不均匀流：关于宇宙整体流、局部观测者和引力
Uneven flows: On cosmic bulk flows, local observers, and gravity
摘要：
利用N体模拟研究了各种系统效应对整体流场和宇宙马赫数的影响。我们考虑两种类型的系统学：那些与调查性质和观察者在宇宙中的位置引起的。在前一类中，我们对稀疏采样、速度误差和调查不完整性进行建模。在后者中，我们考虑本地组（LG）的模拟观测器，放置在宇宙网络内的特定位置，满足各种观测标准。我们区分这样的LG观察员从哥白尼的，谁是在随机位置。我们报告了稀疏采样、速度误差和径向不完整性对所测BF和CMN的强系统效应。对于BF，这些效应中的大多数超过尺度$R\leq100h^{-1}$ Mpc的10%。对于CMN，这些系统中的一些可以是灾难性的大（$>50\%$），也可以在更大的尺度上。此外，我们发现，观察者在宇宙网络中的位置显着影响本地测量的BF（CMN），与$\sim20\%$（$30\%)$在$R\leq50h^{-1}$ Mpc的LG类观察者相比，随机的影响。该效应与样本方差相当。为了突出这些系统学的重要性，我们还研究了一个模型的修改重力（MG）与$\sim15\%$增强的增长率。我们发现，系统效应可以模仿修改后的重力信号。最坏的情况下，实现了一个LG样的观察员的情况下，当由本地结构引起的影响退化与增强的增长率由MG培养。我们的研究结果表明，专用的约束模拟和现实的模拟星系目录将是绝对必要的，以充分受益于即将到来的奇特的速度数据的统计能力，如TAIPAN，WALLABY，宇宙流-4和SKA的调查。
英文摘要：
Using N-body simulations we study the impact of various systematic effects on the bulk flow (BF) and the Cosmic Mach Number (CMN). We consider two types of systematics: those related to survey properties and those induced by observer's location in the Universe. In the former category we model sparse sampling, velocity errors, and survey incompleteness. In the latter, we consider Local Group (LG) analogue observers, placed in a specific location within the Cosmic Web, satisfying various observational criteria. We differentiate such LG observers from Copernican ones, who are at random locations. We report strong systematic effects on the measured BF and CMN induced by sparse sampling, velocity errors and radial incompleteness. For BF most of these effects exceed 10\% for scales $R\leq100h^{-1}$Mpc. For CMN some of these systematics can be catastrophically large ($>50\%$) also on bigger scales. Moreover, we find that the position of the observer in the Cosmic Web significantly affects the locally measured BF (CMN), with effects as large as $\sim20\%$ ($30\%)$ at $R\leq50h^{-1}$Mpc for a LG-like observer as compared to a random one. This effect is comparable to the sample variance. To highlight the importance of these systematics, we additionally study a model of modified gravity (MG) with $\sim15\%$ enhanced growth rate. We found that the systematic effects can mimic the modified gravity signal. The worst-case scenario is realized for a case of a LG-like observer, when the effects induced by local structures are degenerate with the enhanced growth rate fostered by MG. Our results indicate that dedicated constrained simulations and realistic mock galaxy catalogs will be absolutely necessary to fully benefit from the statistical power of the forthcoming peculiar velocity data from surveys such as TAIPAN, WALLABY, Cosmic Flows-4 and SKA.
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