the amount of a bubble’s surface area—implies three-dimensionality.
 

 The universe is not just space, though. As Russian-German mathematician Hermann Minkowski
 <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Minkowski.html">
 demonstrated
 </a>
 , Einstein’s special theory of relativity, postulated to explain how light moves at a constant speed relative to all observers, can best be expressed in four dimensions. Instead of considering space and time independently, he proposed a unified vision of spacetime. In his general theory of relativity, Einstein made use of the concept and described gravity using a dynamic four-dimensional model.
 

 Light stems from electromagnetic interactions, one of the four natural forces. For many decades, physicists have been searching for methods to unite that force with the others—the strong nuclear force, weak nuclear force, and, thorniest of all, gravity—to create a single, elegant theory of fundamental forces. Two of the earliest schemes (before the strong and weak nuclear forces were identified) were independently developed by German mathematician
 <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Kaluza.html">
 Theodor Kaluza
 </a>
 and Swedish physicist
 <a href="http://www-history.mcs.st-andrews.ac.uk/Biographies/Klein_Oskar.html">
 Oskar Klein
 </a>
 . Though we now know that their approaches were inaccurate, each proposed to unify electromagnetism and gravity by extending general relativity by an extra dimension. Klein’s contribution best addressed the question of why such a fifth dimension would not be observable—consistent with Ehrenfest’s conclusion that space appears three-dimensional. In an idea known as compactification, Klein envisioned that the higher dimension would be rolled up into a tiny, compact loop on the order of 10
 
 -33
 
 centimeters. Thus, while it would supply (in theory, if not in practice) a means of unification, it would be undetectable—like a curled up pill bug camouflaged as a dot on a leaf.
 

 Klein’s contemporaries in the late 1920s, shaping the fundamentals of quantum mechanics, chose to explore the possibility of internal (pertaining to an abstract, mathematical space) dimensions, rather than physical ones that supplement spacetime. They developed their theories in Hilbert space, a mathematical construction that makes use of the infinite number of mathematical dimensions to allow for an indefinitely large assortment of quantum states. Aside from Einstein and his assistants Peter Bergmann and Valentine Bargmann, few physicists investigated the notion of unseen extra dimensions in the physical universe. (In the late 1930s and early 1940s, Einstein, Bergmann and Bargmann tried unsuccessfully to extend general relativity’s four-dimensional spacetime by an extra physical dimension to incorporate electromagnetism.)
 

 In the 1970s and 1980s, Kaluza-Klein theory experienced a revival thanks to the emergence of
 <a href="http://superstringtheory.com">
 superstring theory
 </a>
 and its cousin supergravity: the idea that the fundamental components of nature are vibrating strands of energy. Mathematically, superstring theory turned out to be viable only in ten dimensions or more. Consequently, researchers began contemplating ways in which the extra six or more dimensions could be compactified.
 

 Superstring theory evolved in the 1990s into a more general approach, called M-theory, that incorporated energetic membranes, nicknamed “branes,” along with strings. M-theory included the possibility of a large extra dimension, supplementing the ten essential dimensions in which superstrings could live. “Large” in that context meant “potentially observable,” rather than minuscule and compact.
 

 Soon researchers realized that the large extra dimension could potentially solve a conundrum called the hierarchy problem. That dilemma involves the striking weakness of gravity compared to the other forces of nature, such as electromagnetism. A simple experiment illustrates that imbalance. Pick up a steel thumbtack with a tiny kitchen magnet, and see how its attraction overwhelms the gravitational pull of the entire earth.
 

 In the “brane world” scenario, first proposed by physicists Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali (a collaboration abbreviated as “ADD”), and later developed by Lisa Randall, Raman Sundrum, and others, reality consists of two branes, separated by a higher-dimensional gap called the bulk, in a configuration something like the Grand Canyon. Like timid tourists perched on a canyon rim, most particles cling to one of the branes. Consequently, the familiar physical world is situated there. Stalwart hikers that they are, gravitons, the carriers of gravity, are offered an exception and are able to explore the bulk in between. Because gravity’s agents spend much less time interacting with our familiar brane, gravity seems much weaker than the other forces.
 

