本帖最后由 seanwillian 于 2011-4-5 21:24 编辑
How Hydrogen Teaches Us the Temperature of Dark Matter! 氢，暗物质的温度计 Category: Astronomy • Dark Matter • Physics 隶属： 天文学，暗物质，物理 "Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." -Frank Zappa “某些科学家宣称氢是宇宙的基础，理由是氢最多。我才不信，宇宙中有那么多的愚蠢，这才是宇宙的基础。” ——Frank Zappa Oh yeah, Zappa? Well, let me show you what hydrogen can do! Zappa，给力？好吧，让我们看看氢是干什么的！
With just one proton and one electron, hydrogen is not only the simplest of all the atoms in the Universe; it's also the most abundant: over 90% of all the atoms in the Universe are hydrogen atoms!
氢，由一个质子和一个电子构成，是宇宙中最简单的原子。不仅如此，它还是最多的原子，占到宇宙中原子数的90%。
How could such a small thing possibly have anything interesting to tell us about the entire Universe?
但，这么个小东西怎么可能向我们展示整个宇宙？
Well, for starters, hydrogen has very specific energy levels where its electron is allowed to live. Hit it with just the right amount of energy, and it will absorb it, and the electron will jump up to a higher energy level.
首先，氢原子有特定的可供电子运行的能级轨道。适量的能量撞击电子时，电子将吸收能量并跳跃至高能级轨道。
Or, if you let it sit there in a higher energy state, it'll look at those lower -- and more stable -- energy levels, and spontaneously jump down there.
或者，在高能级轨道时，电子会自发地跃至一个低能级轨道，以达到更稳定的状态。
And when they do, they emit radiation! If you jump down to the first energy level, you emit ultraviolet light and belong to the Lyman series; if you jump down to the second energy level, you emit (mostly) visible light and belong to the Balmer series; while if you jump down to the third, you give off infrared light and belong to the Paschen series.
如果向低能级轨道迁跃，电子就会释放辐射。迁跃到一级轨道，辐射是紫外线，属于拉曼谱系；二级轨道，辐射就是可见光（绝大多数情况），属于巴尔默谱系；三级轨道，辐射是红外线，属于帕邢谱系。
In fact, the jump from the third to the second -- the Balmer alpha line -- is so strong that if you look through a telescope at a galaxy that's forming stars: it's that line that causes the galaxy to glow red! In the case of the Whirlpool Galaxy, above, you can see exactly where along its great spiral arms it's presently forming stars, just from this red glow of the hydrogen!
事实上，电子从三级轨道迁跃至二级辐射出的巴尔默α线是最易实现的，以至于如果你通过望远镜观察星系恒星形成区，你会发现那里是呈现红色的。对于漩涡星系，如上图所示，通过氢发出的红色光，你很容易就能区分出旋臂内的恒星形成区。
But most of the hydrogen in the Universe isn't in some exciting, star-forming region. Most of it's in the cool, boring depths of space, sitting around in its lowest energy state.
可惜，宇宙中只有很少一部分氢原子存在于引人注目的恒星形成区。绝大部分都只是游荡于寒冷寂寞的深空中，在最低能级状态下终日无所事事。
And if you're hydrogen in its ground state, you're just waiting for some light of just the right energy to come along and -- ever so briefly -- to give you a ride up to the next exciting energy level!
如果你是处于基态的氢原子，能做的就是等待，等待一束光，一束特定能量的光线。哪怕是短暂的一瞬，它就能让你鲤跃龙门，进入高能级。
(Image credit: Joanne Cohn.)
What's hydrogen's best friend in this case? Ultraviolet light of a wavelength 1216 Angstroms, known as the Lyman-alpha line, or the right amount of energy to kick it up from the ground state to the first excited state! Of course, there are other excited states, and they make absorption lines too, but the Lyman-alpha line is the strongest one.
此时，氢原子最佳损友是谁？是波长为1216埃（1埃等于1亿分之一厘米）的紫外光，也就是上文提到的莱曼α线，或者说是能把电子从基态一脚提到一级激发态的特定能量。当然，还有其他的激发态，也能产生吸收线，只不过莱曼α线是最佳的。
So why should I care? And moreover, how can something as mundane as this super-simple atom emitting and absorbing light teach me anything about dark matter?!
那么，我干嘛要关心呢？此外，氢原子吸收与辐射光线这些超级简单的玩意能告诉我关于暗物质的什么信息呢？
Well, we know where light comes from in the Universe: from stars and galaxies!
