arXiv 2510.268832025-10-30gr-qcastro-ph.COhep-ph

论宇宙学中粒子气体的绝热初始条件

On the adiabatic initial conditions for a particle gas in cosmology

Guillem Domènech

AI总结：

针对宇宙学中暗辐射的作用，本文澄清了膨胀宇宙气体动理论中的绝热初始条件，提出了强、弱两种绝热初始条件，并证明二者均与分离宇宙方法一致，同时强调使用粒子局部动量定义该概念，指出引力子气体可具备绝热初始条件。

摘要：

鉴于近期对宇宙学中“暗”辐射（如宇宙引力波、惰性中微子和暗光子）作用的关注，我们澄清了膨胀宇宙中气体动理论里绝热初始条件的定义。在不假设相空间分布函数任何形式的情况下，我们确定了两种可能性：强绝热初始条件和弱绝热初始条件。强条件对应标准绝热初始条件，而弱条件通过内部等曲率扰动与强条件相关联。我们表明，这两种绝热初始条件均与分离宇宙方法一致，尽管后者需要初始内部等曲率。顺便指出，我们强调在相空间中使用粒子局部动量来定义绝热初始条件概念的重要性。这样做，我们澄清了引力子气体可以具有绝热初始条件。

英文摘要：

In view of recent interest in the role of "dark" radiation in cosmology, such as cosmic gravitational waves, sterile neutrinos, and dark photons, we clarify the definition of adiabatic initial conditions in the kinetic theory of gases in an expanding universe. Without assuming any form for the phase space distribution function, we identify two possibilities: a strong and a weak adiabatic initial condition. The strong one corresponds to the standard adiabatic initial conditions, while the weak one is related to the strong via internal isocurvature fluctuations. We show that both types of adiabatic initial conditions are consistent with the separate universe approach, although the latter requires initial internal isocurvature. In passing, we stress the importance of using the particle local momentum in the phase space to define the notion of adiabatic initial conditions. Doing so, we clarify that a gas of gravitons can have adiabatic initial conditions.

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