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 

The first real surprise in the study of dark matter lay in the outermost parts of galaxies, known as galaxy halos. Here there is negligible luminosity, yet there are occasional orbiting gas clouds which allow one to measure rotation speeds and distances. The rotation speed is found not to decrease with increasing distance from the galactic center, implying that the mass distribution of the galaxy cannot be concentrated, like the light distribution. The mass must continue to increase: since the rotation speed satisfies v^2=GM/r, where M is the mass within radius r, we infer that M increases proportionally to r. This rise appears to stop at about 50kpc, where halos appear to be truncated. We infer that the mass-to-luminosity ratio of the galaxy, including its disk halo, is about 5 times larger than estimated for the luminous inner region, or equal to about 50. Many people believe that the galactic halos are composed of particle dark matter. The Center Direct Detection Group is actively searching for evidence of this dark matter. 



暗物质研究第一真正令人惊奇地方在于银河最远部份,以银河晕轮而闻名。这里具有忽略的发光度,也有允许测量旋转速度和距离的偶然旋转气云。旋转速度被发现不随逐渐增加银河中心的距离减少, 暗示银河质量分配像光的分配那样不可能是集中的。质量应当连续增加:因为旋转速度满足v^2=GM/r,在这里M是半径 r里面质量,我们推论出M随着r比例增加。这个增加似乎在大约50kpc停止,在这里晕轮似乎被切掉了。我们推论出银河质量-与-发光度比,包括它的圆盘晕轮,是比内部区域发光度估计大大约5倍,或约等于50。许多人相信银河晕轮由暗物质粒子组成。中心直接探测团体正积极地搜索暗物质存在证据。 





Galaxy Clusters 


银河星团 



While Oort was carrying out his observations of stellar motions, Fritz Zwicky of Caltech discovered the presence of dark matter on a much larger scale through his studies of galactic clusters. A galactic cluster is an group of galaxies which are gravitationally bound. Our own galaxy, the Milky Way, is a member of a small cluster known as the Local Group. Using the same method employed by Oort, Zwicky determined the Doppler shifts of individual galaxies in one particular system, the Coma cluster--about 300 million light years away. Zwicky found nearly 10 times as much mass as observed in the form of visible light was needed to keep the individual galaxies within the cluster gravitationally bound. It was clear to Zwicky, as it had been to Oort, that a large sum of mass was extant which was simply not visible. At the time, astronomers referred to the material as "missing mass". However, this was deemed a misnomer as the mass was clearly present, but simply not visible. Hence, the more appropriate term "dark matter" came to supercede "missing mass". Since Zwicky's efforts, more recent measurements have found that certain galaxy clusters (and binary galaxies) have M/L ratios up to 300. 


当Oort正完成他的恒星运动观察的时候,加州理工学院Fritz Zwicky通过他的银河系研究在一个很大的比值方面发现了暗物质存在。银河星团是一组受引力约束的银河。银河,我们自己的银河,是以本星系群闻名的小星团成员。利用Oort, Zwicky提供的相同方法在一个特别的系统中决定个别的银河多普勒变换, Coma星团--大约3亿光年远。Zwicky几乎发现了10倍的以可见光形式所观察的同样大的可以需要在群引力范围维持单个银河的质量。Zwicky对此很肯定,如同Oort已经知道那样，大量的质量是存在的只是不能见而已。在那时，天文学家把这物质称为“短缺的质量”。然而，这被认为是一个错误的名字因为质量是明白无误地存在的,但只是不能看见。因此,更恰当的术语“暗物质”就取代了“短缺的质量”。因为Zwicky的努力，更多新近的测量已经发现某些银河群(及二元银河)的M/L(质量与光比值)比值高达300。 



The mass-to-light ratio can also be evaluated by studying galaxy pairs, groups, and clusters. In each case, one measures velocities and length-scales, leading to a determination of the total mass required to provide the necessary self-gravity to stop the system from flying apart. The inferred ratio of mass-to-luminosity is about 100 in galaxy pairs, which typically have separations of about 100 kpc, and increases to 300 for groups and clusters of galaxies over a length scale of about 1 Mpc. 


