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Name: Bruce Frazer
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 1999 - 2000


Question:
I've searched the archives for this question, with no luck. I understand that light from distant objects in the universe has taken up to 15 billion years to reach earth. In other words we are effectively looking back in time. My question is:

If light reaching earth is 15 billion years old, it was presumably emitted at or just after the big bang when all matter, including the source of the light and the particles that now make up the earth were quite close together. Unless the the universe is expanding at close to the speed of light the light emitted from a source that is now 15 billion light years away would have reached the particles that now make up the earth very shortly after emission, rather than taking 15 billion years. How can we now see this light?

I am a teacher by training but must confess to being lapsed. My wife, however, still teaches and she, too, would be interested in your answer. I also have teenage daughters at high school.


Replies:
Space itself is expanding. 10 billion years ago the Milky Way and Galaxy X were a billion light years apart, let us say. Light started out from Galaxy X towards us. Does it take 1 gigayear to get here?

No, because the space between Galaxy X and the Milky Way is continuously getting bigger. In a billion years the light covers 1 billion light years, but, alas, the space has increased to, let us say, 1.9 billion light years, and the light must press on. After another 0.9 billion years, the light has covered 1.9 billion years, but now the space has increased to 2.71 billion light years and the light has another 0.81 billion light years to go. And. . .
time    light has covered distance is now light must still cover
 Gy           Gly              Gly                 Gly
---------------------------------------------------------------
 0          0                  1.0                  1.0
 1.0        1.0                1.9                  0.9
 1.9        1.9                2.71                 0.81
 2.71       2.71               3.439                0.729
 3.439      3.439              4.0951               0.6561
 4.0951     4.0951             4.68559              0.59049
   .          .                 .                    .
   .          .                 .                    .
   .          .                 .                    .
10.0000    10.0000            10.00000              10.0000

You'll note that this behaviour means a definite relationship exists between how far away something is and how fast it is moving away. This is Hubble's Law!

Dr. C. Grayce



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