Neil Turok

The Ekpyrotic universe: Colliding branes and the origin of the hot big bang

We propose a cosmological scenario in which the hot big bang universe is produced by the collision of a brane in the bulk space with a bounding orbifold plane, beginning from an otherwise cold, vacuous, static universe. The model addresses the cosmological horizon, flatness and monopole problems and generates a nearly scale-invariant spectrum of density perturbations without invoking superluminal expansion (inflation). The scenario relies, instead, on physical phenomena that arise naturally in theories based on extra dimensions and branes. As an example, we present our scenario predominantly within the context of heterotic M theory. A prediction that distinguishes this scenario from standard inflationary cosmology is a strongly blue gravitational wave spectrum, which has consequences for microwave background polarization experiments and gravitational wave detectors.

Halo formation in warm dark matter models

Discrepancies have emerged between the predictions of standard cold dark matter (CDM) theory and observations of clustering on subgalactic scales. Warm dark matter (WDM) is a simple modification of CDM in which the dark matter particles have initial velocities due either to their having decoupled as thermal relics or to their having been formed via nonequilibrium decay. We investigate the nonlinear gravitational clustering of WDM with a high-resolution N-body code and identify a number of distinctive observational signatures. Relative to CDM, halo concentrations and core densities are lowered, core radii are increased, and large halos emerge with far fewer low-mass satellites. The number of small halos is suppressed, and those present are formed by top-down fragmentation of caustics, as part of a cosmic web connecting massive halos. Few small halos form outside this web. If we identify small halos with dwarf galaxies, then their number, spatial distribution, and formation epoch appear in better agreement with the observations for WDM than they are for CDM.

From big crunch to big bang

We consider conditions under which a universe contracting towards a big crunch can make a transition to an expanding big bang universe. A promising example is 11-dimensional M-theory in which the eleventh dimension collapses, bounces, and re-expands. At the bounce, the model can reduce to a weakly coupled heterotic string theory and, we conjecture, it may be possible to follow the transition from contraction to expansion. The possibility opens the door to new classes of cosmological models. For example, we discuss how it suggests a major simplification and modification of the recently proposed ekpyrotic scenario.

A Cyclic Model of the Universe

We propose a cosmological model in which the universe undergoes an endless sequence of cosmic epochs that begin with a “bang” and end in a “crunch.” Temperature and density at the transition remain finite. Instead of having an inflationary epoch, each cycle includes a period of slow accelerated expansion (as recently observed) followed by contraction that produces the homogeneity, flatness, and energy needed to begin the next cycle.

Kasner and mixmaster behavior in universes with equation of state w >~ 1

We consider cosmological models with a scalar field with equation of state

Looking for a cosmological constant with the Rees-Sciama effect.

w\ge 1

Local conserved densities and zero-curvature conditions for toda lattice field theories

that contract towards a big crunch singularity, as in recent cyclic and ekpyrotic scenarios. We show that chaotic mixmaster oscillations due to anisotropy and curvature are suppressed, and the contraction is described by a homogeneous and isotropic Friedmann equation if

Power spectra in global defect theories of cosmic structure formation

w>1

Electroweak baryogenesis in the two doublet model

. We generalize the results to theories where the scalar field couples to p-forms and show that there exists a finite value of

Generating Ekpyrotic Curvature Perturbations Before the Big Bang

w