About Time

by Adam Frank

  In Steinhardt and Turok’s views, all the problems that killed previous oscillating universe models were solved naturally within the framework of the model. No theoretical bells and whistles were needed. Whatever questions arose, the new cyclic model was always poised to answer them. As the two scientists described it:

  The model did not need even one iota of change. It was only necessary to discover what was already there all along, waiting to be noticed. This eerie experience is typical. . . . Whenever a problem has arisen, it’s turned out that model already contains the ingredients necessary to address it. Not a single new element has been added to the picture since it was first envisioned.15

  This kind of “eerie experience” is what makes a good theory in a scientist’s eyes. The new cyclic model’s prescience in dealing with problems convinced Steinhardt and Turok they’d stumbled upon a valid theory for the universe that had nothing to do with bangs or beginnings.

  CONFRONTING REALITY: INFLATION, THE CYCLIC MODEL AND DATA

  Attempts to imagine alternatives to the Big Bang did not end with the discovery of the cosmic microwave background. A small contingent of astronomers and physicists had remained unconvinced and determinedly worked on alternative cosmologies. As the decades wore on, the problem they faced was the growing mountain of evidence supporting the Big Bang’s basic image of a universe expanding from a hot condensed state. To challenge the Big Bang, all those separate lines of evidence needed to be addressed: the expansion of space, the abundance of light elements, the existence of the CMB, the pattern of temperature and density fluctuations in the CMB, the distribution of galaxy clusters in space. Each line of evidence points to a universe that is evolving from a hot dense state, and each line of evidence is interwoven with the others. The distribution of galaxy clusters, for example, can be directly tied to the pattern of temperature and density fluctuations in the CMB, as lumps seen in the CMB are what grew into the clusters we see today. Any plausible alternative theory of cosmology must recover these connections in a natural way. But most models haven’t been up to the task.

  The brane-world, cyclic model of Steinhardt and Turok has, to an impressive extent, been an exception to this rule. It makes a series of concrete, testable predictions that stand up well to the data cosmologists already have in hand. Most of these predictions are the same as those for inflation. Like inflation, the ekpyrotic cyclic model predicts a universe with flat space.16 And, like inflation, it anticipates a tight relationship between CMB fluctuations and the distribution of galaxy clusters. Thus, inflation and the ekpyrotic cyclic model appear on equal footing. As Steinhardt and Turok put it, “Despite the basic differences between the inflationary and the cyclic pictures, there are surprising mathematical symmetries that ensure the two agree very closely.”17

  This ability to address the majority of experimental tests in a natural way with no special fine-tuning makes the cyclic model unusual in terms of alternative cosmologies. In a sense, this is more than inflationary models yield. They must add dark energy and the inflationary quantum field as separate, unrelated entities rather than as natural parts of a unified model. There is, however, a future test using gravity waves that should be able to distinguish between these two very different stories of cosmic history. Gravity waves are moving ripples in the fabric of space-time. Like the ripples expanding away from a stone dropped in a pond, gravity waves are generated whenever mass-energy moves through the space-time background. While they have never been directly observed, exotic astronomical objects such as binary neutron stars provide indirect evidence for their existence.18 Scientists are currently building sensitive detectors to pick up these waves from gigantic events nearby, like the merger of black holes. Space-time ripples are also expected to have been generated en-masse during the chaotic first moments of cosmic history. Predicting the form and strength of these waves constitutes a critical future test for all cosmological models. Inflation makes very different predictions for these fossil gravity waves than the cyclic model does. The detection of these waves is likely decades away, as it will require extended space-based instruments. But if and when the gravity waves are found, they will determine which vision of time and cosmic evolution must be abandoned and which can lay some claim to the truth.

  RAJAGAHA, INDIA · SIXTH CENTURY BCE

  The old Brahmin adjusts his tunic and waits for the class to settle down. The students are noisy and distracted, chatting among themselves amidst the wide-leaved plants of the garden. It was going to be a hot day in the city but the old man is willing to wait. He wants today’s lesson to sink in before he sends them back to their families and their studies.

