You might think the answer is obvious - the sun isn't up! But the only reason the sky

你可能認為答案很明顯--太陽沒有升起!但天空的唯一原因是

looks blue during the day is that sunlight scatters off of the atmosphere - if we didn’t

白天看起來是藍色的，是因為陽光從大氣層中散射出來--如果我們不這樣做的話

have an atmosphere (like on the moon), the sky would always be dark, even when the sun

有大氣層的時候，即使在太陽底下，天空也是一片漆黑

is shining. So let’s rephrase the question - why is space dark?

是發光的。所以，我們換個問題--為什麼空間是黑暗的？

I mean, space is full of stars - countless stars which are all about as bright as the

我的意思是，太空中到處都是星星--無數的星星，它們的亮度都和

sun, and in an infinite eternal universe, no matter what direction you picked, if you

太陽，在無限的永恆宇宙中，無論你選擇什麼方向，如果你

looked far enough in that direction, you would see a star or galaxy. So the whole sky should

從這個方向看去，你會看到一顆恆星或星系。所以整個天空應該

be as bright as the sun, night and day!

燦若驕陽，夜以繼日!

And since it's not, does the darkness of the night sky mean that there's some distance

既然不是，那麼夜空的黑暗是否意味著有一些距離呢？

away from us when stars and galaxies just… stop? A boundary between something and nothing?

當恆星和星系... 停下來的時候，我們就離開了？在什麼和什麼之間的界限？

An "edge" to the universe?

一個"邊緣"對宇宙的?

Not exactly - all of our evidence seems to indicate that space has no edge. But the universe

不完全是--我們所有的證據似乎都表明，空間沒有邊緣。但宇宙

itself does - not a spatial edge, but a temporal one: as far as we know, the universe had

本身也是如此--不是空間邊緣，而是時間邊緣：據我們所知，宇宙有

a beginning. Or at least, a time about 13.7 billion years ago when the universe was so

一個開始或者至少是在137億年前 宇宙是如此的脆弱不堪

small and crumpled-up with itself that our standard notion of space and time breaks down.

渺小而又皺巴巴的自己，我們的標準空間和時間概念被打破了。

And since only a finite amount of time has passed since this so-called beginning, that

由於這個所謂的開始只過去了有限的時間，所以，這

means that some of the stars necessary to fill up the "brightness in every direction"

意味著一些恆星必要充實"各個方向的亮度&quot。

are so far away that light from them plain hasn't had time to reach us yet… it's as

是如此之遠，從他們平原的光還沒有來得及到達我們... ... 它是作為作為

if the universe were a big thunderstorm and we're still waiting to hear the thunder from

如果宇宙是一個大雷雨，我們'仍然等待聽到雷聲從

the really distant stars.

真正遙遠的星星。

But wait, it’s better than that - since light takes time to travel across the universe,

但是，等等，比這更好的是--因為光在宇宙中旅行需要時間。

when we point our telescopes at something really far away, we're actually seeing that

當我們把望遠鏡對準遠處的東西時，我們實際上看到的是...

part of universe as it was when the light was emitted. So when we look at 13.5 billion-year-old

宇宙的一部分，因為它是 當光被髮射。所以當我們看135億年前的宇宙時

light, it's not that we don't see stars just because light from them hasn't gotten to us

光，它'不是我們不'看不到星星只是因為從他們的光沒有得到我們。

yet - we don't see any stars because we’re getting a peek at the universe before any

然而 - 我們不'看到任何恆星，因為我們得到一個窺視宇宙之前任何

stars had formed! A star-less universe! Now that sounds to me like a pretty good reason

恆星已經形成!一個沒有恆星的宇宙！現在，這聽起來像是一個很好的理由

why we look up and see a dark night sky.

為什麼我們抬頭看到的是一片漆黑的夜空。

But… it’s not. I mean, it is true that we can find points in the sky where there

但是... 不是這樣的我的意思是，這是真的，我們可以找到點 在天空中，那裡有...

aren’t any stars by looking past the earliest stars and thus farther back in time. But even

沒有任何恆星，通過看過去最早的恆星，從而更遠的時間。但即使是

when we point our telescopes past the earliest stars, we still see light. Not starlight,

當我們把望遠鏡對準最早的恆星時，我們仍然能看到光。不是星光

but the light left over from the big bang. And we detect this “cosmic background radiation”

但大爆炸後留下的光。而我們檢測到的這種 "宇宙背景輻射"

coming more or less evenly from all directions, forming a background beyond the stars. So,

從四面八方或多或少地均勻湧來，形成了恆星之外的背景。所以

I guess the night sky ISN’T actually dark to begin with.

我想，夜空其實並不是一開始就黑暗的。

Right... so if our telescopes tell us that the night sky isn’t dark, then why does

好吧... 所以如果我們的望遠鏡告訴我們夜空並不黑暗，那麼為什麼... ...

it look dark?

它看起來很黑？

Here’s a clue to the real answer: when the Hubble telescope photographed the distant

這裡有一個真正答案的線索：當哈勃望遠鏡拍攝到遙遠的。

stars of the astoundingly beautiful Hubble extreme deep field, it took the picture using

恆星的驚人美麗的哈勃極深場，它拍攝的照片使用的是

an infrared camera. Why? Well, distant stars and galaxies are moving away from us because

紅外線攝影機。為什麼呢? Why? 遙遠的恆星和星系正在遠離我們 Well, distant stars and galaxies are moving away from us because.遙遠的恆星和星系正在遠離我們，因為...

the universe is expanding. So the same way a record slowing down lowers the pitch of

宇宙正在膨脹是以，同樣的方式，一個記錄慢下來降低了音調的

my voice, the doppler effect causes stars moving away from us to become redder, and

我的聲音，多普勒效應會使遠離我們的恆星變得更紅，並且...

the farther away they are, the faster they move away from us and the redder they become,

它們離我們越遠，它們離我們越快，它們變得越紅。

until they become… infrared. And then we can't see them any more. At least not with

直到它們變成... 紅外線然後我們就看不到它們了 And then we can't see them any more.至少不是用...

our human eyes - and that’s why the night sky appears dark!

我們人類的眼睛--這就是為什麼夜空看起來很黑的原因!

In summary: If we lived in an infinite, unchanging universe, the entire sky would be as bright

總結：如果我們生活在一個無限的、不變的宇宙中，那麼整個天空就會像

as the sun. But the sky is dark at night, both because the universe had a beginning

作為太陽。但夜晚的天空是黑暗的，這既是由於宇宙有一個起點

so there aren't stars in every direction and more importantly because the light from super

所以並不是每個方向都有恆星，更重要的是，因為光從超級

distant stars (and the even-more-distant cosmic background radiation) gets red-shifted away

遙遠的恆星（以及更遙遠的宇宙背景輻射）會被紅移開來

from the visible spectrum by the expansion of the universe, so we just plain can't see

從可見光光譜由宇宙的擴張， 所以我們只是普通的可以不看

it.

它。

Finally! We’ve shed some light on why the night sky is dark. And why it isn’t.

終於！我們終於明白了為什麼夜空是黑暗的。還有為什麼它不是。