10 ms·

The Case Against Dark Matter

172 points by bilifuduo 10y ago

mirimir 10y ago
> One problem, said Brian Swingle of Harvard and Brandeis universities, who also works in holography, is that Verlinde lacks a concrete model universe like the ones researchers can construct in AdS space, giving him more wiggle room for making unproven speculations. “To be fair, we’ve gotten further by working in a more limited context, one which is less relevant for our own gravitational universe,” Swingle said, referring to work in AdS space. “We do need to address universes more like our own, so I hold out some hope that his new paper will provide some additional clues or ideas going forward.” Woah. How can AdS model universes have less "wiggle room for making unproven speculations"? There is no AdS universe to observe or experiment in. How could speculations in such universes be "proven"? Or rather, disproved?
- simonh 10y ago
  I think they're talking about mathematical proofs. AdS models are mathematically simpler than the physical universe containing matter, so it's easier to construct mathematical proofs.
  - mirimir 10y ago
    OK. But proofs about mathematical objects are entirely distinct from testable hypotheses about the actual universe. So it seems odd to compare them. OTOH, there is much "theoretical physics" that's not much more grounded in reality than AdS models ;)
marze 10y ago
From the final paragraph: “If somebody were to come to you and say, ‘The solar system doesn’t work on an inverse-square law, really it’s an inverse-cube law, but there’s dark matter that’s arranged just so that it always looks inverse-square,’ you would say that person is insane,” he said. “But that’s basically what we’re asking to be the case with dark matter here.” Awesome point. The McGaugh paper is really problematic for dark matter proponents. Amazing that that work wasn't done earlier.
- hvidgaard 10y ago
  They do state: "They say the amount of dark matter in a galaxy’s halo would have precisely determined the amount of visible matter the galaxy ended up with when it formed". It's not a far fetched idea, but it is problematic because we know of zero such interactions.
antognini 10y ago
As a general principle I would advise a grain of salt whenever a physicist claims to have solved an outstanding astrophysical problem. Verlinde is a great physicist --- I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. But someone with a background in pure physics often lacks the background in astronomy to know whether their ideas will work. (I remember hearing a visiting physicist present his ideas about stars producing axions, and having an astronomy professor point out that if he was right, Cepheid variables would not pulsate at the observed frequencies.) The article mentions the Bullet Cluster as one observation that MOND has difficulty explaining. Another observation that I'm not sure how MOND would explain is the dramatic difference in dynamics between a globular cluster and a dwarf spheroidal galaxy. Globular clusters are observed to have virtually no dark matter, whereas some dwarf spheroidal galaxies have somewhere between 1000 to 10,000 times as much dark matter as ordinary matter [1]. These two kinds of objects have comparable numbers of stars and are roughly the same size (though dwarf spheroidals are admittedly larger by about an order of magnitude). This discrepancy can be explained with cold WIMPs [2], but it's hard to understand how some MOND-like process would produce such incredibly different dynamics between the two kinds of galaxies. I'll admit that I have not looked very carefully at Verlinde's paper. But the evidence for dark matter is much more than galaxy rotation curves these days. A credible alternative to WIMPs has to explain more than that. All that aside, from a theoretical perspective the work is very interesting because it provides a much-needed framework to understand why something like MOND could exist. But I don't think that any of the direct detection experiments should pack it in quite yet. [1]: https://arxiv.org/abs/1510.03856 https://arxiv.org/abs/1510.03856 [2]: Well, sort of. Galaxy formation is still a big open problem, and globular cluster formation is probably even less well understood. But with WIMP dark matter it's very plausible that different formation mechanisms would produce objects with different amounts of dark matter.
- bokononon 10y ago
  > I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. I remember reading this too. Did anything ever become of this theory?
- m_mueller 10y ago
biggerfisch 10y ago
So is this really anything except an interesting way of deriving MOND? The article says > Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine. but doesn't actually show it doing that. As that's a major differentiator between his system and MOND, shouldn't that have been looked at? Seems to easy to otherwise dismiss this as a weird way of supporting MOND
deleted 10y ago
[deleted]
c3d 10y ago
Dark matter and dark energy are ad-hoc adjustments to explain discrepancies between observations and theory. The problem with such adjustments is that, being mostly unobserved, they end up with too many degrees of freedom (e.g. a distribution of invisible stuff where the density can be made "just right" to fit the observations), so they are quite hard to falsify. It's a bit like the "landscape" : when you construct a theory that can predict anything, you end up predicting nothing. In my opinion, we leave "science" as defined by Popper when we start doing that. There is, however, an underlying assumption in all the existing theories of physics, which I see too rarely challenged, and which is easily falsified. It's the idea that the variable "x" in the theory is independent of the physical measurement process being used. In other words, the "x" in MOND is the same as the "x" in GTR and the "x" in quantum mechanics. The topology of a space-time defined by a solid rod of metal in the Pavillon de Sevres (pre-1960 definition