[论文解读] Revealing the Primordial Irreducible Inflationary Gravitational-Wave Background with a Spinning Peccei-Quinn Axion

Yann Gouttenoire, Géraldine Servant+1 more|arXiv (Cornell University)|Jan 1, 2021

Cosmology and Gravitation Theories被引用 19

一句话总结

本文提出，自旋的Peccei-Quinn轴子可在早期宇宙中触发短暂的kination时期，将暴胀产生的原初引力波背景增强为可探测的峰。该峰源于物质到kination的瞬态状态方程变化，产生LISA、爱因斯坦望远镜和宇宙探索者可探测的独特光谱特征。

ABSTRACT

The primordial irreducible gravitational-wave background due to quantum vacuum tensor fluctuations produced during inflation spans a large range of frequencies with an almost scale-invariant spectrum but is too low to be detected by the next generation of gravitational-wave interferometers. We show how this signal is enhanced by a short temporary kination era in the cosmological history (less than 10 e-folds), that can arise at any energy scale between a GeV and the inflationary scale $10^{16}$ GeV.We argue that such kination era is naturally generated by a spinning axion before it gets trapped by its potential.It is usually assumed that the axion starts oscillating around its minimum from its initial frozen position at rest.However, the early dynamics of the Peccei-Quinn field can induce a large kinetic energy in the axion field, triggering a kination era, either before or after the axion acquires its mass, leading to a characteristic peak in the primordial gravitational-wave background. This represents a smoking-gun signature of axion physics as no other scalar field dynamics can trigger such a sequence of equations of state in the early universe.We derive the resulting gravitational-wave spectrum, and present the parameter space that leads to such signal as well as the detectability prospects, in particular at LISA, Einstein Telescope, Cosmic Explorer and Big Bang Observer.We show both model-independent predictions and present as well results for two specific well-motivated UV completions for the QCD axion dark matter where this dynamics is built-in.

研究动机与目标

• 识别早期宇宙标量场动力学中一种新颖的、模型无关的引力波特征。
• 证明轴子样粒子（ALPs），特别是QCD轴子，可自然地产生瞬态kination阶段。
• 确立该kination驱动的峰是轴子物理的决定性信号，可与其它宇宙学引力波源区分开来。
• 绘制该信号在下一代引力波探测器中的可探测参数空间。
• 评估该信号在QCD轴子的两种良好动机的UV完备理论中的可观测性：传统的非对称性对齐与ZN扩展。

提出的方法

• 模拟Peccei-Quinn标量场在振荡前具有大初始动能的早期动力学。
• 利用修正的瞬态kination阶段的标准暴胀引力波谱公式推导引力波谱。
• 应用Friedmann方程计算频率相关的谱指数（kination时β = 1，物质时β = -2）及特征频率fΔ、fKD、fM。
• 使用峰振幅公式ΩGW,KD ≈ 2.84 × 10−13 × (V_inf^{1/4}/10^16 GeV)^4 × exp(2NKD)/22000来量化信号强度。
• 从kination与辐射能量密度之比ρKD/ρΔ计算kination的e-folding数NKD。
• 通过将峰振幅与LISA、爱因斯坦望远镜、宇宙探索者及大爆炸观测者灵敏度曲线比较，评估可观测性。

实验结果

研究问题

• RQ1 轴子动力学是否能在早期宇宙中产生瞬态kination时期，从而增强原初引力波背景？
• RQ2 由此产生的引力波谱形状与振幅如何？与标准暴胀引力波背景有何不同？
• RQ3 哪些未来的引力波探测器可观测到该增强信号？在何种参数条件下？
• RQ4 QCD轴子的具体UV完备理论——如传统非对称性对齐与ZN扩展——如何实现该kination机制？
• RQ5 BBN、畴壁或有效场论有效性限制了可观测参数空间的哪些约束？

主要发现

• 由轴子动力学触发的kination时期在原初引力波谱中产生一个特征峰，低频区斜率为+1，高频区斜率为-2。
• 峰频率fKD ≈ 1.07 × 10−3 Hz × G1/4(TΔ) × (ρKD^{1/4}/10 TeV) × e^{NKD/2}，随kination持续时间NKD而变化。
• 峰振幅达到ΩGW,KD ≈ 2.84 × 10−13 × (V_inf^{1/4}/10^16 GeV)^4 × exp(2NKD)/22000，使其可能被LISA和爱因斯坦望远镜探测到。
• 对于传统QCD轴子，当kination时期在大爆炸核合成前结束且非对称角较大时，该信号在(fa, Tc)平面上可观测。
• QCD轴子的ZN扩展可实现更大的可观测参数空间，尤其当轴子质量Ma较大且初始非对称角显著时。
• 该峰特征是轴子样粒子动力学的决定性信号，因为没有其他标量场演化能产生如此瞬态的kination阶段。