科学家研究宇宙的“配方”

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千百万年来，人类仰望星空并思索：“世间万物从何而来？”对此，宇宙科学家们还没有一个固定的答案。不过，过去的十年中，科学家们不但推演出了一份精确的关于宇宙内含物体的“配方”，而且还说明了这些内容物聚合起来的方式。这一进展把原本的宇宙论从“主要是关于性质的推定”提升到了一种容许其他观念有限度存在的标准的理论，即使它显然还有很多问题没法回答。无论你喜不喜欢，宇宙学家们现在已经或多或少地坚持这种理论了。

If you are cooking along at home, you'll need three ingredients: ordinary matter such as that in stars and planets, mysterious “dark matter” whose gravity binds the galaxies, and even weirder “dark energy” that's stretching space and accelerating the expansion of the universe. According to the latest numbers, the proportions are 4.56% ordinary matter, 22.7% dark matter, and 72.8% dark energy.

自己在家烹饪的时候，你需要三种材料，普通的，神秘的，还有不可思议的。宇宙也一样：普通的物质，如存在于恒星和行星中的那些元素；神秘的“暗物质”用强大的引力把星系捆绑在一起，而不可思议的“暗能量”则起着拉伸空间，加速宇宙扩张的作用。据最新的计算数据，三者间的比例为：普通物质占4.56%，暗物质占22.7%，而暗能量占据了72.8%的最大比例。

Here's how they went together: The universe emerged, superhot and ultradense, in the instant of the big bang. For a tiny fraction of a second it expanded at faster-than-light speed. Known as “inflation,” that growth spurt stretched tiny quantum fluctuations in the density of the newborn universe to colossal scales. The gravity of these “overdense” regions pulled in dark matter, which drew along with it ordinary matter, initially a soup of subatomic particles. Slowly, a vast “cosmic web” of filaments and clumps of dark matter formed, and within them, the galaxies.

以下是它们组合在一起的方式：宇宙浮现之初，在大爆炸发生的瞬间，温度极高，密度极大。在极短的时间内，宇宙超光速地延展开来。这就是“膨胀”，它在密度上延展了新生宇宙那原本细微的量子涨落，使其自身爆发式地增长到巨型的级别。这些“过高密度”区域的引力牵拉着暗物质，它也同时牵拉着普通物质，这便形成了最初的那锅“亚原子粒子汤”。慢慢的，一张巨大的丝状和块状交结的暗物质“宇宙网”形成了，星系便在其中。

Before the turn of the millennium, cosmologists had narrowed the list of cosmic ingredients. Many had suspected the presence of dark matter for decades. Dark energy made a splashy entrance in 1998, when astronomers used stellar explosions called type Ia supernovae to trace the rate of expansion of the universe and found that it was speeding up. The idea of inflation emerged in the 1980s to solve various conceptual puzzles. But only recently have observations enabled scientists to weave these threads into a rigorous theory.

公元2000年以前，宇宙学家们就已经把宇宙成分的范围缩小了。不过在很长一段时间内，许多人都怀疑其中暗物质所占的比例。1998年，暗能量引人注目地进入了这份名单里。当时天文学家们正在观察名为Ia型的超新星爆发，以此来探查宇宙扩张的速率——他们发现速度正在增大。“膨胀”论出现于1980年代，最早是为了回答一系列概念上的迷思。不过，最近的一些观察发现使得科学家们可以把这些信息线索统一到一个严整的理论当中。

Much of the progress has come through studies of the afterglow of the big bang, radiation known as the cosmic microwave background (CMB). In 2000, an experiment known as Balloon Observations of Millimetric Extragalactic Radiation and Geophysics measured the CMB in detail in patches of the sky; a year later, so did the ground-based Degree Angular Scale Interferometer at the South Pole. Then in 2003, NASA's space-based Wilkinson Microwave Anisotropy Probe (WMAP) mapped the CMB across the sky, producing an exquisite baby picture of the cosmos.

这一进展多半来自对大爆炸“余辉”——也就是为人所熟知的，名为“宇宙微波背景辐射”（CMB）的辐射能——的研究。2000年，一项名为“河外微辐射与地球物理气球观测”的实验详细测量了局部天空中的CMB；一年以后，位于南极的陆基角度度量干涉仪也做了同样的测量。之后的2003年，NASA在太空中的“威尔金森微波各向异性探测器”（WMAP）为穿越天空的CMB绘制了映射图，为早期宇宙制做了一幅精确的照片。

It's the myriad dimples in that picture that count. The temperature of the CMB varies by about one part in 100,000 from point to point on the sky, with the hotter spots corresponding to the denser regions in the primordial universe. By measuring the distribution of the spots' sizes and fitting it with their theoretical model, scientists can probe the interplay and amounts of ordinary and dark matter in the early universe. They can also measure the geometry of space. That allows them to deduce the total density of energy and matter in the universe and infer the amount of dark energy.

