Strange but true: dark matter grows hair around stars and planets (Synopsis)

"Few enterprises of great labor or hazard would be undertaken if we had not the power of magnifying the advantages we expect from them." -Samuel Johnson

Dark matter may make up 27% of the Universe's energy density, compared to just 5% of normal (atomic) matter, but in our Solar System, it's notoriously sparse. In particular, there's just a nanogram's worth per cubic kilometer, which makes the fact that we've never directly detected it seem inevitable.

Image credit: J. Cooley, Phys.Dark Univ. 4 (2014) 92-97, via Image credit: J. Cooley, Phys.Dark Univ. 4 (2014) 92-97, via

But recent work has demonstrated that Earth and all the planets leave a 'wake' of dark matter where the density is enhanced by a billion times or more. Time to go put those dark matter detectors where they belong: in the path of these dark matter hairs.

Image credit: NASA/JPL-Caltech. Image credit: NASA/JPL-Caltech.

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Is there some reason to think the (undetectable!) dark matter particles have a prefered travel direction near our solar system? If not, then there would be no "hairs" - just a haze around massive bodies with only slightly increased concentration.


I think you are misunderstanding. Because dark matter is still affected by gravity, as a planet moves through the diffuse dark matter cloud, it would pull some along with it. A large amount of that dark matter would get focused in a trail behind the plant.

By Sinthetic (not verified) on 25 Nov 2015 #permalink

Gary, the idea is that this is a similar thing to gravittional lensing, where gravity makes a stream of particles bend their path, refracting to a focal point.

Our sun moves through the local neighborhood at about 10km/s and the Earth spins around the sun at 30km/s. The dark matter of our galaxy should be spinning around the center roughly like the average of the stars.

By JollyJoker (not verified) on 26 Nov 2015 #permalink

Given the claim isn't that it's 30kps, and that "roughly the same" gives scope for SOME kps, and given that you even say "average" therefore by very definition, there are differences FORM That average, what, exactly, is supposed to be proposed by your post? What are we meant to know after it we should be considering we did not before?

Layman question. I'm wondering why Gary Prézeau's paper deals only with compact, planetary masses in his calculations - the centre of Sol is given [via a Google search I did] as having a density of 160 g/cm^3 whereas that of Earth's inner core is 12 g/cm^3 & that of Jupiter is 25 g/cm^3

So shouldn't this focusing effect for our solar system be greatest at some point in the wake of Sol's orbit around the Milky Way centre? If so, what 'magnification' of the dark matter stream might we expect & at what radius from Sol centre?

My guess as to why Sol was excluded from the calcs: The root of the hair would be located below the 'surface' of the sun - not amenable to experimental confirmation of the effect...

By Michael Fisher (not verified) on 02 Dec 2015 #permalink

One reason is that a diffuse object, like a small and badly formed lens, focusses much much less than a well defined lens or material body. And the sun has a lot greater variation through its larger volume, which would wash out the focusing.

what i am curious about is why this prediction would only affect cold dark matter and not, for example, lukewarm DM...the reasoning just seems so straightforward to me, wouldn't any gravitationally interacting body be formed like this?

I think that these results need to be analyzed from the point of view of signal processing and information theory. A useful analogy is ocean waves which have a power spectrum of two spatial dimensions and one dimension of velocity (i.e. wave length). In the case of dark matter we have 3 spatial dimensions and one velocity dimension. The distribution of water waves is described by its power spectrum - the same for dark matter velocity. The "hairs" are equivalent to a filter being applied to the power spectrum distribution. Only if there is a "peak" in the power spectrum will there be any enhancement in the output of the filter. That is, if dark matter is flowing equally from all directions (think white noise), all filters will produce the same results. I do not know if anyone have tried to model the velocity distribution of dark matter.

However, we can make some good assumptions in the case of two colliding galaxies. Would the massive central black holes generate dense hairs where dark matter might show some physical effect??

By Gerald Grube (not verified) on 06 Dec 2015 #permalink

"I think that these results need to be analyzed from the point of view of signal processing and information theory. "

Use of information theory led to the idea of a holographic universe.

This has not had good news recently:…

So be careful how you extend information theory into the realm outside of its natural habitat.

"what i am curious about is why this prediction would only affect cold dark matter and not, for example, lukewarm DM…"

To be warm it would radiate and not be dark any more...

Actually, it's that the cold means it's not moving fast, therefore it would not be isotropic and you'd get a streamer, not hairs, and the streamer would concord with the DM velocity. If that velocity were random, then that would wipe out any reasonable increase in density, making the event rather unusable.