「宇宙的起源」內容全文 -- 霍金

威尼斯的狼

  [英文版]

  According to the Boshongo people of central Africa, in the beginning there was only darkness, water, and the great god Bumba. One day Bumba, in pain from a stomach ache, vomited up the sun. The sun dried up some of the water, leaving land. Still in pain, Bumba vomited up the moon, the stars, and then some animals: the leopard, the crocodile, the turtle, and, finally man.

  This creation myth, like many others, tries to answer the questions we all ask. Why are we here? Where did we come from? The answer generally given was that humans were of comparatively recent origin, because it must have been obvious, even at early times, that the human race was improving in knowledge and technology. So it can't have been around that long, or it would have progressed even more. For example, the Book of Genesis placed the creation of the world at 4 pm on October the 10th, 4,004 BC. On the other hand, the physical surroundings, like mountains and rivers, change very little in a human life time. They were therefore thought to be a constant background, and either to have existed for ever as an empty landscape, or to have been created at the same time as the humans.

  Not everyone however, was happy with the idea that the universe had a beginning. For example, Aristotle, the most famous of the Greek philosophers, believed the universe had existed forever. Something eternal is more perfect than something created. He suggested, the reason we see progress was that floods, or other natural disasters, had repeatedly set civilization back to the beginning. The motivation for believing in an eternal universe was the desire to avoid invoking divine intervention, to create the universe, and set it going. Conversely, those who believed the universe had a beginning, used it as an argument for the existence of God, as the first cause, or prime mover of the universe.

  If one believed that the universe had a beginning, the obvious question was: What happened before the beginning? What was God doing before He made the world? Was He preparing Hell for people who asked such questions? The problem of whether or not the universe had a beginning, was a great concern to the German philosopher, Immanuel Kant. He felt there were logical contradictions, or Antinomies, either way. If the universe had a beginning, why did it wait an infinite time before it began? He called that the thesis. On the other hand, if the universe had existed forever, why did it take an infinite time to reach the present stage? He called that the antithesis. Both the thesis, and the antithesis, depended on Kant's assumption, along with almost everyone else, that time was Absolute. That is to say, it went from the infinite past, to the infinite future, independently of any universe that might or might not exist in this background.

  This is still the picture in the mind of many scientists today. However in 1915, Einstein introduced his revolutionary General Theory of Relativity. In this, space and time were no longer Absolute, no longer a fixed background to events. Instead, they were dynamical quantities that were shaped by the matter and energy in the universe. They were defined only within the universe, so it m><uld be like asking for a point south of the South Pole. It is not defined.

  If the universe was essentially unchanging in time, as was generally assumed before the 1920s, there would be no reason that time should not be defined arbitrarily far back. Any so-called beginning of the universe, would be artificial, in the sense that one could extend the history back to earlier times. Thus it might be that the universe was created last year, but with all the memories and physical evidence, to look like it was much older. This raises deep philosophical questions about the meaning of existence. I shall deal with these by adopting what is called the positivist approach. In this, the idea is that we interpret the input from our senses in terms of a model we make of the world. One can not ask whether the model represents reality, only whether it works. A model is a good model if it first, interprets a wide range of observations, in terms of a simple and elegant model. And second, if the model makes definite predictions that can be tested, and possibly falsified by observation.

  In terms of the positivist approach, one can compare two models of the universe. One in which the universe was created last year, and one in which the universe existed much longer. The model in which the universe existed for longer than a year can explain things like identical twins that have a common cause more than a year ago.

  On the other hand, the model in which the universe was created last year can not explain such events. So the first model is better. One can not ask whether the universe really existed before a year ago, or just appeared to. In the positivist approach, they are the same.

  In an unchanging universe, there would be no natural starting point. The situation changed radically however, when Edwin Hubble began to make observations with the hundred inch telescope on Mount Wilson in the 1920s.

  Hubble found that stars are not uniformly distributed throughout space, but are gathered together in vast collections called galaxies.

