There is perhaps no current problem of greater importance to astrophysics and cosmology than that of ''''dark matter''''.
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.
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.
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.
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:
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.
Luminous regions of galaxies
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
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.
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.
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.
The rotation curve for the galaxy NGC3198 from Begeman 1989