# Light propagates faster than the rate of expansion of the universe?

1. Sep 2, 2014

### iDimension

I know the universe doesn't expand at a speed but rather a rate over distance but if we take two objects on opposite sides of the observable universe, would they be moving away from each other at a greater rate than light?

IE if object A was a star and object B was a planet, would the light from the star still be able to reach the planet or would they be moving away from each other faster than light can catch up?

And that got me thinking, what if the univere is finite and a galaxy on the "edge" of the universe were to emit light, that light would travel away from the galaxy but only at some point there literally wouldn't be anymore universe for the light to travel into so what happens?

To me at least everything we know about the universe (which isn't very much to be honest) but enough for me to think that an infinite universe just makes so much more sense than a finite one. Isn't it much more likely that the universe is infinite because if the universe is everything that exists then it must, by it's own definition, be unbounded and infinite?

Last edited: Sep 2, 2014
2. Sep 2, 2014

### Staff: Mentor

Yes and no. Their distance increases faster than the speed of light, but they are not actually moving away from each other.
That depends on the evolution of the expansion rate over time. In general, it can be possible - we now see things where the distance increases faster than the speed of light, but if the expansion keeps accelerating, then there are things we will never see.

A finite universe does not have to have an edge - it could be shaped like the surface (!) of earth, for example. Finite, but without an edge. I'm not aware of any reasonable model with an actual edge that could somehow be reached.

No.

3. Sep 2, 2014

### iDimension

The problem I have with this view though is sure the Earth doesn't have a 2D edge but it does have a 3D edge... straight up and you will come away from the surface of Earth but space is like a "sphere" in all directions otherwise if you travelled "up" you'd eventually look down and see the universe which as far as we know that wouldn't happen.

Unless our universe really is like a sphere but it's SO stupendously large that our instruments cannot measure the curavture because it's so faint.

4. Sep 4, 2014

### iDimension

Can someone clear up my last post please as I still don't fully understand the concept of spherical universe.

5. Sep 4, 2014

### Bandersnatch

Universe is not a sphere. The thing about analogies, is that they're never exactly the same as the thing they descibe.

When we point out to the surface of a sphere(e.g. Earth) being an example of a finite space(2-dimensional) without an edge(in those two dimensions that comprise the space), we do so because it's easy for a human mind to picture the sphere thanks to us being spatially 3-dimensional beings.

We use our imagination to embed the 2d surface in the 3d space we're familiar with, but such embedding is not mathematically necessary. That is, there is nothing mathematically speaking, preventing you from having a curved, closed 2d surface without the 3rd dimension at all.

All you need to make sure, is that any two locally parallel lines will eventually intersect, and angles in triangles add up to something more than 180°.

Similarly, with our universe being spatially 3-dimensional as it is, it can have curvature without there being a fourth dimension to be embedded in. All you need to have, is parallel lines intersecting and the triangles thing.

And indeed, if our universe were to be closed, we could look in one direction to eventually see the backs of our heads - at least in principle. But so far all observations suggest that the universe is remarkably close to flat. There's always the possibility of the universe being simply very, very large, so that any curvature is simply hidden in our error bars. The last time I checked(was a while ago), data from WMAP pushed the minimum curvature radius(remember, in the fourth spatial dimension, - which might not physically exist, but is useful as a descriptive tool) to 130-ish billion light years.

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