spectroscope

The Magic of Reality by Richard Dawkinsを読んでいます。
When and how did every thing begin?の章は世界の始まりについての説明でした。ビッグバンが世界の始まりだという事は聞いたことがありますが、これまで、何故そんな事が分かったのか、どれ程その説に信憑性があるのか全く知りませんでしたが、 Richard Dawkinsの説明は非常に分かり易く、納得しました。
Dawkinsは前章の虹の話に関連する "spectroscope" から始め、そして "Doppler effect" を説明し、それで "big bang" の発生に至ることを丁寧に説明します。抜粋して引用します。
I was once asked to contribute a chapter to a book in which scientists were invited to nominate the most important invention ever.
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I chose the spectroscope.
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A spectroscope is a rainbow machine. If it is attached to a telescope, it takes the light from one particular star or galaxy and spreads it out as a spectrum, just an Newton did with his prism. But it is more sophisticatated than Newton's prism, because it allows you to make exact measurements along the spread-out spectrum of starlight.
そして "Doppler effect" により次のことが分かります。
All the galaxies in the universe are rushing away from each other, which means that they are rushing away from us too. It doesn't matter which direction you point your telescope in, the more distant galaxies are moving away from us (and from one another) at ever-increasing speed. The entire universe -- space itself -- is expanding at a colossal rate.

宇宙は広がっている事が観測されたのですが、これをさかのぼると、ビッグバンにたどり着くことになるのです。
Backwards to the big bang
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That's how they know it was somewhere between 13 and 14 billion years ago. That was the moment when the universe itself began -- the moment called the 'big bang'.
抜粋なのでこれでは良く分からないという方は、是非オリジナルのThe Magic of Realityを読むことをお薦めします。