Dipole repeller - what is it?

  • #1

Main Question or Discussion Point

Recently, I read a few popularized articles portraying a study and associated calculations using astronomical measurements that posits a characterization of the direction and acceleration (or velocity) of our galaxy. The researchers assert not only is our galaxy being attracted by massive concentrations of mass, but it is also being pushed, impelled, by a region as empty of mass as the massive superclusters attracting us are dense with it. They called the 'impelling' region a "dipole repeller." Here is one of the couple of articles I read, https://phys.org/news/2017-01-galaxy-space.html

None of the articles presented a characterization of the 'pushing' force. Perhaps the authors of the article, to be published in Nature Astronomy, don't presume to characterize it but just argue that it exists. Though the paper has yet to appear, I wish to ask if anyone can imagine and suggest some explanation or foundation for such an entity or phenomenon.
 

Answers and Replies

  • #2
Ken G
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The "dipole repeller" is not any new physics, and the term should not be taken too seriously. All that is happening is, you start with a constant mass density everywhere, a la the cosmological principle, and then our galaxy would be what is known as "comoving" (if it didn't have any additional velocity, but that's not the point here, acceleration is). What it means to be comoving is that it's not really going anyway, just staying put in the Hubble expansion (even though that expansion is itself accelerating, that's not the kind of special acceleration being talked about here). However, measurements show that our galaxy is not simply comoving, because the mass distribution is not uniform on the nearby scales of galaxy clusters. So we think in terms of a perturbation from the constant homogeneous density, and when we think that way, we see not only a mass enhancement in one direction, but also a mass deficit in the other direction. Either of these, by themselves, would cause the galaxy to accelerate in the same direction, so the two add up for even more acceleration. That's the "dipole repeller." But note the deficit of mass is not producing a force itself, it is failing to counteract the force from the constant density that exists everywhere else outside the deficit. So it's all in the way you picture the deviations from a uniform mass distribution, not any kind of new force that "repels." Perhaps it is an unfortunate terminology, you can certainly see how much confusion it is already causing!
 
  • #3
The "dipole repeller" is not any new physics... Either of these, by themselves, would cause the galaxy to accelerate in the same direction, so the two add up for even more acceleration.
It's the magnitude of the objects and their masses that warrants treating a relative absence of mass as a contributing cause for a galaxy's acceleration, correct? However, it seems a fallacy to treat the absence of a thing as if it were an actual, positive, contributing cause. The low-mass area isn't pushing our galaxy away, it's merely not offsetting the attraction of the surrounding masses. Perhaps the merit and legitimacy of treating it this way develops from the the mathematical treatment.
 
  • #4
Ken G
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I agree the terminology can lead to confusion, but it's not unprecedented. For example, consider the concept of "holes" moving around in semiconductors-- those aren't things either, they are the absence of things. But the absence of something, embedded in an otherwise continuous background of that thing, can take on a "thinginess" of its own as a kind of convenience.
 
  • #5
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Thanks to all the previous commenters, very helpful for all of us who are curious and intrigued by this. Here are a few more questions. How big is this massless area in comparison with the known superclusters, and what is it's shape, and what determines the shape of a massless object? I believe it was last year when scientists proposed rough estimates on the size of the Universe, are there suspected or expected to be more areas like this dipole repeller, are they thought to be common? Also, I get that they are massless, are these voids also lacking dark matter? Or made of it to some degree? I'm trying to imagine what kind of organism the Universe might appear to be if it were to be scaled down. A large stigmatic eyeball perhaps, with veins of superclusters and vast areas of nothingness in-between? Lastly, does anyone know of a publicly available 3-dimensional universe map that includes the known superclusters? Most of what is available online is very limited in scope. Thank ahead of time for any and all responses!
 
  • #6
jim mcnamara
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Click on the link Shellsunde gave us. The graphic at the start is what you want to look at carefully.
 
  • #7
Bandersnatch
Science Advisor
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Lastly, does anyone know of a publicly available 3-dimensional universe map that includes the known superclusters? Most of what is available online is very limited in scope
Hayden planetarium's Digital Unvierse is the best one there is. Available here:
http://www.amnh.org/our-research/hayden-planetarium/digital-universe
Learning the interface is a bit of a hurdle, but not exceedingly so (reading the manual is a must).
 

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