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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.
如果你把镜头拉得足够远，你会发现宇宙中所有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，你可能会想，它是否会以某种方式不断膨胀，
or eventually stop and crunch in on itself because of the attractive force of gravity.
或者最终会因为万有引力的吸引而停止膨胀并自我收缩。
Well, all you have to do is point your telescope at a distant galaxy,
你所要做的就是把你的望远镜对准一个遥远的星系，
measure how fast it's moving away from us, wait about a million years,
测量它离我们有多远并等上大概一百万年，
and then do the same thing again to see if the galaxy is moving faster or slower than before.
然后把相同的事情再做一遍以查看星系移动的速度是比以前快还是慢。
But if you don't have a million years to spare, here's what you do. You look at different galaxies.
但如果你没有一百万年的空闲时间，你可以这样做。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.
因为光到达我们需要时间，当我们观察一个遥远的星系时，我们看到的是很久以前的景象。
So looking at a bunch of galaxies different distances away
所以观察一堆不同距离的星系
is kind of a rough proxy for looking at the same one at different times.
是在不同时间观察同一星系的简易替代。
The light from the stars in a distant galaxy can tell us two basic things.
来自遥远星系恒星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
首先，是它的速度：任何远离你的物体发出的光会变得更红
(kinda like how police sirens get lower in pitch when moving away).
（有点像警笛在移动时音调变低）。
This red shifting of light tells us exactly how fast stars are moving away from us.
红光的移动告诉了我们恒星远离我们的速度有多快。
If the light from a star is 5 percent redder than it should be,
如果恒星发出的光比正常情况下红5%，
it means the star is moving away from us at 5 percent the speed of light.
这意味着恒星正在以5%的光速远离我们。
Light can also tell us how far away a star (and thus its parent galaxy) is.
光也能告诉我们一颗恒星（以及它的母星系）离我们有多远。
When you're driving at night, you can tell how far away a motorcycle is by comparing
夜间开车时，你可以通过对比摩托车前灯的亮度
how bright its headlight seems to you, to how bright it would be if you were right in front of it.
和你站在摩托面前时它的亮度来判断一辆摩托车离你有多远。
The same trick works with stars.
恒星也同样适用。
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一样
– it's about 5 billion times brighter than our sun, and, crucially, it's very consistent in its brightness.
—它比我们的太阳亮50亿倍，而且关键是，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,
这种情况每千年只会在一个星系中发生几次，但在宇宙中有足够的星系，
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.
如果你仔细观察广阔的天空，一个月内就有可能看到十几个星系爆炸。
By comparing the brightness as seen here on Earth with how bright we know each of them should be,
通过对比我们在地球上看到的亮度和它们应有的亮度，
we can work out how far away each supernova is. The dimmer its light, the farther away it is.
我们可以计算出每个超新星的距离。光线越暗，距离越远。
So the color of a type Ia supernova tells us how fast it's moving away from us, and its brightness is like a ruler,
Ia型超新星的颜色告诉我们它正在远离我们，并且它的亮度就像一把尺，
telling us how far away it is – or, because light takes time to get here, how long ago we're measuring its speed.
告诉我们它的距离—又或者因为光到达这里需要时间，我们也能知道多久以前我们测量过它的速度。
By measuring the speed of different supernovae at different distances – aka different times in the past –
通过测量不同距离上不同超新星的速度——也就是过去不同的时间——
we can plot the expansion of the universe over time.
我们可以绘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正在以恒定的速度膨胀，曲线意味着它在减速或减速。
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，我们都认为这条线是直的，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,
但当宇航员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.
令他们吃惊的是，他们发现宇宙现在比很久以前膨胀得更快。
The universe's expansion is accelerating! p.s.
宇宙的膨胀正在加速！另外，
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
最简单且最受欢迎的想法之一是到处都是负压渗透的真空能量，
which we've never detected in any way other than by measuring the expansion of the universe...so we call it dark energy.
除了通过测量宇宙的膨胀，我们从未以任何方式探测到过。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
感谢NASA戈达德空间飞行中心的WFIRST项目
and the Space Telescope Science Institute for supporting this video.
以及太空望远镜科学研究院对本节目的支持。
WFIRST is planned to launch in the mid-2020s, and will, among other things,
WFIRST计划在本世纪20年代中期发射，
allow us to see a much wider swath of the sky at Hubble-like resolutions and even farther distances,
它将使我们能够通过哈勃望远镜那样的分辨率看到更广阔的天空，甚至更远的距离，
and thus hunt for more supernovas many billions of light years away.
并从数十亿光年之外寻找更多的超新星。
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.
或者它是否随着时间而改变，这将帮助我们弄清暗能量到底是什么。