质量-与-光比值也能够由研究银河对，群和星团来评估。在每个情况里,一种测量速度和长度-刻度,导致总的质量必需提供必要的自我引力阻止系统飞离。推断在银河对中质量-与-光的比值大约是100,典型地有大约100kpc的分离比值,而对银河群和系而言在大约1Mpc的长度刻度里增加到300。 





Superclusters 


超星系团 



The largest scale on which the mass density has been measured with any precision is that of superclusters. A supercluster is an aggregate of several clusters of galaxies, extending over about 10 Mpc. Our local supercluster is an extended distribution of galaxies centered on the Virgo cluster, some 10-20 Mpc distant, and our Milky Way galaxy together with the Andromeda galaxy forms a small group (the Local Group) that is an outlying member of the Virgo Supercluster. The mass between us and Virgo tends to decelerate the recession of our galaxy, as expected according to Hubble's law by about ten percent. This effect is seen as a deviation from the uniform Hubble expansion of the galaxies and provides a measure of the mean density within the Virgo Supercluster. One again finds a ratio of mass-to-luminosity equal to 300 over this scale, which amounts to about 20Mpc. 


已经用任何精密测量的质量密度的最大值是超星系团的数值。超星系团是几个银河群集合体,伸展大约10Mpc。我们本地超星系团是室女(星)座为中心银河广大延伸,约10-20 Mpc远,而我们的银河和仙女座银河一起形成一个小的团体(本星系群)以是室女座超星系团的一个边远成员。我们和室女座之间质量趋于减慢我们银河后退,如预期的依照大约百分之十的哈勃定律那样。这作用被视为背离银河哈勃膨胀的统一而提供对室女座超星系团内平均密度的测量。再一次找到等于质量-与- 发光度比300以上的数值, 总数大约为20Mpc。 





 

[此贴子已经被作者于2005-7-13 19:40:37编辑过]

Dark matter has important consequences for the evolution of the Universe. According to standard cosmological theory, the Universe must conform to one of three possible types: open, flat, or closed. A parameter known as the "mass density" - that is, how much matter per unit volume is contained in the Universe - determines which of the three possibilities applies to the Universe. In the case of an open Universe, the mass density (denoted by the greek letter Omega) is less than unity, and the Universe is predicted to expand forever. If the Universe is closed, Omega is greater than unity, and the Universe will eventually stop its expansion and recollapse back upon itself. For the case where Omega is exactly equal to one, the Universe is delicately balanced between the two states, and is said to be "flat". 


暗物质为宇宙的进化占有重要的作用地位。根据标准宇宙理论，宇宙一定遵照三种可能类型之一:开放的,扁平的,或封闭的。以“质量密度”闻名的一个参数-也就是,在宇宙里含有多少单位体积物质–确定三种可能性哪一种适用于宇宙。 在一开放的宇宙里，质量密度(以希腊字母欧米加表示)是小于平均数，且宇宙被预言永远地膨胀下去。如果宇宙是封闭的,欧米加大于平均数，而且宇宙最终会停止它的膨胀及重新往回坍缩自己。对于欧米加正好等于平均数的情形，宇宙微妙地在二种情形之间平衡, 就被说成是“扁平”。 






发送图片到手机  



In the figure above we show graphically some of the measurements of the density of the universe which we have discussed above. What is plotted is the density of the universe, both visible matter and the inferred "dark matter", as a function of the "scale" at which the measurement was made, from the local neighbourhood up to the largest scales. On the smallest scales, probed by Oort, the visible matter and three times as much dark matter give Omega about 1/1000. As we go to larger and larger scales the inferred value of Omega increases, although the measurements become harder and progressively more uncertain. The next point to the right is the mass in galaxies, which moves to the position of the higher dot if we include the dark matter inferred from rotation curves. Then on larger scales we have the measurements from the motions of clusters of galaxies and the cosmic microwave background. The yellow band indicates the amount of matter that can reside in "normal" matter, or baryons, as inferred from Nucleosynthesis. If there is more matter in the universe than this, as the measurements appear to be telling us, then it must be made up of some strange particle which is not familiar to us here on earth. 


