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tommac
2009-Dec-02, 03:24 PM
I have a question about the evidence that we have for redshifts greater than C due to cosmic expansion.

Anything that we see expanding away from us with redshifts that suggest speeds greater than C how is that calculation done? Lets say we were 1,000,000,000 ly away from something. Are we saying that it is currently moving at speed greater than C or that at some point in the past billion year that it was moving greater than C away from us or that on the average it was moving greater than C away from us. Could we calculate how close it was to us 1,000,000,000 years ago?

antoniseb
2009-Dec-02, 03:42 PM
Surely by now someone here has given you the link (http://www.astro.ucla.edu/~wright/CosmoCalc.html) to Ned Wright's Cosmological Calculator. From there he has explanatory links.

gzhpcu
2009-Dec-02, 03:56 PM
I have a question about the evidence that we have for redshifts greater than C due to cosmic expansion.

Anything that we see expanding away from us with redshifts that suggest speeds greater than C how is that calculation done? Lets say we were 1,000,000,000 ly away from something. Are we saying that it is currently moving at speed greater than C or that at some point in the past billion year that it was moving greater than C away from us or that on the average it was moving greater than C away from us. Could we calculate how close it was to us 1,000,000,000 years ago?
Formula 1 in this document shows how to do the calculation (page 5): http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf

The caculated speed is the speed the object had at the time the light left it towards us. These velocity is measured with respect to the comoving observer who observes the receding object to have measured redshift.

tommac
2009-Dec-03, 12:02 AM
Formula 1 in this document shows how to do the calculation (page 5): http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf

The caculated speed is the speed the object had at the time the light left it towards us. These velocity is measured with respect to the comoving observer who observes the receding object to have measured redshift.

Thanks very useful information. Now my follow up. Relativity states that two objects can not be moving away from each other at greater than the speed of light correct? However, we the observer have followed a different path to detect this light. The light is redshifted based on the speed of the star that sent it in the opposite direction of the point where it was detected plus the speed of us the observers moving away from that point.

Does relativity prohibit the sum of these two values from being greater than the speed of light? Is there really any proof that us and the star ever travelled away from each other at greater than the speed of light?

What if its movement away from us was temporary ( and at the same time we were moving in a direction where the distance between us never expanded quicker than c ) then both of us changed directions and are now both receding away from that first spot at less than c .... wouldnt this allow the apparent red shift to be greater than c?

Sorry if my wording is not clear ... however i am just pointing out that a bunch of time when by and both the path of the star and of us could have changed between then and now.

WayneFrancis
2009-Dec-03, 03:57 AM
tommac go read http://en.wikipedia.org/wiki/Redshift I can't believe you don't even have the most rudimentary understanding on what the cosmological redshift and how it differs from a normal Doppler redshift but yet you'll argue, in another thread, that it could be "anti-gravity"

slang
2009-Dec-03, 08:09 AM
Not to mention that this question is answered here every couple of weeks.. tommac, I think you'd do well to follow other question threads here, not just your own. In case of this question, there is currently a thread right below this one, that has a superb explanation by speedfreek.

Why is there a CMB? (http://www.bautforum.com/space-astronomy-questions-answers/97517-why-there-cmb.html)

gzhpcu
2009-Dec-03, 02:06 PM
Does relativity prohibit the sum of these two values from being greater than the speed of light?

Special relativity prohibits speed greater than c, assuming a non-expanding spacetime between the two objects. However, General Relativity allows space to expand at greater than c, this is why galaxies can have superluminal recession speed.