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

In summary: Which means, even if the universe were to have a curvature beyond our current understanding, it would still only occupy a tiny fraction of the total universe's existence.In summary, the two objects would be moving away from each other faster than light can catch up, but they are not actually moving away from each other.
  • #1
iDimension
108
4
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?
 
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  • #2
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?
Yes and no. Their distance increases faster than the speed of light, but they are not actually moving away from each other.
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?
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.

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?
No.
 
  • #3
mfb said:
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.

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 traveled "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
Can someone clear up my last post please as I still don't fully understand the concept of spherical universe.
 
  • #5
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.
 

1. How is it possible for light to travel faster than the expansion of the universe?

Light is not actually traveling faster than the expansion of the universe. The speed of light is a constant and cannot be exceeded. The apparent discrepancy arises from the fact that the universe itself is expanding, causing the distance between objects to increase. This means that the light from distant objects has to travel a longer distance to reach us, giving the impression that it is traveling faster.

2. Does this mean that the speed of light is not constant?

No, the speed of light is a fundamental constant of the universe and cannot be changed. The apparent discrepancy in the speed of light is due to the expansion of the universe, not a change in the speed of light itself.

3. How does the expansion of the universe affect the speed of light?

The expansion of the universe does not directly affect the speed of light. However, as the universe expands, the distance between objects increases, causing the light to have to travel a longer distance to reach us. This gives the appearance that the light is traveling faster than the rate of expansion, but in reality, it is still traveling at the constant speed of light.

4. Can anything travel faster than the expansion of the universe?

No, nothing can travel faster than the expansion of the universe. The rate of expansion is determined by the properties of space and time, and it is not possible for any object to exceed this speed. The apparent discrepancy with the speed of light is due to the expansion of the universe, not any object actually traveling faster.

5. How does the expansion of the universe affect our understanding of the universe?

The expansion of the universe has greatly influenced our understanding of the universe. It has led to the development of the Big Bang theory, which explains the origins and evolution of the universe. It has also provided evidence for the existence of dark energy and dark matter, which play important roles in shaping the universe. The expansion of the universe is a fundamental aspect of our understanding of the universe and continues to be a topic of ongoing research and exploration.

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