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View Full Version : What if two points in the universe were not expanding relative to each other?



Megatarius
2008-Mar-01, 01:00 PM
This is a weird question, but bear with me.


We all know the universe is expanding, and not a single point in it is relatively still or equal or approaching another point. It's all moving away from us, as we are from it, thus the cosmic background radiation.

But my question is, what if there was a place that was not. What if there was a star that moved parallel to our own sun, matching it's exact movements? What if there was even a planet orbiting it that matched Earth's?

It would be pulled from its own galaxy pretty easily, but would other stars and galaxies approach and "pass" it?

I've heard the theory that it is not just physical material that is exapanding, but space itself, the 3rd dimension itself expanding and growing. I would think then that it would be impossible for these two points to be still in relation to each other, the expansion of space itself not allowing it.

But imagine if it was. Would there be some kind of strange relativity time paradox?

Specifically I'm thinking of a wormhole from Earth to another planet, and the two ends would not be able to be moving relative to each other, so that then creates the problem of a point that is not expanding with the rest of the universe.


I know how off the wall this is. Thanks for reading.

Ken G
2008-Mar-01, 02:17 PM
It's really a matter of distance and speed. The Hubble law says that the time it would take the distance to double between objects that are following the "average motion" of their part of the universe is about 10 billion years right now. If you interpret that increasing distance as a speed (which is of limited value at large distance but works at medium distances), then you are talking about 70 km/s for every million parsecs away. Now, 70 km/s is not very fast, cosmologically speaking, but neither is a million parsecs very far. If you want your "two worlds" only that far away, perhaps here to the Magellanic clouds, you would have no trouble finding stars that are not moving away from each other. But if you wanted them to be ten times farther than that, the average motion is now 700 km/s, and you very rarely find stars moving that fast relative to the matter around it. You do find gas moving that fast, so we still have no "paradox" if there is no increase in relative separation. If you go ten times farther still, and imagine 100 million parsecs away (still a small step in the whole universe), you need local speeds of 7,000 km/s to not be separating from us. Such speeds are found in the gas ejected in supernovae, or other energetic events, but there will not be any stars or planets at that speed. So you see, it's not a matter of what is possible, it is all possible-- it is just a matter of what is plausible.

And by the way, the idea that "space itself" is expanding is just a useful picture to understand how general relativity works in cosmology. It's not intended to be taken literally, as we have no unique way to say what "space" is doing.

novaderrik
2008-Mar-01, 06:03 PM
are all the points within your body- which is indeed a part of the universe- moving away from each other?

Jeff Root
2008-Mar-01, 07:38 PM
are all the points within your body- which is indeed a part of the
universe- moving away from each other?
Certainly not my body, and not yours, either, unless you are
exploding or on fire.

The expansion of the Universe was discovered from the redshift of
distant galaxies, and distant galaxies are what are moving apart from
one another. Galaxies which are close to each other are not
moving apart from one another. Stars within a galaxy are not moving
apart from one another. Planets within a solar system are not moving
apart from one another unless the star they orbit is losing mass.

It is just well-separated clusters of galaxies that are moving apart
from one another. That's all. If two galaxies are much more than
ten million light-years apart, then the distance between them is likely
increasing. If two galaxies are much less than ten million light-years
apart, then they are likely gravitationally bound to each other, and
thus both part of the same cluster, and not moving away from one
another in the long term.

-- Jeff, in Minneapolis

Cougar
2008-Mar-01, 09:59 PM
And by the way, the idea that "space itself" is expanding is just a useful picture to understand how general relativity works in cosmology. It's not intended to be taken literally...
Is there anything in your book that is intended to be taken literally?

Ken G
2008-Mar-01, 10:48 PM
Yes-- the measured result of any objectively repeatable experiment is intended to be taken literally. You know, that which science is based on?

Megatarius
2008-Mar-02, 06:15 PM
Thanks guys. Your answers have been very helpful.