At roughly the same time as Penzias and Wilson were investigating noise in their detector, two American physicists at nearby Princeton University, Bob Dicke and Jim Peebles, were also taking an interest in microwaves. They were working on a suggestion, made by George Gamow (once a student of Alexander Friedmann), that the early universe should have been very hot and dense, glowing white hot. Dicke and Peebles argued that we should still be able to see the glow of the early universe, because light from very distant parts of it would only just be reaching us now. However, the expansion of the universe meant that this light should be so greatly red-shifted that it would appear to us now as microwave radiation. Dicke and Peebles were preparing to look for this radiation when Penzias and Wilson heard about their work and realized that they had already found it. For this, Penzias and Wilson were awarded the Nobel Prize in 1978 (which seems a bit hard on Dicke and Peebles, not to mention Gamow!).
Now at first sight, all this evidence that the universe looks the same whichever direction we look in might seem to suggest there is some-thing special about our place in the universe. In particular, it might seem that if we observe all other galaxies to be moving away from us, then we must be at the center of the universe. There is, however, an alternate explanation: the universe might look the same in every direction as seen from any other galaxy too. This, as we have seen, was Friedmann’s second assumption. We have no scientific evidence for, or against, this assumption. We believe it only on grounds of modesty: it would be most remarkable if the universe looked the same in every direction around us, but not around other points in the universe! In Friedmann’s model, all the galaxies are moving directly away from each other. The situation is rather like a balloon with a number of spots painted on it being steadily blown up. As the balloon expands, the distance between any two spots increases, but there is no spot that can be said to be the center of the expansion. Moreover, the farther apart the spots are, the faster they will be moving apart. Similarly, in Friedmann’s model the speed at which any two galaxies are moving apart is proportional to the distance between them. So it predicted that the red shift of a galaxy should be directly proportional to its distance from us, exactly as Hubble found. Despite the success of his model and his prediction of Hubble’s observations, Friedmann’s work remained largely unknown in the West until similar models were discovered in 1935 by the American physicist Howard Robertson and the British mathematician Arthur Walker, in response to Hubble’s discovery of the uniform expansion of the universe.
Proportional 成比例的
大約同時，在附近的普林斯頓的兩位美國物理學家，羅伯特·狄克和詹姆士·皮帕爾斯也對微波感興趣。他們正在研究喬治·伽莫夫（曾為亞歷山大·弗利德曼的學生）的一個見解：早期的宇宙必須是非常密集的、白熱的。狄克和皮帕爾斯認為，我們仍然能看到早期宇宙的白熱，這是因為光是從它的非常遠的部分來，剛好現在才到達我們這兒。然而，宇宙的膨脹使得這光被如此厲害地紅移，以至於現在只能作為微波輻射被我們所看到。正當狄克和皮帕爾斯準備尋找這輻射時，彭齊亞斯和威爾遜聽到了他們所進行的工作，並意識到，自己已經找到了它。為此，彭齊亞斯和威爾遜被授予1978年的諾貝爾獎（狄克和皮帕爾斯看來有點難過，更別提伽莫夫了！）
現在初看起來，關於宇宙在任何方向看起來都一樣的所有證據似乎暗示，我們在宇宙的位置有點特殊。特別是，如果我們看到所有其他的星系都遠離我們而去，那似乎我們必須在宇宙的中心。然而，還存在另外的解釋：從任何其他星系上看宇宙，在任何方向上也都一樣。我們知道，這正是弗利德曼的第二個假設。我們沒有任何科學的證據去相信或反駁這個假設。我們之所以相信它只是基於謙虛：因為如果宇宙只是在我們這兒看起來各向同性，而在宇宙的其他地方並非如此，則是非常異的！在弗利德曼模型中，所有的星系都直接相互離開。這種情形很像一個畫上好多斑點的氣球被逐漸吹脹。當氣球膨脹時，任何兩個斑點之間的距離加大，但是沒有一個斑點可認為是膨脹的中心。並且斑點相離得越遠，則它們互相離開得越快。類似地，在弗利德曼的模型中，任何兩個星系互相離開的速度和它們之間的距離成正比。所以它預言，星系的紅移應與離開我們的距離成正比，這正是哈勃所發現的。儘管他的模型的成功以及預言了哈勃的觀測，但是直到1935年，為了響應哈勃的宇宙的均勻膨脹的發現，美國物理學家哈瓦·羅伯遜和英國數學家阿瑟·瓦爾克提出了類似的模型後，弗利德曼的工作在西方才被普遍知道。
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