  By measuring the light from galaxies, Hubble could determine their velocities. He was expecting that as many galaxies would be moving towards us, as were moving away. This is what one would have in a universe that was unchanging with time. But to his surprise, Hubble found that nearly all the galaxies were moving away from us. Moreover, the further galaxies were from us, the faster they were moving away. The universe was not unchanging with time, as everyone had thought previously. It was expanding. The distance between distant galaxies was increasing with time.

  The expansion of the universe was one of the most important intellectual discoveries of the 20th century or of any century. It transformed the debate about whether the universe had a beginning. If galaxies are moving apart now, they must have been closer together in the past. If their speed had been constant, they would all have been on top of one another about 15 billion years ago. Was this the beginning of the universe?

  Many scientists were still unhappy with the universe having a beginning, because it seemed to imply that physics broke down, and an outside agency which for convenience one can call God, determined how the universe began. They therefore advanced theories in which the universe was expanding at the present time, but didn't have a beginning. One was the Steady State theory, proposed by Bondi, Gold, and Hoyle in 1948.

  In the Steady State theory, as galaxies moved apart, the idea was that new galaxies would form from matter that was supposed to be continually being created throughout space. The universe would have existed forever and would have looked the same at all times. This last property had the great virtue, from a positivist point of view, of being a definite prediction, that could be tested by observation. The Cambridge radio astronomy group, under Martin Ryle, did a survey of weak radio sources in the early 1960s. These were distributed fairly uniformly across the sky, indicating that most of the sources lay outside our galaxy. The weaker sources would be further away, on average.

  The Steady State theory predicted the shape of the graph of the number of sources, against source strength. But the observations showed more faint sources than predicted, indicating that the density of sources was higher in the past. This was contrary to the basic assumption of the Steady State theory, that everything was constant in time. For this, and other reasons, the Steady State theory was abandoned.

  Another attempt to avoid the conclusion that the universe had a beginning was the suggestion that there was a previous contracting phase, but because of rotation and local irregularities, the matter would not all fall to the same point. Instead, different parts of the matter would miss each other and the universe would expand again with the density remaining finite. Two Russians, Lifshitz and Khalatnikov, actually claimed to have proved that a general contraction without exact symmetry would always lead to a bounce with the density remaining finite. This result was very convenient for Marxist Leninist dialectical materialism because it avoided awkward questions about the creation of the universe. It therefore became an article of faith for Soviet scientists.

  When Lifshitz and Khalatnikov published their claim, I was a 21 year old research student looking for something to complete my PhD thesis. I didn't believe their so-called proof, and set out with Roger Penrose to develop new mathematical techniques to study the question. We showed that the universe couldn't bounce. If Einstein's General Theory of Relativity is correct, there will be a singularity, a point of infinite density and space-time curvature, where time has a beginning.

  

[ 本帖最后由 威尼斯的狼 于 12-8-2006 12:57 AM 编辑 ]

威尼斯的狼

  Observational evidence to confirm the idea that the universe had a very dense beginning came in October 1965, a few months after my first singularity result, with the discovery of a faint background of microwaves throughout space. These microwaves are the same as those in your microwave oven, but very much less powerfull. They would heat your pizza only to minus 271 point 3 degrees centigrade, not much good for defrosting the pizza, let alone cooking it. You can actually observe these microwaves yourself. Set your television to an empty channel. A few percent of the snow you see on the screen will be caused by this background of microwaves. The only reasonable interpretation of the background is that it is radiation left over from an early very hot and dense state. As the universe expanded, the radiation would have cooled until it is just the faint remnant we observe today.

  Although the singularity theorems of Penrose and myself predicted that the universe had a beginning, they didn't say how it had begun. The equations of General Relativity would break down at the singularity. Thus Einstein's theory can not predict how the universe will begin, but only how it will evolve once it has begun. There are two attitudes one can take to the results of Penrose and myself. One is that God chose how the universe began for reasons we could not understand. This was certainly the view of Pope John Paul. At a conference on cosmology I attended in the Vatican, the Pope told the delegates that it was alright to study the universe after it began, but they should not inquire into the beginning itself because that was the moment of creation and the work of God. I was glad he didn't realize I had presented a paper at the conference suggesting how the universe began. I didn't fancy the thought of being handed over to the Inquisition like Galileo.

