Yet so strong was the belief in a static universe that it persisted into the early twentieth century. Even Einstein, when he formulated the general theory of relativity in 1915, was so sure that the universe had to be static that he modified his theory to make this possible, introducing a so-called cosmological constant into his equations. Einstein introduced a new “antigravity” force, which, unlike other forces, did not come from any particular source but was built into the very fabric of space-time. He claimed that space-time had an inbuilt tendency to expand, and this could be made to balance exactly the attraction of all the matter in the universe, so that a static universe would result. Only one man, it seems, was willing to take general relativity at face value, and while Einstein and other physicists were looking for ways of avoiding general relativity’s prediction of a nonstatic universe, the Russian physicist and mathematician Alexander Friedmann instead set about explaining it.
Friedmann made two very simple assumptions about the universe: that the universe looks identical in whichever direction we look, and that this would also be true if we were observing the universe from anywhere else. From these two ideas alone, Friedmann showed that we should not expect the universe to be static. In fact, in 1922, several years before Edwin Hubble’s discovery, Friedmann predicted exactly what Hubble found!
The assumption that the universe looks the same in every direction is clearly not true in reality. For example, as we have seen, the other stars in our galaxy form a distinct band of light across the night sky, called the Milky Way. But if we look at distant galaxies, there seems to be more or less the same number of them. So the universe does seem to be roughly the same in every direction, provided one views it on a large scale compared to the distance between galaxies, and ignores the differences on small scales. For a long time, this was sufficient justification for Friedmann’s assumption - as a rough approximation to the real universe. But more recently a lucky accident uncovered the fact that Friedmann’s assumption is in fact a remarkably accurate description of our universe.
Static 靜態的
然而，靜態宇宙的信念是如此之強，以至於一直維持到了20世紀的早期。甚至愛因斯坦於1915年發表其廣義相對論時，還是如此之肯定宇宙必須是靜態的，以使得他在其方程中不得不引進一個所謂的宇宙常數來修正自己的理論，使靜態的宇宙成為可能。愛因斯坦引入一個新的“反引力”，這力不像其他的力那樣，不發源於任何特別的源，而是空間——時間結構所固有的。他宣稱，空間——時間有一內在的膨脹的趨向，這可以用來剛好去平衡宇宙間所有物質的相互吸引，結果使宇宙成為靜態的。當愛因斯坦和其他物理學家正在想方設法避免廣義相對論的非靜態宇宙的預言時，看來只有一個人，即俄國物理學家和數學家亞歷山大·弗利德曼願意只用廣義相對論著手解釋它。
弗利德曼對於宇宙作了兩個非常簡單的假定：我們不論往哪個方向看，也不論在任何地方進行觀察，宇宙看起來都是一樣的。弗利德曼指出，僅僅從這兩個觀念出發，我們就應該預料宇宙不是靜態的。事實上，弗利德曼在1922年所做的預言，正是幾年之後埃得溫·哈勃所觀察到的結果。
很清楚，關於在任何方向上宇宙都顯得是一樣的假設實際上是不對的。例如，正如我們所看到的，我們星系中的其他恆星形成了橫貫夜空的叫做銀河系的光帶。但是如果看得更遠，星系數目就或多或少顯得是同樣的。所以假定我們在比星系間距離更大的尺度下來觀察，而不管在小尺度下的差異，則宇宙確實在所有的方向看起來是大致一樣的。在很長的時間裡，這為弗利德曼的假設——作為實際宇宙的粗糙近似提供了充分的證實。但是，近世出現的一樁幸運的事件所揭示的事實說明了，弗利德曼假設實際上異常準確地描述了我們的宇宙。
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