标题： 最小磁生机制：暴胀扰动与类轴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无处不在的磁场本质的努力似乎都引导我们追溯到它的原始起源。 然而，对于大尺度磁场而言，它们的强度和长度尺度并不一定起源于单一的原始机制，即通常文献中流行的暴胀磁生机制。 在本文中，我们提出了一种极简情景，在这种情景下，大尺度磁场可以通过暴胀扰动生成，而无需任何非共形耦合。 由于这些原始场起源于暴胀标量谱，因此它们本质上很弱，其强度受到标量涨落幅度较小的影响。 然后，我们考虑这种大尺度弱原始磁场与一个质量为 $<10^{-28}$eV 的轻轴子之间的耦合，假设该轴子冻结在一个误对齐的状态，直到光子解耦。 在解耦之后，当宇宙进入暗时代时，轻轴子相干振荡。 通过适当调整轴子-光子耦合参数$\alpha$，我们证明了确实可以通过快中子共振生成足够强度的大尺度磁场。 我们进一步表明，所产生的磁场会诱导二次引力波的独特谱，具有多个峰值，即将到来的CMB-S4可以通过B模式偏振探测到这一特性。 其强度足以违反PLANCK 对张量-标量比值$r \lesssim 0.036$的约束。 这种违反会导致对$\alpha \lesssim 80$的限制。 根据这个耦合的极限值，我们发现现今的磁场强度可以高达$10^{-10}$高斯（Gauss），在$\Mpc$尺度上，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.