Why is the sky dark at night?夜空為什么是黑的？

You might think the answer is obvious - the sun isnt up! But the only reason the sky looks blue during the day is that sunlight scatters off of the atmosphere - if we didnt have an atmosphere (like on the moon), the sky would always be dark, even when the sun is shining. So lets rephrase the question - why is space dark?你可能認為答案很簡單——因為沒有太陽嘛！但白天的天空之所以是藍色的，完全因為大氣層把陽光散射了。而如果沒有大氣層，就比如月球，那么天空就一直都是黑的，即便有太陽照射，那就讓我們換種問法——為什么太空是黑色的？

I mean, space is full of stars - countless stars which are all about as bright as the sun, and in an infinite eternal universe, no matter what direction you picked, if you looked far enough in that direction, you would see a star or galaxy. So the whole sky should be as bright as the sun, night and day!太空里充滿了各種各樣的恒星，每一顆的亮度都和太陽一樣，而在無窮無盡的宇宙中，不管你選擇任何一個方向，只要你走得足夠遠，你都能遇到一顆恒星，或者一個星系，于是天空就應該是亮如白晝的，不論白天還是黑夜。And since its not, does the darkness of the night sky mean that theres some distance away from us when stars and galaxies just… stop? A boundary between something and nothing?但是事實卻并非如此，那么，黑色的夜空是否告訴了我們，在一定的距離以外，就沒有恒星或者星系了嗎？是不是存在一個有星星和沒有星星的邊界？

An edge to the universe?宇宙有邊際嗎？

Not exactly - all of our evidence seems to indicate that space has no edge. But the universe itself does - not a spatial edge, but a temporal one: as far as we know, the universe had a beginning. 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.并非如此。幾乎所有的現有證據都表明，宇宙空間是沒有邊界的。但宇宙本身卻有邊界——并非是空間上的邊界，而是時間上的。 宇宙是有一個起源的，至少137億年前的宇宙是很小，高度壓縮的。以至于我們現在對時間和空間的概念都不適用了。And since only a finite amount of time has passed since this so-called beginning, that means that some of the stars necessary to fill up the brightness in every direction are so far away that light from them plain hasnt had time to reach us yet… its as if the universe were a big thunderstorm and were still waiting to hear the thunder from the really distant stars.而從這個所謂的起源開始，僅僅經過了有限的時間，因此要造就一個明亮的夜空所需要的有些恒星距離我們實在太遠，他們發出的光亮還沒有到達地球呢。如果把宇宙比作一個巨大的雷陣雨，那么我們還在等待著那些遙遠恒星的雷聲，它們還沒達到地球呢。But wait, its better than that - since light takes time to travel across the universe, when we point our telescopes at something really far away, were actually seeing that part of universe as it was when the light was emitted. So when we look at 13.5 billion-year-old light, its not that we dont see stars just because light from them hasnt gotten to us yet - we dont see any stars because were getting a peek at the universe before any stars had formed! A star-less universe! Now that sounds to me like a pretty good reason why we look up and see a dark night sky.但是稍等，似乎還有更好的解釋，既然光穿越宇宙需要時間。當我們把望遠鏡對著一些很遙遠的星星時，我們看到的宇宙，其實還是在光發出時候的樣子，因此當我們看到135億年前發出的光，并不是由于恒星本身不存在，而只是它發出的光還沒有達到地球而已。我們看不到任何星星，那是因為我們窺探到的宇宙，那些星星都還沒有形成呢。一個沒有星星的宇宙！這看似是一個很好的解釋，解釋了為什么夜空是黑色的。But… its not. I mean, it is true that we can find points in the sky where there arent any stars by looking past the earliest stars and thus farther back in time. 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. And we detect this “cosmic background radiation” coming more or less evenly from all directions, forming a background beyond the stars.但是當我們把望遠鏡對著這些古老的星星時，我們卻的確能看到光，不是星光而是大爆炸時留下的光，并且我們可以觀察到宇宙的背景輻射。從各種方向幾乎完全均勻地發射，形成了所有星星的背景。So, I guess the night sky ISNT actually dark to begin with. Right... so if our telescopes tell us that the night sky isnt dark, then why does it look dark?因此我覺得，夜空從一開始就不是黑的，而既然望遠鏡告訴我們，夜空不是黑的，那它為什么看上去是黑的呢？Heres a clue to the real answer: when the Hubble telescope photographed the distant stars of the astoundingly beautiful Hubble extreme deep field, it took the picture using an infrared camera. Why? Well, distant stars and galaxies are moving away from us because the universe is expanding. 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, and the farther away they are, the faster they move away from us and the redder they become, until they become… infrared. And then we cant see them any more. At least not with our human eyes - and thats why the night sky appears dark!這里有個提示：當哈勃望遠鏡拍下那些最古老的恒星時，當它拍下那些極其壯美的極深場圖片時，它使用的是紅外線照相機，為什么呢？我們知道，星星都在向遠離我們的方向移動著。因為宇宙是在膨脹的。就如同如果慢放，我的聲音就會變低沉，多普勒效應使得那些遠離我們的恒星變得更紅離我們越遠的恒星，移動速度越快，因此也就越紅。知道它們發出的光都變成了紅外線，因此我們看不見它們。至少用肉眼是看不到的，因此夜空是黑的！In summary: If we lived in an infinite, unchanging universe, the entire sky would be as bright as the sun. But the sky is dark at night, both because the universe had a beginning so there arent stars in every direction and more importantly because the light from super distant stars (and the even-more-distant cosmic background radiation) gets red-shifted away from the visible spectrum by the expansion of the universe, so we just plain cant see it.總結一下，如果我們生活在一個無限的，不變的宇宙里面，那么夜空就一定是完全亮的。但夜空是黑的，因此我們推斷，宇宙有一個起源。所以并不是所有方向上都有星光。而更重要的是，那些來自遙遠恒星——甚至更遙遠的宇宙的背景輻射，由于宇宙的膨脹，都在可見光譜上發生了紅移，因此我們看不見這些光。Finally! Weve shed some light on why the night sky is dark. And why it isnt.至此，我們簡單解釋了為何夜空看上去是黑的，但它卻其實卻并不是黑的。

