hzwill 发表于 2011-8-22 14:31


Cosmology 101: Dark matter

Astronomy杂志的副主编 Liz Kruesi 将会给我们讲解关于暗物质的概念,这种神秘的物质组成了我们宇宙大约90%的质量。

点此看视频:http://gebaini.com/fan/140/   (视频为英文版)

In this series I give you an overview of important ideas in the area of cosmology. This video is the second in the series, and focuses on dark matter.

Using various detectors and research methods, astronomers have determined that the stuff we see in space — stars, gas, and dust — amounts to less than 5 percent of the universe. This stuff is ordinary matter, and it's made up of protons, neutrons, and electrons. Scientists call ordinary matter "baryonic matter" because protons and neutrons are subatomic particles called baryons.

So if ordinary matter is only about 5 percent of the universe, what's the other 95 percent? Well, about 23 percent is something called dark matter. The remaining 72 percent is a mysterious pervasive force called dark energy. I'll explain more about dark energy in my next video.

Alright, back to dark matter. This mysterious mass is a type of matter that doesn't emit, absorb, or reflect any type of light (so, for example, it doesn't emit X rays or absorb infrared radiation). Dark matter is therefore invisible. If it's invisible, how do astronomers have any idea it's there? Dark matter interacts with ordinary matter through gravity. Its gravitational interaction is how astronomers first found out dark matter exists.

In 1933, a Swiss astrophysicist by the name of Fritz Zwicky first proposed dark matter's existence. While studying the Coma galaxy cluster, he found that the galaxies' gravity alone was much too small to hold the cluster together.

The next round of evidence came about 40 years later, in the 1970s. Astronomers charted the velocity of stars at various distances from the center of a galaxy, and plotted the velocity versus the distance. This plot is called a rotation curve. They expected the velocities to reach a maximum and then decrease farther from the center — but the data showed otherwise. The velocities reach a maximum and then plateau. With velocities of this magnitude at the outer edge of galaxies, the stars should be flung out of their orbits. But they aren't. So some sort of mass that we can't detect must hold these outer stars in orbit.

大规模的星系簇同样预示着暗物质的存在。一个超级天体(比如星系簇)可以将它后方星系发出的光波弯曲或扩大。这种超级天体如同一个引力透镜。在这幅包含了Abell 2218星系簇的图片里,你们可以看到许多的蓝光弧,这些就是星系中光纤被星系簇重力弯曲和扩大的情景。天文学家通过研究这些弧光的尺寸和形状可以得出星系簇的质量。然后通过和那些只计算发光天体(星系)的质量进行对比,他们就能够得出星系簇中有多少暗物质的存在了。
Massive galaxy clusters also show evidence of dark matter. A very massive object — such as a galaxy cluster — can bend and magnify light from galaxies behind it. That massive object acts as a gravitational lens. In this image of galaxy cluster Abell 2218, you'll see numerous blue arcs. Those are background galaxies distorted and magnified by the cluster's gravity. Astronomers study the sizes and shapes of those arcs to determine the cluster's mass. By comparing that calculated mass to the mass that comes from only luminous objects (the galaxies), they can determine how much dark matter is in the cluster.

Today, astronomers are pretty confident that dark matter does in fact exist. They've created numerous "dark matter maps" using different techniques, including gravitational lensing of galaxy clusters. The next step is finding out what it is. As I said earlier, dark matter doesn't emit, absorb, or reflect any type of light. So, it's likely some sort of mysterious non-baryonic particle, meaning it's not made up of the same stuff as ordinary matter (protons and neutrons).

Astronomers split non-baryonic dark matter into two categories: hot and cold. These titles don't mean that if you touch them, you'll feel something that's hot or cold. Hot means that early in the universe, these particles traveled very, very fast — almost at the speed of light. Cold means that early in the universe the particles traveled slower.

How does particle speed relate to figuring out what dark matter is? Well, slower particles will bunch up into small structures earlier in the universe. Those small structures will eventually collide and merge, forming larger structures. Astronomers believe this is how structure forms and evolves in our universe — smaller structures eventually merge into the massive superclusters and filaments we observe today. Astronomers simulate structure evolution with cold dark matter, and create models that resemble today's universe. This simulation shows dark matter distribution. Brighter areas represent more dense regions.



原文地址:www.astronomy.com/News-Observing ... 0Dark%20matter.aspx (需登陆阅读)



hzwill 发表于 2011-8-22 14:33


gohomeman1 发表于 2011-8-22 17:45


oudeman 发表于 2011-8-22 18:38


fengjihu8 发表于 2011-8-23 10:52


Sadalsuud 发表于 2011-8-25 10:29


positron 发表于 2011-8-28 16:56

Astronomers split non-baryonic dark matter into two categories: hot and cold.

原作者偷懒,把 hot dark matter,cold dark matter 简写成 hot and cold。

Q5968661 发表于 2011-9-1 09:35

本帖最后由 Q5968661 于 2011-9-1 10:11 编辑

Coma galaxy cluster = Coma Cluster = 后发星系团

With velocities of this magnitude 已经是“保持这种速率”,又何来“在如此重量(星球)级别下”?


Q5968661 发表于 2011-9-1 11:34


There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.

Q5968661 发表于 2011-9-1 11:36

本帖最后由 Q5968661 于 2011-9-2 08:26 编辑

In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable.

gohomeman1 发表于 2011-9-1 21:07


neveruse 发表于 2011-9-2 21:39

本帖最后由 neveruse 于 2011-9-2 21:40 编辑

页: [1]
查看完整版本: 【翻译】宇宙大指南之暗物质