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gm1970
2015-Dec-21, 06:51 AM
Why doesn't dark matter in the solar system cause pluto or neptune to orbit at a greater than predicted speed, as dark matter affects stars on the edge of the galaxy?

antoniseb
2015-Dec-21, 11:59 AM
Why doesn't dark matter in the solar system cause pluto or neptune to orbit at a greater than predicted speed, as dark matter affects stars on the edge of the galaxy?
The usual model is that there isn't any concentration of dark matter in the solar system (more or less uniformly distributed), and so it has no impact on the orbits of the planets. Dark matter seems to be collected in galaxy-sized spheroids.

01101001
2015-Dec-21, 02:02 PM
Does Dark Matter affect the motion of the Solar System? (http://scienceblogs.com/startswithabang/2013/07/03/does-dark-matter-affect-the-motion-of-the-solar-system/)

Calculation: The effect is about 100000 times smaller than we can measure.

borman
2015-Dec-21, 03:54 PM
A place to search for the effect is in the orbit of Mercury where its accumulated effect could be measured. The results are null, consistent with no dark matter. This is one of the main reasons that Milgrom leaned towards MOND, or Modified Newtonian Dynamics, as a hypothesis alternative to dark matter particles. In the main MOND view the effect does not come until gravity gets very weak, over 5600 AU out for our sun. Otherwise to be deeper in our solar system the effect would need to be screened if it were not absent to reproduce the null results.

Grey
2015-Dec-21, 06:05 PM
Otherwise to be deeper in our solar system the effect would need to be screened if it were not absent to reproduce the null results.No, as the link that 01101001 posted points out, we can calculate the effect we'd expect on the orbits of planets based on the calculated density of dark matter in our neighborhood. It turns out to be several orders of magnitude smaller than we could currently measure.

borman
2015-Dec-21, 08:21 PM
The article makes a big assumption of a "sea of dark matter particles". So far many of the detectors such as the various LUX experiments has yet to detect a candidate if there are WIMP particles. In tension with this are detections from DAMA-LIBRA where "something" is seen at 8 sigma. There is one paper that tries to resolve this by suggesting a proton coupling. Another is that the environment around the DAMA experiment is descreening and allowing discovery where the descreening is not happening at the LUX sites. The ANNAIS experiment may help to sort this out if performed both at the DAMA site (possibly getting positive results) and another different site (getting null results).

The Wide binary results show the dark matter effect of the proper magnitude between binary stars if they are sufficiently far apart but absent if they are too close together. This implies dark matter halos surround just two stars only if they are far enough apart but otherwise absent if they orbit too close. This is anathema to the proposition of a sea of constant density of dark matter particles assumed in the article.

Shaula
2015-Dec-21, 08:53 PM
Can you provide a reference for the Wide Binary results? I've not seen anything conclusive or recent that puts such strong constraints on dark matter as you have stated.

borman
2015-Dec-21, 09:48 PM
Wide binaries as a critical test of Classical Gravity
http://arxiv.org/abs/1105.1873
There was an old astronomy thread where reservations about the results were expressed:
http://cosmoquest.org/forum/showthread.php?117915-MOND-observed-between-binary-stars
Of course, a follow up study can now be started now that GAIA data is being released. This should give improvements over Hipparcos data. The best improvement would come from using GAIA data to locate good data candidates and follow up with observations of these candidates with the thirty meter telescopes.

borman
2015-Dec-21, 10:15 PM
If the MOND idea has some traction, then observation of bodies on an a very large eccentric orbit would show the dark matter effect. Mainly these would be asteroids or comets from the Oort cloud whose orbit takes them out beyond the 5600 AU. Their speeds would be bunched closer together than expected and their incoming speeds would appear hyperbolic. John Anderson computed that if the MOND a_0 is there, the Pioneer's orbits will be closed and they will return after a million years or so.

Shaula
2015-Dec-22, 05:02 AM
Wide binaries as a critical test of Classical Gravity
http://arxiv.org/abs/1105.1873.
So the results in that paper don't support what you have said as far as I can see. The paper shows a possible deviation from Newtonian gravitational predictions. It doesn't have dark matter turning on and off depending on the separation, dark matter is not factored into the calculations at all. I think the confusion comes in the introduction where they invoke dark matter as the only possible explanation for any deviation from Newtonian mechanics in the current model. They certainly have not done any dark matter simulations and shown that they replicate the results shown.

But thank you for also linking to the thread with Tensor's comments - it is not often you see a fan of a theory presenting a link to the argument against it so people can read both. That is to be applauded as a very fair and non-partisan approach.

Reality Check
2015-Dec-22, 10:57 PM
This is anathema to the proposition of a sea of constant density of dark matter particles assumed in the article.
Does Dark Matter affect the motion of the Solar System? (http://scienceblogs.com/startswithabang/2013/07/03/does-dark-matter-affect-the-motion-of-the-solar-system/) starts with a back of the envelope calculation with the assumption that dark matter is isotropic and gets "a good factor of 100,000+ away" from any effects being measurable. That is a reasonable approximation for scales of the solar system. But it goes further, borman.
Constraints on Dark Matter in the Solar System (http://arxiv.org/abs/1306.5534) is cited which looks at the data for planets and spacecraft and finds constraints on the density of dark matter at various distances from the Sun.
Dark Matter in the Solar System (http://arxiv.org/abs/0806.3767) is cited as "a safe upper limit on the amount of dark matter that the Solar System could conceivably have" and there is still not enough dark matter for effects to be observable.

borman
2015-Dec-23, 04:18 AM
So the results in that paper don't support what you have said as far as I can see. The paper shows a possible deviation from Newtonian gravitational predictions. It doesn't have dark matter turning on and off depending on the separation, dark matter is not factored into the calculations at all. I think the confusion comes in the introduction where they invoke dark matter as the only possible explanation for any deviation from Newtonian mechanics in the current model. They certainly have not done any dark matter simulations and shown that they replicate the results shown.

