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Why is the sky dark at night? You might think the answer is obvious - the sun isn't up!
为什么夜晚的天空很暗？你可能会认为，答案很明显-因为太阳落山了！
But the only reason the sky looks blue during the day is that sunlight scatters off of the atmosphere - if we didn't have an atmosphere (like on the moon),
但是太空在白天看起来是蓝色的唯一原因是阳光从大气中散射——如果没有大气（如在月球上）
the sky would always be dark, even when the sun is shining. So let's rephrase the question - why is space dark?
那么天空将一直是黑暗的，即便太阳升起也是如此。所以让我们换个说法，为什么太空是黑暗的？
I mean, space is full of stars - countless stars which are all about as bright as the sun, and in an infinite eternal universe,
我是说，太空中那么多行星——数不清的行星和太阳一样亮，并且在一个无限永恒的宇宙中，
no matter what direction you picked, if you looked far enough in that direction, you would see a star or galaxy.
不论你选择那个方向，如果你朝那个方向看得足够远，你就会看到一颗恒星或星系。
So the whole sky should be as bright as the sun, night and day!
所以无论白天黑夜，整个天空都应该像太阳一样明亮！
And since it's not, does the darkness of the night sky mean that there's some distance away from us when stars and galaxies just…stop?
既然不是，那么夜空的黑暗是否意味着，遥远的恒星和星系到某个地方就戛然而止了？
A boundary between something and nothing? An 'edge' to the universe?
那里就是万物和虚无的界限？宇宙的边际？
Not exactly - all of our evidence seems to indicate that space has no edge.
其实不是——我们所有的证据似乎表明太空没有边界。
But the universe itself does - not a spatial edge, but a temporal one: as far as we know, the universe had a beginning.
但宇宙本身有——不是空间上的边界，而是时间上的边界：就我们所知，宇宙有一个开端。
Or at least, a time about 13.7 billion years ago when the universe was so small and crumpled-up with itself that our standard notion of space and time breaks down.
或者至少在约137亿年前，宇宙还挤在炽热致密的一团，以至于我们对于空间和时间的标准概念都被打破了。
And since only a finite amount of time has passed since this so-called beginning,
由于从所谓的开始到现在只有一段有限的时间，
that means that some of the stars necessary to fill up the 'brightness in every direction'
那意味着一些本该照亮夜空的恒星的光
are so far away that light from them plain hasn't had time to reach us yet… it's as if the universe were a big thunderstorm and
因距离地球太远还没来得及到达地球，就好像宇宙下了一场暴风雨
we're still waiting to hear the thunder from the really distant stars. But wait, it's better than that - since light takes time to travel across the universe,
而我们还在等着听遥远星空传来的雷声。但等等，我们的情况并没那么惨——因为光线穿过空间需要时间
when we point our telescopes at something really far away, we're actually seeing that part of universe as it was when the light was emitted.
当我们的望远镜瞄准远处时，我们实际上观测到的是这束光刚被发出时的状态。
So when we look at 13.5 billion-year-old light, it's not that we don't see stars just because light from them hasn't gotten to us yet -
所以我们看到一束135亿年的光时，我们看不到恒星不是因为它的光还没有传播到地球——
we don't see any stars because we're getting a peek at the universe before any stars had formed!
而是因为我们观测到的宇宙就是所有星星的形成之前的样子！
A star-less universe! Now that sounds to me like a pretty good reason why we look up and see a dark night sky. But… it's not.
一个没有行星的宇宙！我觉得这是一个很好的理由来解释为什么我们会抬头看到漆黑的夜空。但是…并不是。
I mean, it is true that we can find points in the sky where there aren't any stars by looking past the earliest stars and thus farther back in time.
我是说，我们的确可以越过最早的恒星，找到天空中没有任何恒星的地方。
But even when we point our telescopes past the earliest stars, we still see light. Not starlight, but the light left over from the big bang.
但即使我们望过了最早的恒星，我们还是能看到光。不是恒星的光而是宇宙大爆炸的余光。
And we detect this "cosmic background radiation" coming more or less evenly from all directions, forming a background beyond the stars.
我们发现这种“宇宙背景辐射”在各个方向几乎都是均匀的，形成了恒星外的背景。
So, I guess the night sky ISN'T actually dark to begin with. Right... so if our telescopes tell us that the night sky isn't dark, then why does it look dark?
所以我想夜空一开始就不是黑的。那既然望远镜告诉我们夜空不是黑的，为什么它看起来是黑的呢？
Here's a clue to the real answer: when the Hubble telescope photographed the distant stars of the astoundingly beautiful Hubble extreme deep field,
以下是正确答案的线索：当哈勃望远镜在给遥远的恒星拍摄哈勃深空摄影时，
it took the picture using an infrared camera. Why? Well, distant stars and galaxies are moving away from us because the universe is expanding.
用的是红外线摄像机。为什么？其实因为宇宙膨胀，那些遥远的行星和星系正离我们远去。
So the same way a record slowing down lowers the pitch of my voice, the doppler effect causes stars moving away from us to become redder,
就像唱片播放越慢声音音调降低一样，多普勒效应使得远离我们的恒星光谱红移
and the farther away they are, the faster they move away from us and the redder they become, until they become… infrared.
并且离我们越远的恒星，远离我们的速度越快，最终其发出的光红移变成红外线
And then we can't see them any more. At least not with our human eyes - and that's why the night sky appears dark!
然后我们就再也看不见了。至少用肉眼看不见，这就是夜空黑暗的正解！
In summary: If we lived in an infinite, unchanging universe, the entire sky would be as bright as the sun.
总结：如果我们在一个永恒无限且一成不变的宇宙中，整个天空都会像太阳一样明亮。
But the sky is dark at night, both because the universe had a beginning so there aren't stars in every direction
但晚上天会变黑，因为宇宙有开端，所以不是每个方向都有恒星
and more importantly because the light from super distant stars (and the even-more-distant cosmic background radiation)
更重要的是。因为超远距离的恒星的光（和更远处的宇宙背景辐射）
gets red-shifted away from the visible spectrum by the expansion of the universe, so we just plain can't see it.
随着宇宙膨胀红移出了可见光谱，所以肉眼看不见。
Finally! We've shed some light on why the night sky is dark. And why it isn't.
最终我们弄清楚了为什么夜空是黑的以及为什么不是黑的。
