The Dipole Repeller

An interesting bit of local cosmology news has been hitting the headlines over the last few days. The story relates to a paper by Yehuda Hoffman et al. published in Nature Astronomy on 30th January. The abstract reads:

Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity 1 of VCMB = 631 ± 20 km s−1and participates in a bulk flow that extends out to distances of ~20,000 km s−1 or more 2,3,4 . There has been an implicit assumption that overabundances of galaxies induce the Local Group motion 5,6,7 . Yet underdense regions push as much as overdensities attract 8 , but they are deficient in light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow 9 . We show here that repulsion from an underdensity is important and that the dominant influences causing the observed flow are a single attractor — associated with the Shapley concentration — and a single previously unidentified repeller, which contribute roughly equally to the CMB dipole. The bulk flow is closely anti-aligned with the repeller out to 16,000 ± 4,500 km s−1. This ‘dipole repeller’ is predicted to be associated with a void in the distribution of galaxies.

The effect of this “void in the distribution of galaxies” has been described in rather lurid terms as “Milky Way being pushed through space by cosmic dead zone” in a Guardian piece on this research.

If you’re confused by this into thinking that some sort of anti-gravity is at play, then it isn’t really anything so exotic. If the Universe were completely homogeneous and isotropic – as our simplest models assume – then it would be expanding at the same rate in all directions.  This would be a pure “Hubble flow“, with galaxies appearing to recede from an observer with a speed proportional to their distance:


But the Universe isn’t exactly smooth. As well as the galaxies themselves, there are clusters, filaments and sheets of galaxies and a corresponding collection of void regions, together forming a huge and complex “cosmic web” of large-scale structure. This distorts the Hubble flow by inducing peculiar motions (i.e. departures from the pure expansion). A part of the Universe which is denser than average (e.g. a cluster or supercluster) expands less  quickly than average, a part which is less dense (i.e. a void) expands more quickly than average. Relative to the global expansion rate, clusters represent a “pull” and voids represent a “push”. That’s really all there is to it.

The difficult part about this kind of study is measuring a sufficient number of peculiar motions of galaxies around our own to make a detailed map of what’s going on in the local velocity field. That’s particularly hard for galaxies near the plane of the Milky Way disk as they tend to be obscured by dust. Nevertheless, after plugging away at this for many years, the authors of the Nature paper have generated some fascinating results. It seems that our Galaxy and other members of the Local Group lie between a dense supercluster (often called the Shapley concentration) and an underdense region, so the peculiar velocity field around us has an approximately dipole structure.

They’ve even made a nice video to show you what’s going on, so I don’t have to explain any further!




18 Responses to “The Dipole Repeller”

  1. From the Guardian article:

    Michael Rowan-Robinson, an astronomer at Imperial College London, said that while the scientists were right to stress how voids can apparently repel galaxies as much as clusters of galaxies attract, they may have over-emphasised the importance of the Shapley attractor and the so-called dipole repeller. In 2000, his team used galaxy surveys from the Infrared Astronomical Satellite to show that there are scores of superclusters and voids of similar size affecting the flow of galaxies through the cosmos.

    • telescoper Says:

      I saw that comment. I don’t really agree with it. The point is that the selection function for IRAS galaxies falls very rapidly with distance, so large-scale structure around the Local Group is not well traced by them. There’s circumstantial evidence that structures lurking beyond the depth of the IRAS sample may be much more important than that study suggests. That is probably the reason, for example, that the IRAS dipole measurement significantly overestimated Omega….

  2. ..distances of ~20,000 km s−1 or more

    first year students would lose marks for that…

    • It is not uncommon to express distances in terms of velocities. It’s just cz. For small redshifts (where one doesn’t have to worry about what sort of distance), it is perhaps easier to juggle in one’s mind than a few zeros before the significant digits after the decimal point. What would you require of first-year students? Mpc? The advantage of the velocities is that they don’t depend on the cosmological model. OK, you can introduce h, but you can’t easily scale with lambda and Omega.

