In 1965 two American physicists at the Bell Telephone Laboratories in New Jersey, Arno Penzias and Robert Wilson, were testing a very sensitive microwave detector. (Microwaves are just like light waves, but with a wavelength of around a centimeter.) Penzias and Wilson were worried when they found that their detector was picking up more noise than it ought to.
The noise did not appear to be coming from any particular direction. First they discovered bird droppings in their detector and checked for other possible malfunctions, but soon ruled these out. They knew that any noise from within the atmosphere would be stronger when the detector was not pointing straight up than when it was, because light rays travel through much more atmosphere when received from near the horizon than when received from directly overhead. The extra noise was the same whichever direction the detector was pointed, so it must come from outside the atmosphere. It was also the same day and night and throughout the year, even though the earth was rotating on its axis and orbiting around the sun. This showed that the radiation must come from beyond the Solar System, and even from beyond the galaxy, as otherwise it would vary as the movement of earth pointed the detector in different directions.
In fact, we know that the radiation must have traveled to us across most of the observable universe, and since it appears to be the same in different directions, the universe must also be the same in every direction, if only on a large scale. We now know that whichever direction we look, this noise never varies by more than a tiny fraction: so Penzias and Wilson had unwittingly stumbled across a remarkably accurate confirmation of Friedmann’s first assumption. However, because the universe is not exactly the same in every direction, but only on average on a large scale, the microwaves cannot be exactly the same in every direction either. There have to be slight variations between different directions. These were first detected in 1992by the Cosmic Background Explorer satellite, or COBE, at a level of about one part in a hundred thousand. Small though these variations are, they are very important, as will be explained in Chapter 8.
Microwave 微波
Variation 變化
1965年，美國新澤西州貝爾電話實驗室的阿諾·彭齊亞斯和羅伯特·威爾遜正在檢測一個非常靈敏的微波探測器時（微波正如光波，但是它的波長大約為1英寸），他們的檢測器收到了比預想的還要大的噪聲。彭齊亞斯和威爾遜為此而憂慮，這噪聲不像是從任何特別方向來的。首先他們在探測器上發現了鳥糞並檢查了其他可能的故障，但很快就排除了這些可能性。他們知道，當探測器傾斜地指向天空時，從大氣層裡來的噪聲應該比原先垂直指向時更強，因為光線在沿著靠近地平線方向比在頭頂方向要穿過更厚的大氣。然而，不管探測器朝什麼方向，這額外的噪聲都是一樣的，所以它必須是從大氣層以外來的，並且在白天、夜晚、整年，也就是甚至地球繞著自己的軸自轉或繞太陽公轉時也是一樣的。這表明，這輻射必須來自太陽系以外，甚至星系之外，否則當地球的運動使探測器指向不同方向時，噪聲必須變化。
事實上，我們知道這輻射必須穿過我們可觀察到的宇宙的大部分，並且由於它在不同方向都一樣，至少在大尺度下，這宇宙也必須是各向同性的。現在我們知道，不管我們朝什麼方向看，這噪聲的變化總是非常小。這樣，彭齊亞斯和威爾遜無意中非常精確地證實了弗利德曼的第一個假設。然而，由於宇宙並非在每一個方向上，而是在大尺度的平均上相同，所以微波也不可能在每一個方向上完全相同。在不同的方向之間必須有一些小變化。1992年宇宙背景探險者，或稱為COBE，首次把它們檢測到，其幅度大約為10萬分之1。儘管這些變化很小，正如我們將在第八章解釋的，但它們非常重要。
閱讀更多 聰哥記單詞 的文章
