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Re: The Inquiring Mind Wants To Know

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Posted by YANNIRU/">YANNIRU on May 25, 1999 13:02:59 UTC

: : Hello Greg: The error is to think that the last scattered light came from a point. It actually came from an infinite universe in the sense that a circle never ends. Think of us as a point on the circle. At some time in the distant past, the circle began to radiate light and at the same time the circle was expanding. So by the time the first light reaches us, the circle has expanded so much that we see microwaves rather than visible light. But since this first light just goes round and round the circle we will continue to receive it forever. That situation is identical to the universe except it's in 2-D instead of 4-D. Suppose the circle were 20 billion years old and our point came into existence 5 billion years ago. Then our point would see the first light when it was already 15 billion years old. And we see it today when its 20 billion years old. And we will continue to see it in the future because the first light just goes round and round the circle. It may be that the circle is so big that the first light has not yet made one complete revolution. And it may never make one revolution if the circle is big enough because points on the circle are going away from our point faster than the speed of light. But do we know the actual size of the circle? We can guess how big the circle is by assuming it started at a point and expanded at the speed of light until first light. Then from physics we know the temperature of the universe when light was first allowed to propagate. That was 3000 degrees K and now it's 3 degrees K. So the universe expanded by a factor of 1000 since light first propagated. The universe was 300,000 years old at first light. So now its size is only 300 million light-years and the first light has gone around the circle many times. Of course in the actual universe inflation made the size of the universe at first light very much larger than #00,000 light-years. So we do not know its actual size after all.

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