A multi-probe strategy to pin down the nature of gravity and dark energy

2011年诺贝尔物理学奖授予了一项令人困惑的发现，即宇宙不仅在膨胀，而且膨胀的速度越来越快，而在宇宙学的标准图景中，宇宙膨胀的速度预计会减慢。这种加速可能是由于一种称为暗能量的新的奇异物质成分，它是排斥性的，并将形成宇宙的主要成分。另一方面，爱因斯坦的广义相对论所预言的引力定律在宇宙学尺度上可能并不完全正确，需要加以修正。确定暗能量的性质并在宇宙学尺度上检验广义相对论的最佳方法之一是研究宇宙中物质的大尺度分布及其演化，以及它们之间的引力相互作用。在一个探索这种分布的新方法中，我将同时分析数百万个遥远星系的位置、速度和形状，星系在空间中的聚集提供了一幅潜在的物质分布图，因为星系跟踪物质密度。然而，这幅图像是有偏差的，因为星系优先居住在高密度区域，这种星系偏差限制了从星系团中提取的宇宙学信息。通过星系速度测量，人们可以推断出星系的相干运动，这是由于大尺度引力通过所谓的红移空间扭曲造成的，当星系被大质量吸引时，会导致明显的超密度。这种效应需要从星系的实际聚集中解脱出来，星系的实际聚集又受到星系偏置的阻碍，因此仅凭红移空间扭曲来约束宇宙学参数的能力也是有限的。广义相对论预言，大质量的物质可以像放大镜一样偏转光线--因此引力透镜效应这个术语就被创造出来。遥远星系的图像被这些星系和地球之间的宇宙大尺度结构的引力透镜扭曲了。这些“宇宙剪切”扭曲直接映射了干涉物质的分布，而不依赖于星系的偏差。此外，宇宙剪切也探测宇宙的几何形状，因为它取决于光源、透镜和观察者之间的距离。这个潜在的非常强大的宇宙学探测器受到星系形状的内在排列的困扰，这些形状模仿了宇宙剪切的扭曲特征，因此限制了宇宙学测量的准确性。克服这三个探测器局限性的关键是联合分析它们，并将它们的互相关纳入分析。这将解除参数之间的简并，例如由星系偏差引起的，并校准系统效应，如宇宙剪切的内在对齐污染。有了这项技术，我将获得比任何单独探测器更好的对暗能量和引力定律的约束，这种方法需要一个图像质量很好的成像巡天来测量微弱和小星系图像的形状，这与一个红移巡天重叠，可以精确测量所需的星系距离。来测量聚集和红移空间失真。我将领导的研究小组可以获得一对独特的此类调查。联合数据集的质量和大小是如此之好，以至于我可以同时瞄准不同类型的星系，从而应用更先进的技术来消除测量中的系统效应。因此，联合科学分析将对暗能量的性质和引力的性质产生前所未有的精度和准确性。

The Nobel Prize in Physics 2011 was awarded to the puzzling discovery that the Universe is not only expanding, but doing so with ever increasing speed, whereas in the standard picture of cosmology it was expected to slow down. This acceleration could either be due to a new exotic matter component called dark energy which is repellent and would form the major ingredient of the cosmos. Alternatively, the laws of gravity as predicted by Einstein's general relativity may not be quite correct on cosmological scales and need to be modified.One of the best ways to pin down the properties of dark energy and test general relativity on cosmological scales is to study the large-scale distribution of matter in the Universe and its evolution, together with its gravitational interactions. In a novel approach to probe this distribution, I am going to simultaneously analyse the positions, the velocities, and the shapes of millions of distant galaxies.The clustering of galaxies in space provides a picture of the underlying matter distribution since galaxies trace the matter density. However, the picture is biased because galaxies preferentially reside in high-density regions, and this galaxy bias limits the cosmological information that can be extracted from clustering.Using galaxy velocity measurements, one can infer the coherent motions of galaxies due to the large-scale gravitational forces via so-called redshift-space distortions, causing apparent overdensities of galaxies when these are attracted by a large mass. This effect needs to be disentangled from the actual clustering of galaxies, which is again hindered by galaxy bias, so that the power of redshift-space distortions alone to constrain cosmological parameters is limited as well.General relativity predicts that large masses can deflect light rays similar to a magnifying glass - therefore the term gravitational lensing was coined for this effect. The images of distant galaxies are distorted by the gravitational lensing of the large-scale structure of the Universe between these galaxies and Earth. These 'cosmic shear' distortions directly map the intervening matter distribution, without any dependence on galaxy bias. In addition, cosmic shear also probes the geometry of the cosmos as it depends on the distances between the light sources, the lenses, and the observer. This potentially very powerful cosmological probe is plagued by intrinsic alignments of galaxy shapes which mimic the distortions characteristic of cosmic shear and hence limit the accuracy of cosmological measurements.The key to overcome the limitations of the three probes is to analyse them jointly and also include their cross-correlations into the analysis. This will lift degeneracies between parameters as for instance caused by the galaxy bias, and calibrate systematic effects such as the intrinsic alignment contamination of cosmic shear. With this technique I will obtain significantly better constraints on dark energy and the laws of gravity than from any individual probe, and the results will additionally be much less susceptible to systematic errors.This approach requires an imaging survey with excellent image quality to measure the shapes of faint and small galaxy images which overlaps with a redshift survey that allows for the accurate measurement of galaxy distances which are needed to measure clustering and redshift-space distortions. The research team that I am going to lead has access to a unique pair of such surveys. The quality and size of the joint data set is so good that I can target different types of galaxies at the same time and thus apply even more advanced techniques to eliminate systematic effects from the measurements. The joint scientific analysis will therefore yield unprecedented precision and accuracy on the properties of dark energy and the nature of gravity.