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Time at the edge of the universe

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poeteye
#1
Sep7-10, 03:54 PM
P: 40
If a distant galaxy is traveling away from us at the speed of light (due to the expansion of space), does this affect the passage of time? Can we really say that such a distant galaxy is receding at the speed of light and increasing speed the more distant it is? If it's recession is due to spacial expansion is it really velocity? If it is velocity, as the red shift would seem to indicate, does this mean that time is frozen at the horizon of our sight or (relative to our observation) moving backward in time?
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granpa
#2
Sep7-10, 04:10 PM
P: 2,258
no its not real velocity. that galaxy may not be moving at all. its space itself that is expanding and causing the redshift
Chronos
#3
Sep8-10, 12:23 AM
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The distant galaxy thinks we are the ones receeding at great velocity. Distant objects are severely redshifted, but, not frozen in time. It's like the product of 1/n. It never quite reaches zero.

granpa
#4
Sep8-10, 09:05 AM
P: 2,258
Time at the edge of the universe

it never reaches zero within the observable universe
JDoolin
#5
Sep8-10, 10:57 AM
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Quote Quote by poeteye View Post
If a distant galaxy is traveling away from us at the speed of light (due to the expansion of space), does this affect the passage of time? Can we really say that such a distant galaxy is receding at the speed of light and increasing speed the more distant it is? If it's recession is due to spacial expansion is it really velocity? If it is velocity, as the red shift would seem to indicate, does this mean that time is frozen at the horizon of our sight or (relative to our observation) moving backward in time?
I don't know about the expansion of space, but if the galaxy were traveling away at literal speeds at nearly 100% of the speed of light, then Lorentz factor affects how it ages in our reference frame.

[tex]\gamma=\frac{1}{\sqrt{1-(\frac{v}{c})^2}}[/tex]

With a high enough Lorentz factor, you wouldn't expect the distant galaxy to be a galaxy at all; it's time is going so slow, it wouldn't have formed yet. Perhaps you would see a slow-motion plasma; the material of the galaxy long before it formed, as it was just after hydrogen recombination. You should, in fact, expect to see a continuous background of finite intensity if you assumed that there were galaxies flying away from us at such speeds in every direction from a singular event.
RLutz
#6
Sep16-10, 08:43 AM
P: 19
Not an expert, but I do know that there is a difference between a galaxy moving through space at close to the speed of light and the space between two galaxies expanding at close to the speed of light.

If the galaxy were moving through space at some large fraction of c, then yes, all the usual relativistic coolness applies.
However, if galaxies are pretty close to stationary and the space between them is expanding at some decent percentage of c, then there are no weird relativistic effects that are going to be going on.

The galaxies we observe 10+ billion light years away are not moving away from us through space at speeds close to c, rather they are pretty close to stationary and the space between us is expanding at a fast rate.
nismaratwork
#7
Sep16-10, 04:16 PM
P: 2,284
I'm just going to throw this out there as well: the universe has no "edge". Otherwise, see previous answers.
poeteye
#8
Sep17-10, 03:23 PM
P: 40
Here's a puzzler: If the Big Bang is all about the rapid expansion of space, causing all objects to move away from each other at speeds greater than can be achieved through space itself, why isn't this the only motion we see? Why should any galaxy collide with another in a Universe dominated by superluminal expansion?
poeteye
#9
Sep17-10, 03:32 PM
P: 40
Also, independent of expansion, how fast can a galaxy move through space. I'm guessing nowhere near c. If they are pretty close to stationary and only gravity pulls them together at speeds well below c, does this mean that expansion is weaker than local gravity? Is expansion a force? Is there an expanitron?
RLutz
#10
Sep17-10, 03:37 PM
P: 19
Quote Quote by poeteye View Post
Here's a puzzler: If the Big Bang is all about the rapid expansion of space, causing all objects to move away from each other at speeds greater than can be achieved through space itself, why isn't this the only motion we see? Why should any galaxy collide with another in a Universe dominated by superluminal expansion?
I'm no scientist, but I think I can answer that one.

if space itself is expanding, then that means the further apart two objects are, the faster the space between them is growing. If two galaxies are pretty close by (relatively speaking) then they are not being separated extremely quickly by the universe's expansion, heck if they are close enough their gravitational attraction will actually bring them together (make them collide).

Moreover, it's only on really huge scales that the universe's expansion distances things. My nose doesn't fly off my face because of this expansion, nor do planets or galaxies get ripped apart. The force of gravity is enough to hold things together vs the expansion (at least for now).

Basically, the space between us and the farthest things in the observable universe is growing really quickly when compared to say the space between us and the andromeda galaxy (because there's less space that is undergoing expansion between us and the andromeda galaxy, and the gravitational attraction between us is much stronger than say the gravitational attraction between us and something 10 billion light years away, since the gravitational attraction scales with respect to the square of the distance between two objects. Two times farther away = 1/4 the force, 3 times farther = 1/9, 4 times = 1/16 the force).
RLutz
#11
Sep17-10, 03:40 PM
P: 19
Quote Quote by poeteye View Post
Also, independent of expansion, how fast can a galaxy move through space. I'm guessing nowhere near c. If they are pretty close to stationary and only gravity pulls them together at speeds well below c, does this mean that expansion is weaker than local gravity? Is expansion a force? Is there an expanitron?
Yes, it does mean that (at least for now--if the expansion is accelerating, then some point way into the future our galaxy will be ripped apart, then the solar system, then the earth, then this expansive force will even become more powerful than the strong force (gravity is weak sauce compared to the strong force) and rip the nuclei of atoms apart).

Gravity is strong enough now to hold things like galaxies together, but if the expansion continues to accelerate, then that won't be the case forever.


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