View Full Version : The Dipole Repeller?

Ara Pacis
2017-Feb-03, 07:18 PM
I'm not entirely sure what to think of this recent study published in Nature (http://www.nature.com/articles/s41550-016-0036) a few days ago. There's a less technical article and video illustrating the concept at IFLScience (http://www.iflscience.com/space/the-milky-way-is-running-away-from-an-extragalactic-void/all/). It models flow to the Shapley Attractor and concludes that there is a repeller that is pushing the local group.

I thought it was merely a statistical model from this wording:

When describing the gravitational dynamics in co-moving coordinates, by which the expansion of the Universe is factored out, underdensities repel and overdensities attract. The velocity field is represented here by means of streamlines (see Methods), the sources and sinks of which are the attractors and repellers of the large-scale structure

But some of the wording makes me think there's a physical object with repulsive properties:

We conclude that the dipole repeller is not a fictitious structure induced by an ‘edge of the data’ effect, and that subsets of the data, chosen either by distance or galaxy type, uncover a basin of repulsion that ‘pushes’ the Local Group in the direction pointed by the CMB dipole.


2017-Feb-03, 11:02 PM
It seems that the universe wants to do what it won't let us do.

The universe expands faster and faster--but won't give us free energy.
If there is anything to the EM drive, it seems weak.

Here though--there seems quite a push.


Ken G
2017-Feb-04, 05:34 PM
There's a thread on this in Q&A, but the upshot is, the repelling "object" is merely an underdensity in the mass distribution. It only looks like it is "repelling" when you contrast with a uniform mass distribution, which in comoving coordinates, produces no motion at all. So relative to no motion, an underdensity acts like a "repeller." Calling that an "object" is a bit of a stretch, but then we can kind of think of a "hole" in the electron distribution in a semiconductor as like an object also.

Ara Pacis
2017-Feb-05, 03:54 AM
Thanks. I figured it was an artifact of the underlying model, but their wording was weird.

I bet there's lots of pseudoscience grabbing and running with this.

Ken G
2017-Feb-17, 03:04 PM
Yes, it's a criterion that astronomy jargon usually doesn't think about: don't unleash a maelstrom of pseudoscience!