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Although it might not seem obvious when you look at the night sky, we live in a universe that's expanding faster by the instant.
虽然看向夜空的时候可能不那么明显，但我们的宇宙每分每秒都在快速膨胀着。
Every day, stars fall over the horizon of what we can see, as the space between us stretches faster than their light can reach us.
每天都会有很多颗恒星越过地平线，因为我们与横线之间的距离变化地比其发出的光抵达我们的速度要快。
And we can never know what exists past that horizon.
我们永远也不会找到地平线那头是怎样的存在。
So you might imagine, or you might have heard about, a far-off future, where space is stretching faster and faster, and where all of the stars and galaxies are over that edge.
无论是我们自己想象，还是听他人所说，这世界上有一个遥远的未来。在那里，宇宙膨胀的速度不断加快，所以所有的恒星和星系都越过了地平线。
A future where Earth will be left with a dark, empty sky.
在这样的未来里，地球的天空将空空如也，一片漆黑。
But luckily for us, or, at least, for hypothetical future earthlings, that's not actually the case.
但所幸，至少未来假设中的地球人来说，这是不可能看到的。
Because the universe is expanding… but not all of it.
因为宇宙是不断膨胀的，不过并不是所有部分都在膨胀。
We've known that the universe is expanding since the 1920s, but we only discovered that the expansion is accelerating in the 1990s, thanks to the Hubble Space Telescope.
我们都知道，宇宙自20世纪20年代以来就一直在膨胀，但直到90年代，我们才发现这一事实，而这多亏了哈勃望远镜。
Hubble was the first tool to measure really precise distances to supernovas out near the edge of the observable universe.
哈勃望远镜是首个能够测量可观测宇宙边界的超新星距离我们精确距离的工具。
And it showed us that out there, ancient galaxies, and the supernovas in them, are zooming away from us faster than anywhere else.
借助它的帮助，我们发现，宇宙中的古老星系和其内的超新星远离我们的速度比其他地方要快。
In fact, astronomers realized that they were flying away even faster than expected.
实际上，天文学家已经意识到它们远离我们的速度比预期的还要快。
Which, at first, didn't make sense. At the time, we thought the universe was dominated by gravity, which pulls things together.
但起初，我们并不明白原因。那时候，我们以为宇宙是由引力控制的，而引力可以将万物聚拢在一起。
So seeing everything accelerate apart was weird.
所以，要是说看到什么物体加速原理彼此的话，简直是怪谈。
It would kind of be like if you kicked a ball uphill and saw it speed up instead of coming back down to you.
就好像我们将一直皮球踢上了山坡，不但没看到它返回来，反而看到它加速向上前进一样。
Because of this, scientists concluded that there had to be something else going on, something pushing these galaxies apart.
正是因为这个原因，科学家下结论称：一定在发生着什么才使得各星系逐渐远离的。
They came to call that thing dark energy. Decades later, dark energy is still really mysterious, and there's a lot we don't understand about it.
后来，科学家将这个进程成为暗能量。几十年来，暗能量依然神秘莫测，我们对其知之甚少。
One explanation is that it's a property of empty space.
有一种说法是：暗能量是真空区域的特性。
This means that space itself, with no stuff in it at all, has dark energy.
也就是说，如果刨去各种物质，宇宙本身是有暗能量的。
And that energy pushes space apart, creating new space, which in turn has dark energy, which pushes space apart, creating new space, which in turn has dark energy, which You get it.
正是这种暗能量将宇宙分割开来，创造出新的空间，而新的空间又有暗能量，又将宇宙分隔开来，又创造出新的空间，新的新的空间又有暗能量，周而复始。
If dark energy is a property of space, that also means you can't dilute it.
如果暗能量是宇宙的特性，那么暗能量就是无法削弱的。
Its density will always be the same, no matter how much space expands. Of course, that density is also pretty small.
暗能量的密度是常量，不论宇宙如何膨胀都不会改变。当然了，暗能量的密度也微乎其微。
If you borrow Einstein's “E=mc2” trick and express energy as mass, it's equivalent to about one grain of sand in a space the size of the entire Earth.
借用爱因斯坦的质能等价的公式E=mc2，如果将能量看为质量，那么暗能量在地球这么大的空间里，只相当于一粒沙子。
But if you average that over the whole universe, which is mostly empty space, there's more dark energy than anything else.
但鉴于宇宙大部分都是真空区域，所以暗能量比其他物质都要多。
So it dominates, and the universe as a whole expands.
所以暗能量主宰着整个宇宙，而宇宙整体是不断膨胀的。
That's why the most ancient galaxies are also moving away the fastest: It's taken a long time for their light to reach us, so the universe has had more time to stretch.
