# How can we see galaxies behind other galaxies?

1. Jul 10, 2015

### theironmaiden

How can we see galaxies behind other galaxies?

2. Jul 10, 2015

### Staff: Mentor

Sometimes, by exploiting gravitational lensing.

3. Jul 10, 2015

### Staff: Mentor

Galaxies aren't opaque objects. There is an immense amount of space between stars and planets. The only time something truly blocks our line of sight is if it is a large dust cloud (and sometimes we can just switch to another wavelength to image at and see past it).

The only reason galaxies look like they are large, solid objects is because of the way light works when you focus it down onto a detector. It doesn't get concentrated into a perfect spot, but into a small 'blur'. So when you see a bright, colorful galaxy in an image, you're seeing the slightly blurred light from a trillion stars. With the proper software you can actually remove the light from the image and see certain details that you cannot see otherwise. For example, I can take an image of the Andromeda galaxy, run it through my image processing software, and remove much of the galaxy's glow, allowing me to see the full extent of its dust lanes that you wouldn't normally be able to see.

4. Jul 10, 2015

### theironmaiden

Sorry, I am not exactly sure how to phrase my question.
If the universe began at the big bang, wouldn't all galaxies be arranged sort of on the surface of "an expanding balloon", so to speak?
Yet, I have read about galaxies behind other galaxies, at great distance! This geometry doesn't seem to me to support this. Even acounting for "local" skewing of distribution of matter.

5. Jul 10, 2015

### Borg

You might want to read this page from one of our members (Phinds) about the Balloon Analogy. It isn't intended to be an absolute description of the universe.

6. Jul 10, 2015

### theironmaiden

Didn't clear this up.

7. Jul 10, 2015

### Borg

The galaxies are not arranged "on the surface" as you've stated. The expanding balloon analogy is only meant to describe in layman's terms how objects can appear to move relative to each other as the universe expands. The analogy only describes obects on the surface of an arbitrary sphere. That sphere could be located anywhere in the universe. The point that it tries to make is that all objects at a given distance from any observer, anywhere in the universe, have the same redshift. The balloon analogy has nothing to do with how galaxies were distributed.

8. Jul 10, 2015

### Staff: Mentor

The surface of the balloon is a 2d surface, so it obviously cannot be an accurate description of reality, nor is it intended to be. It is just an analogy, a way of explaining how expansion works. In addition, the big bang theory does not have a point of origin. We do not see galaxies flying away from a single point in space.

9. Jul 10, 2015

### Staff: Mentor

10. Jul 18, 2015

### stedwards

But it's close in analogy, given an FLRW universe, which is $S^3$, rather than $S^2$. So with an extra dimension of freedom over $S^2$ space, the occlusion of one galaxy by another would be less.

[Hmm. Actually, what I've described would be a deSitter universe, which could be an FLRW universe with positive curvature. I'm still learning.]

Last edited: Jul 18, 2015