Our universe may have arisen from seeds preserved in a universe that existed before the big bang - all thanks to dark energy. One of the models put forward to explain how the universe began proposes that it is just the latest phase in a never-ending cycle. Proposed in 2002 by Paul Steinhardt of Princeton University and Neil Turok from the University of Cambridge, the model argues that our universe exists on a 3D region called a "brane" separated from similar branes by a fourth spatial dimension. Under the right conditions, these branes collide, triggering a big-bang-like event. After the collision, the branes bounce apart, before another collision occurs many billions of years later.

Title: Ekpyrotic and Cyclic Cosmology Authors: Jean-Luc Lehners

Ekpyrotic and cyclic cosmologies provide theories of the very early and of the very late universe. In these models, the big bang is described as a collision of branes - and thus the big bang is not the beginning of time. Before the big bang, there is an ekpyrotic phase with equation of state w=P/rho >> 1 (where P is the average pressure and rho the average energy density) during which the universe slowly contracts. This phase resolves the standard cosmological puzzles and generates a nearly scale-invariant spectrum of cosmological perturbations containing a significant non-gaussian component. At the same time it produces small-amplitude gravitational waves with a blue spectrum. The dark energy dominating the present-day cosmological evolution is reinterpreted as a small attractive force between our brane and a parallel one. This force eventually induces a new ekpyrotic phase and a new brane collision, leading to the idea of a cyclic universe. This review discusses the detailed properties of these models, their embedding in M-theory and their viability, with an emphasis on open issues and observational signatures.

Title: The New Ekpyrotic Ghost Authors: Renata Kallosh, Jin U Kang, Andrei Linde, Viatcheslav Mukhanov (Version v3)

The new ekpyrotic scenario attempts to solve the singularity problem by involving violation of the null energy condition in a model which combines the ekpyrotic/cyclic scenario with the ghost condensate theory and the curvaton mechanism of production of adiabatic perturbations of metric. The Lagrangian of this theory, as well as of the ghost condensate model, contains a term with higher derivatives, which was added to the theory to stabilize its vacuum state. We found that this term may affect the dynamics of the cosmological evolution. Moreover, after a proper quantisation, this term results in the existence of a new ghost field with negative energy, which leads to a catastrophic vacuum instability. We explain why one cannot treat this dangerous term as a correction valid only at small energies and momenta below some UV cut-off, and demonstrate the problems arising when one attempts to construct a UV completion of this theory.

A six-year-old controversial theory that told of a time before the Big Bang is undergoing a resurgence, only to be lampooned again by its original critics.

The theory of ekpyrotic cosmology was first put forward as an alternative to the standard inflation model of the universe. Inflation supposes that just after the Big Bang the universe underwent a brief period of rapid expansion. This amplified tiny density perturbations, which evolved into the stars, galaxies and galaxy clusters we see today. Although there is currently no way to prove that inflation ever occurred, the fact that it provides such a simple explanation for cosmic structure and the flatness of the universe has cemented it in cosmological doctrine since it was outlined in the early 1980s.

The ekpyrotic universe or ekpyrotic scenario is a cosmological theory of the origin of the universe. The name comes from a Stoic term for "out of fire". The ekpyrotic model of the universe is an alternative to the standard cosmic inflation paradigm, both of which accept that the standard big bang Lambda-CDM model of our universe is an appropriate description up to very early times. The ekpyrotic model is a precursor to, and part of the cyclic model. Brane cosmology assumes that the visible universe lies on a three-dimensional brane which moves in higher dimensional space. Our brane may be one of innumerable others moving through these extra dimensions. The ekpyrotic scenario was proposed by Khoury, Ovrut, Steinhardt and Turok in 2001. It suggests that the visible universe was empty and contracting in the distant past. At some time, our brane collided with another, parallel "hidden" brane, which caused the contracting universe to reverse and begin expanding. Hot matter and radiation was created in the collision, which started the hot big bang from which the present-day universe originated. The brane collision, from the four-dimensional perspective of the visible brane, looks like a big crunch followed by a big bang.

The scenario is appealing because it replaces cosmic inflation with a theory that achieves many of the same successes in a framework that seems compatible with string theory. An important distinction between the ekpyrotic scenario and cosmic inflation is that in the ekpyrotic scenario, the primordial nearly scale invariant spectrum of quantum vacuum fluctuations, which is the seed for all structure in the universe today, is generated in a contracting universe, before the big crunch. In cosmic inflation they are generated immediately after the big bang, in an expanding universe.

Title: The New Ekpyrotic Ghost Authors: Renata Kallosh, Jin U. Kang, Andrei Linde, Viatcheslav Mukhanov

The new ekpyrotic scenario attempts to solve the singularity problem by involving violation of the null energy condition in a model which combines the ekpyrotic/cyclic scenario with the ghost condensate theory and the curvaton mechanism of production of adiabatic perturbations of metric. The Lagrangian of this theory, as well as of the ghost condensate model, contains a term with higher derivatives, which was added to the theory to stabilize its vacuum state. We found that this term may affect the dynamics of the cosmological evolution. Moreover, after a proper quantisation, this term results in the existence of a new ghost field with negative energy, which leads to a catastrophic vacuum instability.

Title: New Ekpyrotic Cosmology Authors: Evgeny I. Buchbinder, Justin Khoury, Burt A. Ovrut (Version v4)

In this paper, we present a new scenario of the early Universe that contains a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy condition without developing any ghost-like instabilities. Thus the contracting universe goes through a non-singular bounce and evolves smoothly into the expanding post big bang phase. The curvature perturbation acquires a scale-invariant spectrum well before the bounce in this scenario. It is sourced by the scale-invariant entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism recently proposed by Lehners et al. Since the background geometry is non-singular at all times, the curvature perturbation remains nearly constant on super horizon scales. It emerges from the bounce unscathed and imprints a scale-invariant spectrum of density fluctuations in the matter-radiation fluid at the onset of the hot big bang phase. The ekpyrotic potential can be chosen so that the spectrum has a "red'' tilt, in accordance with the recent data from WMAP. As in the original Ekpyrotic scenario, the model predicts a negligible gravity wave signal on all observable scales. As such "New Ekpyrotic Cosmology" provides a consistent and distinguishable alternative to inflation to account for the origin of the seeds of large scale structure.