用神经元自动机模拟宇宙结构演化,精度高且可微分。
Emulating Cosmic Structure Formation with a Lagrangian Neural Cellular Automaton

- 在拉格朗日框架下构建局部迭代模型,随物质流移动计算图。
- 在 $k \lesssim 0.5\,h\text{Mpc}^{-1}$ 范围内功率谱误差低于百分级。
- 支持连续时间积分,适合从观测数据反推宇宙初始条件。
从星系巡天中进行场级宇宙初始条件推断,需要一个在非线性区域准确、计算高效且完全可微的前向模型。传统N体模拟虽精确但迭代推断时计算代价过高;而近似求解器如拉格朗日摄动理论(LPT)无法捕捉晚期宇宙网中复杂的晕形成动力学。我们提出拉格朗日神经元自动机(LNCA),一种混合深度学习框架,可在共动网格上以局部、迭代的方式模拟结构形成过程。与固定密度场映射的标准欧拉卷积网络不同,LNCA在拉格朗日框架下运行,使计算图随质量流动而移动。通过仅训练网络学习对泽尔多维奇近似残差位移的修正,实现高保真非线性物理模拟,并保证大尺度精度。进一步采用等变元胞自动机架构,约束模型输出完整轨迹而非仅终态,通过递归迭代内部状态生成动态演化历史。所提模型严格局部,具备平移与旋转等变性,自然支持连续时间积分,是重建光锥数据中宇宙初始条件的可靠可微前向模型。训练后模型在非线性区域($k \lesssim 0.5\,h\text{Mpc}^{-1}$)功率谱与交叉谱达到百分级精度,且参数量仅为同类可解释动态规则模型的约 $10^4$ 分之一。
原文摘要 · Abstract (English)
Field-level inference of cosmological initial conditions from galaxy surveys requires a forward model that is simultaneously accurate in the non-linear regime, computationally efficient, and fully differentiable. Traditional N-body simulations are accurate but computationally prohibitive for iterative inference, while approximate solvers like Lagrangian Perturbation Theory (LPT) fail to capture the knotty halo-forming dynamics of the cosmic web at late times. We introduce the \textit{Lagrangian Neural Cellular Automaton} (LNCA), a hybrid deep learning framework that can be applied to emulate structure formation as a local, iterative dynamical process on a comoving lattice. Unlike standard Eulerian Convolutional Neural Networks (CNNs) which map fixed density fields, the LNCA operates in the Lagrangian frame, advecting the computational graph itself to follow the flow of mass. By training the network to learn only the \textit{residual} displacement corrections to the Zeldovich approximation, we achieve high-fidelity emulation of the non-linear physics while guaranteeing accuracy at large scales. We further constrain our model to produce complete trajectories, not just final states, by adopting an equivariant cellular automaton architecture, which recurrently iterates on its internal states to yield a dynamic history. The resulting model is strictly local, translationally and rotationally equivariant, and naturally supports continuous time integration, making it a reliable differentiable forward model for reconstructing the initial conditions of the universe from lightcone data. Our trained model supports percent-level precision in the power and cross spectra well into the non-linear regime ($k \lesssim 0.5 \, h \text{Mpc}^{-1}$), while requiring $\sim10^4$ times fewer learned parameters than comparable models which take the form of an interpretable internal dynamic rule set.
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