of the metre) is the same as the topology of space-time defined by the movement of light in the vacuum (today's definition). And so on. I believe that our current understanding of physics now has become so precise that we need to take into account subtle differences between this and that measurement process, and rewrite physics in the context of a "more general" relativity, where the laws of physics are to be described independently not just from, say, the state of movement, but more generally independently of the physical process being used. Just like GTR forced us to deal with quantities such as metrics, this more-general relativity would force us to deal with arbitrary transformations between two methods to measure the same thing. A simple example is when we relate time as measured by the rotation of the earth (definition of "days"), the rotation of earth around the sun (definition of "years"), the decay of populations of C14 ("radiocarbon dating"), and our current cesium-decay based atomic clocks (the definition of "second"). The four laws are obviously correlated, they all measure time. But the "fit curve" between them is a distribution in itself, similar to the "metric field" in GTR. I may be wrong, but I suspect that if we introduce this kind of complexity, the problem of "dark matter" will have not one, but multiple solutions, possibly different equations, depending on the measurement you are considering. This does not mean we should not look for equations that predict observations, but rather that we should be cautious when correlating measurements that correspond to different physical processes. It is not obvious to me, for example, that we should take for granted that we can safely deduce the distribution of masses from the distribution of luminosities (even if, I'll admit, I don't see any good alternative...) In quantum mechanics, we do little else with renormalization. When you shift from one scale to another, you need to renormalize to fit the data, because the raw equations would otherwise yield to infinities or other abnormalities. "Shut up and calculate" is frustrating, but if we rephrase that as "observe the universe, it's the ultimate test", it really means the same thing, but is philosophically more grounded. In the end, MOND may just be the shape of the relation between two measurements of mass at large scale, and Verlinde's paper one first step in understanding how this relationship emerges. Or not. There are some interesting objections to "statistical emergence" of gravity, or "entropy-based gravity". One of them is how interference patterns are not destroyed by gravity, at scales where, if we were to trust Verlinde's approach, interactions along different paths would affect probabilities enough to be sensible in interference patterns. I've not done the calculations myself to validate if this objection is solid, though. Intuitively, it seems like there is a flaw (the effect of per-state populations on interference should be at most of the same order as the effect of gravitation itself, which is in the "barely observable" category), but I need to spend more time to understand both sides.
basicplus2 10y ago
The concept of "Dark Energy" is as nebulous as "Dark Matter"
- stefantalpalaru 10y ago
  almost: https://en.wikipedia.org/wiki/Cosmological_constant_problem https://en.wikipedia.org/wiki/Cosmological_constant_problem
bambax 10y ago
> Physicists do not like magic; when other cosmological observations seemed far easier to explain with dark matter than with MOND, they left the approach for dead. But how is "dark matter" not magic? To my untrained and pretty ignorant eye, dark matter looks like filler: the observation does not match the theory, and so, instead of looking for a better theory, we decided the observation was wrong and couldn't see what was there (and not just a little bit, but the greatest part of it). Isn't that the definition of superstition? Invisible forces that explain events, in mysterious ways?
- acqq 10y ago
  It's not as "magic" as you believe it to be. The dark matter is at the moment is not only the simpler explanation (that is, you have formulas that can produce the results) of the observations (Occam razor etc) but the only one that explains different effects that are still fully unexplained (in the sense, there are no formulas that can even match anything) by MOND. And since 2006 there are the actual photos of "dark mater effects": http://www.forbes.com/sites/startswithabang/2016/09/19/dark-matter-proved-real-by-colliding-galaxy-clusters/ http://www.forbes.com/sites/startswithabang/2016/09/19/dark-... Now, how the MOND handles it? "Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine." Note "thinks" not that anybody managed. The dark matter approach could be declared really problematic even before anybody manages if some observations simply can't be explained with the dark matter. But at the moment it's still not so. And all the observations will remain: https://medium.com/starts-with-a-bang/10-facts-everyone-should-know-about-dark-matter-1ce2b222cf08 https://medium.com/starts-with-a-bang/10-facts-everyone-shou... "It’s not going away You will not wake up one day and hear physicists declare it’s not there at all. (Well, you will, but those claims are rare, and those physicists are wrong.) The evidence is overwhelming: Weak gravitational lensing demonstrates that galaxies have a larger gravitational pull than visible matter can produce. Additional matter in galaxies is also necessary to explain why stars in the outer arms of galaxies orbit so quickly around the center. The observed temperature fluctuations in the cosmic microwave background can’t be explained without dark matter, and the structures formed by galaxies wouldn’t come out right without dark matter either. Even if all of this was explained by a modification of gravity rather than an unknown type of matter, it would still have to be possible to formulate this modification of gravity in a way that makes it look pretty much like a new type of matter. And we’d still call it dark matter." Then, not as the underlying principle, but as a simpler handling for all the observations up to now that look like "the invisible-matter-like effects there."
