Localized big bang stability for the Einstein-scalar field equations

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.