  The other interpretation of our results, which is favoured by most scientists, is that it indicates that the General Theory of Relativity breaks down in the very strong gravitational fields in the early universe and has to be replaced by a more complete theory. One would expect this anyway because General Relativity does not take account of the small scale structure of matter which is governed by quantum theory. This does not matter normally because the scale of the universe is enormous compared to the microscopic scales of quantum theory. But when the universe is the Planck size, a billion trillion trillionth of a centimeter, the two scales are the same and quantum theory has to be taken into account.

  In order to understand the Origin of the universe, we need to combine the General Theory of Relativity with quantum theory. The best way of doing so seems to be to use Feynman's idea of a sum over histories. Richard Feynman was a colourful character who played the bongo drums in a strip joint in Pasadena and was a brilliant physicist at the California Institute of Technology. He proposed that a system got from a state A to a state B by every possible path or history.

  Each path or history has a certain amplitude or intensity, and the probability of the system going from A to B is given by adding up the amplitudes for each path. There will be a history in which the moon is made of blue cheese, but the amplitude is low which is bad news for mice.

  The probability for a state of the universe at the present time is given by adding up the amplitudes for all the histories that end with that state. But how did the histories start? This is the Origin question in another guise. Does it require a Creator to decree how the universe began? Or is the initial state of the universe determined by a law of science?

  In fact, this question would arise even if the histories of the universe went back to the infinite past. But it is more immediate if the universe began only 15 billion years ago. The problem of what happens at the beginning of time is a bit like the question of what happened at the edge of the world when people thought the world was flat. Is the world a flat plate with the sea pouring over the edge? I have tested this experimentally. I have been round the world and I have not fallen off.

  As we all know, the problem of what happens at the edge of the world was solved when people realized that the world was not a flat plate but a curved surface. Time however seemed to be different. It appeared to be separate from space and to be like a model railway track. If it had a beginning, there would have to be someone to set the trains going.

  Einstein's General Theory of Relativity unified time and space as space-time, but time was still different from space and was like a corridor which either had a beginning and end or went on for ever. However, when one combines General Relativity with Quantum Theory, Jim Hartle and I realized that time can behave like another direction in space under extreme conditions. This means one can get rid of the problem of time having a beginning in a similar way in which we got rid of the edge of the world. Suppose the beginning of the universe was like the South Pole of the Earth with degrees of latitude playing the role of time. The universe would start as a point at the South Pole. As one moves north, the circles of constant latitude, representing the size of the universe, would expand. To ask what happened before the beginning of the universe would become a meaningless question because there is nothing south of the South Pole.

  Time, as measured in degrees of latitude, would have a beginning at the South Pole, but the South Pole is much like any other point, at least I was told so. I have been to Antartica but not to the South Pole.

  The same laws of Nature hold at the South Pole, as in other places. This would remove the age-old objection to the universe having a beginning that it would be a place where the normal laws broke down. The beginning of the universe would be governed by the laws of science.

  The picture Jim Hartle and I developed of the spontaneous quantum creation of the universe would be a bit like the formation of bubbles of steam in boiling water. The idea is that the most probable histories of the universe would be like the surfaces of the bubbles. Many small bubbles would appear and then disappear again. These would correspond to mini universes that would expand but would collapse again while still of microscopic size. They are possible alternative universes but they are not of much interest since they do not last long enough to develop galaxies and stars, let alone intelligent life. A few of the little bubbles, however, grow to a certain size at which they are safe from recollapse. They will continue to expand at an ever increasing rate and will form the bubbles we see. They will correspond to universes that would start off expanding at an ever increasing rate. This is called inflation like the way prices go up every year.

