May 2002The Endless Universe:Introduction to the Cyclic UniverseBy Paul J. SteinhardtActionBioscience.org editors note: The Cyclic Universe model does not replace theBig Bang theory but offers a potential alternative to certain aspects of the Big Bangthat are longstanding problems. Some scientists are excited about its potential whileothers are cautious or skeptical. The Cyclic Universe model incorporates conceptsfrom standard physics, string theory, and M-theory. For anyone who has everwondered what lies beyond the Universe, the Cyclic Universe model may providesome answers. Big Bang: the Universe expanded about 15 billion years ago toseed the formation of galaxies.The big bang/inflationary modelOver the last century, cosmologists have converged on a highly successful theory ofthe evolution of the Universe -- the big bang/inflationary picture.1 According to thispicture: • space and time sprung into being 15 billion years ago in a big bang • when the Universe emerged, it was filled with particles and radiation of nearly infinite temperature and density • instants later, the Universe underwent a period of extraordinarily rapid, superluminal expansion (inflation) which made the Universe homogeneous and flat and which created fluctuations that seeded the formation of galaxies and large-scale structureThe concept of "dark energy" is intended to explain the fact that expansionof the universe is accelerating.In the last decades, cosmological observations have supported the predictions of thebig bang and inflationary theory in exquisite detail.1,5 They have also provided onemajor surprise. It appears that, billions of years after the big bang, following theformation of galaxies, the Universe was overtaken by some form of dark energy thatis causing the expansion rate to accelerate. Although dark energy was unanticipatedand has no particular role in the big bang/inflationary picture, the general view hasbeen that it can simply be added by fiat to the initial make-up of the Universe. Thereis no compelling reason for a new theoretical approach. Quite the contrary, manycosmologists regard the basic cosmic story as being settled.Cyclic Universe: The Big Bang was not the beginning of time because theuniverse undergoes endless cycles of evolution in infinite space and time.The cyclic universe modelIn this context, a new paradigm has been recently proposed by Paul Steinhardt(Princeton) and Neil Turok (Cambridge) -- the cyclic universe -- that turns the
conventional picture topsy-turvy.2 (Perhaps the model should be called an oldparadigm since it reinvigorates ancient cosmic mythologies and philosophies, albeitusing the tools of 21st century physics.) In this picture: • space and time exist forever • the big bang is not the beginning of time; rather, it is a bridge to a pre- existing contracting era • the Universe undergoes an endless sequence of cycles in which it contracts in a big crunch and re-emerges in an expanding big bang, with trillions of years of evolution in between • the temperature and density of the universe do not become infinite at any point in the cycle; indeed, they never exceed a finite bound (about a trillion trillion degrees) • no inflation has taken place since the big bang; the current homogeneity and flatness were created by events that occurred before the most recent big bang • the seeds for galaxy formation were created by instabilities arising as the Universe was collapsing towards a big crunch, prior to our big bangThe new model provides insight into some questions unanswered by the BigBang theory. The prospects for an alternative cosmology that is so different fromthe well-established convention would seem extremely dim. Yet, the cyclic modelrecoups all of the successful predictions of the big bang/inflationary theory and hassufficient additional predictive power to address many questions which the bigbang/inflationary model does not address at all: • What occurred at the initial singularity? • What is the ultimate fate of the Universe? • What is the role of dark energy and the recently observed cosmic acceleration? • Does time, and the arrow of time, exist before the big bang? or after the big crunch?New matter is created during the transition from big crunch to big bang. Inthe new paradigm, each cycle proceeds through a period of radiation and matterdomination consistent with standard cosmology, producing the observed primordialabundance of elements, the cosmic microwave background, the expansion ofgalaxies, etc. For the next trillion years or more, the Universe undergoes a period ofslow cosmic acceleration (as detected in recent observations1), which ultimatelyempties the Universe of all of the entropy and black holes produced in the precedingcycle and triggers the events that lead to contraction and a big crunch. Note thatdark energy is not simply added on -- it plays an essential role. The transition frombig crunch to big bang automatically replenishes the Universe by creating newmatter and radiation. Gravity and the transition from big crunch to big bang keep thecycles going forever. In fact, as will be discussed, the cyclic behavior is a strongattractor. That is, even if the Universe were disrupted from its periodic behavior, itwould rapidly reconverge to the cyclic solution. Superstring theory: the Universecontains nine or ten spatial dimensions.