How can we see stars past 13.5 billion light years away?

  • #1
okay. i understand that stars can 46 million light years away despite the universe only being 13.5 billion years old. The universe is expanding. However what i dont' understand is how can we see a star that is say 20 billion lights years away if it takes 20 billion light years for the light to reach us?
is it just because our telescopes can see at a distance further? so that the distance is cut down, thus less time for us to see it?

im confused.
i appreciate anyone that can answer this.

Answers and Replies

  • #2
However what i dont' understand is how can we see a star that is say 20 billion lights years away if it takes 20 billion light years for the light to reach us?
The star was not 20 billion ly away when its light left.
  • #3
"When you look at the sky you look into the past."
  • #4
The fact is we don't observe them, but they do exist, just not in the observable universe.
  • #5
The motion of photons through the universe as the universe expands can be imagined reasonably well with the following mind picture. The picture is "correct" in the sense that the same mathematical principles apply in the simplified picture and in the real universe, and you can in fact use this to calculate various distances used in cosmology.

Imagine an infinite rubber sheet, with grid lines marked out upon it. Imagine that the sheet is infinitely stretchable; it can be stretched without limit.

For simplicity, assume that it is being stretched uniformly, so that grid lines are moving apart from each other, and that the rate at which any two given grid lines are moving apart from each other is constant. (We can revise this uniform expansion idea later)

Imagine an ant crawling over the sheet at a fixed velocity relative to the sheet at the point where it is crawling.

Now suppose that the rate at which the sheet is expanding is such that two adjacent grid lines are moving apart from each other more slowly than the ant is crawling. More distant grid lines will, of course, be receding from the ant at higher and higher velocities, proportional in fact to how far away they are. (This is the Hubble relation.)

The ant will, however, still manage to reach any grid line, however distant, if you wait long enough. Clearly, it can reach the next grid line, since it is moving faster than the grid line is receding.

Once there, the next grid line is now further off, since the sheet has stretched somewhat, but it can still reach that next grid line, since where ever the ant is on the sheet it is always moving faster than the next grid line it is approaching.

If you have a distant target point on the sheet which the ant is crawling towards, but which is receding from the ant faster than the ant is actually crawling, then the distance between the ant and the target will be increasing. But as the ant crosses more and more grid lines, the rate at which the distance is increasing falls. Eventually, as the ant crosses more and more of the sheet, it comes into regions where the speed of the ant becomes greater than the rate at which the target is receding, and from then on the distance to target starts to reduce. Eventually, the ant will reach the target.

In relating this to cosmology.
  • The grid lines correspond to "co-moving distance".
  • The distance as measured by a piece of string at a point of time between ant and target is called "proper distance".
  • The crawling speed of the ant corresponds to the speed of light.
  • The notion of time assumed in the mental picture without comment corresponds to "proper time".
  • In the real universe, the sheet will also be a bit wrinkled, with local curvature from mass concentrations in the universe, and it may also have a very large scale curvature as well, as if the sheet is stretched over an enormous ball, or saddle. But as matters stand, the large scale "shape" of the universe is flat to within what can be measured.
  • In the real universe, the rate at which the sheet expands is not uniform. Figuring out how the rate of expansion changes with time is a major focus in cosmology.
  • The real universe has three spatial dimensions, not two like a sheet. Imaging a large infinite block of foam being expanded would resolve this, if you really want.

See also Ned Wright's cosmology tutorial, especially Part 2: Homogeneity and Isotropy; Many Distances; Scale Factor. The following picture from that page is more or less a picture of the thought experiment I propose. The vertical axis is time. The horizontal axis is "proper distance". The black lines are locations of expanding grid lines. The red line is the track of the ant, or of a photon in an expanding universe.


Cheers -- sylas

PS. Kevin, you are mistaken. This post is explaining how we DO observe galaxies which are now 46 light years away. This is a confusing point and I'm happy to try and explain it more if I can. There are other advisors here who can go into even more detail on the matter.
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  • #6
The fact of the matter is that we do not see any stars or galaxies as they are now, that are 46 billion light years away. The further we look into our universe, the further back in time we are looking, and so the most distant objects are only seen as they once existed very early on in the universe’ beginning. The furthest we can look is to about 13.5 billion years back into our past.

The fact that these objects are now estimated to be about 46 billion light years away is nice, but we can’t really view them as they are today. They are therefore not really observable in the sense that we can actually see them in true time. We can see them as they once were 13 billion years ago, and when they were once very close to us as the universe was also much smaller back then, but they have since moved on.

We can estimate their distance from us now, but really they are invisible to us in their present state, and always will be. People say that they are observable, but this is not true.
  • #7
Dusty, we all get that, when looking at distant objects, we are really looking at them in their past.

It is all right to way they are observable; we just need to accept that there's a restriction on that.

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