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The universe is expanding.
宇宙正在膨胀。
If you zoom out far enough, you’ll find that everywhere in the universe is moving away from everywhere else.
如果你镜头拉得足够远，你会发现宇宙中的每一个地方都在远离其他地方。
But once you know the universe is expanding, you might wonder if it’s going to keep expanding somehow,
但是一旦你知道宇宙在膨胀，你可能想知道它是否会以某种方式继续膨胀，
or eventually stop and crunch in on itself because of the attractive force of gravity.
或者最终因为重力的吸引力而停止并收缩。
Well, all you have to do is point your telescope at a distant galaxy, measure how fast it’s moving away from us,
你要做的就是把望远镜对准一个遥远的星系，测量它远离我们的速度，
wait about a million years, and then do the same thing again to see if the galaxy is moving faster or slower than before.
等待大约100万年，然后再做同样的事情，看看星系的移动速度是比以前快了还是慢了。
But if you don’t have a million years to spare, here’s what you do.
但如果你没有一百万年的空闲时间，你就得这么做。
You look at different galaxies.
你可以观察不同的星系。
Because light takes time to reach us, when we look at a galaxy far, far away, we see it as it was a long time ago.
因为光到达我们这里需要时间，所以当我们观察一个遥远的星系时，我们看到的是它很久以前的样子。
So looking at a bunch of galaxies different distances away is kind of a rough proxy for looking at the same one at different times.
所以观察一堆距离不同的星系就像是在不同时间观察同一个星系。
The light from the stars in a distant galaxy can tell us two basic things.
遥远星系中恒星发出的光可以告诉我们两件基本的事情。
First, its speed: light from anything moving away from you becomes redder in color (kinda like how police sirens get lower in pitch when moving away).
首先，它的速度: 任何远离你的物体发出的光会变得更红(有点像警笛在远离你时音调变低)。
This redshifting of light tells us exactly how fast stars are moving away from us.
光的红移告诉我们恒星远去我们的速度有多快。
If the light from a star is 5 percent redder than it should be, it means the star is moving away from us at 5 percent the speed of light.
如果一颗恒星发出的光比正常情况下的红5%，这意味着这颗恒星正以5%的光速远离我们。
Light can also tell us how far away a star (and thus its parent galaxy) is.
光也可以告诉我们一颗恒星(以及它的母星系)有多远。
When you’re driving at night, you can tell how far away a motorcycle is by comparing how bright its headlight seems to you,
当你在晚上开车时，你可以通过比较摩托车前灯的亮度来判断它有多远，
to how bright it would be if you were right in front of it.
如果你站在它面前，它会多么明亮。
The same trick works with stars.
同样的把戏也适用于星星。
There’s a rare kind of supernova explosion that’s like a standard interstellar headlight
有一种罕见的超新星爆炸就像一个标准的星际前灯
– it’s about 5 billion times brighter than our sun, and, crucially, it’s very consistent in its brightness.
——它的亮度大约是太阳的50亿倍，关键是，它的亮度非常一致。
These only happen a few times in a galaxy every thousand years, but there are enough galaxies in the universe that,
这在一个星系中每千年只会发生几次，但宇宙中有足够多的星系，
if you look closely at a wide enough patch of sky, it’s possible to see a dozen of them go off in a month.
如果你近距离观察一片足够宽的天空，就有可能在一个月内看到十几次爆炸。
By comparing the brightness as seen here on Earth with how bright we know each of them should be, we can work out how far away each supernova is.
通过比较我们在地球上看到的亮度和我们所知道的每颗超新星的亮度，我们可以算出每颗超新星距离我们有多远。
The dimmer its light, the farther away it is.
它的光越暗，它离我们就越远。
So the color of a type Ia supernova tells us how fast it’s moving away from us,
所以超新星的颜色告诉我们它远离的速度有多快，
and its brightness is like a ruler, telling us how far away it is
它的亮度就像一把尺子，告诉我们它有多远
– or, because light takes time to get here, how long ago we’re measuring its speed.
或者，因为光需要时间才能到达这里，我们多久以前测量了它的速度。
By measuring the speed of different supernovae at different distances
通过测量不同距离上不同超新星的速度
– aka different times in the past – we can plot the expansion of the universe over time.
-也就是过去的不同时间-我们可以画出宇宙随时间的膨胀。
A straight line means the universe is expanding at a constant rate, a curve means it’s speeding up or slowing down.
一条直线意味着宇宙在以恒定的速率膨胀，一条曲线意味着它在加速或减速。
For a long time we thought the line was straight, or slightly curved in the slowing down direction.
在很长一段时间里，我们认为这条线是直线，或者在减速方向上有轻微的弯曲。
But when astronomers finally looked carefully at supernova in super-distant galaxies,
但当天文学家最终仔细观察超遥远星系中的超新星时，
they discovered, to their surprise, that the universe is expanding faster now than it was long ago.
令他们惊讶的是，他们发现现在的宇宙比很久以前膨胀得更快。
The universe’s expansion is accelerating!
宇宙正在加速膨胀!
We have a few basic ideas for why the universe is accelerating, but we still don’t really know why it’s happening.
对于宇宙为什么在加速，我们有一些基本的想法，但我们仍然不知道它为什么会发生。
One of the simplest and most popular ideas is that there’s an everywhere-permeating vacuum energy with negative pressure
一个最简单和最流行的观点是，存在一种渗透各处的负压真空能量
which we’ve never detected in any way other than by measuring the expansion of the universe... so we call it dark energy.
除了测量宇宙膨胀之外，我们从未通过任何方式发现过…… 所以我们称之为暗能量。
Thanks to the Wide Field Infrared Survey Telescope project at NASA Goddard Space Flight Center
多亏了美国国家航空和宇宙航行局戈达德太空飞行中心的广域红外调查望远镜项目
and the Space Telescope Science Institute for supporting this video.
以及太空望远镜科学研究所对本期视频的支持。
WFIRST is planned to launch in the mid-2020s, and will, among other things,
广域红外调查望远镜项目计划在本世纪20年代中期启动，除其他外，
allow us to see a much wider swath of the sky at Hubble-like resolutions and even farther distances,
让我们能够以类似哈勃望远镜的分辨率看到更宽的天空，甚至更远的距离，
and thus hunt for more supernovas many billions of light years away.
从而寻找数十亿光年之外的更多超新星。
That’ll help us understand whether the acceleration of the universe is different in different parts of the universe
这将帮助我们理解宇宙的加速度在宇宙的不同部分是否不同
or whether it has changed over time, which should help us figure out what exactly dark energy is.
或者它是否随时间发生了变化，这将帮助我们弄清楚暗能量到底是什么。