drewhk 10y ago
I am not a physicist, in fact, I am a complete dilettante, but can the transition from a power law to an inverse law simply indicate that on large scale the universe has less dimensionality? I mean, if we take a suitable definition of dimension, likely along the lines of Hausdorff dimension (https://en.wikipedia.org/wiki/Hausdorff_dimension https://en.wikipedia.org/wiki/Hausdorff_dimension) then we can see it as a number that defines how volume/surface changes with changing diamater of an observed "sphere" (I know I oversimplify things here). In other words, as we start to measure cosmic distances on a large enough scale, volumes/surfaces covered don't grow quadratically anymore but with a lower rate. This might be even in line with the assumption of a universe curved into itself, i.e. closed. This would change the gravity power law from having a constant 2 power to having a function as the power.
- qubex 10y ago
  (Constant negative two power to having a function as the power.)
  - drewhk 10y ago
    It depends on whether you imagine a fraction line there or not: https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6ee5510ba3c7d6664775c0e76b53e72468303a https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6e... ;)
- Tloewald 10y ago
  The idea of fractal dimension being significant at galactic scale is intriguing and based on my totally naive reading of the article might resemble this insofar as the guy is arguing that gravity is an emergent property from quantum interactions that dramatically fall off when matter starts interfering with them at a certain point.
benibela 10y ago
Or MiHsC/quantised inertia: http://physicsfromtheedge.blogspot.de/2016/11/critique-of-verlindes-gravity-1.html http://physicsfromtheedge.blogspot.de/2016/11/critique-of-ve...
PhantomGremlin 10y ago
Rotation speed, rotation speed, rotation speed ... that's always been the poster child for dark matter. But do we truly understand how galactic rotation speeds should vary with distance? A typical galaxy has a hundred billion stars in it. Given such a complex system, can we truly simplify the expected rotation speed to something like "distance squared from the galactic center"? The "three-body problem" is difficult enough, do we trust our approximations to properly model the "100,000,000,000 solar system problem"?
- EdHominem 10y ago
  There's a difference between "complex" and "has no closed-form solution". It's actually a very simple calculation. But fwiw, a lot of thought and tests have gone into showing that gravity works the same way with feather and bowling balls. With big masses and many little ones. If you have a reason to think it doesn't, discuss it and propose tests...
mabbo 10y ago
Relevant recent xkcd comic: https://xkcd.com/1758/ https://xkcd.com/1758/
- jkingsbery 10y ago
  I guess the state of the art is more complicated than the Department of Astrophysics in the comic admits.
phkahler 10y ago
I'm gonna pull my hair out. I saw this: >> The researchers analyzed a diverse set of 153 galaxies, and for each one they compared the rotation speed of visible matter at any given distance from the galaxy’s center with the amount of visible matter contained within that galactic radius. That just smelled like they think Keplers law applies (which I've been arguing against for ages) so I googled "radial acceleration relation" and found this paper: https://arxiv.org/pdf/1609.05917v1.pdf https://arxiv.org/pdf/1609.05917v1.pdf The opening paragraph is this: >> The missing mass problem in extragalactic systems is well established. The observed gravitational potential cannot be explained by the stars and gas. A classic ex- ample is that the rotation curves of disk galaxies become approximately at (V=constant) when they should be falling in a Keplerian fashion. To say the gravitational pull at a given radius depends solely on the amount of matter inside that radius is utterly bogus. It is entirely dependent on the distribution of that matter. It is valid if the distribution is a uniform sphere (or a sphere of concentric layers each of uniform density) but that is not the distribution in a galaxy. Likewise, you can ignore all the matter at a larger radius IF it's distributed as a shell (or series of shells) of uniform density - but that is not the case when dealing with rings instead of shells. In my mind, this whole debate has wreaked of a misapplication of the Divergence Theorem from day one. If you thing galactic dynamics have anything to do with Kepler I have a nice bridge to sell you.
- Tloewald 10y ago
  The use of Keplerian simply refers to the inverse square law, and if you think the inverse square law isn't involved then maybe I should take that bridge off your hands. You seriously think these guys don't know high school physics?
  - phkahler 10y ago
    The inverse square law applies to 2-body systems ONLY. I do in fact think they know high school physics and are trying to misapply it. The use of that simple physics can sometimes be used in other systems as a special case of the divergence theorem (third year calculus) IF certain conditions are satisfied. I'm just saying they keep applying by missing a key point in the justification for doing so. Perhaps they're not actually using Kepler, but they sure do keep bringing up his name - and I am correct that it's not OK to use that. As I sometime say - the only dark matter is between the astrophysicists ears. This assumes they're actually using Kepler as a reference for "expected" behavior - some of those guys are smart beyond my comprehension.
TheMagician0 10y ago
Xenon1T's result is much anticipated: http://www.metaculus.com/questions/199/will-the-xenon1t-experiment-discover-wimp-dark-matter/ http://www.metaculus.com/questions/199/will-the-xenon1t-expe...

