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Galaxies are always in motion relative to the Milky Way. My question is, which galaxy is receding the fastest from our viewpoint?

What is the theorised mechanism that causes this?

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How are you claiming gravity as the cause of the expansion of the universe when almost nothing is known about dark energy? Its not even known if dark energy is a real thing, a result of a theory of gravity which we have not yet discovered (just as general relativity helped explain the orbit of mercury over newtonian gravity), or even just the result of our patch of the universe being situated in an unlikely but nevertheless possible gigantic void which would cause it to expand in all directions because of the gravitational pull of objects we can't yet detect. –  Chris Sep 30 at 21:30
    
There are a lot of unknowns about dark energy. However, typically, it is included into Einstein's field equations through the addition of a term called "the cosmological constant". If it is in fact supposed to be included in GR in this particular way (which it is absolutely not clear that it is supposed to be), then the interpretation is that it must have a negative equation of state. That's all I was saying below. Secondly, we have mapped out the large scale universe out to a fair distance - we do not live at the center of a void (unless the size of the void is that of the entire universe). –  astromax Oct 11 at 12:53

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up vote 4 down vote accepted

This question may be a duplicate to: On what scale does the universe expand?

The mechanism for the expansion of the universe is gravity, manifested through what's known as Dark Energy. Dark Energy is a component to the universe which today, composes 68.3% of the energy density of the universe. You may be confused as to how gravity could be responsible for the expansion of the universe, but the equation of state of DE,

w = pressure/density

is negative (w_DE = -1), which means that it has anti-gravitational properties. Additionally, its density remains constant as the coordinates of the universe change, which is the reason why it dominates later on in the universe's history (while all other components, matter (dark or otherwise) and radiation/relativistic matter, become diluted with the expansion of the universe).

To answer your question, because the expansion of the universe is happening at all points in space, it looks like the "center" of expansion is at every point in space (see: Space expansion in layman terms), the galaxies furthest away (highest redshift), are the ones moving quickest away from the Milky Way Galaxy.

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Is there a specific galaxy receding the fastest? –  user8 Oct 6 '13 at 21:43
    
Whatever the furthest one discovered so far - that's the one receding the fastest. Until we discover a further one. So the question to ask is "what is the furthest galaxy discovered" –  Rory Alsop Oct 6 '13 at 22:04
    
@RoryAlsop yes, something like that –  user8 Oct 6 '13 at 22:20
    
Well in that case: I think it's MACS0647-JD –  Rory Alsop Oct 6 '13 at 22:24
    

The galaxy that is receding away from us fastest is whichever galaxy currently holds the record for the highest measured redshift.

This galaxy obviously changes as more and more discoveries are made. This web page that describes redshift, may keep an up-to-date record of this. It currently lists UDFy-38135539, discovered in the Hubble Ultra Deep Field, as the record holder, with a spectroscopically confirmed redshift of $z=8.5549 \pm 0.0002$. That is, lines in the spectrum appear at wavelengths that are a factor of $(1+z)=9.55$ times longer (redder). But there is apparent controversy over the claimed detection of spectral features in this galaxy. e.g. http://adsabs.harvard.edu/abs/2013MNRAS.430.3314B

Bramnmer et al. (2013) report the "tentative detection" of a possible Lyman alpha emission line at $z=12.12$ for UDFj-39546284, another ultra deep field galaxy.

Either way it seems only a matter of time before $z>9$ galaxies are confirmed, since there are many candidates that have so far been selected only by virtue of their peculiar colours (Lyman break galaxies at near infrared wavelengths, including MACS0647-JD mentioned in comments to other posts). e.g. http://arxiv.org/abs/1211.2230

The reasons that these galaxies are receding from us is simply the expansion of the universe. Galaxies that are further away appear to recede faster. The galaxies discussed above must be $>13$ billion light years away according to the current "concordance" cosmological model.

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