arXiv subjects

Louie Bernhardt

Publications and source records attributed to Louie Bernhardt.

4 recordsLinked to original sources

arXiv2026

Nonlinear stability of Einstein-de Sitter universes

The Einstein-de Sitter universe is the prevailing model used in cosmology to describe the cold dark matter-dominated epoch of the universe. This model is a spatially homogeneous and isotropic spacetime undergoing decelerated expansion, and is linearly unstable under the Einstein-Euler equations with a pressureless fluid equation of state. We show that every initial data set for the Einstein-Euler equations on $\mathbb{T}^3$ with a near-flat metric and positive fluid energy density converges to a flat metric under the Einstein-Euler flow with a polytropic equation of state. This means the metric asymptotes to an Einstein-de Sitter spacetime. In particular, this settles the question of whether the Einstein-de Sitter model can be nonlinearly stable for an appropriate matter model.

gr-qc↗

arXiv2025

Future stability of solutions of the Einstein-nonlinear scalar field system with decelerated expansion

We study solutions to the Einstein equations coupled to a nonlinear scalar field with exponential potential. This system admits Friedmann-Lemaître-Robertson-Walker solutions undergoing decelerated expansion, with $\mathbb{T}^3$ spatial topology and scale factor $a(t) = t^p$ for $1/3 < p < 1$. For each $p \in (2/3,1)$, we prove that the corresponding FLRW spacetime is future-stable as a solution to the Einstein-nonlinear scalar field system. Given initial data on a spacelike hypersurface that is sufficiently close to the FLRW data, we show the resulting solution is future-causal geodesically complete, and remains close to the FLRW solution for all time. Moreover, we show the perturbed metric components and scalar field converge to spatially homogeneous functions as $t \rightarrow \infty$. A key feature of our analysis is the decomposition of the metric and scalar field perturbations into their spatial averages and oscillatory remainders with zero average.

gr-qc↗

arXiv2024

Linear waves on the expanding region of Schwarzschild-de Sitter spacetimes: forward asymptotics and scattering from infinity

We study solutions to the linear wave equation on the cosmological region of Schwarzschild-de Sitter spacetimes. We show that all sufficiently regular finite-energy solutions to the linear equation possess a particular finite-order asymptotic expansion near the future boundary. Specifically, we prove that several terms in this asymptotic expansion are identically zero. This is accomplished with new weighted higher-order energy estimates that capture the global expansion of the cosmological region. Furthermore we prove existence and uniqueness of scattering solutions to the linear wave equation on the expanding region. Given two pieces of scattering data at infinity, we construct solutions that have the same asymptotics as forward solutions. The proof involves constructing asymptotic solutions to the wave equation, as well as a new weighted energy estimate that is suitable for the backward problem. This scattering result extends to a large class of expanding spacetimes, including the Kerr de Sitter family.

math.AP↗

arXiv2024

John's blow up examples and scattering solutions for semi-linear wave equations

In light of recent work of the third author, we revisit a classic example given by Fritz John of a semi-linear wave equation which exhibits finite in time blow up for all compactly supported data. We present the construction of future global solutions from asymptotic data given in arXiv:2204.12870(2022) for this specific example, and clarify the relation of this result of Yu to John's theorem. Furthermore we present a novel blow up result for finite energy solutions satisfying a sign condition due to the first author, and invoke this result to show that the constructed backwards in time solutions blow up in the past.

math.AP↗