Blow-up of waves on singular spacetimes with generic spatial metrics

TL;DR

This paper analyzes the behavior of solutions to the linear wave equation on cosmological spacetimes with Big Bang singularities, showing that rescaled waves tend to a blow-up profile regardless of spatial geometry.

Contribution

It demonstrates that matter blow-up near singularities occurs independently of spatial metric geometry, extending previous results to arbitrary spatial metrics.

Findings

Rescaled waves converge to a blow-up profile near singularities.

Blow-up behavior is independent of spatial geometry.

Open conditions for initial data leading to blow-up are formulated.

Abstract

We study the asymptotic behaviour of solutions to the linear wave equation on cosmological spacetimes with Big Bang singularities and show that appropriately rescaled waves converge against a blow-up profile. Our class of spacetimes includes Friedman-Lema\^itre-Robertson-Walker (FLRW) spacetimes with negative sectional curvature that solve the Einstein equations in presence of a perfect irrotational fluid with $p=(\gamma-1)\rho$. As such, these results are closely related to the still open problem of past nonlinear stability of such FLRW spacetimes within the Einstein scalar field equations. In contrast to earlier works, our results hold for spatial metrics of arbitrary geometry, hence indicating that the matter blow-up in the aforementioned problem is not dependent on spatial geometry. Additionally, we use the energy estimates derived in the proof in order to formulate open conditions on the initial data that ensure a non-trivial blow-up profile, for initial data sufficiently close to the Big Bang singularity and with less harsh assumptions for $\gamma<2$.