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When we observe a galaxy farther than 13 billion light years away, we see that galaxy as it was 13 billion years ago. But back then, that galaxy was much closer to us ,if indeed we live in an expanding and accelerating universe. The question is, why we see it so far when in fact it was very close to us and the time for the light to reach us was much shorter?


marked as duplicate by Rob Jeffries, Jon Custer, John Rennie cosmology Nov 3 '16 at 16:42

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  • $\begingroup$ The Big Bang didn't happen at a point. If the universe is infinite then it has always been infinite so whatever time and distance you choose there are galaxies farther away than that. $\endgroup$ – John Rennie Nov 3 '16 at 8:48
  • $\begingroup$ The nature of expanding space is that, it's not just moving away from us, but we're moving away from it. When a photon was emitted in our direction 13 billion years ago, that photon has to travel through expanding space, so, from the photon's perspective, our galaxy was moving away from it fast enough that it took 13 billion years to reach us, even if the initial distance was much less than that. $\endgroup$ – userLTK Nov 3 '16 at 10:05
  • $\begingroup$ Nothing is "moving" (much) - it is space that has expanded. $\endgroup$ – Rob Jeffries Nov 3 '16 at 12:18
  • 2
    $\begingroup$ Possible duplicate of Are scientists missing the point with distant cosmic objects, or is it just me? $\endgroup$ – Rob Jeffries Nov 3 '16 at 12:19

Have a look at this timeline of the universe:

history of universe

The x axis is the time axis. After the "dark ages" there are galaxies formed, which become diluted in space as time grows.

The question is, why we see it so far when in fact it was very close to us and the time for the light to reach us was much shorter?

Because light has to travel a larger distance than when the universe was more restricted. If we existed at the formation of the two galaxies it would have taken less time to see the galaxy. We exist now, and the photons that reach us now have to travel the larger distance even though they left at a time when it was close to our galaxy because the distance was expanding.

Take the expanding balloon with dots on it. The distance between dots changes with time as the balloon expands. The photons are just a ruler measuring the expanding distance, imo.

  • $\begingroup$ I am afraid that the answers above do not address the issue. Photons travel in vacuum at the speed of light. The age of the universe is 13.7 Billion years old. Back then, the galaxies we see at 13.1 light years away were on top of our galaxy. The dots on the expanding balloon that represent the two galaxies we are talking about, move away from each other at a slower speed than light, so the photons emitted from the faraway galaxy should have already reached us long time ago. The paradox still stands. $\endgroup$ – Benzi Rosenski Nov 3 '16 at 21:37
  • $\begingroup$ velocity c is not instantaneous. In the analogy the balloon distances are measured with a meter ruler, not with photons. Transparency of light came at 10^10 seconds ( hyperphysics.phy-astr.gsu.edu/hbase/astro/unify.html#c1 ). If we were on top of each other we would be close enough now too. It is the ones that were at the edge of the then universe that create your conundrum, (expansion is not so fast after inflation period and dark ages) we would have seen them then but not now. $\endgroup$ – anna v Nov 4 '16 at 4:49
  • $\begingroup$ When they were close to the edge for our observable universe they were emitting photons which reach us at different times, they were emitted at the same time but we see them at different times, as the distance keeps increasing. The photons still come even though the galaxy is not in the visible universe. $\endgroup$ – anna v Nov 4 '16 at 4:55
  • $\begingroup$ This paper explains how things go with special relativity and general relativity and the expansion of the big bang model. arxiv.org/pdf/astro-ph/0310808v2.pdf $\endgroup$ – anna v Nov 4 '16 at 5:02

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