"The Universe's Invisible Hand"

「宇宙を造った見えざる手 暗黒エネルギー」より抜粋

By Christopher J. Conselice C. J. コンセリス
English 日本語 日本語
What took us so long? Only in 1998 did astronomers discover we had been missing nearly three quarters of the contents of the universe, the so-called dark energy--an unknown form of energy that surrounds each of us, tugging at us ever so slightly, holding the fate of the cosmos in its grip, but to which we are almost totally blind. Some researchers, to be sure, had anticipated that such energy existed, but even they will tell you that its detection ranks among the most revolutionary discoveries in 20th-century cosmology. Not only does dark energy appear to make up the bulk of the universe, but its existence, if it stands the test of time, will probably require the development of new theories of physics. なぜこんなにも時間がかかったのか?宇宙の3/4近くを占める暗黒エネルギー(ダークエネルギー)を見逃してきたことに天文学者が気づいたのは,ようやく1998年になってからだ。
Scientists are just starting the long process of figuring out what dark energy is and what its implications are. One realization has already sunk in: although dark energy betrayed its existence through its effect on the universe as a whole, it may also shape the evolution of the universe's inhabitants--stars, galaxies, galaxy clusters. Astronomers may have been staring at its handiwork for decades without realizing it. 暗黒エネルギーとは何か,暗黒エネルギーがどんな意味を持つのか,解明は始まったばかりだ。しかし,徐々にわかってきたことはある。暗黒エネルギーは宇宙全体に及ぼす影響からその存在をうかがわせるのだが,恒星や銀河,銀河団など宇宙の住人たちの進化を形作ってきた面もあるようだ。天文学者たちは暗黒エネルギーがつくり上げた宇宙という“作品”を,それと知らずに何十年も見つめ続けてきたのかもしれない。
Ironically, the very pervasiveness of dark energy is what made it so hard to recognize. Dark energy, unlike matter, does not clump in some places more than others; by its very nature, it is spread smoothly everywhere. Whatever the location--be it in your kitchen or in intergalactic space--it has the same density, about 10-26 kilogram per cubic meter, equivalent to a handful of hydrogen atoms. All the dark energy in our solar system amounts to the mass of a small asteroid, making it an utterly inconsequential player in the dance of the planets. Its effects stand out only when viewed over vast distances and spans of time. 暗黒エネルギーは至るところに存在するが,皮肉なことに,この遍在性が認識を難しくしている。物質と違って,暗黒エネルギーはどこかに集まってほかよりも密になるということがない。そもそも均一にあまねく広がっているものなのだ。銀河間空間であろうと,あなたの家の台所であろうと,どこでも暗黒エネルギーの密度は同じで,質量に換算すると1立方mあたり約10-26kg,水素原子8個ほどになる。
Since the days of American astronomer Edwin Hubble, observers have known that all but the nearest galaxies are moving away from us at a rapid rate. This rate is proportional to distance: the more distant a galaxy is, the faster its recession. Such a pattern implied that galaxies are not moving through space in the conventional sense but are being carried along as the fabric of space itself stretches [see "Misconceptions about the Big Bang," by Charles H. Lineweaver and Tamara M. Davis; Scientific American, March 2005]. For decades, astronomers struggled to answer the obvious follow-up question: How does the expansion rate change over time? They reasoned that it should be slowing down, as the inward gravitational attraction exerted by galaxies on one another should have counteracted the outward expansion. 米国の天文学者ハッブル(Edwin Hubble)以降,ごく近傍のものを除くと,銀河が私たちから高速で遠ざかっていることが知られてきた。そのスピードは距離に比例し,遠方の銀河ほど速く遠ざかっている。こうしたパターンは,銀河が通常の意味で空間のなかを移動しているのではなく,空間そのものが拡大していて,銀河がそれにのって運ばれていることを示している(C. H. ラインウィーバー/T. M. デイビス「ビッグバンをめぐる6つの誤解」日経サイエンス2005年6月号)。
The first clear observational evidence for changes in the expansion rate involved distant supernovae, massive exploding stars that can be used as markers of cosmic expansion, just as watching driftwood lets you measure the speed of a river. These observations made clear that the expansion was slower in the past than today and is therefore accelerating. More specifically, it had been slowing down but at some point underwent a transition and began speeding up [see "Surveying Space-time with Supernovae," by Craig J. Hogan, Robert P. Kirshner and Nicholas B. Suntzeff; Scientific American, January 1999, and "From Slowdown to Speedup," by Adam G. Riess and Michael S. Turner; Scientific American, February 2004]. This striking result has since been cross-checked by independent studies of the cosmic microwave background radiation by, for example, the Wilkinson Microwave Anisotropy Probe (WMAP). 膨張速度の変化に関する初の観測的証拠は,遠方の超新星から得られた。超新星は質量の大きな星が起こす爆発で,流木を見ると川の流速がわかるのと同様,宇宙膨張を知るための目印になる。こうした観測の結果,過去には膨張がゆっくりであり,つまり現在は膨張が加速していることがわかった。
より正確にいうと,初めのうち宇宙膨張は減速してきたのだが,ある時点で加速膨張に転じたのだ(C. J. ホーガンほか「宇宙膨張は加速している」日経サイエンス1999年4月号,A. G. リース/M. S. ターナー「減速から加速へ 宇宙膨張の奇妙な変化」同2004年5月号)。この衝撃的な観測結果は,WMAP(ウィルキンソン・マイクロ波非等方性探査機)などによる宇宙マイクロ波背景放射に関する複数の研究によって確認されてきた。
One possible conclusion is that different laws of gravity apply on supergalactic scales than on lesser ones, so that galaxies' gravity does not, in fact, resist expansion. But the more generally accepted hypothesis is that the laws of gravity are universal and that some form of energy, previously unknown to science, opposes and overwhelms galaxies' mutual attraction, pushing them apart ever faster. Although dark energy is inconsequential within our galaxy (let alone your kitchen), it adds up to the most powerful force in the cosmos. なぜ,膨張が加速しているのか。ひとつの可能性は,銀河を超える巨大な空間スケールでは小スケールの世界とは異なる重力の法則が働き,銀河の重力が宇宙膨張を実質的に妨げなくなるというものだ。