  “Today,” he begins, “I will tell you the story of Indra and the ants so that you might understand where and who you are in this great world.”

  Indra was the king of the gods. Brave, noble, possessed of a compassionate heart, he looked after both the divine and human worlds with the steady hand of a wise father. After defeating a great dragon that had destroyed the city of the gods, Indra called on Vishvakarman, master of the arts, to rebuild the great metropolis. Vishvakarman worked tirelessly and created shining palaces with marvellous gardens, lakes and towers. But Indra was not satisfied. “Give me bigger ponds, trees, towers and golden palaces!” he demanded. Whenever Vishvakarman was done with one thing Indra wanted another. The divine craftsman fell into a deep despair. In desperation he complained to the Brahma, the Universal Spirit, who abides far above the gods. Brahma comforted him: “Go home; you will soon be relieved of your burden.”

  Early the next morning a Brahmin boy appeared at the gate of the palace asking to see the great Indra. “O king of the gods, I have heard of this palace you are building. How many years will it take to finish this rich and extensive residence? Surely no Indra before you has ever succeeded in completing such a task.”

  Indra was amused by the boy. How could this child have known any Indras other than himself? “Tell me, child,” he said, “how many other Indras have you have seen or heard of?”

  The boy replied in a voice as warm and sweet as milk, but with words that sent a chill through Indra’s veins. “My dear child,” said the boy, “I knew your father. And I knew your grandfather. Also I know Brahma, brought forth by Vishnu from a lotus growing from Vishnu’s navel. And Vishnu, too, the Supreme Being, I know.

  “O king of the gods, I have seen the dreadful dissolution of the universe. I have seen it all perish again and again, at the end of each cycle. At that time every single atom dissolves into the primal pure waters of eternity, whence originally all arose. Who will count the universes that have passed away, or the creations that have risen afresh, again and again, from the formless abyss of the vast waters? Who will search through the wide infinities of space to count the universes existing side by side, each containing its own Brahma, its Vishnu and its Shiva? Who will count the Indras in them all?”

  As they were talking a procession of ants had made its appearance in the hall. In military precision, the tribe of ants paraded across the floor. The boy noticed them and suddenly laughed. “See these ants in their long parade? Each of them was once an Indra. Like you, each by virtue of his deeds ascended to the rank of king of the gods. But now through many rebirths each has become again an ant. This army of ants is an army of former Indras.”19

  The king of the gods was speechless. The boy turned and left. After many days alone, Indra called his architect and thanked him for his work. “You have done enough,” Indra said. “You may rest now.”

  The old Brahmin finished his story. He closed his eyes, took a deep breath and exhaled in meditative repose. His students, normally so boisterous and noisy, sat in stunned silence. A small smile appeared at the corners of his mouth. He had opened their imaginations, if only for a moment.

  TIME CYCLES AND CREATION

  A universe of eternal cycles, endlessly resurrected from the ashes of its own destruction, is a seductive vision that has been offered as a narrative in cosmic myth and cosmic s
cience many times. Such a cosmos has no beginning and no end, just endless rebirths stretching from the infinite past to the infinite future. The difference between myth and science, however, lies in science’s dual imperatives of theoretical consistency and empirical evidence. As attractive as Lemaître might have found the “phoenix universe” implied in his and Friedmann’s general relativistic solutions, deeper theoretical explorations of the idea always led to disaster. The buildup of dense entropy from one cycle to another doomed cyclic models, as did the chaotic space-time flailing of a Big Crunch’s Mixmaster phase. Classic general relativity combined with the quantum physics of matter was simply not a framework that allowed for the existence of a cyclic universe. Only by going outside classic relativity could Steinhardt and Turok’s new cyclic model succeed. Only by embracing a string-theory-inspired hyperdimensional brane-world vision of the cosmos could they move past its pitfalls. To embrace the elegance of an eternally regenerating universe, the new cyclic model needed more space. But was this too high a price to be paid?

