9 ms·

The dipole repeller

78 points by perseusprime11 10y ago

sliken 10y ago
Isn't this playing a bit fast and loose with physics? Isn't the the same thing as saying, wow, I'm heavy on the planet earth. The sky is putting a force on me, by not having enough mass to counter the gravity of the earth. Our galaxy is being pushed by a lack of mass on one direction?
- marcosdumay 10y ago
  It's being pushed by a lack of balance of the mass over the different directions. That lack of balance is because there is a void in one direction. Physics is full of those shortcuts, I don't think anybody would get anything done without them.
pmoriarty 10y ago
Invisible force? Aren't all forces invisible? Oh, I see. The title of the HN submission has been creatively edited and is not the title of the original Nature article. Clickbait.
- flukus 10y ago
  > Oh, I see. The title of the HN submission has been creatively edited and is not the title of the original Nature article. Well yeah, they seem to have put it into laymans terms, but as a layman it appears to be an accurate representation of the article. It's certainly more descriptive than "The Dipole Repeller", so not click bait.
- mirimir 10y ago
  Yes, clickbait. The "invisible force" here is gravity. This is the key bit of context: > When describing the gravitational dynamics in co-moving coordinates, by which the expansion of the Universe is factored out, underdensities repel and overdensities attract. So it's basically a coordinates transformation thing.
  - deleted 10y ago
    [deleted]
  - mirimir 10y ago
    Upon reflection, Hoffman et al. (2017) itself may arguably be clickbait. To the extent that I understand it, they're looking at inhomogeneity in cosmic expansion. It seems equivalent to turbulence in fluid dynamics. That is, non-uniform flow. And in considering airflow, for example, one doesn't say that underdensities repel air, or that overdensities attract air. Rather, it's the non-uniform flows that create the underdensities and overdensities. Anyway, it seems to me that Hoffman and coauthors should have stuck to describing correlations, because that's all they have. Using the language of causality is confusing. They don't mention it, but maybe they have an agenda to promote varieties of dark matter with positive and negative gravity.
    - mannykannot 10y ago
      I don't think the analogy holds, because the gravitational force between molecules is not significant in airflow.
      - mirimir 10y ago
        Fair enough. But what about the causality issue?
- djloche 10y ago
  Suggested title: Local Group motion: dominant influence identified as dipole repeller This is about the Local Group *motion
  - mirimir 10y ago
    No, scale is larger than Local Group.[0] 0) http://www.nature.com/articles/s41550-016-0036/figures/1 http://www.nature.com/articles/s41550-016-0036/figures/1
    - djloche 10y ago
      Yeah, its about the motion of our Local Group, but yes. Here's the summary provided in the xml: >The presence of a large underdensity, the dipole repeller, is predicted based on a study of the velocity field of our Local Group of galaxies. The combined effects of this super-void and the Shapley concentration control the local cosmic flow.
      - mirimir 10y ago
        Yes, Local Group motion :) The Dipole repeller and Shapley attractor are ~30000 km/s apart, which is ~1.5 billion light-years.[0] 0) http://www.as.utexas.edu/astronomy/education/spring06/komatsu/secure/lecture13.pdf http://www.as.utexas.edu/astronomy/education/spring06/komats...
- pvg 10y ago
  You can just email the mods about these, they fix them quite quickly.
- tzs 10y ago
  Yup, saying invisible in there was redundant. That's not all, though. Saying force is additionally also redundant too. All things pushed around are pushed around by force. "Milky way pushed around" would have been sufficient to convey the interesting part. It's interesting because at galactic scale isn't it usual to be pulled around rather than pushed around?
  - SmellyGeekBoy 10y ago
    > additionally also redundant too. Not sure if deliberate
- acqq 10y ago
  Mods (dang?) can you please fix the title it's edited to be a clickbait and even more important it is false?
mentalpiracy 10y ago
Is this perhaps evidence of a higher rate of cosmic expansion in low-density "void" regions?
- mirimir 10y ago
  That's what I think. But then, I didn't make it past Physics 2 ;)
codewiz 10y ago
Also watch the "explanatory" video: http://www.nature.com/article-assets/npg/natastron/2017/s41550-016-0036/extref/s41550-016-0036-s2.mp4 http://www.nature.com/article-assets/npg/natastron/2017/s415...
mbenjaminsmith 10y ago
Maybe a physicist can tell me why I'm wrong here: We often visualize the effect of mass on spacetime by using a trampoline and a bowling ball. The outer edges are flat and the closer you get to the bowling ball the more spacetime is warped downward. On the outer edges, the effect is almost nonexistent and the trampoline's surface is nearly flat. What this suggests is that spacetime without mass is flat and that mass warps spacetime in a downward direction. So no mass = flat, mass = curved downward. So the effect is in the range of [0, black hole]. Obviously this is an very loose analogy but bear with me. What if spacetime acts more like a waterbed? Not only does mass warp spacetime "downward" but something (in our analogy the displaced water) pushes the rest of spacetime upward. To use a signal processing analogy, a filter will (nearly?) always affect the passband to some degree, with very high Q filters creating a knee where amplitude is increased significantly before the filter rolls off. That seems counterintuitive (at least to me) but it's nonetheless true. Using another analogy, curved spacetime creates regions of "downhill-ness". What if the space between masses get a little bit of uphill curve as well? That could explain e.g., the Pioneer anomaly and possibly at much larger scales the effect in the article.