 The original ADD conjecture predicted that, when measured at fine scales , gravity should deviate subtly from a perfect inverse-squared distance relationship. However precise torsion balance experiments performed by a team led by Eric Adelberger of the University of Washington placed strict constraints on such a discrepancy down to minute levels. Nevertheless, the idea of extra dimensions has continued to flourish in various proposals for unification of the natural forces.
 

 One of the missions of the Large Hadron Collider (LHC), the behemoth accelerator straddling the French-Swiss border, has been to test the
 <a href="http://home.web.cern.ch/about/physics/extra-dimensions-gravitons-and-tiny-black-holes">
 possibility of unseen extra dimensions
 </a>
 . Since the discovery of the Higgs Boson in 2012, completing the Standard Model of particle physics, the idea of looking at such extensions has become more central.
 

 To establish the existence of extra dimensions with the LHC, there are three major avenues of attack. The first involves finding echo versions of existing particles, called Kaluza-Klein states. These would be like the known particles in all respects, except more massive, like overtones in music. At a proton-proton collision energy of 7 trillion electron volts, searches have been made for Kaluza-Klein gravitons, Kaluza-Klein gluons and others, so far to no avail.
 

 Physicists are also using the LHC to search for evidence of gravitons seeping into higher dimensions. Such signals of otherwise unexplained missing energy would have to be sifted from enormous numbers of collision events, carefully ruling out a plethora of more mundane possibilities, such as escaped neutrinos.
 

 Evidence for extra dimensions could also show up at the LHC in the form of microscopic black holes, predicted by certain higher dimensional theories. Famously, before the LHC opened, alarmists raised a fear of such objects destroying the Earth, despite calculations showing they would harmlessly decay within a tiny fraction of a second. Despite the hopes and warnings, miniature black holes have yet to be detected among the collision data of LHC experiments.
 

 Currently, the LHC is switched off and being revamped in preparation for cranking up its collision energy almost twice as high as the previous run. In 2015 it is expected to reopen and collide protons at 13 trillion electron volts, offering the possibility of producing more massive particles and more unusual events. The upgrade will offer a greater chance to detect evidence of extra dimensions.
 

 Engineers will marvel, no doubt, at its gleaming mechanisms, while mathematicians will be awestruck by the sheer quantity of its collected data and the powerful algorithms sifting through it. And physicists will wait eagerly for possibly the first evidence of a higher-dimensional realm beyond space and time.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 Cosmos:
 <a href="http://www.youtube.com/watch?v=UnURElCzGc0">
 Carl Sagan: The 4th Dimension
 
</a>
 In this scene from the classic “Cosmos” series, Carl Sagan imagines what happens when a three-dimensional character enters a two-dimensional world.
 

<a href="http://www.eldritchpress.org/eaa/FL.HTM">
 Flatland
 </a>
 
 Written in 1884, E.A. Abbott’s classic novella simultaneously satires Victorian culture and imagines life in a two-dimensional world.
 

 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/physics/imagining-other-dimensions.html">
 Imagining Other Dimensions
 </a>
 
 Journey from a two-dimensional “flatland” to the ten- (or more) dimensional world of superstring theory in this illustrated essay.
 
</section>

Discover why physicists think our universe may have ten dimensions&mdash;or more.

The Big Bang’s Identity Crisis

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 Think of the Big Bang, and you probably imagine a moment in time when matter, energy and space itself all burst into existence at once. Yet many astrophysicists now believe that the “Big Bang” was actually two distinct events: first the inaugural instant of space and time, and second the generation of most of the “stuff” that populates that space. So, which really deserves to be called the Big Bang?
 