172 points by bilifuduo 10y ago

> One problem, said Brian Swingle of Harvard and Brandeis universities, who also works in holography, is that Verlinde lacks a concrete model universe like the ones researchers can construct in AdS space, giving him more wiggle room for making unproven speculations. “To be fair, we’ve gotten further by working in a more limited context, one which is less relevant for our own gravitational universe,” Swingle said, referring to work in AdS space. “We do need to address universes more like our own, so I hold out some hope that his new paper will provide some additional clues or ideas going forward.” Woah. How can AdS model universes have less "wiggle room for making unproven speculations"? There is no AdS universe to observe or experiment in. How could speculations in such universes be "proven"? Or rather, disproved?

simonh 10y ago
I think they're talking about mathematical proofs. AdS models are mathematically simpler than the physical universe containing matter, so it's easier to construct mathematical proofs.
- mirimir 10y ago
  OK. But proofs about mathematical objects are entirely distinct from testable hypotheses about the actual universe. So it seems odd to compare them. OTOH, there is much "theoretical physics" that's not much more grounded in reality than AdS models ;)

marze 10y ago

From the final paragraph: “If somebody were to come to you and say, ‘The solar system doesn’t work on an inverse-square law, really it’s an inverse-cube law, but there’s dark matter that’s arranged just so that it always looks inverse-square,’ you would say that person is insane,” he said. “But that’s basically what we’re asking to be the case with dark matter here.” Awesome point. The McGaugh paper is really problematic for dark matter proponents. Amazing that that work wasn't done earlier.

hvidgaard 10y ago
They do state: "They say the amount of dark matter in a galaxy’s halo would have precisely determined the amount of visible matter the galaxy ended up with when it formed". It's not a far fetched idea, but it is problematic because we know of zero such interactions.