  The world record for inflation, was in Germany after the First World War. Prices rose by a factor of ten million in a period of 18 months. But that was nothing compared to inflation in the early universe. The universe expanded by a factor of million trillion trillion in a tiny fraction of a second. Unlike inflation in prices, inflation in the early universe was a very good thing. It produced a very large and uniform universe, just as we observe. However, it would not be completely uniform. In the sum over histories, histories that are very slightly irregular will have almost as high probabilities as the completely uniform and regular history. The theory therefore predicts that the early universe is likely to be slightly non-uniform. These irregularities would produce small variations in the intensity of the microwave background from different directions. The microwave background has been observed by the Map satellite and was found to have exactly the kind of variations predicted. So we know we are on the right lines.

  The irregularities in the early universe will mean that some regions will have slightly higher density than others. The gravitational attraction of the extra density will slow the expansion of the region and can eventually cause the region collapse to form galaxies and stars. So look well at the map of the microwave sky. It is the blue print for all the structure in the universe. We are the product of quantum fluctuations in the very early universe. God really does play dice.

  We have made tremendous progress in cosmology. When I began my research, cosmology was regarded as a pseudo science in which wild speculation was unconstrained by any observations. The situation soon began to change however, both because of observations made possible by new technology and through advances on the theoretical side. The theoretical prediction that the universe had a beginning in a state of high density and curvature was confirmed by the discovery of the microwave background with a thermal spectrum. Similarly, the theoretical prediction that the universe had an early period of inflation, was confirmed by, or is at least consistent with, observations of small variations in the intensity of microwaves from different directions. But not everything is solved. We do not yet have a good theoretical understanding of the observations that the expansion of the universe is accelerating again, after a long period of slowing down. Without such an understanding, we can not be sure of the future of the universe. Will it continue to expand forever? Is inflation a law of Nature? Or will the universe eventually collapse again? New observational results and theoretical advances are coming in rapidly. Cosmology is a very exciting and active subject. We are getting close to answering the age old questions. Why are we here? Where did we come from?

威尼斯的狼

[华文版]



  根據中非Boshongo人的傳說，世界太初只有黑暗，水和偉大的Bumba上帝。一天，Bumba胃痛發作，嘔吐出太陽。太陽灼乾了一些水，留下土地。他仍然胃痛不止，又吐出了月亮和星辰，然後吐出一些動物，豹﹑鱷魚﹑烏龜﹑最後是人。


  這個創世紀的神話，和其他許多神話一樣，試圖回答我們大家都想詰問的問題﹕為何我們在此？我們從何而來？一般的答案是，人類的起源是發生在比較近期的事。人類正在知識上和技術上不斷地取得進步。這樣，它不可能存在那麼久，否則的話，它應該取得更大的進步。這一點甚至在更早的時候就應該很清楚了。


  例如，按照Usher主教的說法，<<創世紀>>把世界的創生定於西元前4004年10月23日上午9時。另一方面，諸如山嶽和河流的自然環境，在人的生命週期裏改變甚微。所以人們通常把它們當作不變的背景。要麼作為空洞的風景已經存在了無限久，要麼是和人類在相同的時刻被創生出來。


  但是並非所有人都喜歡宇宙有個開端的思想。例如，希臘最著名的哲學家亞里士多德，相信宇宙已經存在了無限久的時間。某種永恆的東西比某種創生的東西更完美。他提出我們之所以看到發展處於這個情形，那是因為洪水或者其他自然災害，不斷重覆地讓文明回復到萌芽階段。信仰永恆宇宙的動機是想避免求助於神意的干涉，以創生宇宙並啟始運行。相反地，那些相信宇宙具有開端的人，將開端當作上帝存在的論據，把上帝當作宇宙的第一原因或者原動力。


