AI 中文总结
研究宇宙学相变中泡壁速度，对比流体假设和WallGo两种方法，发现在适度温和相变时二者速度基本相同，纳入散射过程有差异，还研究了流体假设用于强相变时线性化玻尔兹曼方程的局限，强调高精度计算及超越WKB近似处理的必要性。
AI 中文摘要
在宇宙学相变期间，可靠计算泡壁速度需要对膨胀泡附近的非平衡动力学进行适当建模。通过对非平衡粒子分布函数的形状施加一种假设，可以加快计算速度，简化碰撞项并使玻尔兹曼方程在某些非平衡涨落下可解。近期文献中流行两种不同假设：流体假设和在切比雪夫多项式基上的展开（在公开代码WallGo中整合）。本文表明，在适度温和的相变 regime，α≤0.01 时，两种方法产生的泡壁速度基本相同。有趣的是，仅考虑顶夸克湮灭时一致性极佳，但纳入散射过程后出现明显差异。我们还研究了将流体假设应用于更强相变时线性化玻尔兹曼方程的局限性，表明随着α→1，非线性贡献会使预测的终端速度产生显著偏移，尽管与平衡和线性化非平衡部分相比，对泡壁压力的非线性贡献在数量上较小。我们讨论了该结果对两种假设可能产生的后果，同时强调了WKB方法应用于强相变 regime 时自身可能存在的局限性。由于强相变正是未来引力波天文台的主要目标，我们的研究强调不仅需要在半经典方法中对vw进行更高精度计算，还可能需要超越WKB近似的处理方法。
英文摘要
A reliable computation of the bubble wall velocity during a cosmological phase transition requires an adequate modeling of the non-equilibrium dynamics in the vicinity of this expanding bubble. This task can be made computationally faster by imposing an \emph{Ansatz} on the shape of the non-equilibrium particle distribution function, thus simplifying the collision terms and making the Boltzmann equation solvable in terms of some out-of-equilibrium fluctuations. Two different \emph{Ansätze} have prevailed in the recent literature: the so-called fluid \emph{Ansatz} and an expansion in a basis of Chebyshev polynomials, consolidated in the public code \texttt{WallGo}. In this work we show that the two approaches yield essentially the same wall velocity in the regime of reasonably mild phase transitions, $α\lesssim 0.01$. Interestingly, the agreement is excellent when only top-quark annihilation is considered, but a noticeable discrepancy appears once scattering processes are included. We also investigate the limitations of linearizing the Boltzmann equation when the fluid \emph{Ansatz} is applied to stronger phase transitions, showing that non-linear contributions induce significant shifts in the predicted terminal velocity as $α\to 1$, even though the non-linear contribution to the wall pressure remain quantitatively small compared to the equilibrium and linearized non-equilibrium parts. We discuss possible consequences of this result for both \emph{Ansätze}, while also highlighting the possible limitations of the WKB approach itself when applied to the regime of strong transitions. Since strong phase transitions are precisely the primary targets for future gravitational waves observatories, our study emphasizes that not only higher precision computations of $v_w$ in the semi-classical approach are required, but a treatment beyond the WKB approximation may be needed.
Comments35 pages, 6 figures