这张相片上记录有无数的涟漪。约10万块不同的空域，其CBM温度也相应不同。这种变化点对点地发生着，那些温度更高的点对应早期宇宙中密度更高的区域。通过测量这些点的大小分布，并把他们套用进各自的理论模型中，科学家就能推测出早期宇宙中通常物质和暗物质的相互作用和总量。他们还能通过测量空间几何推断出宇宙中能量和物质的总密度，并推算暗能量的总和。

CMB studies show that the universe is flat. That means that if you draw parallel lines in space, then they will remain the same distance apart along their infinite lengths instead of converging or diverging. Such flatness is a central prediction of inflation, so the measurements bolster that wild idea. So does a particular randomness seen in the hot and cold spots. The WMAP team released its latest results in January, 9 months before the satellite shut down.

CMB相关研究证实，宇宙是平的。这意味着如果你在空间中作两条平行线，它们不会聚合或离散，只会一直保持相等的距离无限延伸下去。这种“平坦说”是对“膨胀说”最重要的预言，而各种测量结果也支持这一大胆的想法。CMB图像中热点和冷点独特的随机性也能为这一理论提供支持。最后WMAP项目组宣布了他们最终的研究结果，这距离卫星（也就是WMAP，译注）被关闭还有9个月。

The most jaw-dropping thing about the CMB results is that the cosmologists' model, which has only a half-dozen adjustable parameters, fits the data at all. This isn't some marginal approximation; the theory fits the data like the casing on a sausage. That makes it hard to argue that dark matter and dark energy are not real and to explain them away by, for example, modifying Einstein's theory of gravity. Other cosmological data, such as measurements of the distribution of the galaxies, also jibe nicely with the model.

而CMB研究最让人惊讶的结果是宇宙学家创造的模型，这个模型只有6个可变参数，但却解释了一切。这可不是什么“很接近的推算”，它就像香肠外面的覆膜一样贴合各种数据。这让那种认为暗物质和暗能量不存在的辩驳变得非常困难，更不可能抛开它们去解释任何事情，比如修改爱因斯坦的引力理论。其他宇宙数据，比如星系分布测量等，都和模型保持完美的一致。

All of this leaves cosmologists in a peculiar situation: They have the precise proportions of the ingredients of the universe, but they still do not know what two of the three ingredients—dark matter and dark energy— really are. It's as if they've been handed a brownie recipe that calls for a “sweet granular substance” and a “white powder that mixes with water to make a paste,” instead of specifying sugar and flour.

以上种种让宇宙学家们处于一种特殊的情形中：他们已经掌握了宇宙中各种成分的精确比例，但还是不知道这三种成分中的两种——暗物质和暗能量——到底是什么。这就好比他们手握一份核仁巧克力饼配方，但是上面说，需要一种“甜的颗粒状物质”和一种“能和水混合成为糊状的白色粉末”，而不是明确的“糖和面粉”。

Cosmologists may have a better idea of what they're talking about by the end of the next decade. A particle theory called “supersymmetry” predicts the existence of weakly interacting massive particles (WIMPs), which could be the particles of dark matter. Researchers are optimistic that they may soon detect WIMPs bumping into sensitive detectors deep underground, emerging from high-energy particle collisions at an atom smasher, or producing telltale gamma rays in space. Numerous astronomical surveys will probe space-stretching dark energy, although figuring out what it is could take much longer than another decade.

也许下一个十年，宇宙学家们能够搞清楚他们研究的东西到底是什么。一种被称为“超对称性”的粒子理论预言了“弱作用重粒子”（WIMPs）的存在——它也许就是暗物质粒子。研究人员乐观地认为他们很快就能探测到WIMPs撞击深入地壳的探测器，这种粒子将会出现在粒子加速器内的高能粒子碰撞实验中，或者在宇宙伽马射线里显现出迹象来。大量的天文观测将会探查使宇宙延展的暗能量，即使再花上十数年才有可能搞清它的真面目。

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文中既有中文常用的“天文学家”，也有不常用的“宇宙学家”。因为英语里这两种“家”有所不同。译者不是学天文的，也不是技术宅，还请多指教。

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