  如果人們相信宇宙有一個開端，那麼很明顯的問題是，在開端之前發生了甚麼﹖上帝在創造宇宙之前，他在做甚麼？他是在為那些詰問這類問題的人準備地獄嗎？德國哲學家伊曼努爾 · 康德十分關心宇宙有無開端的問題。他覺得，不管宇宙有無開端，都會引起邏輯矛盾或者二律背反。如果宇宙有一個開端，為何在它起始之前要等待無限久。他將此稱為正題。另一方面，如果宇宙已經存在無限久，為甚麼它要花費無限長的時間才達到現在這個階段。他把此稱為反題。無論正題還是反題，都是基於康德的假設，幾乎所有人也是這麼辦的，那就是，時間是絕對的，也就是說，時間從無限的過去向無限的將來流逝。時間獨立於宇宙，在這個背景中，宇宙可以存在，也可以不存在。


  直至今天，在許多科學家的心中，仍然保持這樣的圖景。然而，1915年愛因斯坦提出他的革命性的廣義相對論。在該理論中，空間和時間不再是絕對的，不再是事件的固定背景。相反地，它們是動力量，宇宙中的物質和能量確定其形狀。它們只有在宇宙之中才能夠定義。這樣談論宇宙開端之前的時間是毫無意義的。這有點兒像去尋找比南極還南的一點沒有意義一樣。它是沒有定義的。


  如果宇宙隨時間本質上不變，正如20世紀20年代之前一般認為的那樣，就沒有理由阻止在過去任意早的時刻定義時間。人們總可以將歷史往更早的時刻延展﹐在這個意義上﹐任何所謂的宇宙開端都是人為的。於是，情形可以是這樣，這個宇宙是去年創生的，但是所有記憶和物理證據都顯得它要古老得多。這就產生了有關存在意義的高深哲學問題。我將採用所謂的實證主義方法來對付這些問題。在這個方法中，其思想是，我們按照我們構造世界的模型來解釋自己感官的輸入。人們不能詢問這個模型是否代表實在,只能問它能否行得通。首先，如果按照一個簡單而優雅的模型可以解釋大量的觀測；其次，如果這個模型作出可能被觀察檢驗，也可能被證偽的明確預言，這個模型即是一個好模型。


  根據實證主義方法，人們可以比較宇宙的兩個模型。第一個模型，宇宙是去年創生的，而另一個是宇宙已經存在了遠為長久的時間。一對孿生子在比一年前更早的時刻誕生﹐已經存在了久於一年的宇宙的模型能夠解釋像孿生子這樣的事物。


  另一方面，宇宙去年創生的模型不能解釋這類事件，因此第二個模型更好。人們不能詰問宇宙是否在一年前確實存在過，或者僅僅顯得是那樣。在實證主義的方法中，它們沒有區別。


  在一個不變的宇宙中，不存在一個自然的起始之點。然而，20世紀20年代當埃德溫 · 哈勃在在威爾遜山上開始利用100英寸的望遠鏡進行觀測時，情形發生了根本的改變。


  哈勃發現，恒星並非均勻地分佈於整個空間，而是大量地聚集在稱為星系的集團之中。


  哈勃測量來自星系的光，進而能夠確定它們的速度。他預料向我們飛來的星系和離我們飛去的星系一樣多。這是在一個隨時間不變的宇宙中應有的。但是令哈勃驚訝的是，他發現幾乎所有的星系都飛離我們而去。此外，星系離開我們越遠，則飛離得越快。宇宙不隨時間不變，不像原先所有人以為的那樣。它正在膨脹。星系之間的距離隨時間而增大。


  宇宙膨脹是20世紀或者任何世紀最重要的智力發現之一。它轉變了宇宙是否有一個開端的爭論。如果星系現在正分開運動，那麼，它們在過去一定更加靠近。如果它們過去的速度一直不變，則大約在150億年之前，所有星系應該一個落在另一個上。這個時刻是宇宙的開端嗎？


