标题： NANOGrav及更广泛的非奇异物质反弹宇宙学的引力波特征 显示英文标题

标题： Gravitational wave signatures of non-singular matter bouncing cosmology in NANOGrav and beyond

摘要： 非奇异物质反弹宇宙模型特别引人关注，因为除了处理初始奇点问题外，它们还能在尺度$k<10^{4}\mathrm{Mpc}^{-1}$上产生接近标度不变的曲率功率谱，这得到了宇宙微波背景（CMB）实验的支持。有趣的是，在此类非奇异反弹宇宙模型中，曲率扰动在物质收缩阶段的超 horizon 尺度上会增长。在本工作中，我们考虑了宇宙扰动在过渡到热大爆炸膨胀宇宙期间的演化，最终在膨胀阶段的 horizon 穿越时刻自然地在非常小的尺度上增强了曲率扰动。这些增强的宇宙扰动由于二阶引力相互作用可以产生引力波（GWs），同时也会坍缩形成原初黑洞（PBHs），后者作为最可行的暗物质候选者之一。值得注意的是，我们发现了一个具有普遍红外（IR）频率标度$f^2$的诱导引力波背景，与最近 NANOGrav 合作组发布的$\mathrm{nHz}$引力波数据高度一致，根据当前手头的反弹宇宙参数值，它也可能被其他引力波观测台如 LISA 和 ET 检测到。 显示英文摘要

摘要： Non-singular matter bouncing cosmological setups are of particular interest since apart from adressing the initial singularity problem they can give rise as well to a nearly scale-invariant curvature power spectrum on scales $k<10^{4}\mathrm{Mpc}^{-1}$, favored by Cosmic Microwave Background (CMB) experiments. Interestingly enough, one can find that within such non-singular bouncing cosmological setups, curvature perturbations grow on super-horizon scales during the matter contracting phase. In this work, we account for the evolution of cosmological perturbations during the transition to the Hot Big Bang expanding Universe, finding at the end naturally enhanced curvature perturbations on very small scales at horizon-crossing time during the expanding phase. These enhanced cosmological perturbations can induce gravitational waves (GWs) due to second order gravitational interactions and collapse as well to form primordial black holes (PBHs), with the latter acting as one of the most viable dark matter candidates. Remarkably, we find an induced GW background with a universal infrared (IR) frequency scaling of $f^2$, in excellent agreement with the recently released $\mathrm{nHz}$ GW data by the NANOGrav collaboration, being potentially detectable as well by other GW observatories such as LISA and ET, depending on the values of the bouncing cosmological parameters at hand.