008 太空：宇宙的上下左右在哪裡？

It’s official: You're lost in a directionless universe

官方消息：天哪！我們迷路了！

圖1 找不到上下左右和前後，每個方向都一樣，這就是太空嗎？

看看太空吧！那無窮無盡的蒼穹！哪裡是上？哪裡又是下？

在研究了宇宙大爆炸產生的古老宇宙微波背景輻射(CMB, cosmic microwave background )之後，科學家們還是無法找到宇宙的方向！每個方向看上去都是一樣的。宇宙是各向同性的(isotropic)：沒有旋轉軸！沒有那個方向有什麼特別！

不過，這對宇宙學家來說也許是值得欣慰的，因為他們的宇宙標準進化模型就是建立在這樣的均一性之上的。

當然宇宙如何具有這樣高度的均一性是一個重要的科學問題。

1543年，哥白尼把地球的中心位置讓給了太陽。後來又發展出來了無限且無中心宇宙概念。到了20世紀，愛因斯坦發表了廣義相對論，哈勃在各個方向上觀察到了宇宙的膨脹。這更讓人有理由相信宇宙在每個地方和每個方向上都是相同的。在一些極端的研究團隊的眼裡，宇宙就是完全均質和完全各向同性的。

不過這個原則也有一定的局限。因為太空中的恆星和星系並不是分布得那麼均勻。這種不均勻的現象可以這樣解釋：大爆炸誕生宇宙的時候，產生的均一的亞原子粒子「湯」，在短暫而快速的宇宙暴漲的過程中，一點點的量子漲落會被放大到巨大的宇宙尺度。原來一點點密度的差別會讓星系在不同地方產生。有了這種解釋，星系分布的不均勻性，就並不會真正威脅到「完全均質和完全各向同性」的宇宙論。

但是宇宙為何一定要這樣的？理論上，就算是宇宙每點都是一樣的，還是有可能有特殊的方向。比如鑽石，密度完全是完全均勻的，可鑽石仍然還是有方向的，因為晶格中的原子排列有方向！

其實，在2000年代早期，科學家就找到了一些各向異性跡象。NASA的威爾金森微波方向太空探測器(WMAP)的數據表明宇宙微波輻射的波動暗示了一根「宇宙邪惡軸心」！不過許多研究者並不認為這是真的，認為這可能是統計學上的偶然事件，「宇宙邪惡軸心」並不存在。

歐洲太空總署的普朗克號飛船收集了2009到2013年更為精確的宇宙微波背景輻射數據。考慮到精度有了大幅度的提高，宇宙各向異性會不會出現呢？

圖2 實際上觀測不到（上圖）各向異性的宇宙微波背景應該顯現出的模式（下圖）

比如，空間膨脹的時候可能會在軸的方向上有不同的膨脹速度。這樣的話會觀測到這個方向上空間拉長效應造成的微波波長變長。

或者，宇宙沿著某個軸旋轉，這樣就會形成螺旋型的微波背景圖樣。

再者，新生的宇宙被重力波的扭曲，某些方向上被壓縮得厲害，而另一些方向上卻被拉長了，這樣微波背景就會有更複雜的螺紋。

……（譯者註：科學家的想像力真強大！）

老式電視機屏幕上的雜點信號就有宇宙微波背景的功勞。用上了超級計算機，科學家們企圖從這些雜點中找到那微弱而模糊不清的「電視圖像」。況且，他們還有非常強大的工具——微波背景偏振分析工具（這是普朗克號飛船最新提供的！）。

同時，科學家們把信號的檢出靈敏度也提高了1個數量級（譯者註：就是10倍左右）。

總之，為了要找到宇宙的各向異性（譯者註：也就是各個方向上有差別的意思），科學家發瘋似地在各個方面都達到極限。

但是……

沒能發現各向異性！

每個方向上都是一樣的！

各向異性再一次被排除了！

我們在宇宙中迷路了！

我們找不著北！沒有上！也沒有下！

本研究發表在Physical Review Letters。

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譯者說明：為了在保持科學性的同時達到通俗易懂的目的，文章可能同時參考原新聞稿、科學論文原文及相關資料進行了註解或重寫。

原新聞稿： Science (DOI: 10.1126/science.aah7276)

-----------英語學習------------

詞彙:

a directionless universe：一個沒有方向的宇宙

isotropic：各向同性（各個方向上不一樣）

anisotropy：各向異性（各個方向上沒有不同）

cosmic microwave background (CMB)：宇宙微波背景

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英文原文:

