The Cyclic Theory of the Universe

The cyclic theory of the universe is a radical alternative to the standard big bang/inflationary scenario that offers a new approach for resolving the homogeneity, isotropy, and flatness problems and generating a nearly scale-invariant spectrum of fluctuations. The original formulation of the cyclic model was based on the picture suggested by M-theory in which the observable universe lies on a brane separated by a small gap along an extra dimension from a second brane. The cyclic model proposes that the big bang is a collision between branes that occurs at regular intervals; that each bang creates hot matter and radiation and triggers an epoch of expansion, cooling and structure formation; that there is an interbrane force responsible for drawing the branes together whose potential energy acts like dark energy when the branes are far apart; and that each cycle ends with the contraction of the extra dimension and a collision between branes – a new big bang – that initiates the next cycle. In more recent formulations, the cyclic model is realized with ordinary quantum field theory without introducing branes or extra dimensions. The key innovation common to all these models is the ekpyrotic phase, the period of ultra-slow contraction preceding the big bang. It is the ekpyrotic phase, rather than inflation, that is responsible for explaining the smoothness, flatness and large scale structure of the universe. It is also the ekpyrotic phase that generates the distinctive signatures in the spectrum of primordial gravitational waves and non-gaussian density fluctuations that will be used to test the cyclic model in forthcoming experiments.