Stable Big Bang formation for Einstein's equations: The complete sub-critical regime

Abstract: For $(t,x) \in (0,\infty)\times\mathbb{T}^D$, the generalized Kasner solutions are a family of explicit solutions to various Einstein-matter systems that start out smooth but then develop a Big Bang singularity as $t \downarrow 0$, i.e., curvature blowup along a spacelike hypersurface. The family is parameterized by the Kasner exponents $\widetilde{q}1,\cdots,\widetilde{q}_D \in \mathbb{R}$, which satisfy two algebraic constraints. There are heuristics in the mathematical physics literature, going back more than 50 years, suggesting that the Big Bang formation should be stable under perturbations of the Kasner initial data, given say at $\lbrace t = 1 \rbrace$, as long as the exponents are "sub-critical" in the following sense: $\mathop{\max{I,J,B=1,\cdots,D}}{I < J} {\widetilde{q}_I+\widetilde{q}_J-\widetilde{q}_B}<1$. Previous works have shown the stability of the singularity under stronger assumptions: 1) the Einstein-scalar field system with $D = 3$ and $\widetilde{q}_1 \approx \widetilde{q}_2 \approx \widetilde{q}_3 \approx 1/3$ or 2) the Einstein-vacuum equations for $D \geq 39$ with $\max{I=1,\cdots,D} |\widetilde{q}_I| < 1/6$. We prove that the Kasner singularity is dynamically stable for \emph{all} sub-critical Kasner exponents, thereby justifying the heuristics in the full regime where stable monotonic-type curvature blowup is expected. We treat the $1+D$-dimensional Einstein-scalar field system for $D \geq3$ and the $1+D$ dimensional Einstein-vacuum equations for $D \geq 10$. Moreover, for the Einstein-vacuum equations in $1+3$ dimensions, where instabilities are in general expected, we prove that all singular Kasner solutions have stable Big Bangs under polarized $U(1)$-symmetric perturbations of their initial data. Our results hold for open sets of initial data in Sobolev spaces without symmetry, apart from our work on polarized $U(1)$-symmetric solutions.