基于下一代宇宙学观测研究暗能量的性质

The next generation of cosmological probes will allow two new analyses. First of all,the baryon acoustic oscillation (BAO) surveys will provide a separate determination of the radial and angular size of the baryon acoustic feature at many distinct redshifts. If the absolute size of the acoustic feature is not fixed, the BAO measurements provide purely geometric information about the expansion of the universe, independent of any cosmological assumptions. For example,it is independent of the number of species of light particles and of whether dark matter is conserved. Collaborating closely with BOSS member Gongbo Zhao, we will apply this geometric information to study the poorly understood systematic errors in the analysis of type Ia supernovae and to obtain the first robust constraint on the evolution of the dark energy equation of state. Second, we will attempt to determine to what extent cosmological probes are sensitive to the properties of dark energy besides the equation of state, such as the speed of sound. In the past such studies have only been performed for dark energy models satisfying very restrictive assumptions, such as constant viscosity single fluid models, and in these models it was found that this sensitivity is strongly suppressed when the equation of state is near -1, as it is today. This is a big problem, as there are multiple models yielding each evolution of the equation of state, and so these analyses suggest that the correct model cannot be experimentally determined. However many simple models, such as perfect two fluid models and generalized Galileons, violate the standard assumptions and we intend to show that in such models properties apart from the equation of state can have detectable signatures such as anisotropies which may be observed by the next generation of CMB and weak lensing probes. For this project I will work with Xinmin Zhang, with Hong Li, who has developed a code which will be necessary for determining the observational consequences of such dark energy models and with Planck member Junqing Xia.

基于下一代宇宙学观测可以开展两类重要分析。第一，BAO（重子声波振荡）观测将能够确定不同红移处的径向和角向距离。如果不固定声波视界的大小，我们就能从BAO测量中得到与模型无关的宇宙膨胀几何信息。比如，与中微子种类的多少，暗物质是否稳定无关。我们将与BOSS成员赵公博密切合作，利用BAO结果强烈限制暗能量状态方程的演化，并研究Ia型超新星表观亮度观测的系统误差。 第二，我们将研究宇宙学探针对暗能量除状态方程以外的性质（如声速）的敏感程度。过去研究的暗能量模型涉及很强限制，且得出结论：该种敏感性会随状态方程达到-1而被抑制。这个结论宣告了此类模型不能为观测所区分。然而很多模型，比如理想两流体模型和推广Galileon模型，并不遵从那些限制；我们将研究它们的可观测信号，利用下一代弱引力透镜和CMB观测暗能量声速。我们将与高能所张新民、李虹、夏俊卿合作，他们已经开发了一整套利用数据研究暗能量性质。

寻找标准宇宙模型的扩展理论，使其能够解释各种反常观测，是当今宇宙学中非常重要的研究内容。目前，可供备选的扩展理论极其丰富，但是我们无法确定能否通过猜测的方法中得到正确的理论。本项目致力于提供一种新的解决方法。我们只采用了几何可观测量，在红移大于观测量自身红移的情况下，对几何可观测量的分析研究不需要借助于宇宙模型，因此相应的参数空间更易于处理，特别是减少了需要猜测情况（除了暗物质演化的参数化过程，我们对其进行了系统的研究）。特别是本项目分析了诸多描述宇宙扩张的几何观测量之间的比率，包括重子声学震荡（Baryon Acoustic Oscillation, BAO）尺度，强透镜系统中的时间延迟和超水微波激射（water megamasers）动力学。在本项目中我们把这些几何观测量主要应用到类星体Lyman alpha forest中BAO观测以及哈勃常数的测量，研究发现与普朗克CMB测量的结果有3个sigma的符合度。在上述每种情况中，我们研究了需要对LCDM作出何种修正以使其能解释反常观测。在第一种情况下，我们发现在红移介于0.57与2.3之间时，需要引入动态暗物质。CANDELS提供的高红移超新星数据与BAO数据的吻合要优于与CMB数据的吻合。在第二种情况下，我们发现仅仅考虑动态暗物质是不够的，还需考虑早期宇宙的不同。特别是由此我们发现，由于早期宇宙暗能量或者辐射的影响宇宙重组可能出现的更早，同时宇宙原初等离子中的声速也可能大大降低。此外，我们研究了未来可能的宇宙观测大型装置，特别是超大光学望远镜项目。我们的研究结果已被采纳到未来30米光学望远镜（TMT）研究计划文档中，包括具体科学目标文档以及设备需求文档。