Have a look at the link below:
The Distance Scale of the Universe.
The link explains the different distance measures used in astronomy/cosmology - it is a useful reference for the rest of this post (although the numbers used are a little old and a little high, it explains the principles well).
spacehugo is correct when he says that if something moves away from you, by the time you see it is further away. But astronomers know this and it is factored into the numbers they give.
First we have to consider the distance away an object was when it emitted the light we see. We can work this out using a combination of how dim it looks and how large it looks (luminosity and angular diameter).
But we also want to consider how far away that object is when we finally see the light it emitted. We only see it at the size it was when it emitted the light, so size is no help in predicting where it will have moved to since. We only have the light to go on. For distant galaxies, we find the light is very dim and the spectrum of that light shows a
redshift. The dimmer the light, the higher the redshift, the more distant the galaxy. But what do we mean by distant? We mean that the light has been travelling for a long time.
So, what do those very dim, highly redshifted galaxies look like? Well, they actually have rather large angular diameters, and this is because they were relatively close to us when they emitted the light we are now seeing. The reason that the light took so long to reach us is because the universe is expanding and was expanding very fast early on. The expansion puts more distance between that light and its eventual "target", which in this case is us!
The most distant galaxies in the Hubble Deep field emitted their light around 12.8 billion years ago (light travel time), but they were only around 3.5 billion light years away (angular diameter) when they emitted the light we are now seeing. We think they are now nearly 30 billion light years away (comoving distance) due to the expansion of the universe. The distance that the light had to cross in order to reach us has been changing for the whole time that light was travelling!
Hubble actually did a "north" and "south" Deep Field in opposite directions, and found pretty much the same results. As we look out in all directions we are looking backwards in time towards the Big Bang, a time when everything was really close to us.
We see galaxies that are apparently moving away from us in all directions and the further we look (backwards in time) the faster they seem to recede. The most "distant" galaxies were really close to us and were apparently receding faster than light when they emitted the light we finally see (we can see it because the expansion of the universe was slowing down for a very long time, which allowed that light to finally enter regions of the universe that were receding from us slower than light).
But it would be a mistake to conclude from the above that we are at the centre of the universe. We
are at the centre of our observable universe, which is made up of the history of all the light we can see, but we think that wherever you are in the universe you would see pretty much the same thing. Whatever galaxy you live in, as you look out into space you would see the history of the universe played out backwards at the speed of light, and everything else would seem to get closer to you as you look further and further back in time.
The limit on our observable universe is the limit on how long any light has had in which to travel. The whole caboodle could be many magnitudes larger than the parts we can see. Or perhaps it is infinite?
UFO whistleblowers tell Congress 'we are not alone in the cosmos' (video)
Basic Error: The accelerating Universe conclusion - reason
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