In this letter we investigate the nature of generic cosmological singularities using the framework developed by Uggla et al. We do so by studying the past asymptotic dynamics of general vacuum G2 cosmologies, models that are expected to capture the singular behavior of generic cosmologies with no symmetries at all. In particular, our results indicate that asymptotic silence holds, i.e., that particle horizons along all timelines shrink to zero for generic solutions. Moreover, we provide evidence that spatial derivatives become dynamically insignificant along generic timelines, and that the evolution into the past along such timelines is governed by an asymptotic dynamical system which is associated with an invariant set -the silent boundary. We also identify an attracting subset on the silent boundary that organizes the oscillatory dynamics of generic timelines in the singular regime. In addition, we discuss the dynamics associated with recurring spike formation.PACS numbers: 98.80.Jk, 04.20.Dw, 04.25.Dm, 04.20.Ha The singularity theorems of Penrose and Hawking [1] state that generic cosmological models contain an initial singularity, but do not give any information on the nature of this singularity. Heuristic investigations of this issue led Belinskiǐ, Khalatnikov and Lifshitz [2] (BKL) to propose that a generic cosmological initial singularity is spacelike, local and oscillatory. Uggla et al.[3] (UEWE) reformulated Einstein's field equations (EFEs) by introducing scale-invariant variables which have the property that all individual terms in EFEs become asymptotically bounded, for generic solutions. This made it possible to characterize a generic cosmological initial singularity in terms of specific limits. The numerical study of the picture proposed by UEWE was initiated in Ref.[4], specializing to Gowdy vacuum spacetimes which have a nonoscillatory singularity. This letter presents the results of the first detailed study of the oscillatory asymptotic dynamics of inhomogeneous cosmologies from the dynamical systems point of view introduced in UEWE.Here we focus on vacuum cosmologies with an Abelian symmetry group G 2 with two commuting spacelike Killing vector fields, and the spatial topology of a 3-torus. This is arguably the simplest class of inhomogeneous models that is expected to capture the properties of a generic oscillatory singularity. Numerical investigations of G 2 spacetimes supporting the BKL proposal were carried out by Weaver et al. in Ref. [5,6].UEWE used an orthonormal frame formalism and factored out the expansion of a timelike reference congruence e 0 by normalizing the dynamical variables with the isotropic Hubble expansion rate H of e 0 . This yielded a dimensionless state vector X = (E α i )⊕S, where E α i are the Hubble-normalized components of the spatial frame vectors orthogonal to e 0 ; e α = e α i ∂ i , E α i = e α i /H.The approach to an initial singularity will be said to be asymptotically silent for timelines along which E α i → 0, and asymptotically silent and local for timel...

By applying a standard solution-generating transformation to an arbitrary vacuum Bianchi type II solution, one generates a new solution with spikes commonly observed in numerical simulations. It is conjectured that the spike solutions are part of the generalized Mixmaster attractor.

We produce numerical evidence that spikes in the Mixmaster regime of G2 cosmologies are transient and recurring, supporting the conjecture that the generalized Mixmaster behavior is asymptotically non-local where spikes occur. Higher order spike transitions are observed to split into separate first order spike transitions.Gowdy solutions with the spike property [7,8].

A period of slow contraction with equation of state w > 1, known as an ekpyrotic phase, has been shown to flatten and smooth the universe if it begins the phase with small perturbations. In this paper, we explore how robust and powerful the ekpyrotic smoothing mechanism is by beginning with highly inhomogeneous and anisotropic initial conditions and numerically solving for the subsequent evolution of the universe. Our studies, based on a universe with gravity plus a scalar field with a negative exponential potential, show that some regions become homogeneous and isotropic while others exhibit inhomogeneous and anisotropic behavior in which the scalar field behaves like a fluid with w = 1. We find that the ekpyrotic smoothing mechanism is robust in the sense that the ratio of the proper volume of the smooth to non-smooth region grows exponentially fast along time slices of constant mean curvature.