Astronomers have discovered a giant cosmic void that explains why our Local Group of galaxies is moving through the universe as fast as it is.

local cosmic structure
This visualization is a slice of the local cosmic structure, roughly centered on the Local Group. The black arrows show the "flow" matter follows in this gravitational watershed. Analysis of these flow patterns has revealed that there's probably a large, unseen void (gray-brown at right) that is "pushing" us toward the Shapley Supercluster (green), which is in turn gravitationally pulling us toward it. The yellow arrow is the direction of the so-called cosmic dipole.
Yehuda Hoffman

The Milky Way Galaxy is one of the biggest galaxies in the Local Group, a modest cluster of stellar metropolises. The Local Group, in turn, lies in a filament of the much larger cosmic structure. The galaxy clusters in this cosmic web don’t stay still, but rather slowly gravitate (literally) toward the largest clusters.

Astronomers have known since the 1980s that the Local Group is moving toward what’s called the Great Attractor, a dense collection in the vicinity of the Centaurus, Norma, and Hydra clusters about 160 million light-years away. They’ve also found another, equally influential attractor called the Shapley Supercluster, a huge structure along roughly the same line of sight but four times farther away.

In 2006, when Dale Kocevski and Harald Ebeling (both then of University of Hawai'i) confirmed Shapley’s influence on the Local Group by mapping out how clusters clump together on the sky, they also saw hints of a void in the opposite direction.

Now, using the Cosmicflows-2 catalog of galaxies, Yehuda Hoffman (Hebrew University, Jerusalem) and colleagues have mapped out the movements of more than 8,000 galaxies and confirmed that, yes, the two titans that determine how local galaxies flow through the cosmic web are Shapley and this single, as-yet unmapped void.

Think of the local cosmic structure as a gravitational water park: the twisty slides start high (where the void is) and end up low (where the cluster is), with the natural motion always being down — that is, with gravity. Galaxies toboggan along the gravitational slides.

But how fast the galaxies go depends on how tall the slides are. In the same way, the fact that there’s a big, "high" void in one part of the gravitational landscape makes the Local Group flow faster toward the dense, “low-lying” regions in the other direction than it would otherwise. The net effect is as though the void is pushing in the same direction as the supercluster is pulling. It may even be that the void, which the team labels “the dipole repeller” in their January 30th Nature Astronomy paper, has more of an effect on the Local Group’s motion than the Shapley region does on its own.

This discovery actually may solve a longstanding cosmic conundrum. Astronomers knew that the Local Group moves with respect to the cosmic microwave background (CMB), the ocean of photons suffusing the universe that is left over from the Big Bang. This motion is called the CMB dipole. But the velocity (630 km/s, or 1.4 million mph) was about double what it should be, if Shapley and the other clusters were responsible. The repeller’s effect essentially doubles Shapley’s pull, explaining why the Local Group moves as fast as it does.


Below, you’ll find a movie explaining the result. Don’t mind the jargon: if it fazes you, the illustrations should carry you through.  Credit: Yehuda Hoffman

The Dipole Repeller from Daniel Pomarède on Vimeo.


Yehuda Hoffman et al. “The Dipole Repeller.” Nature Astronomy. January 30, 2017.

Dale D. Kocevski and Harald Ebeling. “On the Origin of the Local Group’s Peculiar Velocity.” Astrophysical Journal. July 10, 2006.

O. Lahav, M. Rowan-Robinson, and D. Lynden-Bell. “The Peculiar Acceleration of the Local Group As Deduced from the Optical and IRAS Flux Dipoles.” Monthly Notices of the Royal Astronomical Society. October 1, 1988.

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Image of Anthony Barreiro

Anthony Barreiro

January 30, 2017 at 3:13 pm

Thanks, this is fascinating. The video is really helpful. I was wondering how the Laniakea supercluster relates to this, and it shows up in the video.

Thinking about this gravitational gradient in Newtonian terms, if there were equally massive objects in both directions, local motion would be zero. The less mass there is in one direction, the faster we move toward more mass in the opposite direction. Is this adequate to explain what's going on? Is there more to this that I'm not understanding?

