Modified gravity and its cosmology

除了讨论修正的重力理论的一般性质外，我们在2017财年的研究特别侧重于澄清我们在2016财年提出的膨胀质量重力模型预测的可能的观测特征。主要结果如下：1.我们指出了大张量-标量-标量耦合在暴涨过程中可能产生的CMB非高斯性的尺度依赖性，并且这种非高斯特征可能已经在WMAP和Planck获得的观测数据中看到。2.我们发现了一类最小修正的引力理论，其中引力子只有两个动力学自由度，挑战了广义相对论作为唯一的无质量自旋2粒子的非线性理论的独特作用。我们研究了膨胀质量引力模型中引力波的演化，在该模型中，引力子在膨胀后经历参数共振。我们发现，由于参数共振，随机引力波背景的能量密度谱有一个尖峰。在不久的将来，地面或天基引力波探测器可能会探测到这一峰值。

In addition to discussing general properties of modified gravity theories, our research in FY2017 focused particularly on clarifying possible observational signatures predicted by an inflationary massive gravity model we proposed in FY2016. The following are the main results:1. We pointed out the possible scale dependence of CMB non-Gaussianity arising from a large tensor-scalar-scalar coupling during inflation, and that this non-Gaussian feature may be already seen in the observational data obtained by WMAP and Planck.2.We found a class of minimally modified gravity theories, of which graviton has only two dynamical degrees of freedom, challenging the singular role of general relativity as the unique non-linear theory of massless spin-2 particles.3. We studied the evolution of gravitational waves in the inflationary massive gravity model in which graviton undergoes a parametric resonance after inflation. We found a sharp peak in the energy density spectrum of stochastic gravitational wave background due to the parametric resonance. This peak may be detected in the near future either by ground-based or space-based gravitational wave detectors.