antognini 10y ago

As a general principle I would advise a grain of salt whenever a physicist claims to have solved an outstanding astrophysical problem. Verlinde is a great physicist --- I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. But someone with a background in pure physics often lacks the background in astronomy to know whether their ideas will work. (I remember hearing a visiting physicist present his ideas about stars producing axions, and having an astronomy professor point out that if he was right, Cepheid variables would not pulsate at the observed frequencies.) The article mentions the Bullet Cluster as one observation that MOND has difficulty explaining. Another observation that I'm not sure how MOND would explain is the dramatic difference in dynamics between a globular cluster and a dwarf spheroidal galaxy. Globular clusters are observed to have virtually no dark matter, whereas some dwarf spheroidal galaxies have somewhere between 1000 to 10,000 times as much dark matter as ordinary matter [1]. These two kinds of objects have comparable numbers of stars and are roughly the same size (though dwarf spheroidals are admittedly larger by about an order of magnitude). This discrepancy can be explained with cold WIMPs [2], but it's hard to understand how some MOND-like process would produce such incredibly different dynamics between the two kinds of galaxies. I'll admit that I have not looked very carefully at Verlinde's paper. But the evidence for dark matter is much more than galaxy rotation curves these days. A credible alternative to WIMPs has to explain more than that. All that aside, from a theoretical perspective the work is very interesting because it provides a much-needed framework to understand why something like MOND could exist. But I don't think that any of the direct detection experiments should pack it in quite yet. [1]: https://arxiv.org/abs/1510.03856 https://arxiv.org/abs/1510.03856 [2]: Well, sort of. Galaxy formation is still a big open problem, and globular cluster formation is probably even less well understood. But with WIMP dark matter it's very plausible that different formation mechanisms would produce objects with different amounts of dark matter.

bokononon 10y ago
> I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. I remember reading this too. Did anything ever become of this theory?
m_mueller 10y ago

biggerfisch 10y ago

So is this really anything except an interesting way of deriving MOND? The article says > Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine. but doesn't actually show it doing that. As that's a major differentiator between his system and MOND, shouldn't that have been looked at? Seems to easy to otherwise dismiss this as a weird way of supporting MOND

deleted 10y ago

[deleted]

c3d 10y ago

Dark matter and dark energy are ad-hoc adjustments to explain discrepancies between observations and theory. The problem with such adjustments is that, being mostly unobserved, they end up with too many degrees of freedom (e.g. a distribution of invisible stuff where the density can be made "just right" to fit the observations), so they are quite hard to falsify. It's a bit like the "landscape" : when you construct a theory that can predict anything, you end up predicting nothing. In my opinion, we leave "science" as defined by Popper when we start doing that. There is, however, an underlying assumption in all the existing theories of physics, which I see too rarely challenged, and which is easily falsified. It's the idea that the variable "x" in the theory is independent of the physical measurement process being used. In other words, the "x" in MOND is the same as the "x" in GTR and the "x" in quantum mechanics. The topology of a space-time defined by a solid rod of metal in the Pavillon de Sevres (pre-1960 definition of the metre) is the same as the topology of space-time defined by the movement of light in the vacuum (today's definition). And so on. I believe that our current understanding of physics now has become so precise that we need to take into account subtle differences between this and that measurement process, and rewrite physics in the context of a "more general" relativity, where the laws of physics are to be described independently not just from, say, the state of movement, but more generally independently of the physical process being used. Just like GTR forced us to deal with quantities such as metrics, this more-general relativity would force us to deal with arbitrary transformations between two methods to measure the same thing. A simple example is when we relate time as measured by the rotation of the earth (definition of "days"), the rotation of earth around the sun (definition of "years"), the decay of populations of C14 ("radiocarbon dating"), and our current cesium-decay based atomic clocks (the definition of "second"). The four laws are obviously correlated, they all measure time. But the "fit curve" between them is a distribution in itself, similar to the "metric field" in GTR. I may be wrong, but I suspect that if we introduce this kind of complexity, the problem of "dark matter" will have not one, but multiple solutions, possibly different equations, depending on the measurement you are considering. This does not mean we should not look for equations that predict observations, but rather that we should be cautious when correlating measurements that correspond to different physical processes. It is not obvious to me, for example, that we should take for granted that we can safely deduce the distribution of masses from the distribution of luminosities (even if, I'll admit, I don't see any good alternative...) In quantum mechanics, we do little else with renormalization. When you shift from one scale to another, you need to renormalize to fit the data, because the raw equations would otherwise yield to infinities or other abnormalities. "Shut up and calculate" is frustrating, but if we rephrase that as "observe the universe, it's the ultimate test", it really means the same thing, but is philosophically more grounded. In the end, MOND may just be the shape of the relation between two measurements of mass at large scale, and Verlinde's paper one first step in understanding how this relationship emerges. Or not. There are some interesting objections to "statistical emergence" of gravity, or "entropy-based gravity". One of them is how interference patterns are not destroyed by gravity, at scales where, if we were to trust Verlinde's approach, interactions along different paths would affect probabilities enough to be sensible in interference patterns. I've not done the calculations myself to validate if this objection is solid, though. Intuitively, it seems like there is a flaw (the effect of per-state populations on interference should be at most of the same order as the effect of gravitation itself, which is in the "barely observable" category), but I need to spend more time to understand both sides.

