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I've never fully understood why we can still see galaxies that are 10 billion light years away. The age of the universe is calculated to be 13.9 billion years old and we live on a planet that it is roughly 4 billion years old. The universe came into existence 13.9 billion years ago and expanded incredibly quickly but those galaxies that were created in those early years are still visible to us via the amazing Hubble telescope (see the latest from Hubble). Those galaxies would surely have long gone so why can we still see the light from so long ago? How can we see further and further back to the beginning of the universe because surely the light from the moment of the big bang and subsequent eons would have past by us long ago?

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Hi Matt, welcome! I think I understand your question, but before to write an answer it is better to clarify your doubt: is it your question about the "death" of such an old galaxies, or more related to how we can see in the past? – Py-ser Jun 6 '14 at 8:06
More a question of how we can see in the past. I'm trying to understand why the light that left these galaxies is still arriving here on Earth. How did we jump ahead of the light that left those galaxies 10 billion years ago? – Matt Jun 6 '14 at 8:45
Are you aware of the Hubble law? :) based on your answer, I can write a proper answer! – Py-ser Jun 6 '14 at 8:52
I'm thinking of the universe as a big balloon where we all started from the same starting point. So why, if I'm on the circumference of the balloon, can I see a galaxy that is back near where I started? If you explain in terms of balloons I will be fine :) Just kidding. – Matt Jun 6 '14 at 8:57
Py-ser, I am aware of Hubble's law but can't say I understand it fully – Matt Jun 6 '14 at 9:13
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How can we see further and further back to the beginning of the universe because surely the light from the moment of the big bang and subsequent eons would have past by us long ago?

A simple intuitive way to understand that part of your question: Bigbang happened everywhere, in all of space. As time passes we see more distant parts of the bigbang, or rather, of the microwave background which is the furthest away phenomena we can see. The bigbang as it happened for example 1 billion years ago has passed by us long ago. Today we see the bigbang as it happened 13.8 billion years ago.

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Those galaxies were 10 billion light years away from Earth. So light would take much more time to reach here and that light which is now 10 billion years older can be seen now. Even light from the Sun takes 8 minutes to reach to us. So if somehow sun disappears suddenly(very unlikely) we wouldn't know for 8 minutes.

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The deepest we look into the space the further in the past we look. So if we take pictures of galaxies that are 10 billion ly away we see them as they were 10 billion years ago.

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We can't see how those galaxies look right now. If a galaxy appears to be 10 billion light years away, that also means that the light took 10 billion years to reach us. It's a bit confusing that "distance" and "time" are sometimes the same thing in astronomy.

So the light which the galaxies emit today (if they still exist) will reach us in another 10 billion years. What we see right now is light that was emitted 10 billion years in the past.

This also means that by looking at the farthest possible distance, we can see light which was created shortly after the big bang. One such source of light is the ubiquitous background radiation which was probably created by the big bang itself. It's easy to see because it fills all the gaps between the celestial objects.

Unfortunately, this also means that we always get an "outdated" view of the universe. If aliens started to blowing stars at the far side of our galaxy, it would take us 100'000 years to see the light.

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So we, in our galaxy, were already 10 billion light years away when the light from those galaxies left them? And so when the big bang happened we were already far away that it would take 13.9 billion years for us to see the light? It is this aspect of time and distance along with the timeline of events that I am struggling with. – Matt Dec 10 '14 at 21:29
That is a pretty complex question. In a nutshell, yes, the galaxies were 10 billion years away when the light was emitted which we see today. But the universe is still expanding, so they were probably closer but as the light traveled towards us, the distance towards us grew a bit. In fact, there are reasons to believe that the universe is much bigger than we can ever see. After all this time, the outer fringes are expanding faster than the speed of light in relation to our position. So no light they will ever emit could reach us. But we might see old versions of those areas. – Aaron Digulla Dec 11 '14 at 9:53

If the speed of light was and still is constant since at least the first moments of the Big Bang then space must have inflated faster than the speed of light at first but then de creased its expansion rate, otherwise light would still be travelling and we would never see the microwave background. If the rate of inflation were less than c then old stars would have died already and we would not be able to see them nowadays. I am struggling with the concept of relativity. Time passes differently depending on the observer and that has been measured, and is consistent with the constant c. But how does inflation affect time? How does everything above fit together, if it makes any sense at all?

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I'm not sure this answers the question. It seems you have new questions to ask. Perhaps you could ask your questions about relativity here, or on the physics stack exchange. – James K Dec 4 '15 at 17:57

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