It’s official: You're lost in a directionless universe

By Adrian Cho

Ever peer into the night sky and wonder whether space is really the same in all directions or if the cosmos might be whirling about like a vast top? Now, one team of cosmologists has used the oldest radiation there is, the afterglow of the big bang. or the cosmic microwave background (CMB), to show that the universe is 「isotropic,」 or the same no matter which way you look: There is no spin axis or any other special direction in space. In fact, they estimate that there is only a one-in-121,000 chance of a preferred direction—the best evidence yet for an isotropic universe. That finding should provide some comfort for cosmologists, whose standard model of the evolution of the universe rests on an assumption of such uniformity.

"It's a much more comprehensive analysis than in previous cases," says Anthony Challinor, a cosmologist at the University of Cambridge in the United Kingdom who was not involved in the work. "The question of how isotropic is the universe is of fundamental importance."

In 1543, Nicolaus Copernicus knocked Earth and humanity from the supposed center of the universe by noting that Earth goes around the sun, not the other way around. That observation gave birth to the Copernican principle, which holds that we have no special place in the infinite, centerless universe. In the early 20th century, with the advent of Albert Einstein's general theory of relativity and the observation that the universe is expanding in all directions, that idea evolved into the cosmological principle, which assumes that the universe is the same everywhere and in every direction. In fancier terms, the universe is both homogeneous and isotropic.

The principle has its limitations. As the existence of stars and galaxies shows, matter is not distributed exactly the same way everywhere. This, they assume, arises because the universe was born as a homogeneous soup of subatomic particles in the big bang. As the universe underwent an exponential growth spurt called inflation, tiny quantum fluctuations in that soup expanded to gargantuan sizes, providing density variations that would seed the galaxies. Yet, the standard model of cosmology rests on the assumption that, on the largest scales, these variations are insignificant, and space is homogeneous and isotropic.

But it doesn't necessarily have to be that way. Theoretically, it's possible that space could be the same from point to point, but still have special directions—much as a diamond crystal has uniform density, but specific directions in which its atoms line up in rows. There were even some hints of such "anisotropy" in the early 2000s, when measurements from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft suggested that some subtle undulations in the motley CMB appeared to line up along a so-called "axis of evil"—which most researchers discount as a statistical fluke.

Now, Daniela Saadeh and Andrew Pontzen, cosmologists at University College London, and colleagues have ruled out special directions with the most stringent test yet. They also use measurements of the CMB, this time taken with the European Space Agency's Planck spacecraft, which collected data from 2009 to 2013 and provided far more precise CMB maps than WMAP. Instead of looking for curious imbalances in the CMB, they systematically worked the other way around. They considered all the ways that space could have a preferred direction and how such scenarios might imprint themselves on the CMB. Then they searched for those specific signs in the data.

For example, space could be expanding at different speeds along different axes. Such differential expansion would cause the radiation from some directions to stretch to longer wavelengths than in others, and the upshot would be a big bull's-eye pattern in the CMB. Or, space could be rotating about a particular axis, which would create a spiral pattern in the CMB. Finally, the newborn universe could have been agitated by distortions in space itself known as gravitational waves, which would stretch the whole cosmos in one direction and compress it in a perpendicular direction. That sort of motion would leave more complex spirals in the CMB. In all, the researchers identify five potential patterns or "modes" in the CMB that would signal some sort of special direction in space.

Using a supercomputer, Saadeh, Pontzen, and colleagues look for evidence of any such patterns lurking faintly behind random variations in the CMB's temperature—a process not unlike trying to pick out a weak picture through extreme static on an old-fashioned TV screen. To give their study even more bite, they also look for accompanying patterns in the polarization of the CMB's microwaves, which Planck also mapped. For three of the five patterns, "polarization data is the killer thing," Saadeh says.

Others had performed similar tests for signs that the universe is spinning, but Saadeh, Pontzen, and colleagues improve the limit on such a signal by an order of magnitude. They also put limits on all other kinds of anisotropy, as they report in a paper in press at Physical Review Letters. "For the first time, we really exclude anisotropy," Saadeh says. "Before, it was only that it hadn't been probed."

But just how significant is that advance? That's hard to judge, Challinor says, because there aren't compelling alternatives to the standard model of cosmology that predict exactly how an anisotropic universe should be. "The problem is, what do you compare it to?" he asks. Still, he notes, "this assumption is fundamental cosmology" so "it's very important to check."