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 Ambiguity has plagued the expression “Big Bang” since its origin. When British astronomer Fred Hoyle coined it during a radio interview in 1948, he meant it as the ultimate put down. Hoyle refused to believe that the universe had a beginning, a first moment of time and a genesis of all matter and energy. Rather, he thought that the cosmos maintained itself in a “steady state” through a slow trickle of particles into reality. He hypothesized a “creation field” that would gradually generate new matter to fill the gaps between galaxies moving away from each other, keeping the overall density of the universe the same.
 

 Hoyle had a point: Science rightly eschews processes without clear mechanisms, and at the time, no one could explain how the whole cosmos could emerge instantly out of nothing. Nevertheless, his alternative steady state approach could not match up with mounting evidence that the observable universe was once extremely hot and dense. In 1964, Bell Laboratory researchers Arno Penzias and Robert Wilson found the smoking gun: cooled-down radiation from the early universe, known as the
 <a href="http://wmap.gsfc.nasa.gov/universe/bb_tests_cmb.html">
 cosmic microwave background
 </a>
 (CMB), which convinced almost everyone in the scientific community that the universe started with a bang.
 

 The discovery of the CMB was a victory for the Big Bang theory, yet it also presented a puzzle: The radiation was strikingly uniform across the sky. While
 <a href="http://map.gsfc.nasa.gov/universe/bb_cosmo_fluct.html">
 later studies
 </a>
 identified tiny variations of less than one part in 10,000, the standard Big Bang model couldn’t justify such uniformity. There simply wasn’t enough time in early cosmic history, when the universe was small, for energy to have traveled across space and evened out its temperature.
 

 Enter the brilliant concept of inflation, proposed in 1981 by
 <a href="http://web.mit.edu/physics/people/faculty/guth_alan.html">
 Alan Guth
 </a>
 , and later modified by Andrei Linde, Paul Steinhardt, Andreas Albrecht, and others. Guth realized that a sudden, ultra-rapid stretching of the universe could take a tiny uniform patch and expand it to a size where it ultimately would grow and become the observable universe. During the fleeting instant of inflation, any irregularities in the primordial cosmos would be propelled beyond detection, offering a kind of blank slate. It is like taking a crinkled tablecloth and stretching it out so quickly that it appears flat on a tabletop and any wrinkles left are off the table and out of view. Only tiny, jiggling quantum fluctuations would disturb the uniformity; these fluctuations would be the seeds of the galaxies and galaxy clusters we see today.
 

 Inflation solved critical problems in cosmology, but it also split the Big Bang into distinct phases: In the inflationary portrait, the creation of almost all of the matter and energy in the universe takes place at the close of the inflationary period, through a process called “reheating,” rather than before inflation. Reheating involves a massive release of energy from inflation’s driving engine: an entity called the “
 <a href="http://www.stanford.edu/~alinde/golden.gif">
 inflaton
 </a>
 ,” thought to be a fluctuating energy field that ignited ultra-rapid cosmic expansion. Theorists think that at the end of inflation, the inflaton field released an enormous reservoir of potential energy into space—which, following Einstein’s famous equivalence between energy and mass, converted into a deluge of particles. Before then, because stretching causes cooling, the universe was actually relatively cold. As the cosmos rapidly expanded, its hot initial temperature dropped by a factor of many thousand (the precise amount depends on the particular model), becoming extraordinarily hot only after reheating. If you feel that an event should be fiery if it’s going to be called the “Big Bang,” then reheating, not the cosmic dawn, was the true “bang.” (Max Tegmark has made this case in
 <a href="http://www.preposterousuniverse.com/blog/2014/04/21/guest-post-max-tegmark-on-cosmic-inflation/">
 a recent blog post
 </a>
 .)
 

