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You may not notice it, but our Milky Way galaxy is cruising along at 630 kilometers (~391 miles) per second. That speed is often attributed to the influence of a single gravitational source. But in a new study, a group of researchers has found that the motions of the Local Group—the cluster of galaxies that includes the Milky Way—are being driven by two primary sources: the previously known and incredibly massive Shapley Supercluster and a newly discovered repeller, which the researchers dub the Dipole Repeller.
Shapley's contribution was already known, but the Dipole Repeller's hadn't been recognized prior to this study.
The researchers plotted the motions of many galaxies in the nearby Universe in a 3D model, using data from the Cosmicflows-2 database. Since the Universe is expanding, most galaxies are moving away from ours, creating a red-shift in the light they emit. But since the researchers were more interested in the other influences on a galaxy's motion, they simply subtracted the expansion's contribution. The resulting plot shows what the motions of galaxies would look like if space wasn't expanding.
The galaxies in that plot all follow different paths—some proceed through the Great Attractor in the middle of the picture, others curve around the periphery, and so on. They all seemed to have a clear destination: the Shapley Supercluster. But they also seem to have a clear origin point: the Dipole Repeller. When the researchers traced the galaxies' paths backwards, they all originate there. It looks a lot like there's something there repelling the galaxies, as if the Repeller and Shapley formed the negative and positive ends of an electrical dipole, and charges were being driven from one to the other.
That's not what's actually happening. Gravity is the dominant force acting on a galaxy, and gravity, unlike electricity, can't repel—it's only an attractive force. So what's going on?
The Dipole Repeller's true identity is probably, well, nothing. It's actually a void with much less mass than the surrounding space. This has the effect of seeming like a repeller because the nearby space has a much denser concentration of matter, creating a gravitational gradient between the two. The low-density void is the only direction from which there's no force pulling on the galaxy, or at least significantly less force than comes from every other direction.
Source:
Journal Reference:
Nature Astronomy, 2017. DOI: 10.1038/s41550-016-0036
(Score: 3, Informative) by AthanasiusKircher on Sunday February 05 2017, @09:02PM
our Milky Way galaxy is cruising along at 630 kilometers (~391 miles) per second
In what frame of reference?
And yet again, we see tons of questions that could be resolved simply by linking to the original research article [nature.com]. As stated in the first sentence of the abstract, the motion is relative to the cosmic microwave background radiation, which is probably the most common frame of reference when talking about motion of galaxies, etc. on a cosmological scale.
If we pick a random point in the universe and study the motion of galaxies compared to that, I'm sure that we would find some equally fascinating results.
Yeah, the problem with this story is that the motion and assumed frame of reference isn't explained very well, because people (even science reporters) don't seem to understand cosmology very well. The entire universe is in motion, expanding after the Big Bang. The CMB serves as a kind of "frame of reference" that expands at the same general rate as the universe. (See here [stackexchange.com] for some explanation.) Any motion with respect to that reference frame is something that requires some explanation in terms of the the position of bodies around it.
This also kind of explains the use of the term "repulsor" and being "pushed" by the "void" here. The "default" state of a body in the universe is to be pulled on gravitationally in all directions. If these forces are roughly "balanced" by uniform distribution in all directions, we'd move in sync with the CMB reference frame. The Shapeley Supercluster has been proposed as a primary explanation for why the Milky Way appears to be moving with respect to the CMB, but it turns out that the relative velocity is also increased by the absence of mass on the other side. The void in this sense represents a non-uniformity, a kind of anomaly -- hence the idea that it is contributing to "pushing" us.