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.
In the olden days of cosmology—that is, before the 1998 discovery that the universe’s expansion is accelerating—there was hope that the universe might bounce back from a “Big Crunch.” 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 discussions with his assistant Raymond Laflamme, 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 endless accordion of expansions and contractions. 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 Ekpyrotic Universe, 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 repeated Big Bounces.
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 reboots in early 2015, 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 conformal cyclic cosmology, 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 new start.
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.
Editor’s picks for further reading
Cycles of Time: An Extraordinary New View of the Universe
Roger Penrose’s book explores the theory of conformal cyclic cosmology.
Endless Universe: Beyond the Big Bang
Paul Steinhardt and Neil Turok make the case for cyclic cosmology in this popular 2007 book.
The Nature of Reality: In the Beginning
Science writer Charles Choi on the cosmologists who dare to ask what happened before the Big Bang.