 So, science is faced with a dilemma. Which moment should the term “Big Bang” refer to: cosmic genesis, when space emerged, or the time of reheating, when most of the matter and energy was created? If the Big Bang denotes post-inflationary reheating, then we’ll have to talk about pre-Big Bang cosmology, which makes cosmologists squirm. Also, reheating occurred at slightly different times in various parts of the universe, making it more of a process than a sudden burst. On the other hand, if the Big Bang refers to the first moment of time, it was not much of a “bang.” That’s because the energy fields created then wouldn’t have been very hot.
 

 Furthermore, in some versions of inflation, the seeds of rapid growth developed from the loam of an
 <a href="http://www.stanford.edu/~alinde/1032226.pdf">
 eternal cosmos
 </a>
 . In that case, there could have been
 
 no
 
 initial moment of creation.
 

 Perhaps it is time to replace the expression “Big Bang” with more precise nomenclature. Could there be a better way of distinguishing the idea of an initial cosmic moment from the concept that a field caused sudden expansion that tapered off and led to the generation of matter and structure?
 

 Unfortunately, past attempts to find a replacement term for “Big Bang” have met with failure. Most famously, in 1993 Sky and Telescope magazine sponsored a contest to identify a new name. Astronomer and science writer Timothy Ferris, who called for a better designation, served as one of the judges. Ferris said in an
 <a href="http://articles.philly.com/1993-10-03/news/25936693_1_big-bang-matter-and-energy-georges-lemaitre">
 interview
 </a>
 that the term was “inappropriately bellicose” and unsuitable for conveying “the event thought to have spawned the starry skies.”
 

 Despite more than 13,000 submissions from participants in 41 countries, including colorful suggestions such as “The Grand Expansion,” “The Hubble Bubble,” and “The First Fireball,” the contest was declared a bust. The judges did not find any of the entries as simple and evocative as “Big Bang.” Consequently, they announced that nobody had won and that
 <a href="http://community.seattletimes.nwsource.com/archive/?date=19940201&amp;slug=1892815">
 the old name would remain
 </a>
 .
 

 More than 20 years later, “Big Bang” still doesn’t seem quite right, especially as cosmology has declared the nascent history of the cosmos more complex than once believed. Perhaps as our picture of the early universe continues to become more refined, new names will emerge to mark the complexity of its initial development. Just as we talk about an ovum, zygote, embryo and fetus in the process of human development, maybe we will someday have precise terms for early phases in the emergence of space, time, energy and matter.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


<a href="http://www.amazon.com/The-Inflationary-Universe-Alan-Guth/dp/0201328402">
 The Inflationary Universe
 
</a>
 Alan Guth’s accessible “diary” of the birth of inflation theory.
 

 Wikipedia:
 <a href="http://en.wikipedia.org/wiki/Chronology_of_the_universe">
 Chronology of the universe
 </a>
 
 10
 
 150
 
 years of cosmic history, from the inflationary epoch to the heat death of the universe, all on one Wikipedia page.
 
</section>

New insights into the early history of the universe have some physicists saying that it is time to rename the Big Bang.

Schrödinger’s Menagerie

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 Erwin Schrödinger loved animals.
 

 That may come as a surprise to those who associate Schrödinger most closely with his cat paradox. Of all of Schrödinger’s accomplishments, including the Nobel-Prize-winning development of the quantum wave equation that bears his name, that hypothetical thought experiment, occupying just a few paragraphs in a single paper, is what sticks in the public memory. It has also earned Schrödinger an
 <a href="http://www.youtube.com/watch?v=JNalMWLnt0o">
 undeserved reputation
 </a>
 for
 <a href="http://www.cartoonstock.com/directory/s/schrodinger_s_cat.asp">
 animal cruelty
 </a>
 .
 

 Yet his family knew better. As related by his daughter Ruth Braunizer, Schrödinger’s dog Burschie (“Laddie”) offered great comfort to the family throughout the dark days of World War II. No family scene was complete without their beloved collie.
 

 Why would a lover of animals imagine something so cruel? For the same reason that, 80 years later, Schrödinger is still branded a cat-hater: preposterous analogies are memorable.
 

