Advancing weak lensing and intrinsic galaxy alignment studies into the era of precision cosmology

如今，宇宙学家可以用一个一致的框架成功地解释宇宙的关键特征。然而，令人困惑的是，95%的宇宙成分似乎是一种奇异的物质形式，其性质仍然不清楚。有人发现，所有的天体，从星系到最大的结构，其重量都比它们发出的光的重量要大。因此，大部分物质必须是一种未知的、不发光的类型，称为暗物质。此外，宇宙的几何形状及其加速膨胀暗示了暗能量的存在，这是一种更奇特的成分，据推测，它占宇宙总能量预算的四分之三。引力透镜是揭示暗物质和暗能量本质的最有力的方法之一。爱因斯坦的相对论告诉我们，大质量的物体会使光偏转，这样星系就可以像放大镜一样聚焦来自背景光源的光——它就像一个引力透镜。类似于靠近放大镜边缘的效果，遥远星系的图像被沿着视线的物质的光的偏转所扭曲。在数十亿个星系中测量这些微小的扭曲，我们可以探测到这些星系和地球之间物质的分布，而这些分布又由暗物质和暗能量的特性决定。这种用统计工具分析大量扭曲星系图像的方法被称为宇宙剪切。我的工作旨在高精度地确定宇宙黑暗组成部分的性质，使用对天空大部分区域测量的宇宙剪切。我将把星系图像失真的测量结果与星系位置、速度和距离的测量结果结合起来，进一步测试我们的引力理论在宇宙尺度上是否仍然正确。为了充分利用即将到来的宇宙学数据集，例如由我参与设计的欧洲欧几里得卫星产生的数据集，我将开发和应用新的测量方法和分析工具，使我们能够访问迄今为止尚未使用的信息。获得高度精确的测量结果是至关重要的，但同样重要的是，它们不会受到所谓的“系统误差”的影响：即与真实结果之间未被发现的偏差，这会产生错误的答案，导致我们得出错误的结论，就像用天平测量质量时，天平的刻度有问题一样。严重的系统误差是由星系在邻近物质的引力作用下变形和排列造成的。这种效应被称为内在对准，它模拟了我们想要测量的宇宙剪切的扭曲，因此如果忽略它，就会导致强烈的系统误差。然而，星系的内在排列不仅是对宇宙剪切测量的一个麻烦，而且可以为我们提供有价值的洞察星系形成和演化的过程，这在很大程度上仍然是未知的。为了提高我们对星系的理解和对宇宙剪切系统误差的控制，我将进行迄今为止最大的努力，测量和分析内在的排列，结合在夏威夷和加那利群岛、阿塔卡马沙漠和澳大利亚等世界上最好的观测地点用望远镜获得的大量数据集。通过联合研究宇宙剪切和内在星系排列，我将能够从这些宇宙探测器中提取最大的信息，从而对黑暗和明亮的宇宙产生独特的见解。

Cosmologists can nowadays successfully explain the key features of the Universe in a consistent framework. However it is puzzling that 95% of the cosmic ingredients appear to be in exotic forms of matter whose nature is still obscure. One finds that all astronomical objects, ranging from galaxies up to the largest structures, weigh more than the amount of light which they emit suggests. Hence, the majority of matter must be of a yet unknown, non-luminous type, termed dark matter. Furthermore, the geometry of the Universe and its accelerating expansion hint at the existence of dark energy, an even more exotic component that supposedly makes up three quarters of the total cosmic energy budget.Gravitational lensing is one of the most powerful ways of revealing the nature of dark matter and dark energy. Einstein's theory of relativity tells us that massive bodies deflect light such that a galaxy can focus the light from a background source like a magnifying glass - it acts as a gravitational lens. Similar to the effects close to the rim of a magnifying glass, the images of distant galaxies are distorted by the deflections of light by the matter along the line of sight. Measuring these tiny distortions on billions of galaxies, we can probe the distribution of matter between those galaxies and Earth, which in turn is determined by the properties of dark matter and dark energy. This method of analysing large numbers of distorted galaxy images with statistical tools is called cosmic shear.My work aims at determining the properties of the dark building blocks of the Universe with high precision, using cosmic shear measured on large fractions of the sky. I am going to combine measurements of galaxy image distortions with those of galaxy positions, velocities, and distances to go even further and test whether our theory of gravity is still correct on cosmological scales. To make the most of upcoming cosmological data sets, as e.g. produced by the European Euclid satellite whose design I have contributed to, I am going to develop and apply new measurement methods and analysis tools which allow us to access information that has remained unused hitherto.Obtaining highly accurate measurements is crucial, but it is equally important that they do not suffer from so-called "systematic errors": undetected deviations from the true result which would produce wrong answers and lead us to incorrect conclusions, much like measuring masses with a balance that has a faulty scale. A serious systematic error is caused by galaxies which are deformed and aligned by the gravitational pull exerted by matter in their neighbourhood. This effect, called intrinsic alignment, mimics the distortions that we want to measure in cosmic shear and thus causes strong systematic errors if ignored. However, intrinsic galaxy alignment is not only a nuisance to cosmic shear measurements, but can provide us with valuable insight into processes of galaxy formation and evolution which are still largely unknown at present. To improve both our understanding of galaxies and the control of cosmic shear systematic errors, I will undertake the hitherto biggest effort of measuring and analysing intrinsic alignments, combining large data sets taken with telescopes at the world's best observational sites on Hawaii and the Canary Islands, in the Atacama desert and Australia.By studying cosmic shear and intrinsic galaxy alignments jointly, I will be able to extract a maximum of information from each of these cosmological probes, yielding unique insights into both the dark and bright Universe.