Dark Matter

暗物质

There is perhaps no current problem of greater importance to astrophysics and cosmology than that of ''''dark matter''''. 


也许再没有比‘暗物质''''对天体物理学和宇宙论而言是现在更为重要的问题了。 



The controversy, as the name implies, is centered around the notion that there may exist an enormous amount of matter in the Universe which cannot be detected from the light which it emits. 


争论,如名字暗示的那样,争论的焦点集中在宇宙可能存在不能够从它发出的光被探测到的一种巨大量的物质。 


This is ''''matter'''' which cannot be seen directly. So what makes us think that it exists at all? Its presence is inferred indirectly from the motions of astronomical objects, specifically stellar, galactic, and galaxy cluster/supercluster observations. 



这是不能够被直接看到的‘物质''''。因此使我们认为它究竟存在与否?它的存在是从天文学物体运动确定的恒星，银河及银河星团/超星系观察被间接的推断出来。 



The basic principle is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects flying apart. When such velocity measurements are done on large scales, it turns out that the amount of inferred mass is much more than can be explained by the luminous stuff. Hence we infer that there is dark matter in the Universe. There are many different pieces of evidence on different scales. And on the very largest scales, there may be enough to "close" the Universe, so that it will ultimately re-collapse in a Big Crunch. 


基本的原则是如果我们在一些地方测量速度,那么在那里必须有阻止所有物体飞离所需引力足够大的质量存在。当测量到速度很大的时候,推断出的质量数比用能发光的东西来解释大很多。因此我们推断出宇宙中存在暗物质。在不同的数值范围存在许多不同种的证据。而且在非常大的数值上,可能足够“结束”宇宙，所以它在大的碎裂声中最终再崩溃。 






Various means of weighing the universe lead us to believe in the presence of dark matter. There is evidence from different astronomical objects, in order of increasing size: 


衡量宇宙的各种不同的方法引导我们相信暗物质的存在。有来自不同的天文学物体证据,逐渐膨胀的体积: 


Stellar Motions: 


恒星运动： 



Dutch astronomer Jan Oort first discovered the presence of dark matter in the 1930''''s when studying stellar motions in the local galactic neighborhood. By observing the Doppler shifts of stars moving near the galactic plane, Oort was able to calculate how fast the stars were moving. Since he observed that the galaxy was not flying apart he reasoned that there must be enough matter around that the gravitational pull kept the stars from escaping, much as the sun''''s gravitational pull keeps the planets in the solar system in orbit. He was able to determine that there must be three times as much mass as is readily observed in the form of visible light. Hence, Oort''''s calculations yielded an M/L ratio of 3 for the region of the immediate galactic neighborhood. The M/L ratio increases by several orders of magnitude as larger astro-physical phenomena come under similar scrutiny. 


当在本银河附近研究恒星运动时候，荷兰天文学家Jan Oort在1930年首先发现暗物质存在。借助观察*近银河面的运动恒星多普勒变换，Oort能够计算出恒星移动多快。因为他观察银河没有飞离开，他断定在附近一定有足够的物质引力吸引阻止恒星逃逸,非常像太阳引力吸引把行星留在太阳系轨道中那样。他能够确定一定有三倍大的质量以可见光的形式被观察到。因此Oort的计算为紧*银河附近的区域产生了3的M/L比。M/L比增加如比较大的astro-实际的现象来自相似的仔细研究之下。 





Luminous regions of galaxies 


银河发光的地方 



The luminous region of a galaxy extends over a radius of about 10 kpc. The sun is on the outskirts of the Milky Way galaxy, and about this distance from the center of the galaxy. One measures the total mass interior to the orbit of the sun from the sun''''s rotation speed around the galaxy and its galac to centric distance: this gives the centrifugal force, which must be balanced by the gravitational force due to all the mass interior to the sun''''s orbit. One finds that this mass is 10^11 M(Sun), while the cumulative luminosity of all the stars in the Milky Way is about 10^10 L(Sun). The ratio of mass to luminosity is therefore equal to 10, so that the average star is about half the mass of the sun. This is not a great surprise: the solar neighborhood contains younger, relatively more massive and luminous stars as do other spiral arm regions as compared with the galaxy as a whole. When we add up the luminosity from ale the stars in all the galaxies in the universe we find that the mass is far less than that required to close the universe. It is also significantly less than the mass density implied by Big Bang Nucleosynthesis. This deficit indicates that there may be "baryonic" dark matter (although not enough to make the universe recollapse), as well as the more exotic "particle" dark matter. 


