作者:物理学家布莱恩·格林

翻译:王烁 王佩 机械工业出版社

史蒂芬·平克在《风格感觉》一书第二章引用,作为古典风格的范例

如果空间今天在扩张,那么从前的宇宙必然比现在要小。在很久很久之前的某一刻,我们今天看到的所有东西(包括组成每个行星、每个恒星、每个星系甚至空间本身的那些成分)必然是先被压缩成一个无限小的点,然后向外扩大膨胀,演变成今天我们所知道的宇宙。

print("Hello World")

大爆炸理论诞生了……但科学家们知道大爆炸理论有个重要缺点。它竟然没解释大爆炸本身。爱因斯坦方程式非常精彩地描述了大爆炸后的瞬间宇宙如何演化,但要是应用到宇宙最早那一刻的极端环境上,方程式就不起作用了(就像你计算器上输入1除以0的时候,计算器会发出报错信息)。所以,大爆炸理论没告诉我们可能是什么引发了爆炸本身。

把球向上抛,地球的引力会拖慢球的上升,同样,每个星系对彼此的引力也一定在拖“慢空间的扩张……(但是)与被拖慢非常不一样,空间的扩张在大约70亿年前换到超速挡,而且从那以来一直在加速。好比将一个球轻轻向上扔,刚开始其上升变慢了一点儿,然后就像火箭一样越升越快。

我们都习惯于引力只做一件事——将物体彼此拉近。但根据爱因斯坦的相对论,引力也可以把物体分开……如果空间包含一种不可见的能量,就像一种不可见的雾一样均匀地分布在空间中,那么这些能量雾造成的引力就会产生排斥效果。

但是,这个暗能量假设指向了又一个谜团:当天文学家们推导有多少暗能量弥漫在宇宙各个角落才解释得了观察到的宇宙加速扩张时,遇到了这个无人能解释的数字:

0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000138

在暗能量数量更多的宇宙中,每当物质要聚集成星系,暗能量的斥力就会强到能分开这些聚集起来的块,星系便无法形成。在暗能量少得多的宇宙中,这些斥力则转为引力,使这些宇宙很快塌陷回去,星系还是无从形成。而没有星系,则没有恒星,也没有行星,因此那些宇宙中,我们这种形态的生命没有机会存在。

我们处在这个宇宙而不是其他宇宙,跟我们处在地球上而不是海王星上,原因差不多——我们这种形态的生命出现的条件在哪里成熟,我们才会在哪里出现。

正如只有货源充足的鞋店才能保证你能找到自己的尺码一样,只有“货源充足”的多重宇宙才能保证其中存在一个恰有这么多暗能量的宜居宇宙。仅靠自身,暴胀宇宙学理论还达不到目标。尽管其无休无止的大爆炸创造出的宇宙数量极大,但许多宇宙的特征是相似的,就好比鞋店里一货架一货架的鞋全是5号和13号的,却没有你要找的尺码。

把暴胀宇宙学理论与弦理论结合起来……可能存在的宇宙的数量多得简直就要爆仓了:通过膨胀,一个接一个的大爆炸将弦理论中数量多得要命的可能存在的宇宙变成实际存在的宇宙。我们所处的宇宙实际上被保证在其中有一席之地。而因为拥有那些我们这种生命形式所必需的特质,那就成了我们栖息的宇宙。

英文:

The Theory of Multiple Universes

If space is now expanding, then at ever earlier times the universe must have been ever smaller. At some moment in the distant past, everything we now see—the ingredients responsible for every planet, every star, every galaxy, even space itself—must have been compressed to an infinitesimal speck that then swelled outward, evolving into the universe as we know it.

The big-bang theory was born…Yet scientists were aware that the big-bang theory suffered from a significant shortcoming. Of all things, it leaves out the bang. Einstein’s equations do a wonderful job of describing how the universe evolved from a split second after the bang, but the equations break down (similar to the error message returned by a calculator when you try to divide 1 by 0) when applied to the extreme environment of the universe’s earliest moment. The big bang thus provides no insight into what might have powered the bang itself.

Just as the pull of earth’s gravity slows the ascent of a ball tossed upward, the gravitational pull of each galaxy on every other must be slowing the expansion of space… [But] far from slowing down, the expansion of space went into overdrive about 7 billion years ago and has been speeding up ever since. That’s like gently tossing a ball upward, having it slow down initially, but then rocket upward ever more quickly.

We’re all used to gravity being a force that does only one thing: pull objects toward each other. But in Einstein’s theory of relativity, gravity can also push things apart. If space contains an invisible energy, sort of like an invisible mist that’s uniformly spread through space, then the gravity exerted by the energy mist would be repulsive.

When the astronomers deduced how much dark energy would have to permeate every nook and cranny of space to account for the observed cosmic speedup, they found a number that no one has been able to explain:

0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000138

In universes with larger amounts of dark energy, whenever matter tries to clump into galaxies, the repulsive push of the dark energy is so strong that the clump gets blown apart, thwarting galactic formation. In universes whose dark-energy value is much smaller, the repulsive push changes to an attractive pull, causing those universes to collapse back on themselves so quickly that again galaxies wouldn’t form. And without galaxies, there are no stars, no planets, and so in those universes there’s no chance for our form of life to exist.

We find ourselves in this universe and not another for much the same reason we find ourselves on earth and not on Neptune—we find ourselves where conditions are ripe for our form of life.

Just as it takes a well-stocked shoe store to guarantee you’ll find your size, only a well-stocked multiverse can guarantee that our universe, with its peculiar amount of dark energy, will be represented. On its own, inflationary cosmology falls short of the mark. While its never-ending series of big bangs would yield an immense collection of universes, many would have similar features, like a shoe store with stacks and stacks of sizes 5 and 13, but nothing in the size you seek.

By combining inflationary cosmology and string theory, the stock room of universes overflows: in the hands of inflation, string theory’s enormously diverse collection of possible universes become actual universes, brought to life by one big bang after another. Our universe is then virtually guaranteed to be among them. And because of the special features necessary for our form of life, that’s the universe we inhabit.

最后修改:2022 年 10 月 29 日
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