 Schrödinger was a master of vivid analogy. He loved literature and drama, particularly the writings of the ancient Greeks and eastern mystics, and drew upon such sources for ways of expressing his ideas. His theatrical lectures captivated audiences with references to commonplace situations with which they might identify and have a laugh.
 

 In fact, Schrödinger’s cat is just one member of a whole zoo of (imagined) animals that Schrödinger pressed into service to bring his analogies to life: Schrödinger’s menagerie!
 


 Schrödinger’s Cat
 

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 The epitome of ridiculousness, Schrödinger’s cat paradox explores the extreme ramifications of quantum entanglement, a term he coined to describe how a single quantum state can encompass multiple, separate objects. Although Schrödinger pioneered the wave version of quantum mechanics, he was flummoxed by how his ideas were reinterpreted by Max Born and others as a probabilistic theory that allows for sudden, discontinuous transitions from one particle state to another. As Werner Heisenberg and John von Neumann suggested, observation of a mixed quantum state—representing, for example, a range of position values–can trigger it to collapse into an unblended state, pertaining to, for instance, a single position value. If the initial state describes an entangled set of particles, then the entire state collapses at once, immediately affecting all of the particles in question.
 

 As a believer in continuous transformations, rather than sudden changes, Schrödinger found preposterous the idea that a scientific measurement could instantly spur a cavalcade of transformations. Therefore he developed the idea of linking a cat’s fate to that of a decaying radioactive sample by making a Geiger counter the trigger for the release of poison. Just as the sample would be in a mixed quantum state before observation, so would be the cat—in a bizarre superposition of life and death.
 

 While Schrödinger hoped that his thought experiment would stimulate a rethinking of quantum assumptions, the collapse idea has remained the mainstream approach, with only a minority of theorists supporting alternative notions such as the Many Worlds Interpretation (in which the cat would be alive in one parallel universe and dead in the other). As for the cat itself, it has become a symbol of ambiguity.
 


 Schrödinger’s Flea
 

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 Schrödinger’s wave equation gathered acclaim because of its ability to predict the energy levels and other properties of electrons in an atom. It was more tangible than Heisenberg’s matrix mechanics approach, which achieved the same aim but was based on more abstract calculations. When Born fused the two ideas into a comprehensive quantum mechanics based on probability waves, he included the idea of instant quantum transitions between energy levels. Mortified by the transformation of his continuous theory into a discontinuous one, Schrödinger reportedly said, “If you have to have these damn quantum jumps then I wish I’d never started working on atomic theory.”
 

 Schrödinger argued that electrons, unlike creatures of the animal kingdom, lack free will and therefore couldn’t “decide” when to jump. Therefore, they must be described through a continuous theory. According to physicist Walter Heitler, who worked for a time with Schrödinger, he was often known to say, “I can’t believe that an electron hops about like a flea.”
 

 Schrödinger sharply distinguished animate objects, such as fleas, from inanimate particles, such as electrons. That’s because, versed in the writings of the Hindu Vedas and other eastern ideas, he firmly believed in a world of the spirit that transcended the material world. While the inanimate world behaved like clockwork, the animate world was subject to choices.
 


 Schrödinger’s Hydra Fusca
 


 Schrödinger had a strong interest in possible connections between physics and biology. In his influential book, “What is Life?,” he speculated that life possessed a unity that was derived from a template. His proposal that life emerged from a aperiodic crystal helped inspire James Watson and Francis Crick to search for the molecular blueprint of life—what became known as DNA.
 

 In another book, “My View of the World,” Schrödinger further stressed the interconnectedness of all living beings through examples involving simple organisms such as planaria and Hydra fusca (a freshwater polyp.) Noting the polyp’s tendency to divide he wrote, “I want the reader to think himself into Hydra fusca. We cannot but attribute some kind of consciousness, however dim and undifferentiated, to our primitive little cousin on the ladder of life. And this consciousness will appear in both sections as the undivided continuation as what was there before.”
 

