Revealing dark matter with small-scales dynamics

自从你开始阅读这篇文章以来，数以百万计的暗物质粒子已经以每秒几百英里的速度穿过你的身体。这些粒子的数量大约是普通重子物质的四倍，但它们的性质仍然是一个令人费解的谜：我的研究旨在了解它。星系和星系团的内部动力学，以及宇宙的大尺度结构，都表明存在一种无处不在的物质形式，它通过引力相互作用，但对电磁辐射不可见。在过去的几十年里，人们提出了几种不同的模型，但我们仍然不知道这种“暗物质”实际上是由什么组成的，也不知道它与粒子物理学的标准模型有什么关系。我们知道星系嵌入在巨大的暗物质晕中，暗物质在控制星系自身形成和演化的所有过程中都起着重要作用。例如，现在正在穿过你身体的粒子是我们银河系的一部分。暗物质粒子的性质可以通过研究星系的结构和动力学来加以限制。经过一系列长期的渐进式进展，未来十年将在这一研究领域取得一些突破。盖亚卫星于2013年12月发射，目前正在测量银河系中数十亿颗恒星的位置和速度，构成了我们银河系前所未有的规模、范围和数据质量的完整运动学地图。这张地图提供了革命性的工具来改变我们对星系如何在宇宙时间内聚集的理解，并最终测试不同暗物质模型的预测。我的研究计划侧重于通过利用盖亚数据以及其他一些即将到来的专门主要调查的数据来实现这两个目标。银河系的暗物质晕通过吸积和吞没几十个较小的卫星星系而分层聚集。盖亚卫星将提供这一过程的完整记录，绘制出这些被破坏的矮星系的碎片流。盖亚的数据包含了大量的运动子结构和相空间超密度，每一个都是由来自一个共同祖先的恒星组成的，直到今天仍然在相似的轨道上运行。有了这些数据，我们将能够重建银河系是如何形成和演化的，这是一块无价的“罗塞塔石碑”，可以解码数百万个其他星系是如何在暗物质引力的驱动下聚集起来的。相互竞争的暗物质模型之间的关键区别之一是星系晕中小尺度子结构的数量和性质。引力束缚的暗物质团块（亚晕）在目前流行的冷暗物质模型中是丰富的，但在许多其他模型中却很少或不存在。确认或否定亚晕的存在，并测量它们的丰度作为质量的函数，确定暗物质的功率谱，从而确定暗物质粒子的本质。亚光晕是看不见的，但可以通过它们的引力扰乱它们碰巧飞过的恒星的运动。由破坏球状星团形成的稀薄恒星流是目前最好的“动态暗物质探测器”。盖亚提供的运动学地图将首次具有检测这些扰动所需的精度。几十年来，暗物质一直是一个谜，在十年内取得如此大的进展是非常令人兴奋的。

Since you started reading this, many millions of dark matter particles have gone through your body, at speeds of a few hundred miles per second. These particles are approximately four times more abundant than ordinary baryonic matter, yet their nature is still a puzzling mystery: my research is aimed to understand it.The internal dynamics of galaxies and galaxy clusters, as well as the large-scale structure of the universe point to the existence of a pervasive form of matter, which interacts through gravity, but is invisible to electromagnetic radiation. Several different models have been proposed during the past decades, but we still do not know what such a 'dark matter' actually consists of, or how it relates to the Standard Model of particle physics. We know that galaxies are embedded in massive dark matter haloes, and that dark matter has a fundamental role in all of the processes that govern the formation and evolution of the galaxies themselves. For instance, the particles that are going through you right now are part of our own galaxy, the Milky Way.The properties of dark matter particles can be constrained by studying the structure and dynamics of galaxies. After a long series of incremental advances, the coming decade will allow a number of breakthroughs in this field of research. Launched in December 2013, the Gaia satellite is currently measuring positions and velocities of billions of stars in the Milky Way, composing a full kinematic map of our galaxy of unprecedented size, extent and data quality. This map provides the instruments to revolutionise our understanding of how galaxies assemble through cosmic time and to finally test the predictions of different dark matter models. My research programme focuses on achieving both of these goals by exploiting Gaia data as well as data from a number of other upcoming dedicated major surveys.The dark matter halo of the Milky Way has assembled hierarchically by accreting and engulfing several tens of smaller satellite galaxies. The Gaia satellite will provide a full record of this process, mapping the streaming debris of each of these disrupted dwarf galaxies. Gaia data contains a wealth of kinematic substructures and phase-space overdensities, each composed of stars originating from a common progenitor, still moving on similar orbits to the present day. With this data we will be able to reconstruct how the Galaxy formed and evolved, an invaluable `Rosetta Stone' to decode how millions of other galaxies assembled, driven by dark matter's gravity.One of the crucial differences between competitive dark matter models is in the amount and properties of small-scale substructure in galaxy haloes. Gravitationally bound clumps of dark matter (subhalos) are abundant in the currently prevailing cold dark matter model, but much rarer or absent in a number of alternative models. Confirming or disproving the existence of subhaloes and measuring their abundance as a function of mass, pinpoints the dark matter power spectrum, and therefore the nature of the dark matter particle. Subhaloes are invisible, but can perturb the motion of the stars they happen to fly by to, through their gravity. The thin stellar streams formed by disrupting Globular Clusters are the best available 'dynamical dark matter detectors'. For the first time, the kinematic map provided by Gaia will have the precision necessary to detect these perturbations. Dark matter has been a mystery for many decades, it is very exciting that so much progress is now possible within ten years.