NASA pictures of dark matter collisions

[wabbit]

Not sure what you mean by QG density? I get the small scale.

I meant not a specific number, but the density scale at which QG must become important, i.e. some multiple of the Planck density. At the Planck density, a Plank-sized volume has enough mass to become a black hole and would thus form a singularity in GR - so if QG is to cure such singularities it must be pretty strong at that density and somewhere above it.

 

[Lord Crc]

Let's take the sigma/M=.5cm/g value and assume a dark matter mass of 1 keV (a larger mass gives a larger cross-section). Then we get ~10^33 m^2 as cross-section. That is several orders of magnitude above the limits for the interaction of dark matter with regular matter, and ~7 orders of magnitude above typical neutrino cross-sections at 1-10 MeV.

Thanks! This result then does not rule out sterile neutrinos as candidates?

 

[mfb]

Certainly not. The upper limit is just too weak.

 

[Jimster41]

That has nothing to do with quantum mechanics.

You're not saying that Saturn and it's rings (the stuff) is "not quantum mechanical" right. You are just saying that the structure we see isn't affected, caused, by any thing that happens to it, during it's movement (as QM stuff) through the quantum mechanical geometry of space-time from one proper instant to the next.

I didn't think there was stuff that was "not quantum mechanical". Approximations of it's behavior aren't don't necessarily have to be QM to function but, all stuff is, only irreducibly QM.

 

[mfb]

You are just saying that the structure we see isn't affected, caused, by any thing that happens to it, during it's movement (as QM stuff) through the quantum mechanical geometry of space-time from one proper instant to the next.

Right. Classical mechanics and gravity is sufficient to describe the rings. It has to be, as there is no way quantum-mechanical effects could be relevant*.

*very indirectly: they are responsible for making the ring particles solid, and this influences how collisions work. But classical mechanics still gives a good approximation.

 

[Jimster41]

Right. Classical mechanics and gravity is sufficient to describe the rings. It has to be, as there is no way quantum-mechanical effects could be relevant*.

*very indirectly: they are responsible for making the ring particles solid, and this influences how collisions work. But classical mechanics still gives a good approximation.

That's helpful. Though I would have said that the irreversible history that has left us with those rings as phenomena, is described only sufficiently via the Entropy.

 

[mfb]

I found
The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62+/-0.48kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section $\sigma/m=(1.7 \pm 0.7)\cdot10^{-4}cm^2/g \cdot (t/10^9yrs)^{-2}$, where t is the infall duration.

(I formatted the formula for readability)
Note: this cross-section estimate is three orders of magnitude below the upper limit in the paper discussed previously.
~3 (astrophysical) sigma, so not really significant, but it looks interesting.