标题： 最小磁生成：暴胀扰动和轴子样粒子的作用及其引力波特征 显示英文标题

标题： Minimal Magnetogenesis: The Role of Inflationary Perturbations and ALPs, and Its Gravitational Wave Signatures

摘要： 任何试图理解当前宇宙中磁场普遍性质的努力似乎都引导我们去探讨其原初起源。 然而，对于大尺度磁场而言，其强度和尺度可能并不一定源自单一的原初机制，即暴胀磁发生机制，这在文献中是一个流行的考虑。 在本文中，我们提出一个最小情景，其中大尺度磁场是从暴胀扰动中产生的，而无需任何非共形耦合。 由于它们起源于暴胀标量谱，这些原初磁场本质上是微弱的，其强度被标量扰动的小振幅所抑制。 然后我们考虑这种大尺度弱原初磁场与质量为$<10^{-28}$电子伏特的轻轴子之间的耦合，该轴子被假定在光子退耦之前处于未对齐状态。 在退耦之后，当宇宙进入黑暗时代时，轻轴子会相干振荡。 通过适当调节轴子-光子耦合参数$\alpha$，我们证明可以通过超光速共振确实产生足够强的大尺度磁场。 我们进一步表明，所产生的磁场会诱导出具有多个次级引力波峰值的独特谱，这一谱可以通过CMB-S4的B模式偏振进行探测。 这种强度足以违反PLANCK对张量-标量比$r \lesssim 0.036$的限制。 这种违反会导致对$\alpha \lesssim 80$的约束。 在这一耦合的极限值下，我们发现现今的磁场强度在Mpc尺度上可以高达$10^{-10}$高斯，这与观测结果一致。 显示英文摘要

摘要： Any attempt to understand the ubiquitous nature of the magnetic field in the present universe seems to lead us towards its primordial origin. For large-scale magnetic fields, however, their strength and length scale may not necessarily originate from a singular primordial mechanism, namely inflationary magnetogenesis, which has been a popular consideration in the literature. In this paper, we propose a minimal scenario wherein a large-scale magnetic field is generated from the inflationary perturbation without any non-conformal coupling. Due to their origin in the inflationary scalar spectrum, these primordial fields are inherently weak, with their strength suppressed by the small amplitude of scalar fluctuations. We then consider the coupling between this large-scale weak primordial magnetic field and a light axion of mass $<10^{-28}$ eV, which is assumed to be frozen in a misaligned state until the photon decoupling. After the decoupling, when the universe enters into a dark age, the light axion coherently oscillates. By appropriately tuning the axion-photon coupling parameter $\alpha$, we demonstrate that a large-scale magnetic field of sufficient strength can indeed be generated through tachyonic resonance. We further show that the produced magnetic field induces a unique spectrum with multiple peaks of secondary gravitational waves, which the upcoming CMB-S4 can probe through B-mode polarization. The strength can be sufficient enough to violate the PLANCK bound on tensor-to-scalar ratio $r \lesssim 0.036$. Such a violation leads to a constraint on $\alpha \lesssim 80$. With this limiting value of the coupling, we find that present-day magnetic field strength could be as high as $10^{-10}$ Gauss at Mpc scale, consistent with observation.