Degeneracy is a method to accommodate exact, low energy vacuum states in scalar-tensor gravitational models despite the presence of an arbitrarily large vacuum energy. However, this approach requires very particular combinations of scalar field and metric couplings in the Lagrangian. In this work we study departures from the restrictive degeneracy condition -- starting from a fiducial model containing an exact Minkowski space solution, we break the degeneracy condition in numerous simple ways to test if the resulting models maintain certain key features -- specifically the dynamical cancellation of a large vacuum energy by the scalar field and the existence of a low energy vacuum state. We highlight the role the tadpole plays in eliminating the fixed points of the dynamical system, generically rendering both the scalar field and metric time dependent. Our results indicate that when violating the degeneracy condition but preserving shift symmetry, the metric maintains an asymptotic Minkowski state, irrespective of the presence of the cosmological constant. In contrast, when shift symmetry is also broken the asymptotic behaviour can radically alter. Regardless, the non-degenerate models in this work share an attractive quality; harboring low energy, late-time asymptotic states that are independent of the vacuum energy. The tadpole allows for a broader class of non-degenerate, self-tuning models than was previously realized.
Breaking a Dark Degeneracy with Gravitational Waves
我们确定了一个标量张量模型嵌入在Horndeski行动的宇宙学背景和线性标量涨落与和谐宇宙学退化。该模型承认在后期的自加速背景膨胀是稳定的，对扰动与声速归因于新的领域，等于光速。当标量涨落退化时，模型的自加速意味着当前宇宙学张量模的传播< 95% of the speed of light with a damping of the wave amplitude that is >效率比广义相对论低5%。我们发现，这些差异是特有的自加速Horndeski理论与退化的大尺度结构，并与宇宙学距离的事件发射的引力波的测量进行测试。因此，引力波宇宙学打破了观测到的大尺度结构中的暗简并性，将这种暗简并性置于两种根本不同的宇宙加速度解释之间--一种是宇宙学常数，另一种是引力的标量张量修正。alIGO仪器最近探测到的引力波事件GW 150914及其与费米GBM实验观测到的弱短伽马射线暴的潜在关联可能提供了这一关键测量。
Breaking the Mass-sheet Degeneracy in Time-delay Cosmology with Lensed and Unlensed Type Ia Supernovae
College of Physics, Hebei Normal University（河北师范大学物理学院）；Shijiazhuang Key Laboratory of Astronomy and Space Science, Hebei Normal University（河北师范大学石家庄天文与空间科学重点实验室）；School of Physics and Technology, Wuhan University（武汉大学物理科学与技术学院）
A no-go theorem in bumblebee vector-tensor cosmology
University of Sheffield（谢菲尔德大学）；Institute for Basic Science (IBS)（韩国基础科学研究院）；Observatoire de Paris, Université PSL, CNRS, LNE, Sorbonne Université（巴黎天文台、巴黎文理研究大学、法国国家科学研究中心、法国国家天文台、索邦大学）；Center for Quantum Spacetime, Sogang University（延世大学量子时空中心）；Institute of Science Tokyo（东京科学大学）；Yonsei University（延世大学）
Abundance of peaks and dips in three-dimensional mass and halo density fields: a test for cosmology
利用宇宙学N体模拟，我们研究了在$10-100$ s Mpcs尺度上晕和暗物质（DM）的平滑分布中发现的局部极大值（峰）和极小值（谷）的丰度。模拟包括高斯和本地型$f_{\rm NL}$非高斯初始条件。在文献中推导出的表达式的丰度（不考虑高度）的峰值高斯字段是令人惊讶的准确的演变晕和DM密度场的所有初始条件考虑。此外，高度分布是非常好的拟线性尺度上的对数正态拟合。丰度作为尺度的函数依赖于宇宙学参数（$H_0$和背景物质密度）通过功率谱的形状，但它是不敏感的集群振幅。此外，平滑晕轮分布中的丰度在非高斯中与高斯模拟基本上不同。在非高斯模型中，这种效应的解释是直接的晕偏差的尺度依赖性。从三维大星系红移巡天中提取的丰富的极值可能是宇宙学参数和初始非高斯性的竞争性探针。它打破了$f_{\rm NL}$和集群振幅之间的简并性，使其与星系团的计数和弱透镜图中的峰值互补。
