The terminal velosity of an expanding universe

[STANDOWN]

Question: If the universe is expanding and accelerating for 13.7 trillion yrs. When will we exceed light speed? What will happen to time if we do? And if we are moving away from a point how do we calculate the time and distance if speed is inverse to time.

 

by standown

[timo]

- The speed depends on the distance of two objects. The relative speed that the distance between two points increases is (according to the all-mighty ... Wikipedia) 75 km/s per megaparsec. From that you can calc out yourself how far two objects must be apart for their distance to increase faster than the distance a beam of light would locally cross in the same small time interval.

 

- This increase in distance is not a velocity. It is conceptually a bit different. The typical picture is along the lines of an ant sitting on the surface of the balloon next to a dot marked on the surface of the balloon. There's two different ways that the distance between the ant and the dot can increase: By the ant walking away from it on the balloon surface or by the ant sitting still and the balloon increasing in size (or combinations, of course). The former is restricted by the maximum speed/velocity of the ant. The increase in balloon size (-> expanding of the universe) is not due to the ant or the dot moving and not restricted by the ant's maximum speed. Btw, you can easily verify that an expanding balloon also satisfies Hubble's Law. The reason why I said that all: Being something different than an object moving around in space, the increase in distance between two points in space (dots) due to cosmic expansion is not restricted by light-speed so there's nothing special to be expected for two points whose increase in distance due to expansion exceeds light speed.

 

- The influence of velocity on time that you seem to have in mind is an influence due to velocities, not due to expansion. So your last two sentences probably don't apply.

 

- While I've typed a few more letters than you have in your question, the answer is still very brief. So don't worry if not everything made sense. I think the key point you are missing is that expansion is somewhat different from movement. The balloon is a very nice and prominent example because it is a non-static background (like spacetime in cosmology is) but still familiar to most (unlike spacetime in cosmology).

[Martin]

Good answer by Atheist.

The technical term for the distance getting larger without the thing moving is recession. It is not like ordinary motion where you actually go somewhere. Recession rates are not real motion velocities and arent limited by special relativity. They have no effect on time. They are just the rate some distance is increasing, which can easily be > c if the distance is large enough to start with.

 

Most galaxies which we can see have redshift z > 1.4. And any such galaxy we are receding from at a rate > c. It has no effect on time or any thing else. Point your telescope at a typical galaxy with redshift 1.4 or better, in any direction, the people in that galaxy see us receding at a rate > c.

The distance we are from them increases by more than a lightyear each year that goes by.

 

But aside for a small random individual motion that doesn't really count (such a small percentage of c) we are not going anywhere. There is no place we are getting closer to at any rate comparable to c.

 

So there is no reason to worry about rates of recession having an effect on time.