宇宙中心在何方?

　　还记得马克博士吗？2006年10月号的“探索区”中，我们就“时间旅行”这个问题请来马克博士，他简洁明了的讲解受到很多读者的喜爱。时隔两年，我们再次请来马克博士。这次的问题可不简单哦――宇宙的中心在哪里？一起听听马克博士的解释吧。 　　 　　Diane Fisher: Marc, today our question � a BIG one � 　　comes from the CATO Rocketry[火箭学] Club in Gales Ferry, Connecticut. Here goes: Is it possible to point to a direction in the sky and say “that way is the center of the universe, where the Big Bang started”? 　　 　　Dr. Marc Rayman: Wow. Well…no. The “center of the universe” has always been a very intriguing[引起兴趣的] 　　idea for humans. Until the 16th century, even learned people thought Earth � and people � were at the center of the cosmos[宇宙]. Then the Polish astronomer[天文学家] 　　Nicolaus Copernicus had a better idea � although still wrong � for explaining the motion of the planets 　　through the night sky. He thought the Sun was 　　actually the center of the universe, with Earth and the other planets circling the Sun. And his theory was widely condemned[谴责] and ridiculed[嘲笑，奚落] at first. Now we know that not only are we humans not at the center of the universe, but there is no center of the universe! 　　 　　Fisher: Now THAT is really hard to imagine! 　　 　　Rayman: It is. But a center just doesn’t fit with what scientists have learned through decades of modern astronomy. The Big Bang is the name scientists give to the events that started the universe. Although the Big Bang is often described as a huge explosion, an 　　explosion has a central point, such as a bomb or a spark. The Big Bang wasn’t like that. 　　 　　Fisher: Why not? 　　 　　Rayman: The Big Bang happened everywhere at once. It’s an expansion of space itself, not the expansion of things in space. That means everywhere in space is 　　moving apart from everywhere else. This has been going on in the entire universe for almost 14 billion years. 　　 　　Fisher: But aren’t we located somewhere with respect to[关于，至于] the rest of the universe? 　　 　　Rayman: We certainly are. We’re right here, and we know where “here” is relative to other objects in the 　　universe! But here is not special, like a center would be, and, for that matter, nowhere is special. That’s like asking � if we had a powerful telescope that could see all the way to the end of the universe, would we find more of the universe on one side of Earth than on the other? No. We would find that it looks the same in all directions. 　　 　　Fisher: So, doesn’t that mean we are still at the center of the universe? 　　 　　Rayman: Well, no, it doesn’t. Observers everywhere in the universe would find the same result. Imagine that you are on one of many dots on a spherical[球形的] 　　balloon. No matter which way you look along the 　　surface of the balloon, the end of your world seems to be the same distance from you. But that doesn’t mean you are at the center of this little world! The fact is your two-dimensional world has no center. 　　 　　Fisher: But what about the continuing expansion of the universe? Doesn’t it have to expand from a center point somewhere? 　　 　　Rayman: Well, suppose your balloon world is being 　　inflated[充气，膨胀]