因此，大多数古老星系也在以最快速度远离：它们发出的光需要很长的时间才能抵达地球，所以宇宙才有更多时间膨胀。
Now, this might all make dark energy seem super strong.
说完这些，大家会觉得暗能量很强势。
After all, it makes up more than two-thirds of all the stuff in the universe, and it's pushing apart entire galaxies.
毕竟，暗能量占宇宙中所有物质的2/3，将所有星系分隔开来。
But it's only powerful because there's a lot of it.
暗能量的强大在于其含量较多。
Within small spaces, especially those full of planets and stars, dark energy is actually pretty weak.
在小空间内，尤其是布满行星和恒星的小空间内，暗能量是很微弱的。
Like, the gravity between the Sun and the Earth, or the Earth and the Moon, is more than enough to overpower the repulsive dark energy between them.
就好像太阳和地球之间的引力，又或者地球和月球之间的引力已经足够强大，足以压制住它们之间起排斥作用的暗能量。
In fact, most of the universe's mass is concentrated in galaxy clusters, and these pockets of matter are completely immune to dark energy. They're simply not expanding.
实际上，宇宙的大部分质量都集中在星系团中，而暗能量对星系团毫无威胁。星系团是不会扩张的。
And I don't mean the expansion is negligible, like how technically your gravity pulls ever-so-slightly on Earth but it's not enough to actually notice.
虽然其膨胀还不可以忽略，就好像地球的引力作用一样，但它依然小到难以察觉。
I mean that, as far as we know, dark energy is truly not stretching our galaxy at all.
我想说的是，就目前我们所了解的情况，暗能量根本不会起到拉扯银河系的作用。
This is because it's not a force like gravity, so it works a little differently.
这是因为，暗能量与引力作用不同，其原理机制有些差异。
To understand how, think about pushing on a heavy door.
要了解其原理机制，就以推动一扇沉重的门为例。
If you push lightly, it won't open. Push a little harder and it still won't.
如果轻轻地推，门就不会开。如果稍微用点力，门还是不会开。
But if you push hard enough, once you cross a certain threshold of pushing, it will open.
但如果足够使劲儿，那么一旦过了阈值，门就会开。
That door is gravity and, within a galaxy, there's just not enough dark energy to push it open.
这扇门其实就是引力。而一个星系内的暗能量是不足以使“门”打开的。
In other words, gravity is too strong. So our galaxy will never expand, because if it can't stretch even a little, then it can't create more space.
换言之，引力太强。所以，银河系不会扩张，因为无法拉扯丝毫的星系是无法创造出新的空间的。
And that means the amount of dark energy inside will never grow.
再往下推，暗能量的总量也不会有任何增长。
Of course, this isn't something we've been able to directly observe, like by looking at other galaxies.
当然了，这并不是我们能直接观测到的结果。无论是观测其他星系还是用什么方法，都不能得到这样的结果。
But multiple observations have shown us what dark energy is like, and they all suggest this should be true.
不过，多次的观察已经让我们能了解暗能量的大致情况，而这些观测都表明我们的推断是正确的。
Eventually, in the really distant future, fewer and fewer galaxies will be visible from Earth.
最后，在遥远的未来，地球上已经看不到几个星系了。
And in 100 billion years or so, deep space will be almost empty.
大约1000亿年后，深空就完全空了。
But if Earth were still around by then, which, admittedly, is pretty unlikely, we'd still have a beautiful night sky.
但是如果地球到那时候还存在（虽然这不太可能），我们就能看到美丽的夜空。
Even as the universe stretches, the glow of our galaxy will still be overhead, and we'll have stars, constellations, and even a handful of galaxies bound by gravity to ours.
即便宇宙在膨胀，但银河系的光晕也会在上空。而且我们依然有很多恒星、星座以及很多星系通过引力的作用与我们相连。
All because dark energy just can't get a foothold around here.
而这都是因为暗能量在上述地方站不住脚。
Of course, this will only last until the heat death of the universe… but that's another story.
当然了，如果太阳消亡了，这一切也就结束了……不过太阳一时半会还在。
Thanks for watching this episode of SciShow Space, especially to our patrons on Patreon!
感谢收看本期的《太空科学秀》，尤其要感谢我们的铁粉！
Because of you, we're able to keep exploring big topics like dark energy, along with all of the other amazing things that make our universe so cool.
因为你们，我们才能持续探索暗能量等重要话题和宇宙里很酷的事情。
If you'd like to support free space education online and help us make more content like this, you can go to patreon.com/scishow.
如果您想要支持免费的太空在线教育并帮助我们只做更多类似这样的节目的话，您可以访问patreon.com/scishow。
