The Sunyaev-Zel'dovich effect and cosmology

最大的星系团由数千个单独的星系组成，是宇宙中质量最大的引力约束天体。将星系团结合在一起的质量主要是由暗物质提供的，但也是一个重要的成分，而普通物质的主要成分是热气体，它通过引力场与星系团结合在一起。热气体最容易被观测到的是它的X射线发射，这是由钱德拉和XMM-牛顿等卫星天文台绘制的，但也可以通过它对穿过星系团的无线电波的影响来探测到。星系团气体中的电子与辐射场中的光子之间的碰撞会加热光子，并导致透过星系团看到的无线电波的强度看起来与在其他方向上看到的无线电波不同。这种效应，即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看到，这是宇宙早期炎热阶段遗留下来的热辐射，在天空中的所有方向上都具有几乎相同的亮度，除了通过星系团的辐射。这项研究计划的目的是测量大量星系团的Sunyaev-Zel‘dovich效应，并利用这些测量结果来研究星系团的大气结构。将Sunyaev-Zel‘dovich效应和来自星系团的X射线进行比较，可以告诉我们它们所包含的热气体与暗物质的比率。如果这个比率随着红移而改变，那么我们就能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，因为距离随着红移(测量宇宙膨胀的量)而变化的方式可以告诉我们整个宇宙是由什么组成的--并可以测试一些关于神秘的暗能量的想法，这种能量被认为是目前宇宙加速膨胀的原因。

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.