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We’re used to thinking of space as the emptiness in which things happen, like an empty warehouse ready to be filled, or a theater stage on which the events of the Universe play out.
我们习惯于认为空间是事物发生的空地, 就像一个等待被填满的空仓库，或者是宇宙事件上演的剧场舞台。
But General Relativity predicts that space is not just emptiness, it’s a physical, dynamic thing, and that prediction has been borne out by many, many experiments.
但是广义相对论预言空间不仅仅是空的，它是一个物理的，动态的东西，这个预言已经被很多很多的实验证实了。
Space can bend because of matter and energy, curving the paths of objects that move inside of it.
由于物质和能量的作用，空间会发生弯曲，从而使在其中移动的物体的路径发生弯曲。
![3PT$R8`AD3~6@L(VI7ZV]}R.png](http://upload.kekenet.com/2021/1021/20931634799416.png "20931634799416.png")
It can ripple with gravitational waves.
它会受到引力波的影响。
And it can expand, creating more and more space in space.
它可以扩展，在空间中创造越来越多的空间。
All of these seemingly different phenomena can be described by one idea: curvature of space (or spacetime).
所有这些看似不同的现象都可以用一个概念来描述:空间(或时空)的曲率。
In flat regions of spacetime (like, if there’s no energy or matter nearby), objects traveling along parallel paths stay along parallel paths.
在时空的平坦区域(比如，如果附近没有能量或物质)，沿着平行路径移动的物体会沿着平行路径移动。
In positively curved regions of space (like near planets or black holes), parallel paths converge, and in negatively curved regions of space parallel paths (or even paths pointed at each other) diverge.
在正向弯曲的空间区域(如行星或黑洞附近)，平行路径会汇聚，而在反向弯曲的空间区域，平行路径(甚至是指向彼此的路径)会发散。
But what about space as a whole?
但是宇宙作为一个整体会怎么样呢?
If space is positively curved everywhere, then there’s only one shape space can be: a giant hyperspace-potato.
如果空间处处都是正弯曲的，那么空间只能是一种形状:一个巨大的超空间土豆。
If you went in one direction for long enough, eventually you’d end up in the same place you started.
如果你朝着一个方向走的时间足够长，最终你会回到开始的地方。
If space is flat everywhere, its shape could be simple: just extend out straight to infinity.
如果空间处处平坦，它的形状可能很简单:直接延伸到无穷远。
Or it could loop around in a periodic way, like in some video games: And if space is negatively curved everywhere, sports would be impossible.
或者它能以周期性的方式循环，就像在一些电子游戏中:如果空间到处都是负弯曲的，运动就不可能了。
So which is it?
到底是哪一个呢?
There are basically two ways to measure the large-scale curvature of the Universe.
基本上有两种方法来测量大尺度的宇宙曲率。
One is to measure the angles inside of triangles.
一种是测量三角形的内角。
If the space is flat, then the angles will add up to 180 degrees.
如果空间是平的，那么两个角的和是180度。
But if the space is curved, those angles will add up to more or less than 180 degrees depending on the type of curvature.
但如果空间是弯曲的，这些角根据弯曲的类型，加起来会大于或小于180度。
Cosmologists have done the equivalent of measuring our Universe’s triangles by looking at a picture of the early Universe, and studying the spatial relationship between different points on that picture.
宇宙学家通过观察早期宇宙的图像来测量我们宇宙的三角形，并研究图像上不同点之间的空间关系。
The second way to measure curvature is to measure the thing that causes space to curve in the first place: the density of energy and matter throughout the Universe.
第二种测量曲率的方法是首先测量导致空间弯曲的物质:整个宇宙的能量和物质的密度。
Which cosmologists have also measured.
宇宙学家也测量过。
It turns out that in both cases, measurements show the Universe to be… pretty much flat (within 0.4% margin of error).
结果表明，在这两种情况下，测量结果都显示宇宙相当平坦(误差在0.4%以内)。
But before you get disappointed that we don’t live in a cool cosmic hyper-potato, let me tell you one big problem.
如果你因为我们不是生活在一个酷酷的宇宙超级土豆中而感到失望的话，在此之前，让我告诉你一个大难题。
The fact that we live in a flat Universe appears to be a GIGANTIC, COSMIC-LEVEL COINCIDENCE.
我们生活在一个平坦的宇宙中，这似乎是一个巨大的、宇宙级的巧合。
If the Universe had just a little bit more mass and energy, space would have curved one way.
如果宇宙的质量和能量再多一点，空间就会向一个方向弯曲。
And if it had just a little bit less mass and energy, space would have curved the other way.
如果它的质量和能量少一点，空间就会向另一个方向弯曲。
But we seem to have just the right amount to make space perfectly flat as far as we can tell.
但据我们所知，我们似乎有足够的量使空间完全平坦。
This perfect amount is the equivalent of five hydrogen atoms per cubic meter of space, on average.
这个完美的数量相当于平均每立方米空间有5个氢原子。
Does big empty parts of space make up for all the atoms we have crammed in here around us?
我们周围的原子都被我们塞在了太空中，而太空的空旷部分就能弥补这一切吗?
If instead there were six hydrogen atoms per cubic meter of space on average, or four, the entire Universe would have been a lot more curved or a lot less.
如果平均每立方米空间有6个或4个氢原子，那么整个宇宙的弯曲度会大得多，或者小得多。
And we so far have no idea why our universe has the mass energy density it does.
到目前为止，我们还不知道为什么宇宙有这么大的质量和能量密度。
When it comes to the curvature of the universe, our knowledge falls flat.
谈到宇宙的曲率，我们的知识就显得苍白无力了。
