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# Relative speed in an expanding universe

ocalhoun
So, in our universe, everything appears to be moving away from us, and the farther away from us something is, the faster it appears to move away from us.

Let's say that we are at point A, and we are looking at three different distant galaxies: galaxy B, C, and D.
C is twice as far away as B, and D is twice as far away as C (which means that D is for times as far from A as B is).
Now, B is so far away that it appears to be moving away at .5 times the speed of light (c).
Going by the normal formula, C should be twice that speed and D should be four times that speed.
But, that would make the apparent speeds of C and D 1c and 2c, respectively!
I don't understand what is wrong here...

My guesses:
1: Not a real violation of special relativity, because space is expanding, nothing is really moving
2a: The equation for finding how fast something should be receding becomes non-linear for long distances
2b: The equation for finding how fast something should be receding needs to account for relativity theory when using such large values in order to get a correct result.

But, if the solution is one of the 2's, wouldn't that mean that (from our point of view, at least) many of the distant objects in the universe would seem to clump together more than they should? (Because the furthest ones are slowed down by the limitations of the speed of light.)

Is it one of my guesses, or something else entirely?
Bikerman
Spacetime itself is not subject to the limitations of Special Relativity and can (and does) expand so that two objects appear to move apart at a rate greater than c. Astronomers routinely observe distant galaxies with very high value redshifts (z)
In general terms any galaxy observed with a redshift (z) greater than 1.7 was 'moving away from us' at speed>c when the photons were emitted. Some galaxies have been observed with z=10.
Voodoocat
You calculated the non-relative velocity which only applies at low velocities. This link explains how to add velocities in special relativity.

http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html
Bikerman
That is true and valid - but only for velocity 'within' spacetime. It doesn't work for the expansion of spacetime itself.
yagnyavalkya
I found an article where the authors say that
"The source of this acceleration is not related to an exotic matter but to a scalar field whose origin can be traced back to geometry of the brane and, specifically, to the curvature scalar View the MathML source and depends on two free parameters, namely α and λc. Hence, an accelerating universe driven by curvature would certainly seem to be a possibility."
what do you guys think
Ref: Atazadeh et al 2008 Accelerating universein View the MathML source brane gravity
Physics Letters B
Volume 660, Issue 4, 28 February 2008, Pages 275-281
Bikerman
What I think is that you should start using quote tags when lifting abstracts from published papers.
yagnyavalkya
 Bikerman wrote: What I think is that you should start using quote tags when lifting abstracts from published papers.

Yes I will but please do let me know what is quote tags
Should put the test in quotes
I have quoted the references of the papers
Actually this is a new type of postings I am trying to do by which I am trying to make the readers aware of the scholarly articles in relation to the topic
If it is against the rules I will refrain from doing so in my future posts
But please do tell me how exactly to "Quote Tags"
Thanks
 Quote: This is an example of the use of quote tags. If you edit this posting you will see how I have enclosed these two sentences in quote tags.
yagnyavalkya
 Quote: Gotcha!