　　with air. All the other dots will be getting farther and farther away from you as the balloon gets 　　bigger. In fact, all dots get 　　farther from each other, so no matter where you are, it looks as if you are at the center of the expansion. The expansion of three-dimensional space is similar. Like the surface of the balloon, there is no center in the universe. 　　 　　Fisher: How do we even know the universe started with a Big Bang? 　　 　　Rayman: One of the ways is that scientists have been able to see the faint radiation[辐射，放射物] left over from shortly 　　after this cosmic birth. Some scientists predicted what this radiation would look like, and others found it using a 　　radio telescope[无线电天文望远镜]. As more measurements have been made from Earth and from space, the Big Bang has 　　continued to provide an excellent description of how the universe has evolved. 　　 　　Fisher: Is there any other way to learn about the Big Bang besides observing its leftover radiation? 　　 　　Rayman: Well, yes, there are several. And scientists would like to be able to study gravitational waves[引力波] left by the Big Bang. Gravitational waves will be very hard to detect, but should tell us a lot about the universe that 　　electromagnetic[电磁的] radiation and matter cannot. 　　Detecting gravitational waves will require some very 　　advanced technologies, but could lead to thrilling[令人兴奋的] new insights into the workings[运动方式，运行] of the universe. 　　 　　Fisher: Our time’s up for now. Marc, thank you for another fascinating discussion. 　　 　　黛安・费希尔：马克，我们今天的问题，一个很广的问题，来自（美国）康涅狄格州盖尔斯费里村的“CATO”火箭俱乐部。问题是这样的：我们能否指着天空的某一个方向，然后说“那里是宇宙的中心，宇宙大爆炸就是从那里开始的”？ 　　 　　马克・雷曼博士：哇，呃……不能。“宇宙中心”这个问题一直吸引着人们的注意力。直到16世纪以前，即使是有学问的人也认为地球――和人类――是宇宙的中心。后来，波兰天文学家尼古拉・哥白尼对行星在夜空中的运动有了更好的想法，虽然他的想法仍然是错误的。他认为太阳才是宇宙的中心，地球以及其它行星围绕着太阳运转。起初，他的理论备受诟骂，遭人奚落。现在我们知道不单我们人类不是宇宙的中心，实际上宇宙根本没有 　　中心！ 　　 　　费希尔：这确实让人难以想象！ 　　 　　雷曼：的确如此。但“中心”这个概念实在不符合科学家多年来通过现代天文学得出的结论。“宇宙大爆炸”是科学家们为宇宙形成的一系列事件起的名字。虽然宇宙大爆炸往往被描述为一次强烈的爆炸，而爆炸往往有一个中心点，例如一个炸弹或火花，但宇宙大爆炸并非那样。 　　 　　费希尔：为什么不是那样呢？ 　　 　　雷曼：宇宙大爆炸是所有地方在同一时间发生的。它是太空自身的膨胀，而不是太空里某些物体的膨胀。也就是说，太空中所有地方都在相互远离。这个过程在宇宙中持续了近140亿年。 　　 　　费希尔：但相对于宇宙的其它地方，难道我们不是处于某个特定的位置吗？ 　　 　　雷曼：当然是的。我们就在这里，我们也知道“这里”相对于宇宙其它物体的位置！然而这里并不是像中心那样特殊的地方，在这个问题上，没有一个地方是特殊的。这就像问――假如我们有一个功能强大的望远镜，可以一直看到宇宙的尽头，我们会否看到地球一边的宇宙比另一边多呢？不会。我们会发现从任何方向看都是一样的。 　　 　　费希尔：那么，这不还是意味着我们处于宇宙的中心吗？ 　　 　　雷曼：噢，不，不是这样的。处在宇宙任何位置的观测者会得到相同的结果。试想一下，一个球形气球有许多个点，你在其中一点上。无论你沿着气球表面的哪个方向看，尽头与你的距离似乎都是一样的。但这不代表你处在这个小小世界的中心！事实上，你的二维世界没有中心。 　　 　　费希尔：但宇宙的不断膨胀又是怎样的呢？难道不是必须得从一个中心开始膨胀吗？ 　　 　　雷曼：噢，假设那个气球世界由于充气而膨胀，随着气球不断变大，所有的点都会离你越来越远。实际上，所有的点都会越离越远，因此不管你在哪里，都会觉得自己就在中心。三维空间的膨胀与此相似。就像气球表面那样，宇宙不存在中心。 　　 　　费希尔：其实我们怎么知道宇宙是由宇宙大爆炸形成的？ 　　 　　雷曼：其中一点是，科学家能够探测到这场宇宙诞生所残余的微弱辐射。一些科学家预测到这种辐射是怎样的，另一些科学家则通过无线电天文望远镜探测到它。随着越来越多对地球和太空的测量，宇宙大爆炸理论不断为宇宙的演变过程提供了很好的 　　描述。 　　 　　费希尔：除了探测残余辐射，还有其它了解宇宙大爆炸的方法吗？ 　　 　　雷曼：呃，有几种方法。科学家希望能够研究宇宙大爆炸留下的引力波。引力波很难探测到，但它应该能够让我们了解到很多电磁辐射及物质未能反映的宇宙状况。探测引力波要求很先进的技术，但可以带我们进入激动人心的全新境界，进一步了解宇宙的运行方式。 　　 　　费希尔：我们的时间到了。马克，谢谢你再次为我们带来这么精彩的讲解。