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Camille M. Carlisle

February 1, 2017 at 10:44 am

Anthony: Yes, you've got it! 🙂
Rick: Yeah, calling it a repeller really causes confusion -- that's why I prefer the water slide analogy. Dark energy isn't involved here (I asked one of the authors just to be sure), and there's no real "push" happening -- it's just an analogy. Anthony's comment pretty much sums it up. Another way of thinking about it is as a teeter-totter. If you put a big sandbag on one end and a small sandbag on the other, the teeter-totter will tilt toward the bigger one, and a ball rolling along the teeter-totter's frame will roll that direction. But if you put a big sandbag on one end and nothing on the other, the tilt will be more severe and the ball will roll faster. By calling the void a repeller, the team is saying that the effect is "as though" the empty end were pushing on the ball and speeding it up.

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Anthony Barreiro

February 1, 2017 at 3:48 pm

Thanks Camille. It's reassuring to know that dear old Newton can still help us understand phenomena playing out over hundreds of millions of light years.

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February 1, 2017 at 1:53 pm

Think of the "dipole repellers" as areas of the universe with low density of mass (few galaxies). These areas allow time and space to unfold more efficiently than areas with dense mass, which the unfolding time and space must curve around (a phenomenon we call gravity). Analogy: Like holes expanding in a rising loaf of bread giving the universe a foamy texture.

Since spacetime is unfolding more freely in these regions, we experience them as a pushing phenomenon; but it's merely continuously expanding spacetime expanding more freely than spacetime encumbered by areas of denser mass. Dark Energy theory is trying to figure out why spacetime acts this way - it may just be its nature.

This is similar in a way to the Hubble Constant; we observe distant galaxies accelerating away from us, but it's really only the intervening time and space continuously expanding which creates this illusion.

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January 31, 2017 at 7:17 pm

Does this really make any sense. They're claiming that the repeller acts like a repeller because there isn't much mass there to exert any pull on the galaxies. Now if you told me that there was some source of energy in the vacuum that was pushing the galaxies away then you could call it a repeller. Isn't dark energy suppose to explain the accelerating expansion of the universe? Couldn't local areas devoid of mass also produce or contain larger amounts of this dark energy?

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Edward Schaefer

February 2, 2017 at 6:21 pm

If the local cluster was surrounded by uniform mass in all directions, we would not have become accelerated with respect to the cosmic microwave background. Increased mass in certain areas creates an attraction, which is pulling us in the direction of the "Great Attractor" and the Shapely Supercluster. However, we have been pulled towards them faster than would be expected in those areas were higher-density regions in an area of otherwise uniform density.

This "Repeller" is an area of unexpectedly low density. Since the expected mass is not there, we are being pulled towards the Great Attractor and Shapely faster than would otherwise be expected.

Overall, you are right. There is no outright repulsion, but instead a lack of attraction. That is not the same thing in terms of the physics. But it is a good way of explaining that difference that otherwise exists between theory and observation.

BTW - Dark energy is not an issue here. All of the available evidence is that it is uniformly distributed throughout spacetime, and this study does nothing to contradict that.

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Peter Wilson

February 2, 2017 at 1:45 pm

The statement, "the Local Group is moving toward what’s called the Great Attractor," is somewhat confusing.

Everything beyond about 10 Mly away is getting farther away. Using a Hubble-constant of 72 km/s/Mpc, the Great Attractor, at 160 Mly, would be moving away from us at @ 3,530 km/s. The 630 km/s of the LG "towards" the GA means the GA is receding at "only" 2,900 km/s. It is still moving away from us at a good clip, just not as fast would be expected in a uniformly expanding universe. The arrows in the picture and video show velocity relative to the average expansion velocity, not actual velocity.

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February 3, 2017 at 10:52 pm

Got time to answer an ignorant layman's question: Is dark matter so evenly distributed at this scale that it has a negligible effect on galaxy and Local Group motions?

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February 4, 2017 at 1:40 am

1. This seems a bit like describing low tide as being caused by the ocean being repelled by the absence of an overhead moon.

2. Since this volume of space does not actually repel, I don't think it a good idea to call it a "Repeller." As seen in other comments above, this term is confusing to people who understand some science. Imagine how misleading it would be to use such a term while describing these motions to the general public!

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Stub Mandrel

February 7, 2017 at 2:33 pm

I have to agree with you Stanr.

It's similar to the misunderstanding that a vacuum sucks air in rather than the pressure outside forcing it in.

I think this could be better worded.

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