basicplus2 10y ago

The concept of "Dark Energy" is as nebulous as "Dark Matter"

stefantalpalaru 10y ago
almost: https://en.wikipedia.org/wiki/Cosmological_constant_problem https://en.wikipedia.org/wiki/Cosmological_constant_problem

bambax 10y ago

> Physicists do not like magic; when other cosmological observations seemed far easier to explain with dark matter than with MOND, they left the approach for dead. But how is "dark matter" not magic? To my untrained and pretty ignorant eye, dark matter looks like filler: the observation does not match the theory, and so, instead of looking for a better theory, we decided the observation was wrong and couldn't see what was there (and not just a little bit, but the greatest part of it). Isn't that the definition of superstition? Invisible forces that explain events, in mysterious ways?

acqq 10y ago
It's not as "magic" as you believe it to be. The dark matter is at the moment is not only the simpler explanation (that is, you have formulas that can produce the results) of the observations (Occam razor etc) but the only one that explains different effects that are still fully unexplained (in the sense, there are no formulas that can even match anything) by MOND. And since 2006 there are the actual photos of "dark mater effects": http://www.forbes.com/sites/startswithabang/2016/09/19/dark-matter-proved-real-by-colliding-galaxy-clusters/ http://www.forbes.com/sites/startswithabang/2016/09/19/dark-... Now, how the MOND handles it? "Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine." Note "thinks" not that anybody managed. The dark matter approach could be declared really problematic even before anybody manages if some observations simply can't be explained with the dark matter. But at the moment it's still not so. And all the observations will remain: https://medium.com/starts-with-a-bang/10-facts-everyone-should-know-about-dark-matter-1ce2b222cf08 https://medium.com/starts-with-a-bang/10-facts-everyone-shou... "It’s not going away You will not wake up one day and hear physicists declare it’s not there at all. (Well, you will, but those claims are rare, and those physicists are wrong.) The evidence is overwhelming: Weak gravitational lensing demonstrates that galaxies have a larger gravitational pull than visible matter can produce. Additional matter in galaxies is also necessary to explain why stars in the outer arms of galaxies orbit so quickly around the center. The observed temperature fluctuations in the cosmic microwave background can’t be explained without dark matter, and the structures formed by galaxies wouldn’t come out right without dark matter either. Even if all of this was explained by a modification of gravity rather than an unknown type of matter, it would still have to be possible to formulate this modification of gravity in a way that makes it look pretty much like a new type of matter. And we’d still call it dark matter." Then, not as the underlying principle, but as a simpler handling for all the observations up to now that look like "the invisible-matter-like effects there."

drewhk 10y ago

I am not a physicist, in fact, I am a complete dilettante, but can the transition from a power law to an inverse law simply indicate that on large scale the universe has less dimensionality? I mean, if we take a suitable definition of dimension, likely along the lines of Hausdorff dimension (https://en.wikipedia.org/wiki/Hausdorff_dimension https://en.wikipedia.org/wiki/Hausdorff_dimension) then we can see it as a number that defines how volume/surface changes with changing diamater of an observed "sphere" (I know I oversimplify things here). In other words, as we start to measure cosmic distances on a large enough scale, volumes/surfaces covered don't grow quadratically anymore but with a lower rate. This might be even in line with the assumption of a universe curved into itself, i.e. closed. This would change the gravity power law from having a constant 2 power to having a function as the power.

