Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Effect of dark matter on solar system dynamics

  1. Oct 12, 2006 #1
    Dark matter plays a significant role on the relation between orbital radius and velocity of stars within galaxies. Does it, however, have a theoretically calculable and eventually measurable effect on planets, like those of our solar system?
     
  2. jcsd
  3. Oct 12, 2006 #2

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Maybe the Pioneer Anomaly, which is approximately equal to cH and of the same order of magnitude as the MOND acceleration, is an indication that cosmological phenomena affect local Newtonian dynamcs?

    Garth
     
  4. Oct 13, 2006 #3
    As good an explanation as I could come up with. Are there any tests looming that will make or break MOND?

    Did you know that I independently surmise the "Hubble acceleration," cH0, on my website below, in the first paragraph of "Matters of Gravity"?
     
  5. Oct 13, 2006 #4

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I guess it depends somewhat on what you mean by 'make or break'! :smile:

    Observations of the bullet cluster have been taken by many as a rather strong nail in MOND's coffin.

    For me, the apparent lack of any effort by MONDians to try to model the rotation curves of M31, the LMC, and M81 is a red flag wrt 'break' - after all, a key criterion for MONDians to model the published data on the rotation curve of a given galaxy is the quality of the "kinematic and photometric data" that exists for that galaxy. These three meet such a criterion, in spades*, yet none of these three are among the 84-0-11 (or whatever) list on their website.

    Could it be that there is a reason for this rather odd omission? Perhaps because all three should have very clear MONDian signals, but that the gross features of the published rotation curves strongly suggest no MONDian regime (e.g. M81's turns down)?

    *Indeed, M31 and M81 were among the very first galaxies to have rotation curves investigated.
     
  6. Oct 16, 2006 #5

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Continuing; there is a mystery to be solved here.

    The PA is a real effect that so far has not been explained by other authors using prosaic effects such as gas emission by the spacecraft or anisotropic radiation pressure.

    It also seems difficult to explain it as a gravitational perturbation by Kupier belt objects as the anomaly effects both Pioneer spacecraft.

    The anomalous sunward acceleration is roughly equal to the Hubble acceleration (~10% greater than cH), which therefore suggests it might be cosmological and not local in nature.

    It is not Hubble Expansion applied to the local solar system as it is in the wrong direction (being sunward), i.e. it would represent a Hubble contraction not expansion.

    The question of whether it might be caused by a modification to Newtonian gravity is an interesting one, discussed by Iorio here. Iorio's answer is no, for such a modification would also show up in the orbits of the Outer Planets.

    Is it an effect of Dark Matter, or Dark Energy, showing up in the Outer Solar System, or of something else?

    Garth
     
    Last edited: Oct 16, 2006
  7. Oct 20, 2006 #6
    I think the issue here is scale. The solar system is just too small and our current technology is not even close to being capable to detecting dark matter effects in such small amounts. Also, the dark matter proposes a halo which increases in mass as it extends outward. Because we are pretty much inside the theoretical baryonic matter dominated area, I dont think dark matter has much of an effect at all. We have to keep in mind that the solar system is miniscule compared to an entire galaxy, or even a small portion of a galaxy.
     
    Last edited: Oct 20, 2006
  8. Oct 20, 2006 #7

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Maybe you are correct, but what then is your explanation for the Pioneer Anomaly?

    Garth
     
  9. Oct 20, 2006 #8

    There are so many alternative theories to pioneer. I believe it is probably a miscalculation in acceleration, or the influence of the Kuiper Belt or dust or solar winds or something to that effect. The thing is, in such a relatively small distance, the pioneers have not sufficently gone through a change in "dark matter' density. That is, the proposed dark matter is imperically of constant density through the small stretch of space these craft have traveled in. If dark matter was the culprit, I dont believe these changes would have occured like they have. Either they would have had more accelerational drag during their entire voyage, or they would be unaffected. I just thought about that off the top of my head so if you see any problems tell me so I can reconsider.
     
  10. Oct 20, 2006 #9

    Garth

    User Avatar
    Science Advisor
    Gold Member

    You may wish to read an important recent paper by Turyshev et al., The Study of the Pioneer Anomaly:New Data and Objectives for New Investigation
    Note:
    Hubble constant in units of sec-1 (using h = 0.73) is: 2.4 × 10−18sec-1.
    Compare with the time acceleration at = (2.92 ± 0.44) × 10−18sec-1 above.

    Garth
     
    Last edited: Oct 21, 2006
  11. Oct 20, 2006 #10
    My speculation: The effects of "dark matter" on large-scale structures are predominantly due to compliance with discretized minimal acceleration, given by the ratio between speed of light squared and the cosmological horizon radius.
     
  12. Oct 21, 2006 #11

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If the same DM that's known and loved by astronomers, wrt galaxies and galaxy clusters, seems to have no bearing on the PA, then surely further discussion of the PA is OT (for this thread)?
     
  13. Oct 22, 2006 #12

    Garth

    User Avatar
    Science Advisor
    Gold Member

    OT: 'Off topic'? 'On Topic'?
    From the context Nereid I assume you mean 'Off Topic'.

    If that is your question then it might be considered 'On Topic' if DM is actually the cause of the PA. Or if MOND is the explanation of DM....

    My point being that we require DM by our Newtonian/GR modelling of galaxy/cluster dynamics yet we seem to have a problem with Newton/GR modelling our own 'local' spacecraft dynamics. Therefore the PA might indeed be pertinent to the OP question.

    OTOH if the effect is cosmological in nature it might instead be an example of 'DE', not DM, which affects solar system dymanics, and then it would be strictly OT.....

    Garth
     
    Last edited: Oct 22, 2006
  14. Oct 22, 2006 #13

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Sorry, yes 'OT' meant 'off-topic'.

    AFAIK (as far as I know), no astronomical observations of our region of the Milky Way require DM (in the standard meaning of DM), in our neighbourhood, in terms of either Newtonian or GR gravity.

    Which is a very different thing from saying there is none; it merely says that any DM is dilute enough, or small enough, to have observable effects that are buried in the error bars.

    The PA is just that, an anomaly ...

    Re MOND: unless Milgrom has changed his mind, the PA cannot be due to MOND ... the standard MOND has a fudge factor (I forget its name) which guarrantees that there are no MONDian regimes within the solar system (or indeed in our local MW neighbourhood) ... so no one could do a test for MOND, in a lab here on Earth, no matter how ingenious or sensitive the experiment.

    I hope that clarifies things ...
     
  15. Oct 23, 2006 #14

    Garth

    User Avatar
    Science Advisor
    Gold Member

    agreed
    That is what makes it so interesting.....
    The 'fudge factor' is the MOND acceleration below which the regime changes from an 1/r2 to a 1/r dependence. The reason why I included it is that this MOND acceleration is OOM the same magnitude as the PA acceleration, (but nowhere near as accurately the same magnitude of the PA as cH).

    Garth
     
    Last edited: Oct 23, 2006
  16. Oct 23, 2006 #15

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think the question has been sufficiently answered. No, dark matter is not dynamically important in the solar system.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?