Time at the edge of the universe

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Discussion Overview

The discussion revolves around the nature of time and motion at the "edge" of the universe, particularly in the context of galaxies receding from us due to the expansion of space. Participants explore concepts related to the effects of this expansion on time perception, the distinction between velocity and spatial expansion, and the implications of relativistic effects on distant galaxies.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question whether the recession of distant galaxies at the speed of light constitutes real velocity, suggesting that it is the expansion of space causing redshift rather than the galaxies moving through space.
  • Others argue that distant galaxies perceive us as receding at high velocities, but they are not frozen in time, likening the situation to a mathematical limit that never reaches zero.
  • A participant introduces the Lorentz factor, discussing how it affects the aging of a galaxy moving at relativistic speeds, suggesting that at extreme speeds, a galaxy might not have formed yet.
  • Another viewpoint emphasizes the distinction between galaxies moving through space and the expansion of space itself, asserting that galaxies are relatively stationary while space expands between them.
  • Some participants express skepticism about the existence of an "edge" to the universe, suggesting that such a concept contradicts previous points made in the discussion.
  • A puzzler is raised regarding why galaxies collide if the universe's expansion is superluminal, with a response indicating that gravitational forces can overcome expansion on smaller scales.
  • Concerns are raised about the relative strength of expansion versus local gravity, with some suggesting that if expansion continues to accelerate, it may eventually overpower gravitational forces.

Areas of Agreement / Disagreement

Participants express multiple competing views on the nature of motion and time at the edge of the universe, with no consensus reached on key points such as the interpretation of redshift, the effects of expansion, and the implications for gravitational interactions.

Contextual Notes

Participants highlight limitations in understanding the effects of expansion on local gravitational forces and the conditions under which galaxies may interact, but these remain unresolved within the discussion.

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Time at the "edge" of the universe

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 spatial 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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no its not real velocity. that galaxy may not be moving at all. its space itself that is expanding and causing the redshift
 


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.
 


it never reaches zero within the observable universe
 


poeteye said:
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 spatial 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.

\gamma=\frac{1}{\sqrt{1-(\frac{v}{c})^2}}

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.
 


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.
 


I'm just going to throw this out there as well: the universe has no "edge". Otherwise, see previous answers.
 


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?
 


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?
 
  • #10


poeteye said:
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).
 
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  • #11


poeteye said:
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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