银河的发光区域伸展半径大约10 kpc。太阳是在银河系的边缘上,这大致就是到银河中心的距离。测量相对太阳围绕银河旋转太阳的轨道内部总体质量和它的银河中心距离: 就得出了离心力,这力对于所有的太阳轨道内部质量必须与重力引力平衡。一项发现这个质量是10^11 M(太阳), 而所有银河里的星星累积的质量大致是10^10 L(太阳)。质量与光比值因就是等于10,所以一般恒星是太阳质量一半。这不是很让人太惊奇:太阳附近包含年代较小的,就整个银河来看相对质量更庞大和更发光的恒星真的就是处于银河旋臂区域。当我们把宇宙所有银河中来自ale恒星的发光度加起来时候, 我们发现质量远远地小于闭合宇宙所需要的。它也同样明显小于大爆炸核合成暗示的质量密度。这缺额显示可能存在“重子”暗物质(虽然不足于使得宇宙重新坍缩)以及更奇异的“粒子”暗物质。 









Galactic Rotation Curves 



银河系自旋轨迹 






 



The galaxy "M51". Messier 51 is also known as NGC 5194 and sometimes called the Whirlpool galaxy. It is the prototypical "Grand Design" spiral (i.e. very symmetrical and regular arms). The galaxy is of type Sc and it is very nearly face-on. The distance to M51 is about 9 Mpc (or about 30 million light years), and it is moving away from us at about 500 km/s. The other object is the lenticular companion galaxy NGC 5195. This particular image was taken at near-infrared wavelengths. Picture provided by Rosa Gonzalez and Jim Graham. 


银河“M51”。比较散乱的51也是以NGC 5194闻名而有时称作漩涡银河。它是原生型的“壮观设计”螺旋。(也就是非常对称的和规则的双臂)。银河是Sc类型，而且它几乎非常面对着。M51的距离大约是9Mpc(或大约3千万光年)，而且它是以大约500 km/s速度移动远离我们而去。另一个物体是透镜状的伴生银河NGC 5195。这个特别的图像以近-红外线波长拍摄。是由罗莎·宫泽勒和吉姆·格雷厄姆提供照片。 


Evidence of dark matter has been confirmed through the study of galactic rotation curves. These measurements are on a smaller scale than the galaxy clusters, but give more detail about the way the dark matter is distributed. 

通过银河系自旋轨迹的研究已经证实暗物质存在的证据。这些测量与银河系相比微不足道, 但是提供了有关暗物质分步情形较多的细节。 


To make a rotation curve one calculates the rotational velocity of stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the galactic center. 


沿着银河的长度通过测量它们的多普勒变换计算星系的旋转速度的旋转曲线,然后绘出它们分别远离银河中心的距离。 


Invariably, it is found that the stellar rotational velocity remains constant, or "flat", with increasing distance away from the galactic center. This result is highly counterintuitive since, based on Newton''''s law of gravity, the rotational velocity would steadily decrease for stars further away from the galactic center. Analogously, inner planets within the Solar System travel more quickly about the Sun than do the outer planets (e.g. the Earth travels around the sun at about 100,000 km/hr while Saturn, which is further out, travels at only one third this speed). One way to speed up the outer planets would be to add more mass to the solar system, between the planets. By the same argument the flat galactic rotation curves seem to suggest that each galaxy is surrounded by significant amounts of dark matter. It has been postulated,and generally accepted, that the dark matter would have to be located in a massive, roughly spherical halo enshrouding each galaxy. 


一般总是发现恒星旋转速度是常数或形状“扁平”,由于远离银河中心距离逐渐增加。这在以前是大大违反直觉的,基于牛顿地心引力定律,旋转速度有规律地减少，因为恒星更加远离银河中心。类似的，太阳系内部的行星比太阳系外部的行星围绕太阳旋转更快。(例如地球以大约100,000公里/小时围绕太阳转而更外层的土星仅是以这速度的三分之一围绕太阳旋转)。加速外部行星的一个方法就是要把更多质量加入太阳系行星之间。根据相同的观点扁平银河系旋转轨迹像是暗示每一个银河被明显大量的暗物质包围。这已经被假定,而通常被接受,暗物质肯定是大量存在, 大概以球形晕轮隐蔽着每个银河。 







 

The rotation curve for the galaxy NGC3198 from Begeman 1989 


1989年贝格曼的银河NGC3198自旋轨迹