[论文解读] Adiabatic or Non-Adiabatic? Unraveling the Nature of Initial Conditions in the Cosmological Gravitational Wave Background

Lorenzo Valbusa Dall'Armi, Alina Mierna+2 more|arXiv (Cornell University)|Jul 20, 2023

Cosmology and Gravitation Theories被引用 16

一句话总结

本文表明来自暴涨的宇宙学引力波背景的初始条件是非绝热的，相对于绝热情况，其CGWB角功率谱约提高一个数量级。

ABSTRACT

The non-thermal nature of the cosmological gravitational wave background (CGWB) makes it difficult to define the initial condition for the graviton distribution, which determines the initial contribution to the CGWB anisotropies. Specifically, unlike Cosmic Microwave Background (CMB) photons, primordial gravitational waves (GWs) are not necessarily subject to adiabatic initial conditions. For the CGWB generated by quantum fluctuations of the metric during inflation the initial conditions are not adiabatic. The violation of adiabaticity arises from the presence of independent tensor perturbations during inflation, which behave as two extra fields that affect the standard single-clock argument. The non-adiabatic initial contribution enhances the total CGWB angular power spectrum compared to the standard adiabatic case. Given the high degree of correlation between the CGWB and Cosmic Microwave Background (CMB) anisotropies, we present the constrained realization maps of the CGWB built using the high-resolution CMB Planck maps for different initial conditions.

研究动机与目标

• 解释为什么由暴涨产生的宇宙学引力波背景(CGWB)的绝热初始条件会失效。
• 从引力波的能动量张量计算CGWB的初始超密度。
• 量化非绝热初始条件如何修正CGWB角幂谱及其与绝热情形的比较。
• 评估对CGWB-CMB相关性以及对早期宇宙额外相对论自由度的敏感性的影响。

提出的方法

• 在泊松规范下定义扰动的FLRW时空，并将张量扰动分离为小尺度的 h_ij 和大尺度的 H_ij 分量。
• 使用短波（几何光学）极限通过分布函数 f_GW 描述引力子并用无碰撞玻尔兹曼方程演化。
• 在短波情形下（Isaacson给定法）计算GW能动量张量 T^{GW}_{\u00026}，并将其与分布函数扰动 Gamma 相关。
• 从GW能动量张量和度量扰动推导 Gamma_I 的初始条件，突出非绝热贡献（来自标量扰动的单极项和来自张量扰动的大尺度四极项）。
• 通过 Gamma(\beta_in, k, q) 的显式表达式比较绝热与非绝热初始条件，并评估对CGWB角幂谱的影响。

实验结果

研究问题

• RQ1 Do inflation-generated GWs admit adiabatic initial conditions like photons, or are non-adiabatic modes inevitable due to independent tensor perturbations?
• RQ2 How do non-adiabatic initial conditions alter the CGWB angular power-spectrum relative to the adiabatic case?
• RQ3 What is the role of the GW energy-momentum tensor in setting CGWB initial overdensities, given subdominant energy density at early times?
• RQ4 How does the CGWB initial condition impact the correlation with CMB anisotropies and the sensitivity to extra relativistic species in the early Universe?

主要发现

• Non-adiabatic initial conditions arise for CGWB produced by inflationary metric fluctuations, due to independent tensor perturbations acting as extra fields.
• The initial GW overdensity Gamma_I contains monopole (from scalar perturbations) and a quadrupole term (from large-scale tensor perturbations H_ij).
• The derived Gamma_I leads to an enhanced CGWB angular power-spectrum, increasing the total signal by about one order of magnitude compared to the adiabatic case (quantified via C_\u0003bb_\u0000e9 terms).
• The enhancement persists across scenarios with all relativistic species coupled or decoupled and depends on the tensor tilt n_gwb and transfer functions T_\u00043c, T_\u00043c, T_H.
• Non-adiabatic initial conditions increase the CGWB's sensitivity to extra relativistic degrees of freedom and potentially modify its correlation with CMB anisotropies at large scales.
• The approach is general and applicable to perturbations from decoupled spectator fields during inflation.