  許多科學家仍然不喜歡宇宙具有開端。因為這似乎意味著物理學崩潰了。人們就不得不去求助於外界的作用，為方便起見，可以把它稱作上帝﹐去確定宇宙如何起始。因此他們提出一些理論。在這些理論中，宇宙此刻正在膨脹，但是沒有開端。其中之一便是邦迪、高爾德和霍伊爾於1948年提出的穩恒態理論。


  在穩恒態理論中，其思想是，隨著星系離開，由假設中的在整個空間連續創生的物質形成新的星系。宇宙會永遠存在，而且在所有時間中都顯得一樣。這最後的性質從實證主義的觀點來看，作為一個可以用觀測來檢驗的明確預言﹐具有巨大的優點。在馬丁 · 賴爾領導下的劍橋射電觀測天文小組在20世紀60年代早期對弱射電源進行了調查。這些源在天空分佈得相當均勻，表明大部分源位於銀河系之外。平均而言，較弱的源離得較遠。

威尼斯的狼

  穩恆態理論預言了源的數目對應於源強度的圖的形狀。 但是觀測表明﹐微弱的源比預言的更多﹐這表明在過去源的密度較高。這就和穩恆態理論的任何東西在時間中都是不變的基本假設相衝突。由於這個﹐也由於其他原因﹐穩恆態理論被拋棄了。


  還有另一種避免宇宙有一開端的企圖是﹐建議存在一個早先的收縮相﹐但是由於旋轉和局部的無規性﹐物質不會落到同一點。相反﹐物質的不同部份會相互錯開﹐宇宙會重新膨脹﹐這時密度保持有限。兩位俄國人利弗席茲和哈拉尼科夫實際上聲稱﹐他們證明了﹐沒有嚴格對稱的一般收縮總會引起反彈﹐而密度保持有限。這個結果對於馬克思主義列寧主義的唯物辯証法十分便利﹐因為它避免了有關宇宙創生的難以應付的問題。因此﹐這對於蘇聯科學家而言成為一篇信仰的文章。


  當利弗席茲和哈拉尼科夫發表其斷言時﹐我是一名21歲的研究生﹐為了完成博士論文﹐我正在尋找一個問題。我不相信他們所謂的證明﹐於是就著手和羅傑 · 彭羅斯一起發展新的數學方法去研究這個問題。我們證明了宇宙不能反彈。如果愛因斯坦的廣義相對論是正確的﹐就存在一個奇點﹐這是具有無限密度和無限時空曲率的點﹐時間在那裡有一個開端。


  在我得到第一個奇點結果數月之後﹐即1965年10月﹐人們得到了確認宇宙有一個非常密集開端的思想的觀察證據﹐那是發現了貫穿整個空間的微弱的微波背景。這些微波和你使用的微波爐的微波是一樣的﹐但是比它微弱多了。它們只能將匹薩加熱到攝氏負270.4度, 甚至無法將匹薩化凍﹐更不用說烤熟它。實際上你自己就可以觀察到這些微波。把你的電視調到一個空的頻道去﹐在螢幕上看到的雪花的百分之幾就歸因於這個微波背景。早期非常熱和密集狀態遺留下的輻射是對這個背景的僅有的合理解釋。隨著宇宙膨脹﹐輻射一直冷卻下來﹐直至我們今天觀察到它的微弱的殘餘。


  雖然彭羅斯和我自己的奇性定理預言﹐宇宙有一個開端﹐這些定理並沒有告訴宇宙如何起始。廣義相對論方程在奇點處崩潰了。這樣﹐愛因斯坦理論不能預言宇宙如何起始﹐它只能預言一旦起始後如何演化。人們對彭羅斯和我的結果可有兩種態度。一種是上帝由於我們不能理解的原因，選擇宇宙的啟始方式。這是約翰保羅教皇的觀點。在梵帝岡的一次宇宙論會議上，這位教皇告訴代表們，在宇宙起始之後，研究它是可以的。但是他們不應該探究起始的本身，因為這是創生的時刻，這是上帝的事體。我暗自慶幸，他沒有意識到，我在會議上發表了一篇論文﹐剛好提出宇宙如何起始。我可不想像伽利略那樣被遞交給宗教裁判廳。