But thank you for also linking to the thread with Tensor's comments - it is not often you see a fan of a theory presenting a link to the argument against it so people can read both. That is to be applauded as a very fair and non-partisan approach.

Yes, the paper looks for deviations from Newtonian gravity. Better telescopes will bring greater clarity to what is happening between wide binaries. The question is whether the deviations are due to Dark Matter from the CDM paradigm or some alternative or modified gravity paradigm. This is a very small scale for CDM which already has some problems at the galaxy level while doing much better at the cluster and larger scales. They don't present a dark matter particle model to account for the observations as it would not be consistent with the current paradigm. Dark matter does not flicker off and on at the stellar level within the current paradigm. Rather it is assumed to spread out smoothly at a specific density throughout the galaxy which may or may not have slight alterations due to local gravitational capture, according to the CDM paradigm. No one has suggested that an alternative dark matter,such as "pink" matter, is at the source of the wide binary deviations only at that scale but not at other scales. If CDM predicted off and on flickering at the wide binary level then it could be modeled here.

The old thread goes back years. What has changed is that the paper has been published. That said, it was about such an unusual topic that it has yet to gather any citations although I would expect a citation if a follow up study is performed even if only using GAIA data.

The same authors wrote another paper, this time focusing on a slightly larger scale. Instead of just two stars, they look at the outer stars of a globular cluster, still much smaller than even a dwarf galaxy at around 10^5 stars, and continue to see the deviations when the ambient acceleration dropped below a_0. Among their eight GCs they chose GC288 which could be modeled from the CDM perspective as a "Cored" GC where the others are more "cuspy". Here CDM theory has to be stretched to allow local concentrations within the galaxy. Again there is the presence of the fourth power function relating velocity to mass that is not forthcoming from CDM theory nor should it be if Dark Matter Particles are independent of baryons.

The outskirts of globular clusters as modified gravity probes
http://arxiv.org/pdf/1108.4021v3.pdf


While I may be presently something of a fan of the discovery of the seeming Milgrom constant, I am not yet confident of its presentation as either MOND or MI. It appears to suffer from a "too big and too little" problem. The "too big " problem was already noted by Milgrom in the 1983 series of papers with respect to the x-ray clusters. On this scale, to rescue the hypothesis where it was desired to keep a_0 constant, dark or hidden barons were needed to be invoked. On the much larger scale where cosmological CDM workds well, the invoking of dark baryons will not suffice perhaps conflicting with BBN. The "too little" problem is more fatal to,at least, the MOND hypothesis. One may recall the "bullet" cluster where more gas mass was stripped away into the "bullet" while the galaxies and weak lensing curves indicating the dark matter effect traveled on ahead in a nearly collisionless manner. MOND states that when a sufficient distance from a gravitating mass where the surface density drops below a threshold, the a_0 makes an appearance as a modification of gravity. But looking at the weak lensing maps in the vicinity of the bullet, there appeared to be no k-curves at a distance from the bullets. This is a much more serious problem for MOND than that the lensing traveled with the galaxies. Absence of signal where it should be as a result of gravity modification undermines the theory. At least the local "too big and too small" problem could be resolved if a cosine function were appended to a_0. But then past history of interactions would intrude into gravitational effects. With GR there is simply energy density on one side and curvature on the other. There is no place for history or cosines on this brane. This causes a divorce form an attempt at modifying Newtonian gravity. The role for a_0 lies somewhere else than CDM, MI, or MOND.

antoniseb
2015-Dec-23, 12:48 PM
... Better telescopes will bring greater clarity to what is happening between wide binaries. ... weak lensing maps in the vicinity of the bullet...
borman, this is now talking about something way outside the scope of the OP. This post is a hijack to talk about a non-mainstream model in the mainstream section. We were tolerant of this with your first few posts because it was simply about the f(R) Gravity that might be detectable in the solar system, but now you are trying to make a wider case for MOND in the Q&A section. So, cease & desist please (this is a zero-point warning), and if you are feeling like you want to talk about observational evidence for or against f(R) Gravity, start a thread in the Astronomy section. If you want to make some theoretical case for MOND, open a tread in ATM.

cjameshuff
2015-Dec-23, 02:25 PM
Does Dark Matter affect the motion of the Solar System? (http://scienceblogs.com/startswithabang/2013/07/03/does-dark-matter-affect-the-motion-of-the-solar-system/) starts with a back of the envelope calculation with the assumption that dark matter is isotropic and gets "a good factor of 100,000+ away" from any effects being measurable. That is a reasonable approximation for scales of the solar system. But it goes further, borman.

He makes a rather major mistake by assuming the sphere of dark matter enclosed by an orbit somehow counts when the identical sphere tangent to the orbit on the far side from the sun does not. This analysis leads to the absurdity of particles being able to orbit any arbitrary point within a dark matter field by having the right motion to orbit the "enclosed mass", even as other particles pass by on straight trajectories with the same velocity.

A dark matter distribution that is isotropic would have no effect whatsoever on planetary orbits. The net gravitational acceleration from such a distribution is simply zero. In reality, the gravitation of the sun would distort the dark matter distribution so there would be some effect, but this would be a tiny fraction of the effect predicted from the "enclosed mass" idea (and much harder to calculate, depending on parameters like the relative speed of the dark matter particles).