- jaddood 10y ago
  Actually, considering the effects of dark matter, it seems (at least to me) to be a great candidate for what you're talking about.
- paulddraper 10y ago
  General relativity, which describes the effect of mass on space-time, is a very elegant theory with beautiful maths. The trampoline and ball have incidental similarity, but they're for illustration only. There's nothing that makes the waterbed theory necessarily wrong, but you'd have to posit some new maths different and more complicated than we have with GR.
  - mbenjaminsmith 10y ago
    Yeah, I understand that the trampoline analogy is inadequate but I believe that we don't consider empty regions of space to have any type of "opposite" curvature or to exhibit negative gravity. I believe curvature is thought to be [0, black hole], not [-black hole, black hole]. Happy to be corrected on that, IANAP. The disparity between what GR predicts and what we observe has led to the cosmological constant / dark energy / dark matter. Of course, those aren't really things in themselves, they're just a measure of that disparity, a fudge factor. I'm suggesting that the disparity could be explained by spacetime "responding" to regions of curvature by mass with an opposite curvature. Unfortunately I'm not qualified to do anything with that idea. I just wanted to throw it out there. It's something that's been bothering me for a long time.
michaelsbradley 10y ago
Maybe Biot–Savart operating at intergalactic scale? cf. http://plasmauniverse.info/Perattpdf/PerattEvidenceCosmic.pdf http://plasmauniverse.info/Perattpdf/PerattEvidenceCosmic.pd...
autocorr 10y ago
Interestingly this is the first issue of Nature Astronomy, I hadn't known until now that they were making a topical journal (similar to Nature Geoscience, etc.) Looking through the different abstracts in the issue, I have to be honest, they don't seem very different than normal papers you find everyday on arXiv. I don't think it's a good thing to have another journal in this case, since Nature is not open access friendly. To be honest, I don't know too much about Nature Publishing Group, but what I do know is that the standard astronomical journal in America, the Astrophysical Journal (ApJ, "ap-jay"), is very well. It's owned by the American Astronomical Society (AAS) and published by the Institute of Physics (IOP) [1] which is itself a non-profit and does great work in education and other fields. They have a lenient policy where articles become open-access after 12-months. My point is, they're not blood-suckers like Elsevier :) And I would be unhappy to see Nature use it's prestige to muscle in a journal with more onerous access policies. [1] https://en.wikipedia.org/wiki/IOP_Publishing https://en.wikipedia.org/wiki/IOP_Publishing
- timthorn 10y ago
  Nature was founded by Norman Lockyer, so it does have a good claim to astronomical papers. :)
vpribish 10y ago
Can someone explain their distance units to me? I'd read "km s-1" as kilometers per second. which can't be right and isn't just a typo as the magnitude is way to small for "km". "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"
- senrj 10y ago
  could be delta-v?
- sanxiyn 10y ago
  It seems to be recessional velocity, from which you can get distance using Hubble's law.
- dexwiz 10y ago
  It's related to the Hubble Velocity. https://en.m.wikipedia.org/wiki/Hubble's_law https://en.m.wikipedia.org/wiki/Hubble's_law Since the universe is expanding and the relative velocity is greater for further objects, you can express the distance from Earth as the relative velocity.
  - phkahler 10y ago
    Seems like cheating. The expansion of the universe is a correlation between distance to things and their red shift. You still need to have a proper measure of the distance to something rather than using it's velocity as a stand-in for distance. Particularly if you're looking at things like flow fields.
    - acqq 10y ago
      It's not. It's honesty, and keeping the measurements in the form that will remain more usable. What we measure directly is the redshift. The estimation of the distance (that is, the exact value of the H0, the Hubble constant) is done using various methods and the results aren't bad, just not precise enough to use it combined with the redshift. That's what makes us preferring the redshift now, as the speed can be exactly calculated from it: https://en.wikipedia.org/wiki/Redshift https://en.wikipedia.org/wiki/Redshift Knowing that the Universe is "flat" ( https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe ) allows us also to do good calculations, even if we still don't have the exact value of H0. In fact, it's also not so bad: the estimates based on the Planck satellite, measuring the state of the Universe 13.7 billion years ago as the galaxies and stars haven't existed, and the ones based on the Hubble satellite data (measuring the state of the Universe much more recently) mismatch only some 9%. It is amazing enough for me. This difference, after more research, can produce some new physics discoveries or make us improve some engineering (and we surely should invest in more research) but it's quite good for everybody else: even if the "emptiness" from this article affecting the Milky Way is not 700 million but 730 million or 670 million light years, it won't affect our plans: for example, the Andromeda galaxy is just some 2.5 million light years from us and we estimate it will collide with the Milky Way in some 4 billion years, give or take. Should we care much if the "emptiness" is 268 or 292 times farther than the Andromeda is at this moment? See my other response here for the way you can calculate the distance from the speed, based on your preferred estimate of the H0, it's simple, just less precise at the moment than simply stating the speed (or the redshift).
Pocketsnakes 10y ago
the imagination of the jewish (recently mutilated baby) cocksucker who invented this preposterous, unbelievable nonsense