  對我們結果的另外解釋，這也是得到大多數科學家贊同的解釋。這個結果顯示，在早期宇宙中的非常強大的引力場中，廣義相對論崩潰了﹐必須用一個更完備的理論來取代它。因為廣義相對論沒有注意到物質小尺度結構﹐而後者是由量子理論制約的﹐所以人們預料總要進行這種取代。在通常情況下，因為宇宙的尺度和量子理論的微觀尺度相比較極為巨大﹐所以是否取代無所謂。但是當宇宙處於普朗克尺度，也就是1千億億億億分之一米時﹐這兩個尺度變成相同，必須考慮量子理論。


  為了理解宇宙的起源，我們必須把廣義相對論和量子理論相結合。里查德 · 費恩曼對歷史求和的思想似乎是實現這個目標的最佳方法。里查德 · 費恩曼是一位多姿多彩的人物。他在帕沙迪那的脫衣舞酒吧裡敲小鼓，又是加州理工學院卓越的物理學家。他提議一個系統從狀態A到狀態B經過所有可能的路徑或歷史。



  每個路徑或者歷史都有一定的振幅和強度。而系統從A到B的概率是將每個路徑的振幅加起來。存在一個由蘭乾酪製成月亮的歷史。但是其振幅很低。這對於老鼠來說不是一個好消息。



  宇宙現在狀態的概率可將結局為這個狀態的所有歷史疊加得到。但是這些歷史如何起始的呢？這是一個改頭換面的起源問題。是否需要一個造物主下達命令，宇宙如此這般起始呢﹖還是由科學定律來確定宇宙的初始條件呢﹖


  事實上，即便宇宙的歷史回到無限的過去，這個問題仍然存在。但是如果宇宙只在150億年前起始，這個問題就更加急切。詢問在時間的開端會發生甚麼，有點像當人們認為世界是平坦的，詢問在世界的邊緣會發生甚麼一樣。世界是一塊平板嗎？海洋從它邊緣上傾瀉下去嗎﹖我已經用實驗對此驗證過。我環球旅行過，我並沒有掉下去。


  正如大家知道的，當人們意識到世界不是一塊平板，而是一個彎曲的面時，在宇宙的邊緣發生甚麼的問題就被解決了。然而，時間似乎不同。它顯得和空間相分離。像是一個鐵軌模型。如果它有一個開端﹐就必須有人去啟動火車運行。

愛因斯坦的廣義相對論將時間和空間統一成時空。但是時間仍然和空間不同，它正像一個通道，要麼有開端和終結，要麼無限地伸展出去。然而，詹姆哈特爾和我意識到，當廣義相對論和量子論相結合時，在極端情形下，時間可以像空間中另一方向那樣行為。這意味著，和我們擺脫世界邊緣的方法類似，可以擺脫時間具有開端的問題。

威尼斯的狼

  假定宇宙的開端正如地球的南極，其緯度取時間的角色。宇宙就在南極作為一個起始點。隨著往北運動，代表宇宙尺度的常緯度的圓就膨脹。詰問在宇宙開端之前發生了甚麼是沒有意義的問題。因為在南極的南邊沒有任何東西。


  時間，用緯度來測量，在南極處有一個開端。但是南極和其他的點非常相像。至少我聽別人這麼講的。我去過南極洲，沒有去過南極。



  同樣的自然定律正如在其他地方一樣，在南極成立。長期以來﹐人們?t宙的開端是正常定律失效之處﹐所以宇宙不應該有開端。而現在﹐宇宙的開端由科學定律來制約﹐所以反對宇宙有開端的論證不再成立。


