The Dark Side of the Milky Way

うねる銀河系 暗黒物質の知られざる働き

By Leo Blitz L. ブリッツ
English 日本語 日本語
Although astronomers only slowly came to realize dark matter’s importance in the universe, for me personally it happened in an instant. In my first project as a postdoc at the University of California, Berkeley, in 1978, I measured the rotational velocities of star-forming giant molecular clouds in the outer part of the disk of our Milky Way galaxy. I worked out what was then the most accurate method to determine those velocities, and I sat down to plot out the results (by hand on graph paper) in the astronomy department lounge. Two other experts on the Milky Way, Frank Shu and Ivan King, happened by. They watched as I filled in the velocities of the outermost clouds, and the pattern we saw made it clear at once that the Milky Way was rife with dark matter, especially in its outermost parts. We sat and scratched our heads, imagining what the nature of the dark matter could be, and all the ideas we came up with turned out in short order to be wrong.  暗黒物質は宇宙の進化や銀河の形成に重要な役割を果たしている。このことは,今でこそ天文学者の間で共通認識となっているが,その重要性は直ちに理解されたわけではない。この認識に収束するまでには長い年月がかかった。しかし,私にとってみれば,初めから疑う余地はなかった。
 そこにたまたま天の川銀河のエキスパートであるシュー(Frank Shu)とキング(Ivan King)が居合わせていて,私の作業を見ていた。私たち3人は完成したグラフを見てすぐに悟った。「天の川銀河,特にその最外部には,暗黒物質が充満している!」 私たちは暗黒物質の正体をあれこれ想像してみたが,その場で浮かんだアイデアはすべて誤りであることがすぐわかった。
This study was one of many in the 1970s and 1980s that forced astronomers to conclude that dark matter—a mysterious substance that neither emits nor absorbs light and reveals itself solely by its gravitational influence—not only exists but is the dominant material constituent of the universe. Measurements with the WMAP spacecraft confirm that dark matter accounts for five times as much mass as ordinary matter (protons, neutrons, electrons, and so on). What the stuff is remains as elusive as ever. It is a measure of our ignorance that the most conservative hypothesis proposes that dark matter consists of an exotic particle not yet detected in particle accelerators, predicted by theories of matter that have not yet been verified. The most radical hypothesis is that Newton’s law of gravity and Einstein’s general theory of relativity are wrong or, at the very least, require unpleasant modifications.  1970年代から1980年代にかけ,天の川銀河の回転運動の様子が詳しく調べられた。私の研究もその1つだが,それらの研究の結果,暗黒物質の存在は動かぬものとなり,それが宇宙の物質の大部分を占めていることもわかった。
Whatever its nature, dark matter is already providing keys to unlock some persistent puzzles about how the Milky Way came to have certain of its features. For example, astronomers have known for more than 50 years that the outer parts of the galaxy are warped like a vinyl phonograph record left on a heater. They could not make a viable model for the warp—until they considered the effects of dark matter.  暗黒物質が何であれ,天の川銀河にまつわるいくつかの厄介な問題を解決してくれることは確かだ。その1つが,天の川銀河の円盤の外縁部に見られる歪み,「ウォープ」だ。実は,このウォープは50年も前からその存在が知られていた。天の川銀河の円盤はヒーターの上に置いたプラスチック製のレコードのように歪んでいる。天文学者はウォープの成因をいろいろと考えたが,結局のところ暗黒物質の影響を考虜しなければ説明できないことに気付いた。