Isn't this playing a bit fast and loose with physics? Isn't the the same thing as saying, wow, I'm heavy on the planet earth. The sky is putting a force on me, by not having enough mass to counter the gravity of the earth. Our galaxy is being pushed by a lack of mass on one direction?

marcosdumay 10y ago
It's being pushed by a lack of balance of the mass over the different directions. That lack of balance is because there is a void in one direction. Physics is full of those shortcuts, I don't think anybody would get anything done without them.

pmoriarty 10y ago

Invisible force? Aren't all forces invisible? Oh, I see. The title of the HN submission has been creatively edited and is not the title of the original Nature article. Clickbait.

flukus 10y ago
> Oh, I see. The title of the HN submission has been creatively edited and is not the title of the original Nature article. Well yeah, they seem to have put it into laymans terms, but as a layman it appears to be an accurate representation of the article. It's certainly more descriptive than "The Dipole Repeller", so not click bait.
mirimir 10y ago
Yes, clickbait. The "invisible force" here is gravity. This is the key bit of context: > When describing the gravitational dynamics in co-moving coordinates, by which the expansion of the Universe is factored out, underdensities repel and overdensities attract. So it's basically a coordinates transformation thing.
- deleted 10y ago
  [deleted]
- mirimir 10y ago
  Upon reflection, Hoffman et al. (2017) itself may arguably be clickbait. To the extent that I understand it, they're looking at inhomogeneity in cosmic expansion. It seems equivalent to turbulence in fluid dynamics. That is, non-uniform flow. And in considering airflow, for example, one doesn't say that underdensities repel air, or that overdensities attract air. Rather, it's the non-uniform flows that create the underdensities and overdensities. Anyway, it seems to me that Hoffman and coauthors should have stuck to describing correlations, because that's all they have. Using the language of causality is confusing. They don't mention it, but maybe they have an agenda to promote varieties of dark matter with positive and negative gravity.
  - mannykannot 10y ago
    I don't think the analogy holds, because the gravitational force between molecules is not significant in airflow.
    - mirimir 10y ago
      Fair enough. But what about the causality issue?
djloche 10y ago
Suggested title: Local Group motion: dominant influence identified as dipole repeller This is about the Local Group *motion
- mirimir 10y ago
  No, scale is larger than Local Group.[0] 0) http://www.nature.com/articles/s41550-016-0036/figures/1 http://www.nature.com/articles/s41550-016-0036/figures/1
  - djloche 10y ago
    Yeah, its about the motion of our Local Group, but yes. Here's the summary provided in the xml: >The presence of a large underdensity, the dipole repeller, is predicted based on a study of the velocity field of our Local Group of galaxies. The combined effects of this super-void and the Shapley concentration control the local cosmic flow.
    - mirimir 10y ago
      Yes, Local Group motion :) The Dipole repeller and Shapley attractor are ~30000 km/s apart, which is ~1.5 billion light-years.[0] 0) http://www.as.utexas.edu/astronomy/education/spring06/komatsu/secure/lecture13.pdf http://www.as.utexas.edu/astronomy/education/spring06/komats...
pvg 10y ago
You can just email the mods about these, they fix them quite quickly.
tzs 10y ago