  詹姆 · 哈特爾和我發展宇宙自發創生的圖景有一點像泡泡在沸騰的水中形成。


  其思想是﹐宇宙最可能的歷史像是泡泡的表面。許多小泡泡出現﹐然後再消失。這些對應於微小的宇宙﹐它們膨脹﹐但在仍然處於微觀尺度時再次坍縮。它們是另外可能的宇宙﹐由於不能維持足夠長的時間﹐來不及發展星系和恆星﹐更不用說智慧生命了﹐所以我們對它們沒有多大興趣。然而﹐這些小泡泡中的一些會膨脹到一定的尺度﹐到那時可以安全地逃避坍縮。它們會繼續以不斷增大的速率膨脹﹐形成我們看到的泡泡。它們對應於開始以不斷增加的速率膨脹的宇宙。這就是所謂的暴脹﹐正如每年的價格上漲一樣。


  通貨膨脹的世界記錄應歸一戰以後的德國。在18月期間價格增大了一千萬倍。但是﹐它和早期宇宙中的暴脹相比實在微不足道。宇宙在比一秒還微小得多的時間裡膨脹了十的30次方倍。和通貨膨脹不同﹐早期宇宙的暴脹是非常好的事情。它產生了一個非常巨大的均勻的宇宙﹐正如我們觀察到的。然而﹐它不是完全均勻的。在對歷史求和中﹐稍微具有無規性的歷史和完全均勻和規則歷史的概率幾乎相同。因此﹐理論預言早期宇宙很可能是稍微不均勻的。這些無規性在從不同方向來的微波背景強度上引起小的變化。利用MAP(微波各向異性) 衛星已經觀察到微波背景﹐發現了和預言完全一致的變化。這樣，我們知道自已正在正確的道路上前進。


  早期宇宙中的無規性，意味著在有些區域的密度，比其他地方的稍高。這些額外密度的引力吸引使這個區域的膨脹減緩，而且最終能夠使這些區域坍縮形成星系和恒星。請仔細看這張微波天圖。它是宇宙中一切結構的藍圖。我們是極早期宇宙的量子起伏的產物。上帝的確在擲骰子。


  在過去的百年間，我們在宇宙學中取得了驚人的進步。廣義相對論和宇宙膨脹的發現，粉碎了永遠存在並將永遠繼續存在的宇宙的古老圖像。取而代之，廣義相對論預言，宇宙和時間本身都在大爆炸處起始。它還預言時間在黑洞裏終結。宇宙微波背景的發現，以及黑洞的觀測，支持這些結論。這是我們的宇宙圖像和實在本身的一個深刻的改變。


  雖然廣義相對論預言了，宇宙來自於過去一個高曲率的時期﹐但它不能預言宇宙如何從大爆炸形成。這樣﹐廣義相對論自身不能回答宇宙學的核心問題，為何宇宙如此這般。然而，如果廣義相對論和量子論相合併，就可能預言宇宙是如何起始的。它開始以不斷增大的速率膨脹。這兩個理論的結合預言，在這個稱作暴脹的時期，微小的起伏會發展，導致星系、恒星以及宇宙中所有其他結構的形成。對宇宙微波背景中的小的非均勻性的觀測，完全證實了預言的性質。這樣，我們似乎正朝著理解宇宙起源的正確方向前進，儘管還有許多工作要做。當我們通過精密測量空間航空器之間距離，進而能夠檢測到引力波，就會打開極早期宇宙的新窗口。引力波從最早的時刻自由地向我們傳播，所有介入的物質都無法阻礙它。與此相比較，自由電子多次地散射光。這種散射一直進行到30萬年後電子被凝結之前。


  儘管我們已經取得了一些偉大成功，並非一切都已解決。我們觀察到，宇宙的膨脹在長期的變緩之後，再次加速。對此理論還不能理解清楚。缺乏這種理解，對宇宙的未來還無法確定。它會繼續地無限地膨脹下去嗎？暴脹是一個自然定律嗎？或者宇宙最終會再次坍縮嗎？新的觀測結果，理論的進步正迅速湧來。宇宙學是一個非常激動人心和活躍的學科。我們正接近回答這古老的問題﹕我們為何在此？我們從何而來？