原子前的光子和重子：紧耦合近似的改进
Photons and Baryons before Atoms: Improving the Tight-Coupling Approximation
摘要：
在复合之前，光子、电子和原子核迅速散射，行为几乎就像一个紧密耦合的光子-重子等离子体。在这里，我们调查的紧耦合近似的准确性通常用于数值发展的重子和光子微扰方程在早期。通过求解精确的扰动方程与刚性求解器开始深的辐射主导的时代，我们发现的不准确性引入诉诸标准的一阶紧耦合近似的水平。我们开发了一个新的二阶近似的逆汤姆逊不透明度扩展，并表明它密切跟踪完整的解决方案，基本上没有额外的数值成本。我们发现，由一阶近似引入的宇宙学参数的估计偏差，对于大多数参数，可以忽略不计。最后，我们表明，我们的二阶近似可以用来减少所需的时间计算宇宙微波背景角谱高达~ 17%。
英文摘要：
Prior to recombination photons, electrons, and atomic nuclei rapidly scattered and behaved, almost, like a single tightly-coupled photon-baryon plasma. We investigate here the accuracy of the tight-coupling approximation commonly used to numerically evolve the baryon and photon perturbation equations at early times. By solving the exact perturbations equations with a stiff solver starting deep in the radiation-dominated epoch we find the level of inaccuracy introduced by resorting to the standard first-order tight-coupling approximation. We develop a new second-order approximation in the inverse Thomson opacity expansion and show that it closely tracks the full solution, at essentially no extra numerical cost. We find the bias on estimates of cosmological parameters introduced by the first-order approximation is, for most parameters, negligible. Finally, we show that our second-order approximation can be used to reduce the time needed to compute cosmic microwave background angular spectra by as much as ~17%.
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