I should say that everything is just right so far. 我要说，到目前为止，一切都恰到好处。

In the long term, gravity may turn out to be a little too strong, 从长远来说，引力也许会变得稍强一点；

and one day it may halt the expansion of the universe and bring it collapsing¹ in upon itself, 有朝一日，它可能阻止宇宙膨胀，让自己将自己压瘪，

till it crushes itself down into another singularity, 最后坍缩成又一个奇点，

possibly to start the whole process over again. 整个过程很可能重新开始。

On the other hand it may be too weak and the universe will keep racing² away forever until everything is so far apart that there is no chance of material interactions, 另一方面，引力也许会变得过弱，那样的话，宇宙会永远地膨胀，直到一切都互相远离，不再可能发生实质性的相互作用，

so that the universe becomes a place that is inert³ and dead, but very roomy. 于是宇宙就成为一个非常空旷呆滞而又没有生命的地方。

The third option is that gravity is just right 第三种可能是，引力恰如其分

"critical density⁴" is the cosmologists' term for it 就是宇宙学所谓的"临界密度"

and that it will hold the universe together at just the right dimensions to allow things to go on indefinitely. 它把宇宙控制在一个恰当的范围，使事物永远继续下去。

Cosmologists in their lighter⁵ moments sometimes call this the Goldilocks effect 宇宙学家有时轻浮地把这称之为"金发姑娘效应"

that everything is just right. 一切都处于恰如其分的状态。

(For the record, these three possible universes are known respectively as closed, open, and flat.) （需要说明的是，这三种可能出现的宇宙分别叫做封闭式宇宙、开放式宇宙和扁平式宇宙。）

Now the question that has occurred to all of us at some point is: 大家迟早会想到一个问题，那就是，

what would happen if you traveled out to the edge of the universe and, as it were, put your head through the curtains? 假设你来到宇宙边缘，把头伸出帘幕，那会发生什么？

Where would your head be if it were no longer in the universe? 你的头会在什么地方(要是它不再是在宇宙里的话)？

What would you find beyond? 你会看到对面是什么？

The answer, disappointingly, is that you can never get to the edge of the universe. 回答是令人失望的：你永远也到不了宇宙的边缘。

That's not because it would take too long to get there 倒不是因为去那里要花很长时间

though of course it would 虽然没错儿，的确要花很长时间

but because even if you traveled outward and outward in a straight line, indefinitely and pugnaciously⁶, 而是因为，即使你沿着一条直线往外走，不停地坚持往外走，

you would never arrive at an outer boundary. 你也永远到不了宇宙的边缘。

Instead, you would come back to where you began 恰恰相反，你会回到起始的地方

(at which point, presumably, you would rather lose heart in the exercise and give up). （到了这种地步，你很可能会灰心丧气，放弃这种努力）。

The reason for this is that the universe bends, in a way we can't adequately imagine, in conformance with Einstein's theory of relativity (which we will get to in due course). 其原因是，按照爱因斯坦的相对论（我们届时将会讲到），宇宙是弯曲的。至于怎么弯曲，我们也不大能想像出来。

For the moment it is enough to know that we are not adrift in some large, ever-expanding bubble. 眼下，你只要知道，我们并不是在一个不断膨胀的大气泡里飘浮，这就足够了。

Rather, space curves, in a way that allows it to be boundless⁷ but finite. 确切点说，空间是弯曲的，恰好使其无限而又有限。

Space cannot even properly be said to be expanding because, 恰当地说，甚至不能说空间在不断膨胀，这是因为，

as the physicist⁸ and Nobel laureate Steven Weinberg notes, 正如诺贝尔奖获得者、物理学家史蒂文·温伯格指出的：

"solar systems and galaxies⁹ are not expanding, and space itself is not expanding." "太阳系和星系不在膨胀，空间本身也不在膨胀。"

Rather, the galaxies are rushing apart. 倒是星系在飞速彼此远离。

It is all something of a challenge to intuition. 这对直觉都是一种挑战。

Or as the biologist J. B. S. Haldane once famously observed: "The universe is not only queerer than we suppose; 生物学家J.B.S.霍尔丹有一句名言："宇宙不仅比我们想像的要古怪，

it is queerer than we can suppose." 而且比我们可能想像的还要古怪。"