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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?

ProfRob
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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. –  Sep 30 '14 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 '14 at 12:53

2 Answers2

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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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astromax
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  • Is there a specific galaxy receding the fastest? –  Oct 06 '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 06 '13 at 22:04
  • @RoryAlsop yes, something like that –  Oct 06 '13 at 22:20
  • Well in that case: I think it's MACS0647-JD – Rory Alsop Oct 06 '13 at 22:24
  • http://www.space.com/18502-farthest-galaxy-discovery-hubble-photos.html – Rory Alsop Oct 06 '13 at 22:28
  • Expansion is not the only mechanism causing galaxies to move away from us. Galaxies have their own momenta, otherwise they would not collide. The galaxies of the Local Group orbit a center of mass between Milky Way and Andromeda (though their orbits may not be radial). See http://arxiv.org/ftp/arxiv/papers/1101/1101.2821.pdf – called2voyage Oct 08 '13 at 13:43
  • This momentum you describe is motion relative to the expansion of the universe (the "Hubble Flow"), and it's deemed peculiar velocity. Yes, it does dictate how galaxies of the same group/cluster move with respect to one another, however his question is asking about galaxies which move the fastest away from the MW. These are certainly non-local galaxies, and the reason is because of the expansion of the universe, not peculiar velocity. – astromax Oct 08 '13 at 15:54
  • @astromax If expansion is discounted, would it be possible to calculate the galaxy moving away from the Milky Way the fastest in peculiar velocity only? – called2voyage Oct 08 '13 at 21:25
  • @called2voyage Sure, if the universe didn't expand, and the motions of galaxies had only to do with orbiting about local centers of mass, then the redshift of all galaxies would be telling you about their peculiar velocities. When we measure redshifts, we're measuring the speed of both the peculiar velocity of its motion about local influences and the bulk flow of the universe. It just so happens that the bulk flow trumps local gravitational influences on large scales. – astromax Oct 08 '13 at 21:28
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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 lists UDFy-38135539, discovered in the Hubble Ultra Deep Field, as a previous record holder (see below), 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. Bunker et al. 2013).

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. Bouwens et al. (2014).

Update Oesch et al. (2016) report HST grism spectroscopy of a high redshift galaxy candidate, GN-z11, identified in the CANDELS/GOODS survey. They confirm the presence of a Lyman break indicating a spectroscopic redshift of 11.1. This appears to be the current record holder.

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.

Translating a redshift to a recession velocity requires knowledge of the cosmological parameters. Fig.2 of Davis & Lineweaver (2004) shows the relationship. For our current best estimates of cosmological parameters, a redshift of 11 corresponds to a recession velocity of 2.3c. That is, this galaxy is currently receding from us at 2.3 times the speed of light.

ProfRob
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