Re: Dark matter vs. modifications of the gravitational inverse-square law. Results from planetary motion in the solar system

In article <e8o45r$994$1@xxxxxxxxxxxxxxxxxxxxx>,
"George Dishman" <george@xxxxxxxxxxxxxxxxx> wrote:

"Joseph Lazio" <jlazio@xxxxxxxxxxxxxxxxx> wrote in message
news:llverak1ai.fsf@xxxxxxxxxxxxxxxxxxxx

astro-ph/0606197
Title: Dark matter vs. modifications of the gravitational
inverse-square law. Results from planetary motion in the solar
system
Authors: M. Sereno (Univ. Zuerich), Ph. Jetzer (Univ. Zuerich)
Comments: 7 pages, 4 figures, accepted for publication in MNRAS

Dark matter or modifications of the Newtonian inverse-square law in
the solar-system are studied with accurate planetary astrometric
data. From extra-perihelion precession and possible changes in the
third Kepler's law, we get an upper limit on the local dark matter
density, rho_{DM} < 3*10^{-16} kg/m^3 at the 2-sigma confidence
level. ...

Fascinating, that is 3*10^{-19} g/cm^3, exactly the same
as the upper bound found from the Pioneer anomaly:

http://www.arxiv.org/abs/astro-ph/0501626

The direction of the Pioneer anomaly also implies DM dust
at that density would need to be at rest wrt the Solar
system since both craft are accelerated towards the Sun.

The mean density suggested from the Milky Way's rotation
curve is given as 2*10^{-25} g/cm^3 so the Solar system
interplanetary density would then be 6 orders higher than
the local interstellar mean yet it would need to be nearly
constant with heliocentric range.

*At most* six orders of magnitude; if all we have is an "upper
limit/bound" we can't even be sure that the DM density is greater around
here than in interstellar space. Absent a lower bound or other
refinement of the estimate, AFAICT it says more about the insensitivity
of our methods WRT this application than it does about the actual
quantity in question.

--
Odysseus
.

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