 Interestingly, physicist Hugh Everett made a similar argument when he proposed what became known as the Many Worlds Interpretation. He suggested that human conscious awareness splits into parallel branches each time a quantum measurement is taken, like the dividing of primitive animals.
 


 Schrödinger’s Rabbits
 

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 In the mid 1940s, Schrödinger joined Einstein in a quest to explain all of the forces of nature in a single unified theory. While Einstein felt confident that the theory would replace quantum mechanics, Schrödinger wasn’t so sure. He once asserted that Einstein was being too ambitious. “You are after big game, as they say in English,” he informed Einstein. “You are on a lion hunt and I’m talking about rabbits.”
 


 Schrödinger’s Iguanodon
 

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 As I describe in my new book “
 <a href="http://www.basicbooks.com/full-details?isbn=9780465075713">
 Einstein’s Dice and Schrödinger’s Cat
 </a>
 ,” in January 1947 Schrödinger declared to the Royal Irish Academy that he had discovered a unified theory of physics—the goal that had eluded Einstein for decades. Privately, in a letter to Einstein, he expressed some self-doubt. Referring to a story by Kurd Lasswitz about a primitive creature vexed by climate change, Schrödinger wrote to Einstein about his theory: “There is simply nothing else sensible… If that is not right, then I’ll let myself be Iguanodon, say ‘Cold, cold cold,’ and stick my head in the snow.”
 

 We see that cats were far from the only creatures in Schrödinger’s menagerie of animal analogies. Far from someone cruel to animals, the great Austrian physicist believed they are connected to us in the grand scheme of life. Next time you hear about Schrödinger’s cat, perhaps you can offer a witty reply about his flea or Iguanodon.
 


 Go Deeper
 
 

 Our picks for further reading
 



 The Conversation:
 <a href="http://theconversation.com/schrodingers-cat-gets-a-reality-check-37278">
 Schrödinger’s cat gets a reality check
 </a>
 
 Physicist Eric Cavalcanti explains a new experiment designed to probe the nature of the wavefunction and answer the question: Is Schrödinger’s cat really dead and alive?
 

 MacTutor:
 <a href="http://www-history.mcs.st-and.ac.uk/Biographies/Schrodinger.html">
 Erwin Rudolf Josef Alexander Schrödinger
 </a>
 
 Discover more about Schrödinger’s life in this concise online biography.
 

 University of Pittsburgh:
 <a href="http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_measurement/index.html">
 The Measurement Problem
 </a>
 
 John Norton of the University of Pittsburgh provides a detailed, accessible primer on Schrödinger’s cat.
 
</section>

You already know Schrodinger’s cat. Now meet the other animals of the famous physicist's thought experiments.

When the End is Just the Beginning: Exploring Cosmic Cycles

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 This is how it all could end: heat death, the moment when the universe expends its last drop of usable energy and settles into lifelessness. Like the ultimate junkyard, only relics of once-burning stars and their planetary companions remain. Wait long enough and even black holes vanish through the slow trickle of Hawking radiation.
 

 That is a bleak and humbling thought. But might nature enjoy an encore or even a revival? A second act for the universe, even if it were many billions of years from now, would offer a source of comfort. We’d know that even if our own civilization—and indeed all possible living worlds in space—blew out like a flame in the wind, flickers of new life could eventually relight.
 
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 In the olden days of cosmology—that is, before the 1998 discovery that
 <a href="http://imagine.gsfc.nasa.gov/docs/features/exhibit/tenyear/darkenergy.html">
 the universe’s expansion is accelerating
 </a>
 —there was hope that the universe might bounce back from a “
 <a href="http://map.gsfc.nasa.gov/universe/uni_fate.html">
 Big Crunch
 </a>
 .” In that scenario, the universe’s sheer heft would eventually cause it to stop expanding, switch gears and shift into reverse. Like a film of the Big Bang played backward, all of space would collapse to a single point. Cosmologist Stephen Hawking once speculated that time itself would switch directions during the contraction phase. (Later, after
 <a href="http://www.theglobeandmail.com/life/facts-and-arguments/my-time-with-stephen-hawking/article4085631/">
 discussions with his assistant Raymond Laflamme
 </a>
 , he changed his mind.) Today, with no end in sight for cosmic expansion, the “Big Crunch” is out of fashion. But new theories are offering some hope that the heat death may not be terminal.
 