Yup, saying invisible in there was redundant. That's not all, though. Saying force is additionally also redundant too. All things pushed around are pushed around by force. "Milky way pushed around" would have been sufficient to convey the interesting part. It's interesting because at galactic scale isn't it usual to be pulled around rather than pushed around?
- SmellyGeekBoy 10y ago
  > additionally also redundant too. Not sure if deliberate
acqq 10y ago
Mods (dang?) can you please fix the title it's edited to be a clickbait and even more important it is false?

mentalpiracy 10y ago

Is this perhaps evidence of a higher rate of cosmic expansion in low-density "void" regions?

mirimir 10y ago
That's what I think. But then, I didn't make it past Physics 2 ;)

codewiz 10y ago

Also watch the "explanatory" video: http://www.nature.com/article-assets/npg/natastron/2017/s41550-016-0036/extref/s41550-016-0036-s2.mp4 http://www.nature.com/article-assets/npg/natastron/2017/s415...

mbenjaminsmith 10y ago

Maybe a physicist can tell me why I'm wrong here: We often visualize the effect of mass on spacetime by using a trampoline and a bowling ball. The outer edges are flat and the closer you get to the bowling ball the more spacetime is warped downward. On the outer edges, the effect is almost nonexistent and the trampoline's surface is nearly flat. What this suggests is that spacetime without mass is flat and that mass warps spacetime in a downward direction. So no mass = flat, mass = curved downward. So the effect is in the range of [0, black hole]. Obviously this is an very loose analogy but bear with me. What if spacetime acts more like a waterbed? Not only does mass warp spacetime "downward" but something (in our analogy the displaced water) pushes the rest of spacetime upward. To use a signal processing analogy, a filter will (nearly?) always affect the passband to some degree, with very high Q filters creating a knee where amplitude is increased significantly before the filter rolls off. That seems counterintuitive (at least to me) but it's nonetheless true. Using another analogy, curved spacetime creates regions of "downhill-ness". What if the space between masses get a little bit of uphill curve as well? That could explain e.g., the Pioneer anomaly and possibly at much larger scales the effect in the article.

jaddood 10y ago
Actually, considering the effects of dark matter, it seems (at least to me) to be a great candidate for what you're talking about.
paulddraper 10y ago
General relativity, which describes the effect of mass on space-time, is a very elegant theory with beautiful maths. The trampoline and ball have incidental similarity, but they're for illustration only. There's nothing that makes the waterbed theory necessarily wrong, but you'd have to posit some new maths different and more complicated than we have with GR.
- mbenjaminsmith 10y ago
  Yeah, I understand that the trampoline analogy is inadequate but I believe that we don't consider empty regions of space to have any type of "opposite" curvature or to exhibit negative gravity. I believe curvature is thought to be [0, black hole], not [-black hole, black hole]. Happy to be corrected on that, IANAP. The disparity between what GR predicts and what we observe has led to the cosmological constant / dark energy / dark matter. Of course, those aren't really things in themselves, they're just a measure of that disparity, a fudge factor. I'm suggesting that the disparity could be explained by spacetime "responding" to regions of curvature by mass with an opposite curvature. Unfortunately I'm not qualified to do anything with that idea. I just wanted to throw it out there. It's something that's been bothering me for a long time.

