# Where are we?

1. Jun 14, 2010

### PrestonMcCall

Are we in the middle of the universe? I mean, imagine the universe is some sperical or odd shaped three dimensional shape. We can 'see' out to some pint, but is it always a similar distance, making us iin the middle? It makes sense that if the outer edge of the universe is expanding at a rate greater than the speed of light, then we would only be able to see a finite distance and that it would be similar in distance from us? The light ohoton would never reach us from anywhere outside of the area speeding faster than the speed of light.

2. Jun 14, 2010

### brother time

The shape of the universe is unknown. The best analogy we have is that the universe is a sphere. thus is due to the light reaching us at the same time.
It is hard to tell where )or how large it is) in the universe we are due to this.
cheers, BT

3. Jun 21, 2010

### Calluuuum

Remember that light has only been travelling for ~13/14 billion years. That means we can only see anything that is up to 13/14 billion light years away. This is not limited in any direction, so we see everything 13/14 billion light years away in all directions, giving the appearance of us being in the centre of a sphere.

4. Jun 21, 2010

### phyzguy

Every observer will see themselves to be in the center of their observable universe, just like when you stand on the surface of the Earth you are in the center of the region defined by your horizon. We don't know, and probably can never know, what is beyond our cosmological horizon.

5. Jun 23, 2010

### seto6

everyone will see as if they are in the middle of the universe because it is infinite..pass 300pc or 300Mpc the universe has no structure

6. Jun 23, 2010

### justwondering

Is it theoretically 'allowed' for Mass A and Mass B to have a closing speed that is greater than C? e.g. Mass A is headed towards Mass B with its 'real' speed greater than .5 C, and Mass B is headed towards Mass A with its 'real' speed greater than .5 C.
Likewise for them to have a separating speed greater than C ?

7. Jun 24, 2010

### Chronos

No, this is covered in introductory relativity text books.

8. Jun 26, 2010

### Dav333

From wiki (observable uni)
The visible universe is thus a sphere with a diameter about 93 billion light-years.

Is this a mistake? Aren't they talking about observable not visible? Which visible means what we can see as in the only up to the cmb?

9. Aug 20, 2010

### justwondering

In a Synchrotron particles, that are each traveling at near the speed of light, are at times traveling towards each other and away from each other at a differential velocity much greater than C. This can't be taking place?

10. Aug 22, 2010

Something without shape or structure can exist? How?

11. Aug 23, 2010

### Janus

Staff Emeritus
"Closing speed" can exceed c, Relative speed can't.

Closing speed is the apparent respective speed as seen by an observer not sharing a rest frame with either object. For example, if I see a particle coming from my left at 0.75c and one coming from my right at 0.75c, then according to me, the closing speed between the two is 1.5 c

Relative speed is the speed of one object with respect to another as measured from the rest frame of either object. It would be the speed either of the particles measures as the difference of velocity between the two, which would equal 0.96c in our example.

Closing speed is no more "real" than relative speed.

12. Aug 24, 2010

### alt

If so, how is it that it is said that the universe is flat ?

13. Aug 25, 2010

### dipstik

it is flat as an approximation, just like a plain on earth

14. Aug 28, 2010

No .