      • telescoper Says:

        I think the point Albert was making is that it’s dimensionally incorrect. To make an actual distance you need to put in a value for the Hubble constant. It is actually very convenient to use the implied velocity instead of doing this, especially if your goal is to estimate Omega.

      • Two points. First, as you say, leaving out the Hubble constant is useful if one is interested in things, such as Omega, which are independent of the Hubble constant. Second, the conversion to appropriate dimensions is known. It’s like saying that something has a mass of 1 GeV, or “natural” units with c=G=1.

      • Indeed, I was commenting on the units. Students need to be shown how to use -and balance- units. It is an essential skill right from the beginning. Of course we all know what this ‘distance’ means. But it is a short-cut and to students it is confusing. It may be better to be more precise in the phrasing (or more pedantic). And GeV is not a unit of mass – that is GeV/c^2. We spend a fair amount of effort drilling that into the students. Perhaps not enough though..

      • telescoper Says:

        GeV is a unit of mass if c=1.

  3. I have a few layman’s questions…(1) why does the density of the universe in any particular area affect its expansion rate?

    It’s intuitive to me why density might affect expansion in 3D space (ie, heavier would be slower), but I was under the impression that the expansion of the universe wasn’t merely the expansion of space itself in 3 dimensions (spreading out and getting bigger), but the expansion of space-time.

    (2) I can’t really visualize what “space-time” means, but there is the popular conception of a fabric that is warped by concentrations of mass, so that a mass like a star causes a sink (metaphorically) that pulls other objects towards it. If this conception is at all accurate, what might the expansion of that fabric be like?

    (3) If this conception is accurate, is the dipole repeller akin to a hill rather than a sink?

    (4) Finally, does dark matter and/or dark energy have anything to do with this?

    Thank you!

    • (1) Because that is what the equations say. The expansion of the universe is the expansion of 3-D space. Yes, there is also space-time, but not everything is space-time; sometimes space is enough, as here. (Both space and space-time can be curved.)

      (2) The analogy you mention involves space, not space-time.

      (3) Not a hill, but rather a region where the slope is less than in neighbouring regions.

      (4) Dark matter is matter, so affects expansion like any other matter. Dark energy also affects the expansion, but differently.

      • Thanks very much for you answer. It’s lead me to research a bit more to try to understand your answer before I ask more, and I’m still working on it!

  4. During one of the first lectures I ever gave on cosmology, a student remarked that she couldn’t make sense of the equation v = hd; was hubble suggesting in 1929 that a galaxy was accelerating?
    After class, I realised she had a point; I guess we should really say v1/v2 = h(d1/d2 ) !

    • telescoper Says:

      The other day I pointed out to my students that if d is the proper distance and v is the proper velocity (ie the rate of change of proper distance with cosmological proper time) then v=hd is exact for any d, even when v>>c.

      They were surprised.

      • If they were surprised, then because they haven’t read Edward Harrison. This is explained in detail in his wonderful textbook Cosmology: The Science of the Universe and in a landmark paper (where he also points out that some big names have made the same mistake).

    • I’m a bit confused as to why she was confused. In the equation v = hD, all quantities are evaluated at a given moment in cosmic time. (None of these quantities is “directly observable”, which confuses some people.)

      Did she think that as distance increases, then as an object recedes due to the expansion of the universe, its velocity increases? If so, this is wrong for several reasons, one of which is that the Hubble constant in general changes with time (and no, that doesn’t make the term an oxymoron; it is called “constant” for a different reason) and another of which is that whether this is the case depends on the cosmological parameters other than the Hubble constant.

  5. I think it’s a good example of the manner in which we often use foreknowledge to interpret an equation, and fail to realise that the equation could be read quite differently. The student, unversed in cosmology, read ‘v’ and ‘d’ as variables for a particular galaxy (as in y = mx). Perfectly understandable, to my mind, and an illustration that equations in physics aren’t always as explicit as we think.

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