qubex 10y ago
(Constant negative two power to having a function as the power.)
- drewhk 10y ago
  It depends on whether you imagine a fraction line there or not: https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6ee5510ba3c7d6664775c0e76b53e72468303a https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6e... ;)
Tloewald 10y ago
The idea of fractal dimension being significant at galactic scale is intriguing and based on my totally naive reading of the article might resemble this insofar as the guy is arguing that gravity is an emergent property from quantum interactions that dramatically fall off when matter starts interfering with them at a certain point.

benibela 10y ago

Or MiHsC/quantised inertia: http://physicsfromtheedge.blogspot.de/2016/11/critique-of-verlindes-gravity-1.html http://physicsfromtheedge.blogspot.de/2016/11/critique-of-ve...

PhantomGremlin 10y ago

Rotation speed, rotation speed, rotation speed ... that's always been the poster child for dark matter. But do we truly understand how galactic rotation speeds should vary with distance? A typical galaxy has a hundred billion stars in it. Given such a complex system, can we truly simplify the expected rotation speed to something like "distance squared from the galactic center"? The "three-body problem" is difficult enough, do we trust our approximations to properly model the "100,000,000,000 solar system problem"?

EdHominem 10y ago
There's a difference between "complex" and "has no closed-form solution". It's actually a very simple calculation. But fwiw, a lot of thought and tests have gone into showing that gravity works the same way with feather and bowling balls. With big masses and many little ones. If you have a reason to think it doesn't, discuss it and propose tests...

mabbo 10y ago

Relevant recent xkcd comic: https://xkcd.com/1758/ https://xkcd.com/1758/

jkingsbery 10y ago
I guess the state of the art is more complicated than the Department of Astrophysics in the comic admits.

phkahler 10y ago

I'm gonna pull my hair out. I saw this: >> The researchers analyzed a diverse set of 153 galaxies, and for each one they compared the rotation speed of visible matter at any given distance from the galaxy’s center with the amount of visible matter contained within that galactic radius. That just smelled like they think Keplers law applies (which I've been arguing against for ages) so I googled "radial acceleration relation" and found this paper: https://arxiv.org/pdf/1609.05917v1.pdf https://arxiv.org/pdf/1609.05917v1.pdf The opening paragraph is this: >> The missing mass problem in extragalactic systems is well established. The observed gravitational potential cannot be explained by the stars and gas. A classic ex- ample is that the rotation curves of disk galaxies become approximately at (V=constant) when they should be falling in a Keplerian fashion. To say the gravitational pull at a given radius depends solely on the amount of matter inside that radius is utterly bogus. It is entirely dependent on the distribution of that matter. It is valid if the distribution is a uniform sphere (or a sphere of concentric layers each of uniform density) but that is not the distribution in a galaxy. Likewise, you can ignore all the matter at a larger radius IF it's distributed as a shell (or series of shells) of uniform density - but that is not the case when dealing with rings instead of shells. In my mind, this whole debate has wreaked of a misapplication of the Divergence Theorem from day one. If you thing galactic dynamics have anything to do with Kepler I have a nice bridge to sell you.

Tloewald 10y ago
The use of Keplerian simply refers to the inverse square law, and if you think the inverse square law isn't involved then maybe I should take that bridge off your hands. You seriously think these guys don't know high school physics?
- phkahler 10y ago
  The inverse square law applies to 2-body systems ONLY. I do in fact think they know high school physics and are trying to misapply it. The use of that simple physics can sometimes be used in other systems as a special case of the divergence theorem (third year calculus) IF certain conditions are satisfied. I'm just saying they keep applying by missing a key point in the justification for doing so. Perhaps they're not actually using Kepler, but they sure do keep bringing up his name - and I am correct that it's not OK to use that. As I sometime say - the only dark matter is between the astrophysicists ears. This assumes they're actually using Kepler as a reference for "expected" behavior - some of those guys are smart beyond my comprehension.

TheMagician0 10y ago

Xenon1T's result is much anticipated: http://www.metaculus.com/questions/199/will-the-xenon1t-experiment-discover-wimp-dark-matter/ http://www.metaculus.com/questions/199/will-the-xenon1t-expe...