The Sunyaev-Zel'dovich effect and cosmology

最大的星系团由数千个独立的星系组成，是宇宙中最大的引力约束物体。将星系团结合在一起的质量主要由暗物质提供，但这是一个重要的组成部分，而“普通物质”的主要组成部分是通过引力场与星系团结合的热气体。高温气体最容易通过其X射线发射被发现，这是由钱德拉和XMM-牛顿等卫星天文台绘制的，但也可以通过其对穿过星团的无线电波的影响来检测。团簇气体中的电子和辐射场中的光子之间的碰撞会加热光子，并导致通过团簇看到的无线电波的强度与在其他方向看到的无线电波不同。这种效应，Sunyaev-Zel'dovich效应，可以通过观察几乎均匀的宇宙微波背景辐射来观察，这是早期宇宙热阶段遗留下来的热辐射，在天空的各个方向上几乎都有相同的亮度，除了辐射来自星系团的地方。该研究方案的目的是测量大量星系团的Sunyaev-Zel'dovich效应，并利用测量结果研究星系团的大气结构。比较Sunyaev-Zel'dovich效应和来自星系团的X射线可以告诉我们它们所包含的热气体与暗物质的比例。如果这个比例随着红移而变化，那么我们就可以了解星系团是如何形成的。Sunyaev-Zel'dovich效应和X射线数据的不同比较可以用来测量星系团的距离。这很有趣，因为距离随红移（测量宇宙膨胀的量）变化的方式可以告诉我们宇宙作为一个整体是由什么组成的-并且可以测试一些关于神秘的“暗能量”的想法，这些想法被认为是宇宙当前加速膨胀的原因。

The largest clusters of galaxies are made up of thousands of individual galaxies, and are the most massive gravitationally-bound objects in the Universe. The mass that binds the clusters together is principally provided by dark matter, but a significant component, and the dominant component of 'ordinary matter' is in hot gas which is bound to the cluster by the gravitational field. The hot gas is most easily seen through its X-ray emission, which is mapped by satellite observatories such as Chandra and XMM-Newton, but can also be detected by its effect on radio waves passing through the cluster. Collisions between electrons in the cluster gas and photons in the radiation field heat up the photons, and cause the intensity of radio waves seen through the cluster to appear different from radio waves that are seen in other directions. This effect, the Sunyaev-Zel'dovich effect, can be seen by looking at the almost-uniform cosmic microwave background radiation, which is the left-over thermal radiation from an early, hot, phase of the Universe and has almost the same brightness in all directions on the sky, except where the radiation comes through clusters of galaxies. The purpose of this research programme is to measure the Sunyaev-Zel'dovich effects of a large number of clusters of galaxies, and to use the measurements to study the structures of the atmospheres of the clusters. Comparisons of the Sunyaev-Zel'dovich effects and the X-rays from clusters can tell us about the ratio of hot gas to dark matter that they contain. If this ratio changes with redshift, then we learn something about how clusters of galaxies form. A different comparison of Sunyaev-Zel'dovich effect and X-ray data can be used to measure the distances of clusters of galaxies. This is interesting because the way that distance changes with redshift (which measures the amount that the Universe has expanded) can tell us what the Universe as a whole is made of - and can test some ideas about the mysterious 'dark energy' that is thought to be responsible for the current accelerated expansion of the Universe.