Can galaxies separate at speeds faster than the speed of light?

[iamBevan]

Can the speed at which galaxies separate exceed the speed of light?

 

[dentedduck]

No, as with all objects, as the relative speed between the two increases, relativistic effects (such as increased mass and time dilation) work to create a limit on their speeds.

Dave

 

[Cold Winter]

Sure, if the universe is expanding as per the "big bang", then effectively the separation would be "superluminal".

However, in inertially defined space, the answer is still no...

Clear as mud eh?

 

[dentedduck]

There is an article that I once read talking about faster than light particles that could have been created in the big bang, but would be relegated to always being faster than light. They wouldn't be able to slow down and would be trapped on the other side of the asymptote. I think this is the ref... Don't have time to read it now.

http://prola.aps.org/pdf/PR/v159/i5/p1089_1

 

[bcrowell]

General relativity doesn't provide a uniquely defined way of measuring the velocity of objects that are far away from one another. There is no well defined value for the velocity of one galaxy relative to another at cosmological distances. You can say it's some big number, but it's equally valid to say that they're at rest, and the space between them is expanding. Neither verbal description is preferred over the other in GR. Only local velocities are uniquely defined in GR, not global ones.

 

[marcus]

Can the speed at which galaxies separate exceed the speed of light?

B. Crowell pretty well said it. Beyond that, you should realize that the speed at which MOST galaxies we know of are receding exceeds the speed of light---assuming it is the speed and distance that you see in the Hubble Law
v = Hd

In the Hubble Law, the distance d is the freezeframe distance (as if you could stop the expansion process and measure with radar or yardstick, any conventional means, the distance at that moment) and v is the rate at which that distance d is increasing.

Most of the galaxies we can see have redshift > 1.5 and it is just a fact that any such galaxy is at this moment receding faster than light. As per Hubble Law.

We see galaxies with redshifts out past 5, 6, 7 and more. Most of the universe which we study is redshift > 1.5 and therefore receding > c.

I have a cosmology calculator link in my signature that gives recession speeds:
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

You have to give it 3 model parameters on the LHS margin (.27, .73, 71) and THEN the redshift. Press calculate and it gives the distances and recession rates. Try redshift 1.7, you may be surprised! Ask if you have any questions.