Dark Matter

Suppose the dark areas in the picture of the center of our Milky Way galaxy below represented just as much mass as the bright areas. Does that sound plausible? The results of the WMAP assessment of mass density in the universe indicate that 27% of the effective density of the universe is in mass, but that only 4.4% is ordinary "baryonic matter". The remainder is "dark matter", the composition of which is unknown to us. Although we don't know the geometry of the dark matter distribution, the picture below was outlined so that the total area is about six times the area of the bright visible matter, the ratio of the total mass density to the detectable ordinary matter of the galaxy.

假设立方体中银河系中心的暗区所代表的物质量与亮区相同，这听起来合理吗？WMAP对宇宙中物质密度的评估结果表明，宇宙有效密度的27%是物质，但其中只有4.4%是普通的‘质子物质’。其余部分是‘暗物质’，其组成尚不清楚。虽然我们不知道暗物质的分布几何形状，但下面的图示是按照总区域大约是可见物质区域六倍的比例绘制的，即总质量密度与可探测的普通物质的比例。

The dark matter is undetectable by any electromagnetic radiation at our disposal - we can't see it in the visible range, can't detect it by radio waves at the low end of the spectrum or by the xrays at the high end of the spectrum. It appears not to interact by the electromagnetic force. The obvious question is then "How do we know it is there?" For some 80 years we have known the answer to that - it interacts by gravity and thereby produces measurable results.

暗物质无法通过任何现有的电磁辐射来探测——我们无法在可见光范围内看到它，也无法通过低频无线电波或高频X射线来探测它。它似乎不与电磁力相互作用。那么自然会提出一个问题：‘我们是如何知道它存在的？’对于这个问题，我们已经知道答案有80年了——它通过引力相互作用，从而产生可测量的结果。

When the orbital velocity of material about the center of our galaxy is examined, it is found that there is much more mass than can be accounted for by observation at any wavelength. The orbital velocity is a measure of the mass within the orbit, and once the orbit of any object is outside the collection of mass that attracts it, the orbit velocity diminishes with the inverse of the square root of the orbit radius in the so-called Keplerian orbit.

当研究银河系中心周围物质的轨道速度时，发现其质量远超过任何波长观测所能解释的范围。轨道速度是轨道内质量的度量，一旦物体的轨道位于吸引它的质量集合之外，其轨道速度与轨道半径的平方根成反比，这在所谓的开普勒轨道中是成立的。

The Sun lies about 8.5 kpc from the galactic center of the Milky Way galaxy, and the visible spiral arms and globular clusters extend out to about 15 kpc. Radio frequency methods should detect gas and dust past this radius, but not much is found. It was expected that the orbital velocity of that matter which is detected should diminish, but it stays more or less constant well beyond any significant detectable mass concentrations. The orbital velocity data clearly indicates the presence of gravitational mass, and the term "dark matter" is used to describe it.

太阳位于银河系中心约8.5 kpc处，可见的螺旋臂和球状星团延伸到约15 kpc。无线电频率方法应在这个半径之外探测到气体和尘埃，但并没有发现很多。人们预期，探测到的物质的轨道速度应减小，但即使在任何显著可探测的质量集中区域之外，轨道速度仍保持大致不变。轨道速度数据清楚地表明了引力质量的存在，因此用“暗物质”一词来描述它。

On a larger scale, the measurement of the speeds of galaxies within clusters gives a value for the total mass of the cluster. Again the inferred total mass seems to be several times larger than the visible mass of the galaxies. This was first observed with the Coma Cluster by Franz Zwicky in 1933.

在更大的尺度上，测量星团内星系的速度可以给出星团的总质量。再次，推断出的总质量似乎比星系的可见质量大几倍。这一现象最早由弗朗兹·泽维克（Franz Zwicky）在1933年用Coma星团观测到。

There have been observations of gravitational lensing which forms multiple images of distant galaxies around a closer galactic cluster. When that lensing is modeled, the apparent mass of the lensing cluster is much larger than the implied mass of the visible constituents.

已观测到引力透镜效应，这种效应使遥远星系在更近的星系团周围形成多个图像。当对这种透镜效应进行建模时，透镜星系团的表观质量远大于可见组成部分所暗示的质量。

Two types of proposed constituents of the dark matter have been proposed and investigated, and have been given the labels WIMPS and MACHOS. WIMP is an acronym for weakly interacting massive particle and MACHO is an acronym for massive compact halo object. WIMPS would be new kinds of particles interacting by the weak interaction, and efforts have been made to observe them using techniques used for detecting neutrinos. MACHOS could be planet-sized objects, primordial black holes, brown dwarfs, etc., which just cannot be detected out in the halo. Some candidates have been observed, but at present not nearly enough of them to account for a mass so much larger than the currently visible ordinary matter. A mystery for the WIMP hypothesis is why the dark matter should be out in the halo beyond the visible matter. If they were particles produced in the big bang and are gravitationally bound, one would expect more concentration toward the center of galaxies.

两种暗物质的候选构成物已被提出并研究，分别被命名为WIMPS和MACHOS。WIMP是“弱相互作用大质量粒子”的缩写，MACHO是“大质量致密晕天体”的缩写。WIMPS应是通过弱相互作用相互作用的新种类粒子，人们已尝试利用探测中微子的技术来观测它们。MACHOS可能是行星大小的天体、原始黑洞、红矮星等，只是无法在晕中被探测到。一些候选者已被观测到，但目前尚不足以解释比目前可见普通物质质量大得多的暗物质。对于WIMP假说而言，一个谜题是为什么暗物质应该位于晕中，超出可见物质之外。如果它们是在大爆炸中产生的粒子并且被引力束缚，人们会期望它们在星系中心有更多集中分布。