We prove the nonlinear stability in the contracting direction of Friedmann-Lema\^itre-Robertson-Walker (FLRW) solutions to the Einstein-scalar field equations in $n\geq 3$ spacetime dimensions that are defined on spacetime manifolds of the form $(0,t_0]\times \mathbb{ T}^{ n-1}$, $t_0>0$. Stability is established under the assumption that the initial data is \textit{ synchronized}, which means that on the initial hypersurface $\Sigma= \{ t_0\}\times \mathbb{ T}^{ n-1}$ the scalar field $\tau= \exp\bigl(\sqrt{ \frac{ 2(n-2)}{ n-1}}\phi\bigr) $ is constant, that is, $\Sigma=\tau^{ -1}(\{ t_0\})$. As we show that all initial data sets that are sufficiently close to FRLW ones can be evolved via the Einstein-scalar field equation into new initial data sets that are \textit{ synchronized}, no generality is lost by this assumption. By using $\tau$ as a time coordinate, we establish that the perturbed FLRW spacetime manifolds are of the form $M = \bigcup_{ t\in (0,t_0]}\tau^{ -1}(\{ t\})\cong (0,t_0]\times \mathbb{ T}^{ n-1}$, the perturbed FLRW solutions are asymptotically pointwise Kasner as $\tau \searrow 0$, and a big bang singularity, characterised by the blow up of the scalar curvature, occurs at $\tau=0$. An important aspect of our past stability proof is that we use a hyperbolic gauge reduction of the Einstein-scalar field equations. As a consequence, all of the estimates used in the stability proof can be localized and we employ this property to establish a corresponding localized past stability result for the FLRW solutions.
Categories: gr-qc math.AP