在上面的图里我们用草图显示了我们在上面已经讨论过的宇宙密度的一些测量。被绘出来的东西就是宇宙密度，既有可见物质又有可推论的“暗物质”, 作为一个“刻度”功能，从本地邻近直到最大的数值范围在此数值范围做了测量。在最小数值范围里,由Oort探查的可见物质和三倍多的暗物质给出欧米加(希腊字母最后一个)值约为1/1000。当我们转到越来越大的数值范围欧米加推导出来的数值加大了,虽然测量变得更难和更日益增加不确定性。右边下个点是银河的质量如果我们包括从旋转曲线推论出暗物质移到更高点的位置上。然后在更大的刻度上我们可以测量来自银河群的运动和宇宙背景微波。黄色部分显示以“常态”物质存在的物质数量, 或从核合成推论出的重子。如果与这比较宇宙有更多物质,如测量似乎告诉我们那样,那么它一定由地球这里不是我们熟悉的一些奇异粒子组成。 


There is also a somewhat philosophical idea that makes Omega=1 attractive. The point is that as the Universe expands the value of Omega changes. In fact the value 1 is unstable, and the Universe would prefer to evolve towards one of the two natural values: 0, if the expands forever further apart until the Universe is almost totally empty ; and infinity, if the matter recollapses to a state of higher and higher density. Then the observation that Omega is fairly close to 1 today, means that it must have been even closer to 1 in the past. It is unsatisfying to believe that we just happen to live at the time when Omega is just starting to depart from 1 by a small factor. It is much more appealing to consider that we do not live at a special epoch, so that Omega is still close to 1 today. But then we need to explain why Omega started out very close to 1 in the early universe. The theory of inflation provides just such a justification - it predicts that the early Universe was driven extremely close to flat, and that it is still very close to flat today. If this is so, then at least 90% of the mass of the Universe is dark. Dark matter (DM) candidates are usually split into two broad categories, with the second category being further sub-divided: 


同样也有些使欧米加=1更加吸引人的哲学思想。该点是作为宇宙扩张欧米加变化值。事实上1值是不稳定的，而宇宙会宁愿向二自然值之一进化:0,如果膨胀永远更进一步分离直到宇宙几乎完全地是空的；而无限大,如果物质坍缩为越来越高的密度状态。那么欧米加相当地*近今天1的观察,意谓它过去一定是甚至更*近1。它是不令人满意地相信我们正好碰巧生活在这一时刻，*着一个小的因素欧米加是正好开始从1离开的时候。它是更加引起兴趣认为我们不生活在一个特别的新纪元,所以欧米加仍然接近今天的1。但是另一方面我们需要解释为什么欧米加在早期宇宙中开始非常接近1。膨胀理论正好提供这样一个解释-它预言早期宇宙极端地受到驱使接近扁平,而且它仍然非常接近今天的扁平。如果真是这样,那么至少宇宙90%的质量是暗的。暗物质(DM)候选者通常分裂为二个主要种类，由于第二个种类更是进一步的子分离: 


-          Baryonic 


-          重子 



-          Non-Baryonic - hot dark matter (HDM) and cold dark matter (CDM), 


-          非重子-热的暗物质(HDM)及冷的暗物质(CDM)， 



depending on their respective masses and speeds. CDM candidates have relatively large mass and travel at slow speeds (hence "cold"), while HDM candidates include minute-mass, rapidly moving (hence "hot") particles. 


取决于它们各自的质量和速度。CDM对象相比较质量大，速度慢(因此“冷的”),而HDM对象包括微小的质量,快速地移动的( 因此“热的”)粒子。 


-          Encyclopedia Britannica 


-          大英百科全书 



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