Why is the sky dark at night?夜空為什么是黑的？

You might think the answer is obvious - the sun isnt up! But the only reason the sky looks blue during the day is that sunlight scatters off of the atmosphere - if we didnt have an atmosphere (like on the moon), the sky would always be dark, even when the sun is shining. So lets rephrase the question - why is space dark?你可能認為答案很簡單——因為沒有太陽嘛！但白天的天空之所以是藍色的，完全因為大氣層把陽光散射了。而如果沒有大氣層，就比如月球，那么天空就一直都是黑的，即便有太陽照射，那就讓我們換種問法——為什么太空是黑色的？

I mean, space is full of stars - countless stars which are all about as bright as the sun, and in an infinite eternal universe, no matter what direction you picked, if you looked far enough in that direction, you would see a star or galaxy. So the whole sky should be as bright as the sun, night and day!太空里充滿了各種各樣的恒星，每一顆的亮度都和太陽一樣，而在無窮無盡的宇宙中，不管你選擇任何一個方向，只要你走得足夠遠，你都能遇到一顆恒星，或者一個星系，于是天空就應該是亮如白晝的，不論白天還是黑夜。And since its not, does the darkness of the night sky mean that theres some distance away from us when stars and galaxies just… stop? A boundary between something and nothing?但是事實卻并非如此，那么，黑色的夜空是否告訴了我們，在一定的距離以外，就沒有恒星或者星系了嗎？是不是存在一個有星星和沒有星星的邊界？

