# Us in the center

1. Aug 26, 2008

### Yoni

Can anyone explain why is it said that looking at other gallaxies from our Milky Way it would seem that we are at the center of the universe?

2. Aug 26, 2008

### Staff: Mentor

Due to the finite speed of light, it appears from anywhere in the universe that you are at the center of the universe.

3. Aug 26, 2008

### Fredrik

Staff Emeritus
What does the speed of light has to do with it?

I would just say that it's because all the distant galaxies are moving away from us. (You would however make the same observation from any galaxy in the universe. Distant galaxies are moving away from you no matter where you are).

4. Aug 26, 2008

### Chronos

We are equidistant from the 'surface of last scattering' - i.e., the redshift of the CMB is exactly the same in all directions. This, however, does not mean we are in a 'special' location. Any observer anywhere else in the universe would make the same observation.

5. Aug 26, 2008

### Staff: Mentor

As I understand it, there is a limit to how far we can see due to the finite speed of light. We are at the center of a sphere of what we can see.

6. Aug 26, 2008

### K.J.Healey

Its similar to any place on the earth being able to call itself the "top of the earth". Sans the spinning, there would be no way of distinguishing, and it would all be the same.

7. Aug 28, 2008

### OS Richert

I think this is why we originally thought we were in the center. When we first noticed that the universe was expanding, every direction we looked it was moving away from us like we were in the center! Boyah! But then we looked closer and noticed that objects 2X from us moved twice as fast away from us as objects located 1X from us. This means if someone were at the location of the object 1X distance from us, they would see us, and the object at 2X moving away from them! Indeed, any point in the universe that we have been able to see would look as if every other point of the universe were moving away from them. We are not expanding from a single point, such that everything to the right of this point will agree that everything else is also moving right along with it. Every point is moving away from every other point simultaneously.

Qualification: I am a total amateur and probably just slaughtered that.

8. Aug 29, 2008

### schroder

Take a balloon and infalate it about half way. Now, with your indelible ink magic marker, with a soft point, make dots all over the surface of the balloon. Now inflate it further. You will notice that as the balloon expands, all the dots move farther away from each other. No two dots move closer. If you consider that we are one of those dots, it would appear to us that everything was moving away as if we were at the center of something. Of course, it also appears the same from all those other dots. Only from the vantage point of the balloon owner is it apparent that we are very mistaken and we are just one more dot among billions of others.

9. Aug 29, 2008

### DaveC426913

The OP is specifically asking about observation of galaxies, rather than the sphere of the observable universe.

OSRichert described the reality:

Simply put, all galaxies seem to be - when averaged - moving away from us. The farther a galaxy is from us, the faster it seems to be moving away. The effect is that is seems that we are at the centre of an expanding universe of galaxies.

Schroder gave an analogy (the venerable balloon analogy) which describes why this appears to be the case.

10. Aug 29, 2008

### Leopold

The CMB is not isotropic; we're moving at about 600 km/s relative to it: http://apod.nasa.gov/apod/ap010128.html

11. Aug 29, 2008

### Janus

Staff Emeritus
I'm going to interject a little historical perspective here. People originally believed that we(the Earth) were at the center of the universe because we were special. Besides, it "looked" like everything went around the Earth.

Then we discovered that the Earth revolved around the Sun.

Hey, but at least the Sun was at the center of the Universe. After all when you looked out at the night sky, the Milky Way, which represents the disk of the Galaxy looks even in all directions, and if we were nearer one edge than the other it should look thinner in that direction.
Then we discovered that it was dust clouds obscuring most of the galaxy from view that caused it to look so even, and that we were actually quite a ways out from the center of the Galaxy.

By the time we got around to noting the recession of the galaxies, the reaction was one more of suspicion than confirmation. Instead of "Hey, We're special after all!" it was "Wait a minute, why should we be special?" After all, every observation that put us at the center had been wrong so far, why not this?

12. Sep 2, 2008

### Yoni

I always thought it had to do something with the theory of relativity.

Also, I have a question about the analogy of the inflating balloon: Is it analogous to real expansion of the universe in the regular 3 dimensions, or are you talking about the expansion of the universe in higher dimensions, such as space-time or a four dimension plain?
A regular 3-dim expansion predicts we see some galaxies moving sideways compared with other galaxies.
On the otherhand if light travels only on the mantle of the space-time sphere than we wont see this, and it can explain the stronger shift for closer galaxies.
But how do we know that the stronger shift (which indicates a higher velocity) is not induced by the fact that we are observing a farther object and thus an earlier object, which might just move faster than closer objects that had time to deccelerate (disregarding dark energy)