标题： 梯度展开形式中轴子暴胀的引力波 I：纯轴子暴胀 显示英文标题

标题： Gravitational waves from axion inflation in the gradient expansion formalism I: Pure axion inflation

摘要： 轴子膨胀是一种具有丰富现象学特征的宇宙膨胀的合理模型。 在轴子膨胀期间，规范场的大量产生显著地产生了随机引力波（GW）背景信号，这使得人们希望未来的GW搜索有可能探测该模型。 在本文中，我们在梯度展开形式（GEF）中仔细研究了轴子膨胀期间的GW产生，GEF是一种强大的数值技术，在轴子梯度趋于零的极限下能够捕捉系统的非线性动力学。 我们专注于与纯阿贝尔规范场耦合的轴子膨胀，即纯轴子膨胀（PAI），并对接近强反作用开始时的阿贝尔PAI模型中的GW产生进行了首次全面的参数扫描。 值得注意的是，我们发现未来GW干涉仪能够探测到的GW信号只能在导致强反作用且与$\Delta N_{\rm eff}$的上限，即暗辐射允许的能量密度相冲突的参数区域内实现。 这一观察结果为未来对轴子膨胀的格点研究设定了明确的目标，这些研究可能会确认或改进我们GEF基准的预测。 显示英文摘要

摘要： Axion inflation is a well-motivated model of cosmic inflation with a rich phenomenology. The abundant production of gauge fields during axion inflation notably sources a stochastic gravitational-wave (GW) background signal, which nourishes the hope that future GW searches might have a chance to probe the model. In this paper, we scrutinize GW production during axion inflation in the gradient expansion formalism (GEF), a powerful numerical technique that captures the nonlinear dynamics of the system in the limit of vanishing axion gradients. We focus on axion inflation coupled to a pure Abelian gauge sector, i.e., pure axion inflation (PAI), and perform the first-ever comprehensive parameter scan of GW production in the Abelian PAI model close to the onset of strong backreaction. Remarkably enough, we find that GW signals within the reach of future GW interferometers can only be realized in parameter regions that also lead to strong backreaction and that are in conflict with the upper limit on $\Delta N_{\rm eff}$, i.e., the allowed energy density of dark radiation. This observation defines a clear target for future lattice studies of axion inflation that may confirm or improve the predictions of our GEF benchmark.