  By the end of the twenty-first century’s first decade, enthusiasm for string theory and its revolutions had waned for many physicists.20 Earlier in the decade, string researchers had discovered that their still-emerging theory would not lead to a unique specification for cosmic evolution. Instead of a single all-embracing string theory prediction for what our universe should look like, there were many theorists who began to talk about a “landscape” of many universes.21 In the new picture, our universe would not be uniquely determined by string theory but would become one instance of a vast range of possibilities. According to best estimates, the number of potential universes in the landscape would be around 10500, which for all intents and purposes was about infinity. For some, string theory’s apparent failure to explicitly predict the features of this one universe we inhabit was the last straw. Instead of a theory of everything, some critics began calling it a “theory of nothing”.

  Steinhardt and Turok’s cyclic model never required the full machinery of string theory. But by beginning with the hyperdimensional brane-world idea, it accepted certain key assumptions about reality (the extra dimensions) that lay above and beyond what we directly experience.

  Of course, string theory is not the only way to create a route to quantum gravity. Other ideas provide rich and fertile ground for theorists to explore. These other paths have not received as much attention or effort as string theory but progress has been made. A theory called loop quantum gravity is perhaps the best-known of these.22 Loop quantum gravity seeks to work out the details of quantized space-time in the form of elemental atoms of existence. Originally developed by researchers such as Abhay Ashtekar, Ted Jacobson, Carlos Rovelli and Lee Smolin, the field has grown more slowly than string theory. In the wake of the general disappointment over string theory’s perceived failings—that is, the string landscape—more attention is now being paid both to loop quantum gravity and to other routes to quantum gravity. Loop quantum gravity in particular has even matured to the point where researchers have begun looking at its cosmological alternatives. Loop quantum cosmologists such as Martin Bojowald of Pennsylvania State University have even found evidence for their own version of cyclic models, as atomized space-time may yield routes through the dreaded singularity of a Big Crunch. Bojowald’s popular book Once Before Time, published in 2010, offered a layman’s description of loop quantum cosmology.23

  There are other versions of cyclic cosmologies as well. In 2009, University of North Carolina physicist Paul Frampton published a popular book, Did Time Begin?, on his own version of a cyclic model. In the same year as Bojowald’s book came out, none other than Roger Penrose announced what he claimed was evidence for previous cycles existing within the CMB. His ideas would also appear in a popular book called Cycles of Time.24 Clearly a tide of new theories reaching back to before the Big Bang had begun to wash into the popular imagination from the domains of academic cosmology.25

  Thus the ancient dream of endless cycles is very much alive as a possibility for tomorrow’s cosmology. The Vedic and Stoic visions of creation followed by destruction, followed by creation again and forever, remains a very viable alternative. Whether it’s Steinhardt and Turok’s colliding brane-worlds or another form of quantum gravitational theory providing the framework, the ancient mythic call to cycles remains and cannot yet be discounted.

  Chapter 10

  EVER-CHANGING ETERNITIES: THE PROMISE AND PERILS OF A MULTIVERSE

  Eternal Inflation, Arrows of Time and the Anthropic Principle

  LONDON • FEBRUARY 4, 1950, 8:50 P.M.

  It was a cold night in the middle of a long cold winter. But the terrible temperature outside made staying home and listening to the radio that much more delicious. And it wasn’t just any radio programme she was giving up a Saturday night for. It was that astronomer Fred Hoyle and his wonderful lectures on space and the stars and the whole universe. Fred Hoyle was worth so much more than that skinny boy, Tommy McEwan, her friend Lilly wanted to set her up with tonight.

