全超轻质量范围内CMB中的暴胀轴子等曲率扰动
Inflationary Axion Isocurvature in the CMB across All Ultralight Masses
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中文总结 AI 辅助
该研究扩展AxiECAMB的ETA方法，分析全超轻质量范围轴子等曲率扰动，给出解析公式与约束，指出特定质量区域的信号特征及共存窗口，或缓解S8张力。
中文摘要 AI 辅助
如果超轻轴子的Peccei-Quinn对称性在暴胀结束前破缺，轴子量子涨落会产生等曲率扰动，并将其与张量标量比r关联。我们扩展了玻尔兹曼代码AxiECAMB的有效时间平均（ETA）方法，以准确演化这些扰动，覆盖从暗能量型（ma≲H0）到暗物质型（ma≫10^-28 eV）的全轴子质量范围。我们针对给定初始场值φini，给出了轴子丰度的解析拟合公式，对ma≫H0及允许的暗物质占比fdm，精度达亚百分比级别。在暗物质区域，普朗卫星对CDM等曲率的约束要求r·fdm < 0.08(ma/10^-27 eV)^-1/2，当ma·fdm²≳10^-26.4 eV时，该约束比当前BICEP的张量约束更强。对于10^-32≲ma/eV≲10^-28，金斯抑制打破了与CDM等曲率的简并，留下独特信号；在暗能量区域（ma≲H0），等曲率信号被进一步抑制，仅在CMB四极矩处达到峰值。我们为这两种信号提供了标度关系。结合张量约束，在这两个最轻质量区域中若探测到原初CMB信号，将表明等曲率模式的非暴胀起源或标准冻结场错位场景的失效。在ma~10^-25 eV且fdm≳0.1附近，存在一个共存窗口，其中轴子等曲率模式与张量模式均可在当前约束以下被探测到，同时可缓解S8张力。
英文摘要
If the Peccei-Quinn symmetry of an ultralight axion is broken before the end of inflation, axion quantum fluctuations seed isocurvature perturbations, linking them to the tensor-to-scalar ratio $r$. We extend the effective time average (ETA) approach of the Boltzmann code ${\rm AxiECAMB}$ to accurately evolve these perturbations across the full axion mass range from dark energy ($m_a \lesssim H_0$) to dark matter ($m_a \gg 10^{-28}$ eV) types. We provide analytic fitting formulae for the axion abundance given the initial field value $ϕ_{\rm ini}$, accurate at sub-percent level for $m_a \gg H_0$ and allowed dark matter fraction $f_{\rm dm}$. In the dark matter regime, the Planck bound on CDM isocurvature requires $r\ f_{\rm dm} < 0.08\,(m_a/10^{-27}\,{\rm eV})^{-1/2}$, which becomes stronger than the current BICEP tensor bound for $m_a f_{\rm dm}^2 \gtrsim 10^{-26.4}\,{\rm eV}$. For $10^{-32} \lesssim m_a/{\rm eV} \lesssim 10^{-28}$, Jeans suppression breaks the degeneracy with CDM isocurvature, leaving unique signatures, and in the dark energy regime ($m_a \lesssim H_0$), the isocurvature signal is even more highly suppressed, peaking only at the CMB quadrupole. We provide analytic scalings for both signatures. Given the tensor bound, any primary CMB detection in these two lightest regimes would indicate a non-inflationary origin of the isocurvature modes or a breakdown of the standard frozen-field misalignment scenario. A window of coexistence opens near $m_a \sim 10^{-25}$ eV and $f_{\rm dm} \gtrsim 0.1$ where both axion isocurvature and tensor modes could be discovered just below current bounds while simultaneously alleviating the $S_8$ tension.
