Expert: Courtney Seligman - 8/13/2011

Question
QUESTION: How is it possible that the light from the cosmic horizon is older than the age of our universe?

ANSWER: It isn't possible. Wherever you heard that has to be wrong. The cosmic horizon represents the time when the Universe became transparent to the light struggling through it, which was more than a hundred thousand years after the beginning of the Universe. So the light from the cosmic horizon is at least that much younger than the Universe.

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QUESTION: right...I think I should not have used the word 'age'.  Is light visible from a distance of more than 13.7 bly? I have read that astronomers can detect light from a distance of 42 billion light years. Has this light traveled for 42 billion years?

Answer
The 42 billion light years refers to the present distance of objects that we see near the cosmic horizon. At the time their light was emitted, they were only a few tens or hundreds of thousands of light years away, but the space between here and there was expanding at nearly the speed of light, so it took the light 13.7 billion years to traverse the space originally between us, and the "new" space resulting from the expansion of that space during the light-travel time. In other words, the 13.7 billion years refers to (1) the "age" of the Universe at the end of the Cosmic Fireball and (2) the time it took the light from the cosmic horizon to reach us. The actual distance of the galaxies we see as they were at that time was only a few tens or hundreds of thousands of light years (in other words, they were much closer than the 13.7 billion light-travel-time seems to imply). (In case it makes any difference, their actual distance is referred to as their "angular distance", because it is related to how big they look. For most objects, the further away they are the smaller they look; but for very distant objects, since we see them as they were when they were much closer, they can actually look bigger than nearer galaxies (!!!).)

So what is the 42 billion light years? As stated above, it is the estimated distance of those same objects at the present time. During the 13.7 billion years it took the light to get here, the space between us and the original position of the galaxies expanded to nearly 13.7 billion light years. But the space where the galaxies are has expanded even more than that, because being further away than the light that reached us, there was more space between us and them than between us and their light, and if that space expanded at the same rate per unit of distance it must have expanded by far more than the 13.7 billion light years that the "original" space expanded. In other words, they must now be more than twice as far away as the age of the Universe, and probably considerably farther away.

Just how far they now are depends upon certain assumptions about the geometry of space-time -- whether it is "flat", "spherical", or "hyperbolic". Each assumption gives a different estimate of the current distance of the ancient galaxies we see near the beginning of the Universe. The 42 billion light year distance corresponds to the assumption that space-time is more or less "flat", which is the most popular view at the moment. But they could be a bit closer or a bit further away, if some other assumption about the shape of space-time is correct. However, as explained in the second paragraph, no matter what the shape of the Universe is, they must be considerably further than 2 times 13.7 or 27.4 billion light years away. As it happens a "flat" space-time assumption puts them at about half again that distance, or about 42 billion light years away.

I might note that we will never ever be able to see them as they are now. Since even at their original distance they were moving away from us at nearly the speed of light, and as they moved further away there was more space between us and them, which of necessity must have expanded even faster than the original space, they have long been in the region "outside" the "observable Universe" where their light is traveling toward us (at the speed of light) more slowly than the space between us and them is expanding. So we can see them as they were near the dawn of time, but no matter how long we wait we will never be able to see them as they were after that.

I suspect the above may need a bit of "cleaning up" to be as clear as possible, but I have to leave for an engagement in a few minutes, and I wanted to send you an answer as soon as possible, instead of making you wait until late tonight or tomorrow. If there is any part of this answer that you would like explained in a different way, just let me know and I'll be happy to oblige (although, as noted, probably very late tonight or tomorrow).