The (somewhat) recent paper "Probing the Dark Flow signal in WMAP 9 yr and PLANCK cosmic microwave background maps" (submitted in 2014) by a team lead by the researchers at NASA's Goddard Space Flight Center seems to convincingly contradict the initial analysis of the ESA's Planck project, strengthening the case for "dark flow".

Where the initial Planck publication suggested the project's data seemingly contradicted NASA's earlier WMAP results, this reanalysis suggest that supposed conflict is correct and that both data sets when correctly filtered show a systematic dipolar effective motion of the expanding universe -- commonly known as "dark flow". The authors further provide evidence that this finding is too large to be accounted for by experimental uncertainties/known errors.

Do these results suggest that we're moving toward some massive concentration of matter (per our current observations, let's say a massive supercluster)? And if so are there any papers discussing what kind of object might be overriding expansion and creating that pull? (Ultramassive Black hole? Super string like object? Alien superstructure? (Just kidding on the last one... but hey, who knows...))

Per my previous reading this dark flow is faster than expansion, right? So our supercluster is actually going to effectively travel towards this region of space -- which per my understanding is outside our Hubble sphere?

Further what does "an isocurvature component of the primordial density field" mean in context of the below passage from the work:

KABKE termed this the “dark flow” speculating that it may be reflective of the effective motion across the entire cosmological horizon. If true, this is equivalent to at least a part of the all-sky CMB dipole being of primordial origin, a possibility that requires an isocurvature component in the primordial density field (Matzner 1980, Turner 1991, Mersini-Houghton & Holman 2009).

Also is the discussed quadrupole suggesting dark flow in some other direction (and that there's some sort of multi-dimensional symmetry to these flows)?

Additionally, have any attempts been made to visualize these proposals for the benefits of us laymen? i.e. How might this reshape the classic conic visualization of the evolution of the expanding open universe?

conic depiction of expanding universe

So far the only one I found was this one... not sure the orig. source or how accurate it was:

Dark flow artist depiction

And does this dark flow provide strong evidence of an inhomogeneous universe?

Lastly, do these findings have any impact on the newer idea of a "holographic universe" (that seems to be growing in popularity in the physics community from what I've gathered)?


1 Answer 1


Since no one answered this, I wanted to post what I believe to be the best current answer, which I stumbled across.

First, according to a new study the answer is "yes" per the latest evidence which from a layman's perspective sounds fairly compelling and comprehensive.

The new work [per my understanding] suggests the local group and other neighborly mass is indeed moving towards the overdense region of the "cosmic dipole", aka the "Shapely attractor" / "great attractor" / etc. in part due to a gravitational "repeller" of sorts that has long been observed (as the dipole) in the CMB, but was largely dismissed as an artifact of the early evolution of the universe. Now it appears it in fact may be an evolving superstructure of sorts -- essentially a void that acts in a repulsive sense to increase the potency of the Shapely attractor.

In terms of illustrations I found the equi-gravitational potential and velocity 3D flow renders to be wonderful in depicting this:

enter image description here

enter image description here

Also, I found this Forbes article by one of the authors to be particularly good at explaining in layman's terms the question of what a "repeller" means, when gravity is purely attractive.

The concept of a low-mass region of mostly void pushing things struck me as peculiar until I read that and realized the current thinking (per the new work) wasn't so much that the repeller is truly "pushing" (a violation of how gravity is understood to work) but rather as incrementally removing the resistance to the pulling, that amounts to a similar (and synergistic!) outcome to the more familiar "pulling" we see from objects of mass (orderly movement).

  • $\begingroup$ So, they think they found some local (from a cosmological point of view) structure where mass concentrates and others where it's a lower than average density. Since gravity attracts that's not surprising. What is the new information that merits talking about it? Oh, and btw, it must be hard to answer your own question and grade yourself on it. Sounds like you wanted to tell something $\endgroup$
    – Bob Bee
    May 11, 2017 at 2:41
  • $\begingroup$ I appreciate the comment, although it seems a bit negative. "What is the new information that merits talking about it?" The study uses Cosmicflows-2 -- one of the best 3D flow field (movement of galaxies in the universe) based on redshift surveys -- as its basis. What makes this meta-analysis important, to my understanding is its strong indication that the underdensity in the CMB dipole may not be an artifact of a primordial universe but instead appears to be a true, growing void -- an underdense region paralleling the overdense "great attractor", hence termed the "great repeller". $\endgroup$ May 19, 2017 at 4:09
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    $\begingroup$ Jason, thank you. I was indeed too negative. Probably biased due to you answering your own question. The findings are interesting, and important. I'd seen the 2014 paper, and more recent papers such as Hoffman's et al letter at NATURE Astronomy at nature.com/articles/s41550-016-0036. The finding do seem to be confirming the great attractor (more importantly the Shapley attractor) and great repeller. I had thought, maybe incorrectly, that it was always thought to be due to local, though at larger scales, gravitational effects (eg, the great attractor), and not. -- see next $\endgroup$
    – Bob Bee
    May 20, 2017 at 0:16
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    $\begingroup$ Inherently a new major cosmologically sourced inhomogeneities. But we still don't know if the inflation reside of Planck inhomogeneities could have eventually led to it or it needed bigger inhomogeneities - at least I have not seen a calculation that specifically predicts their reasonable possibilities. So I see a great (apparently Shapley) repeller and attractOr have been found, but no evidence for a cosmological conclusion or not. That's what I was questioning about 'something new'. If have have anything on that I'd be interested and so would others, and maybe I just missed it. $\endgroup$
    – Bob Bee
    May 20, 2017 at 0:21
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    $\begingroup$ I don't think this answers the question. Your original question was about dark flow, which describes a possible motion of large regions of our observable universe. Your answer and the paper discuss movements of the local group and local superclusters. Isn't there several orders of magnitude between those concepts? $\endgroup$
    – quant
    Dec 1, 2018 at 13:22

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