 Cosmic cycles have a long tradition in philosophy and religion. From the ancient Chinese to the Mayans, numerous cultures embraced the idea of time as an ever-turning wheel. Many ancient peoples imagined that a cosmic “spring” would follow each cosmic “winter” in a perpetual sequence of phases, just like the seasons on Earth. Scientific cosmology followed suit. Shortly after Edwin Hubble discovered that galaxies were receding from each other—an observation that beautifully matched Belgian scientist and cleric Georges Lemaître’s hypothesis that the universe is expanding—physicist Richard Tolman began to investigate the possibility that the cosmos was eternally oscillating through cycles of creation and destruction. The idea was that each Big Crunch would be followed by a new Big Bang, tracing an
 <a href="http://teachastronomy.com/astropedia/article/Alternatives-to-the-Big-Bang-Model">
 endless accordion of expansions and contractions
 </a>
 . Unfortunately, Tolman later realized that the second law of thermodynamics, which states that entropy in a closed process can’t decrease, would force each cycle to be longer with less usable energy, a scenario that seems less like renewal and more like modulated decay.
 

 But it’s not all bad news for cosmic cycles. Believers in eternal renewal can take heart that researchers have advanced several cyclic cosmologies that are consistent with the latest discoveries about cosmic acceleration and dark energy.
 

 One such model is the cyclic universe (originally and more poetically known as the
 <a href="http://www.physics.princeton.edu/~steinh/vaasrev.pdf">
 Ekpyrotic Universe
 </a>
 , after the ancient Greek name for the fiery interludes between cycles) first proposed by astrophysicists Paul Steinhardt and Neil Turok and string theorist Burt Ovrut, later joined by Steinhardt’s student Justin Khoury. The trio (sans Khoury) hatched the idea on a train ride back to London from a conference they were attending together in Cambridge, while discussing the cosmological implications of collisions between hypothetical entities called D-branes, or branes for short. In string theory, the universe is composed of strings—energetic strands whose vibrations are hypothesized to produce particle properties—and branes, pulsating surfaces to which open strings can attach like floppy spaghetti. That special relationship has suggested the concept of braneworlds, models of the universe in which all particles in the Standard Model, from electrons and quarks to photons and gluons, are represented by open strings, gravitons (the carriers of gravity) by closed strings, and the observable universe by a region on a brane.
 

 The researchers imagined an oscillatory cosmos in which our brane repeatedly collides with a neighboring one along a higher dimension. This model explains how some particular features of our cosmos, like the formation, distribution, and recession of galaxies, could be produced by repeated infusions of energy from inter-dimensional smash-ups. Each time the branes collide, everything from the previous cycle is wiped out and new structures form. Part of the collision energy becomes what we know as dark energy, driving the acceleration of three-dimensional space. Ultimately matter and energy spread out until the universe is increasingly dilute. The branes collide once more and the cycle renews. A solitary Big Bang becomes replaced with
 <a href="http://www.physics.princeton.edu/~steinh/brane3.html">
 repeated Big Bounces
 </a>
 .
 

 One can think of the cyclic cosmology as something akin to a desert plain that experiences a tremendous burst of rain once a year. During the deluge, an enormous pool of water starts to spread across the dry land, continuing to dissipate even after the rain stops. Gradually the water seeps completely into the soil until the land becomes parched once more. Eventually a fresh torrent heralds the start of the next cycle. Similarly, while the cause of cosmic cycles hits us from beyond our brane, the resulting expansion takes place within it. (Unlike a growing pool of rainwater, though, space does not expand into anything; rather, locales such as galaxies move farther and farther away from each other.)
 

