Could Dark Matter Explain the Mystery of Shear Viscosity in Neutron Stars?

[ImaLooser]

The situation is this:

In a neutron star the Coriolis force induces Rossby waves, just like on Earth. These are waves with very long wavelength -- like halfway around the Earth -- and very large volume but very little amplitude, like fifty meters. On Earth they have a big effect on climate, with El Nino and so forth.

In a neutron star Rossby waves cause the emission of gravity waves. Not only that, the gravity waves reinforce the Rossby waves with a positive feedback. This would result in so much gravity waves that rotational energy would be lost quickly, but this does not seem to be the case. The best bet is that shear viscosity dampens the waves, but a superfluid core is not very viscous and there does not seem to be enough viscosity.

It has been hypothesized that dark matter could supply the viscosity. Dark matter has a long free path, which results in shear viscosity. Dark matter would of course tend to concentrate in neutron star cores. If no other explanation can be found...

I've got the referneces ... none of this is original with me ... but I have a dental appointment so that is going to have to wait.

 

[Chronos]

Dark matter is collisionless for all practical purposes. It would not be captured by a neutron star.

 

[ImaLooser]

Dark matter is collisionless for all practical purposes. It would not be captured by a neutron star.

Wikipedia

In astrophysics, weakly interacting massive particles or WIMPs, are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak force and gravity, and possibly through other interactions no stronger than the weak force.​

It seems to me that there is plenty of weak force in a neutron star.

I don't see that it makes any difference whether the dark matter is captured, all it has to be is present in the core. Gravity attracts dark matter so surely the concentration is higher in a neutron star then elsewhere. What matters is whether the mean free path of the dark matter is fairly high but less than the radius of the star. Then the desired shear viscosity results.

I don't claim to understand this, so here is the promised reference. I hope my summary is accurate.

Dark matter transport properties and rapidly rotating neutron stars
C.J. Horowitz1,
1 Department of Physics and CEEM, Indiana University, Bloomington, Indiana 47405, USA
(Dated: May 17, 2012)
http://arxiv.org/pdf/1205.3541.pdf

The ratio of WIMPS to nucleons necessary to get the viscosity is estimated as one to ten billion.

 

[Chronos]

The high density of neutron stars would improve the chances for interaction of dark matter particles with baryonic matter - depending on how 'collisional' dark matter is with baryonic matter. Opinion varies on this count. There is a school of thought that dark matter may be weakly collisional with baryonic matter, another that dark matter may be weakly collisional with itself, and various ideas somewhere in between. The evidence to date is not compelling for any particular case. The author of the paper you cite suggests it is collisional with baryonic matter. I agree to the extent a neutron star would be the ideal laboratory for detection of such events, although how you would go about detecting such a thing is unclear.

 

[twofish-quant]

Really, really cool paper.

The big problem with it is that there is so much that is unknown about high density nuclear physics that it's not hard to come up with some other source of viscosity.

Hmmm... I wonder how WIMPS would affect the explosion.

 

[ImaLooser]

The laboratory is rather inconveniently located. That's how it goes in astrophysics. The problem now is that we don't have a good idea how large/dense the star is, and that makes a big difference.The focus there is on detecting gravitational waves. Using that along with X and gamma ray data will narrow down the equation of state a lot so results will be considerably sharper.

 

[twofish-quant]

The laboratory is rather inconveniently located. That's how it goes in astrophysics. The problem now is that we don't have a good idea how large/dense the star is, and that makes a big difference.

I think the limits for neutron star size/density are pretty firm. Within reasonable equations of state you can get the central density out to at least an order of magnitude. The big unknowns are interaction/reaction rates.

The focus there is on detecting gravitational waves. Using that along with X and gamma ray data will narrow down the equation of state a lot so results will be considerably sharper.

I think the big EOS improvements are going to be made through Earth based nuclear experiments and better QCD calcuations. Also, one thing that the paper didn't mention was viscosity due to magnetic eddy currents which are likely to be important.

But it's a really cool paper anyhow because it gets you to look for something you didn't think of looking for...