  She was fifteen years old and, in spite of all the things everyone told her to the contrary, she longed to be an astronomer. She devoured everything she could find on physics and maths at her school’s library and she never stopped pleading with her parents for a telescope. When her teacher, the only person who encouraged her scientific ambitions, told her about Professor Hoyle’s Saturday night BBC lectures, the radio became her new religion. Tonight was one of Hoyle’s last programmes. Her head was already spinning. He was talking about the science of the entire universe, a field he called cosmology. He obviously was not happy with a theory other scientists seemed to believe. She loved the way he explained it all in his crooked Yorkshire accent:

  The assumption [is] that the universe started its life a finite time ago in a single huge explosion. . . . This big bang idea seemed to me to be unsatisfactory even before detailed examination showed that it leads to serious difficulties.

  After that Hoyle began talking about his own theory. He called it a steady-state model. The universe was expanding, just as that American Edwin Hubble had discovered, but the expansion had no beginning and no end. Even though space expanded, Hoyle said of his theory, the universe would always look the same. He asked her to imagine a film of the universe taken frame after frame over billions of years.

  What would the film look like? Galaxies would be observed to be continually condensing out of the background material. The general expansion of the whole system would be clear, but . . . there would be a curious sameness about the film . . . The overall picture would stay the same because of the compensation whereby the galaxies that were constantly disappearing through the expansion of the universe were replaced by newly forming galaxies. The casual observer who went to sleep during the showing of the film would find it difficult to see much change when he awoke. How long would a film show go on? It would go on for ever.1

  FIGURE 10.1. Voice of the Cosmos. Fred Hoyle’s BBC radio lectures on astronomy in 1950 captivated listeners. It was during his lecture on cosmology that Hoyle invented the name “Big Bang” as a term of derision. Hoyle favoured his own “steady state” version of cosmology.

  Forever. The sound of it rang in her head. She threw a coat over her nightgown and ran outside into the frigid night air, her slippers crunching on the week-old snow. There weren’t many stars to be seen from the back garden of their London council flat, but it didn’t matter. The bright city lights could not drown out the beauty of the ones she could see. She stood looking at the black and the stars for a long time, superimposing her own vision of Hoyle’s steady-state universe on them: galaxies expanding, new galaxies forming forever.

  “Forever,” she said out loud, watching the words turn to mist in the cold air. “How long is forever?”

  THE LURE OF MULTIPLE UNIVERSES

  Eternity can wear many masks. A universe running through endless cycles of creation and destruction offers one route beyond, and before, the Big Bang. It is an el
egant, aesthetically pleasing solution to the dilemma of time’s origin. But it is not the only solution.

  The desire for a timeless universe that changes locally but is framed within a changeless cosmic architecture also holds an aesthetic appeal. From Aristotle to Newton to Einstein, philosophers and scientists have all been drawn to visions of a universe in eternal repose. But Hubble’s 1930 discovery of an expanding universe, now coupled with the 1999 discovery of accelerating expansion, makes the old vision of stasis impossible. For those seeking to replace time’s origin with eternity running forward and backwards, stasis must be replaced by a steady state—a universe that is always changing yet always looks the same.

  Fred Hoyle’s tireless attempts to create a steady-state cosmology in the 1950s were driven by this scientific aesthetic. But Hoyle and his collaborators’ efforts were killed when the world spoke so clearly for itself. Material engagement in the form of astronomically tuned microwave antennas provided an unambiguous signal, selecting between competing cosmic histories. Once the cosmic microwave background was discovered, it became clear that the universe now looks nothing like the universe 13.7 billion years ago. The universe is not in a steady state. It is always changing and does not always look the same. Thus Hoyle’s steady-state model could not provide a scientifically supportable theory for a universe free of time’s beginning.

  The siren call of eternity did not, however, disappear—it was waiting to find a new expression. It is no small irony that new possibilities for eternity and a steady-state universe appeared from within the very theory intended to save the Big Bang. Just a few years after the original formulation of inflationary cosmology, some of its most creative advocates found a way out of the Big Bang’s paradox of before and after. Inflation became eternal inflation and the one universe became a multiverse.