 The cyclic scenario avoids the entropy problem that plagued Tolman’s model, Steinhardt explains, because in the cyclic cosmos, “entropy density does not accumulate. What’s happening in ours is that you are producing entropy, but our three dimensions don’t re-collapse, so it just remains thinned out after the period of expansion. Now when you collide you produce new entropy at very high density. The old stuff is of such low density that it’s just irrelevant at the end of the next cycle. So you can go right into the next cycle with essentially the same density you’ve had before. The total entropy has increased, but the entropy density goes to negligibly small each cycle.”
 

 One weakness in the cyclic scenario is that there is no experimental evidence for extra dimensions beyond our perceived space and time. Fortunately, when the Large Hadron Collider
 <a href="http://home.web.cern.ch/about/updates/2014/06/cern-announces-lhc-restart-schedule">
 reboots in early 2015
 </a>
 , we will be able to test some models that predict extra, unseen dimensions.
 

 Another cyclic approach eschews higher dimensions in favor of a topological twist. Known as
 <a href="https://accelconf.web.cern.ch/accelconf/e06/PAPERS/THESPA01.PDF">
 conformal cyclic cosmology
 </a>
 , it was proposed by Oxford mathematician Roger Penrose, based on the work of his former student K. Paul Tod, as a way of explaining why entropy is so low at the dawn of time and so high at its dusk—which is another way of explaining why eggs splat when they fall out of the carton but never spontaneously reassemble themselves, and, indeed, why time only goes one direction. Penrose postulated that the geometry of space itself could be changing over time, becoming ever more tangled, like an unkempt, snarled head of hair. In conformal cyclic cosmology, this tangliness is described by a mathematical object called the Weyl curvature tensor, which starts at zero at the beginning of time and grows larger and larger over the eons.
 

 The cyclic part enters via a mathematical sleight-of-hand called conformal invariance. Conformal invariance is a symmetry between geometries of different scales, transforming size but keeping shape intact. It acts as a kind of mathematical magnifying glass. Penrose noted that many billions of years from now—after all the stars in the universe are extinguished, all black holes are drained of their energy through Hawking radiation, and all matter completely decayed—space would be perfectly uniform – a reservoir of useless energy. Thus, except for scale and degraded (high entropy) energy, it would be identical to the cosmic beginning. Suppose, then, that a conformal transformation connects the emptiness at the end of time with the emptiness at the beginning, like a strip of paper glued into a ring. Degraded particles would become vibrant (zero entropy) once more. The demise of the universe would become a
 <a href="https://www.youtube.com/watch?v=sM47acQ7pEQ">
 new start
 </a>
 .
 

 Only a few years ago, many researchers thought that cycles were so over, banished by the fall of the Big Crunch scenario. But like retro fashions, they keep returning, brought back by savvy designers who tailor them to the needs of contemporary cosmological conditions—and the cycle continues.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


<a href="http://www.randomhouse.com/book/129417/cycles-of-time-by-roger-penrose">
 Cycles of Time: An Extraordinary New View of the Universe
 </a>
 
 Roger Penrose’s book explores the theory of conformal cyclic cosmology.
 

<a href="http://www.physics.princeton.edu/~steinh/endlessuniverse/index2.html">
 Endless Universe: Beyond the Big Bang
 </a>
 
 Paul Steinhardt and Neil Turok make the case for cyclic cosmology in this popular 2007 book.
 

 The Nature of Reality:
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/06/in-the-beginning/">
 In the Beginning
 </a>
 
 Science writer Charles Choi on the cosmologists who dare to ask what happened before the Big Bang.
 
</section>

Discover why some theorists think our universe could one day be reborn, in an endless cycle of creation and destruction.

Paul Halpern

Articles