其实吧，我们也就知道宇宙中的光是哪来的：恒星和星系。
(Image credit: R. Windhorst, S. Driver, W. Keel, and NASA.)
Well, my Universe isn't just empty; there ought to be clumps of this hydrogen gas all over the place! And wherever my light from these distant galaxies passes through these clumps of gas, that neutral hydrogen will leave its mark by absorbing that 1216 Angstrom light.
我们的宇宙也不是看起来那样的空洞无物，起码到处都是氢气构成的云块。无论光从何而来，有多远，都要穿越氢气云。必然的，基态的氢原子就会吸收波长是1216埃的光线进入激发态。
(Image credit: Ned Wright.)
But because of the expansion of the Universe, this light gets redshifted! In other words, you place cold, neutral hydrogen gas at different distances away from us, in between us and a distant galaxy, and it will leave absorption lines at different wavelengths!
然而，由于宇宙的扩张，光线出现了红移。换句话说，你待的地方冷，处于我们与遥远星系之间的基态氢原子随着不同的距离形成的吸收线波长是不一样的。
What we basically do is take a spectrum of distant galaxies using a super-powerful telescope (like Hubble), and see where we have clumps of hydrogen gas along the way.
我们主要的工作就是使用超级望远镜（比如哈勃太空望远镜）收集星系的光谱，看看在观察方向上那些氢气云都在哪。
If you look at something nearby, you'll only have a few clumps of gas in between you and the object you're observing. But if you look at something very far away, you're likely to get a whole slew of absorption lines! For very distant objects, there are so many clumps of gas that the lines we see are known as the Lyman-alpha forest!
如果距离很近，那肯定只有很少的氢气云会被观察到；如果距离遥远，你能观察到一堆吸收线。在特定的距离上，我们能获得的吸收线就是莱曼α森林。
(Image credit: Bob Carswell.)
Now, here's where it gets interesting! Because when we look at things that are farther away, we're also looking back in time! And if we want to get these big, deep absorption lines happening far away, we need to have dense, collapsed clouds of gas.
现在，你知道有意思的地方了吧。因为观察一个遥远的物体时，我们实际也是在回顾时间。若想知道更多更深层的吸收线，我们需要浓密的已经收缩的气体云。
Guess what? That tells you something about your dark matter! Because if you want to make something that's dense and collapsed, it can't be moving too quickly. In astrophysics, if you're moving quickly, we call you hot, and if you're moving slowly, we call you cold.
猜猜看，这说明了什么？暗物质。因为如果物质密实收缩，那么它就不能移动的很快。在天体物理学中，如果移动的快，我们称之为“热”，相反，慢的就是“冷”。
For dark matter, the cosmic microwave background doesn't care whether you're hot or cold. But structure does, and the Lyman-alpha forest is very sensitive to it! If dark matter were hot (or even if it were too warm), the forest would be too shallow; in other words, hot dark matter makes it too hard to form small-scale structure at early times.
对于暗物质，宇宙微波背景辐射才不管你的冷热。但结构需要，而且莱曼α森林对此极为敏感。如果暗物质是热的（或者即便只是有点暖），森林就会很暗淡，也即由于热的暗物质存在，宇宙早期很难形成小尺度结构。
But we see the evidence of this small-scale structure directly in the Lyman-alpha forest! What does this tell us?
可是，现实是根据莱曼α森林，我们看到的更多的是小尺度结构。这代表了什么？
(Image credit: Benedetta Ciardi.)
It tells us that dark matter can be WIMPs (like from supersymmetry), because they're too massive to move quickly, or they can be particles that are born cold, like axions or (some) sterile neutrinos, because they started off moving slowly. But they can't be regular neutrinos or hot sterile neutrinos, among others, because this small-scale structure -- and hence the hydrogen lines that we see -- would get washed out at early times!
这意味着暗物质是一种大质量且运动缓慢的弱相互作用重离子（Weakly Interacting Massive Particle）（就像超对成性），也可能是天生就“冷”的一类粒子，就像轴子或惰性中微子，因为它们从一开始移动的就很慢。但它不可能是常规或者热惰性中微子，如果是则小尺度结构在宇宙早期就已经被淘汰，而我们也不可能观测到现在的氢吸收线。（这一段翻译的对不对我也不知道）
So just like that, from looking at hydrogen, we can tell how cold our Universe's dark matter has to be. And that's how hydrogen teaches us the temperature of dark matter!
就像这样，通过观察氢原子，我们能够知道宇宙中暗物质有多冷，也即指示暗物质的温度。 |
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