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Most Distant Object, Age Of Universe, Etc.

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Posted by Sander on November 22, 2002 20:29:25 UTC

I am confused about something that I am having difficulty resolving by searches through this forum and elsewhere. Specifically, what is the relationship between the most distant object that can be seen and the size and age of the universe?

That is, given an ever-expanding universe (I'm not an astronomer, so I'll assume the standard model of "Big Bang", expansion still underway, etc.) and an estimated age of 15 billion years (let's say), how is it possible that we can see objects anywhere near the distance of 15 billion light years? Since the light takes that long to reach us, wouldn't those objects have to be at this point in time at LEAST 30 billion light years away (with the universe's age that much greater)?

Should I assume that when astronomers report that when they have discovered such objects what they REALLY mean is that they have discovered objects whose light (red-shift?) indicates that BY NOW they must be that distant? When these reports are made it sounds as though they are actually LOOKING 15 billion (say) light years into space ( which is impossible, no?), One exception I found was at http://www.er.doe.gov/feature_articles_2001/June/Decades/44.html,
where it does spell out:
"Because the universe is expanding, the quasar was only about 4 billion light years from Earth when the light seen now was emitted"
which makes sense to me, except that they also state that "This quasar had the highest redshift ever seen, corresponding to a distance of 27 billion light years from Earth", which would contradict the widely-accepted age of the universe!

So, another way of asking my "question" (to the extent that it can be coherently characterized as such!) is how far back into space/time can we PRESENTLY see? My theory is that if we assume a universe age of 15 billion, and that space has been expanding at an ever-increasing rate since its birth, then the most distant light source we could possibly see could only appear PRESENTLY to be four to six billion light years distant. Any "sightings" of greater distance are inferring the distance.

This is probably very basic astronomy (or cosmology, perhaps) question , so I apologize for my lack of sophistication, but I really have had difficulty resolving this succinctly.

Kindly enlighten me!


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