An edge to the universe?宇宙有邊際嗎？

Not exactly - all of our evidence seems to indicate that space has no edge. But the universe itself does - not a spatial edge, but a temporal one: as far as we know, the universe had a beginning. 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.并非如此。幾乎所有的現有證據都表明，宇宙空間是沒有邊界的。但宇宙本身卻有邊界——并非是空間上的邊界，而是時間上的。 宇宙是有一個起源的，至少137億年前的宇宙是很小，高度壓縮的。以至于我們現在對時間和空間的概念都不適用了。And since only a finite amount of time has passed since this so-called beginning, that means that some of the stars necessary to fill up the brightness in every direction are so far away that light from them plain hasnt had time to reach us yet… its as if the universe were a big thunderstorm and were still waiting to hear the thunder from the really distant stars.而從這個所謂的起源開始，僅僅經過了有限的時間，因此要造就一個明亮的夜空所需要的有些恒星距離我們實在太遠，他們發出的光亮還沒有到達地球呢。如果把宇宙比作一個巨大的雷陣雨，那么我們還在等待著那些遙遠恒星的雷聲，它們還沒達到地球呢。But wait, its better than that - since light takes time to travel across the universe, when we point our telescopes at something really far away, were actually seeing that part of universe as it was when the light was emitted. So when we look at 13.5 billion-year-old light, its not that we dont see stars just because light from them hasnt gotten to us yet - we dont see any stars because were getting a peek at the universe before any stars had formed! A star-less universe! Now that sounds to me like a pretty good reason why we look up and see a dark night sky.但是稍等，似乎還有更好的解釋，既然光穿越宇宙需要時間。當我們把望遠鏡對著一些很遙遠的星星時，我們看到的宇宙，其實還是在光發出時候的樣子，因此當我們看到135億年前發出的光，并不是由于恒星本身不存在，而只是它發出的光還沒有達到地球而已。我們看不到任何星星，那是因為我們窺探到的宇宙，那些星星都還沒有形成呢。一個沒有星星的宇宙！這看似是一個很好的解釋，解釋了為什么夜空是黑色的。But… its not. I mean, it is true that we can find points in the sky where there arent any stars by looking past the earliest stars and thus farther back in time. 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. And we detect this “cosmic background radiation” coming more or less evenly from all directions, forming a background beyond the stars.但是當我們把望遠鏡對著這些古老的星星時，我們卻的確能看到光，不是星光而是大爆炸時留下的光，并且我們可以觀察到宇宙的背景輻射。從各種方向幾乎完全均勻地發射，形成了所有星星的背景。So, I guess the night sky ISNT actually dark to begin with. Right... so if our telescopes tell us that the night sky isnt dark, then why does it look dark?因此我覺得，夜空從一開始就不是黑的，而既然望遠鏡告訴我們，夜空不是黑的，那它為什么看上去是黑的呢？Heres a clue to the real answer: when the Hubble telescope photographed the distant stars of the astoundingly beautiful Hubble extreme deep field, it took the picture using an infrared camera. Why? Well, distant stars and galaxies are moving away from us because the universe is expanding. 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, and the farther away they are, the faster they move away from us and the redder they become, until they become… infrared. And then we cant see them any more. At least not with our human eyes - and thats why the night sky appears dark!這里有個提示：當哈勃望遠鏡拍下那些最古老的恒星時，當它拍下那些極其壯美的極深場圖片時，它使用的是紅外線照相機，為什么呢？我們知道，星星都在向遠離我們的方向移動著。因為宇宙是在膨脹的。就如同如果慢放，我的聲音就會變低沉，多普勒效應使得那些遠離我們的恒星變得更紅離我們越遠的恒星，移動速度越快，因此也就越紅。知道它們發出的光都變成了紅外線，因此我們看不見它們。至少用肉眼是看不到的，因此夜空是黑的！In summary: If we lived in an infinite, unchanging universe, the entire sky would be as bright as the sun. But the sky is dark at night, both because the universe had a beginning so there arent stars in every direction and more importantly because the light from super distant stars (and the even-more-distant cosmic background radiation) gets red-shifted away from the visible spectrum by the expansion of the universe, so we just plain cant see it.總結一下，如果我們生活在一個無限的，不變的宇宙里面，那么夜空就一定是完全亮的。但夜空是黑的，因此我們推斷，宇宙有一個起源。所以并不是所有方向上都有星光。而更重要的是，那些來自遙遠恒星——甚至更遙遠的宇宙的背景輻射，由于宇宙的膨脹，都在可見光譜上發生了紅移，因此我們看不見這些光。Finally! Weve shed some light on why the night sky is dark. And why it isnt.至此，我們簡單解釋了為何夜空看上去是黑的，但它卻其實卻并不是黑的。