# could distant galaxies be moving faster than lightspeed relative to each other?

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Im new to this forum, My wife is fed up with me talking science, and has SUGGESTED I join a science forum, so I did a google search and here I am.

Hi Everyone.

I have a passion for science, but unfortunately I had no such passion when I was at school hence my limited knowledge comes from recent books Iv read and not from an education. With this in mind I hope any experts out there will be gentle and be prepared to answer quite a lot of probably dumb questions I may have on aspects of science I am enjoying learning about. Here comes the first....

If a distant galaxy's red shift is observed to give a speed of 3/4 the speed of light and another galaxy in the opposite direction is also observed to be moving away from us at 3/4 the speed of light, does this mean that galaxy A is traveling faster than light speed relative to galaxy B ?

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The Hubble redshift is not caused by galaxies moving, but rather by space expanding between them. And yes, the distance between them can increase at faster than the speed of light. Incidentally, when something is far enough away that the expansion of space would require you to go faster than the speed of light to ever reach it, you can essentially consider it to be outside of the universe. See below for a presentation about Hubble expansion.

As for adding actual velocities, there is a relativistic formula to use that will allow you to find the velocity one of the moving things would say the other has; see here:

http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

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so, will space ever stop expanding? if so, how long until then? and will life still be a possibility with expanded space?

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...If a distant galaxy's red shift is observed to give a speed of 3/4 the speed of light and another galaxy in the opposite direction is also observed to be moving away from us at 3/4 the speed of light, does this mean that galaxy A is traveling faster than light speed relative to galaxy B ?

Welcome!

The observer in between sees them seperate a 1.5c but to each other the galaxies are seperating at .96c.

A few calculators to help with the math.

http://www.cthreepo.com/cp_html/math1.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html

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so, will space ever stop expanding? if so, how long until then? and will life still be a possibility with expanded space?

At this point the rate of expansion is actually increasing.

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Wow!

So there really is an end to the visible universe!

This raises questions in my mind as to weather there may be galaxies that are blue shifted beyond our visibility! suggesting different regions of the universe expand and contract!

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Welcome!

The observer in between sees them seperate a 1.5c but to each other the galaxies are seperating at .96c.

A few calculators to help with the math.

http://www.cthreepo.com/cp_html/math1.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html

One needs to be very careful in defining the situation. Two objects moving away from a point, each at 0.75c will be moving relative to each other at 0.96c if that is happening in a locally flat space. That's the realm of special relativity.

In general, discussions about galaxies receding from each other is a discussion of general relativity, where the galaxies are not in the same local spacetime. Then, recession speeds can exceed c. Hubble's law is linear, so recessional speeds are proportional to distance. Two galaxies receding from a point at .75c, are twice as far away from each other, and should recede from each other at 1.5c.

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Incidentally, when something is far enough away that the expansion of space would require you to go faster than the speed of light to ever reach it, you can essentially consider it to be outside of the universe.

Here's something funny.

Let's say the first illustration below is the present day universe.

Earth ------------------Visible Universe

Then, let's say the next illustration below is a future date in the universe. By then, the visible universe has shrunken to us. What's funny is both planets might be claiming that the other isn't part of the universe, but they'd be wrong.

Earth ------------------Visible Universe------------------Other planet

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In general, discussions about galaxies receding from each other is a discussion of general relativity, where the galaxies are not in the same local spacetime. Then, recession speeds can exceed c. Hubble's law is linear, so recessional speeds are proportional to distance. Two galaxies receding from a point at .75c, are twice as far away from each other, and should recede from each other at 1.5c.

Good point. Right words for the wrong situation. I knew better if I would have thought about it. I see why they let you stay around.

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Wow!

So there really is an end to the visible universe!

This raises questions in my mind as to weather there may be galaxies that are blue shifted beyond our visibility! suggesting different regions of the universe expand and contract!

Hubble's law says that galaxies are moving away from us at a speed proportional to their distance but this does not always apply to nearby galaxies effected by local gravity. I see no reason why the expansion would not apply to spacetime outside of our visible universe.

"There are about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe. Most of these galaxies are in our own local group, and are all in orbit about each other. Most are dwarf galaxies among them include the Andromeda Galaxy,"

Edited by NowThatWeKnow
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