Quote:
Originally Posted by sman
I've heard of telescopes that see BILLIONS of light years away. Here's my problem: the universe is only ~15 billion years old. So if we are looking at something 15 billion ly away, we are looking 15b years into the past - at that time when whatever we are looking at should've been RIGHT HERE.
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Close, but not quite. There are two different kinds of distance that are important here. Light travel time is the distance you're thinking of. If we see a galaxy and the light took 10 billion years to reach us then you might think it is 10 billion lightyears away. But, the universe is expanding, so while the light from that galaxy traveled for 10 billion years the galaxy from which it came moved away from us.
The comoving distance is how far the galaxy is from us today which can be more than 13.7 billion lightyears because of expansion. If the light travel time is 10 billion years then the comoving distance is about 16.4 billion lightyears. The furthest back we can see is to when the universe was filled with opaque plasma which is today viewed as the
cosmic microwave background.
That was more than 13 billion years ago. The light that we see as the CMB today was only 36 million lightyears from us (or from the matter that would eventually become the Milky Way) when it was released. The universe has expanded about 1300 times over since that happened meaning the matter which emitted that light which we see today is currently about 46 billion lightyears from us.
So, in general, your post is correct. The further back we look the closer everything was. The only reason it seems like a paradox is because you're mixing up the two different types of distance. A comoving distance of 20 or 30 billion lightyears does not mean the light was emitted 20 or 30 billion years ago.
~modest