michaelsbradley 10y ago

Maybe Biot–Savart operating at intergalactic scale? cf. http://plasmauniverse.info/Perattpdf/PerattEvidenceCosmic.pdf http://plasmauniverse.info/Perattpdf/PerattEvidenceCosmic.pd...

autocorr 10y ago

Interestingly this is the first issue of Nature Astronomy, I hadn't known until now that they were making a topical journal (similar to Nature Geoscience, etc.) Looking through the different abstracts in the issue, I have to be honest, they don't seem very different than normal papers you find everyday on arXiv. I don't think it's a good thing to have another journal in this case, since Nature is not open access friendly. To be honest, I don't know too much about Nature Publishing Group, but what I do know is that the standard astronomical journal in America, the Astrophysical Journal (ApJ, "ap-jay"), is very well. It's owned by the American Astronomical Society (AAS) and published by the Institute of Physics (IOP) [1] which is itself a non-profit and does great work in education and other fields. They have a lenient policy where articles become open-access after 12-months. My point is, they're not blood-suckers like Elsevier :) And I would be unhappy to see Nature use it's prestige to muscle in a journal with more onerous access policies. [1] https://en.wikipedia.org/wiki/IOP_Publishing https://en.wikipedia.org/wiki/IOP_Publishing

timthorn 10y ago
Nature was founded by Norman Lockyer, so it does have a good claim to astronomical papers. :)

vpribish 10y ago

Can someone explain their distance units to me? I'd read "km s-1" as kilometers per second. which can't be right and isn't just a typo as the magnitude is way to small for "km". "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"

senrj 10y ago
could be delta-v?
sanxiyn 10y ago
It seems to be recessional velocity, from which you can get distance using Hubble's law.
dexwiz 10y ago
It's related to the Hubble Velocity. https://en.m.wikipedia.org/wiki/Hubble's_law https://en.m.wikipedia.org/wiki/Hubble's_law Since the universe is expanding and the relative velocity is greater for further objects, you can express the distance from Earth as the relative velocity.
- phkahler 10y ago
  Seems like cheating. The expansion of the universe is a correlation between distance to things and their red shift. You still need to have a proper measure of the distance to something rather than using it's velocity as a stand-in for distance. Particularly if you're looking at things like flow fields.
  - acqq 10y ago
    It's not. It's honesty, and keeping the measurements in the form that will remain more usable. What we measure directly is the redshift. The estimation of the distance (that is, the exact value of the H0, the Hubble constant) is done using various methods and the results aren't bad, just not precise enough to use it combined with the redshift. That's what makes us preferring the redshift now, as the speed can be exactly calculated from it: https://en.wikipedia.org/wiki/Redshift https://en.wikipedia.org/wiki/Redshift Knowing that the Universe is "flat" ( https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe ) allows us also to do good calculations, even if we still don't have the exact value of H0. In fact, it's also not so bad: the estimates based on the Planck satellite, measuring the state of the Universe 13.7 billion years ago as the galaxies and stars haven't existed, and the ones based on the Hubble satellite data (measuring the state of the Universe much more recently) mismatch only some 9%. It is amazing enough for me. This difference, after more research, can produce some new physics discoveries or make us improve some engineering (and we surely should invest in more research) but it's quite good for everybody else: even if the "emptiness" from this article affecting the Milky Way is not 700 million but 730 million or 670 million light years, it won't affect our plans: for example, the Andromeda galaxy is just some 2.5 million light years from us and we estimate it will collide with the Milky Way in some 4 billion years, give or take. Should we care much if the "emptiness" is 268 or 292 times farther than the Andromeda is at this moment? See my other response here for the way you can calculate the distance from the speed, based on your preferred estimate of the H0, it's simple, just less precise at the moment than simply stating the speed (or the redshift).

Pocketsnakes 10y ago

the imagination of the jewish (recently mutilated baby) cocksucker who invented this preposterous, unbelievable nonsense