NASA pictures of dark matter collisions

[Jimster41]

Thanks wabbit. Looks like a great thing to add to my stack. I have been looking for something to add to my weak grasp of the subject.

One comment though... don't we know that Newton's mechanics, didn't do it?

 

[wabbit]

One comment though... don't we know that Newton's mechanics, didn't do it?

Sure, there's the famous precession of Mercury's perihelion, so we do know that for some aspects GR corrections come into play - but at the scale of the solar systems given the masses involved, they're fairly small corrections (after all Newtonian predictions aren't that bad even for Mercury) and well known too, so they can be included when needed. I haven't studied that though, so this is just a layman opinion based on a small sample of information.

 

[Jimster41]

I think what I'm confused about, are any of the features of something like Saturn interesting in a different way when viewed as manifestations of a QM gravitation process (not a Newtonian one), like statistical periodicity in space-time structure?

 

[Lord Crc]

I think what I'm confused about, are any of the features of something like Saturn interesting in a different way when viewed as manifestations of a QM gravitation process (not a Newtonian one), like statistical periodicity in space-time structure?

Not sure I fully understand you. AFAIK Saturn and the formation of it's features can be described just fine without quantum gravity. However if you go further back, to the big bang, then quantum gravity is believed to set its mark. This was what BICEP2 was trying to detect. A nice layman-level article about that can be found here: http://profmattstrassler.com/2014/0...t-gravitational-waves-directly-or-indirectly/ (you can skip to the third section if you don't feel like reading the whole shebang).

 

[wabbit]

Why would QG effects be at play here ? Again, there are a huge number of things that can and have been studied about such systems involving only ordinary gravity, which is known to be far far more potent at the scales and densities concerned. Any QG effect would most likely be a very small correction to these.

To take a comparison going to a widely greater scale, in cosmology, QG effects are estimated to be very subtle already a tiny fraction of a secong after the bang or bounce - there are predicted effects that should de observable today when analysing subtle features of the CMB, or perhaps in explaining why this or that feature of our current universe is what it is - but these come from the progapation in time of something that happened near the bang/bounce, not from effects generated afterwards - and this is a far cry from QG effects happening today within the solar system.

Finding detectable predictions of QG is in fact a challenge, if it were detectable so easily we'd probably already have a well established QG theory by now.

 

[Jimster41]

I just don't understand how we can relegate the process of QM gravitation to some small distant corner, when everything that happens in the next proper instant, literally everywhere, somehow has to flow through that process.

Sorry guys, I really appreciate your patience. It's been a real help to be able to voice these questions and confusions. I'm going to go back to studying my Susskind etc and hope I'm not too embarrassed later, when it finally clicks.

 

[wabbit]

It's an interesting question : why is the QG scale so small / the QG density so high ?

Experimentally it's just a fact : it it was much larger, we'd have noticed it as say deviations to NG or GR already - or we'd have labs running experiments and studying those effects like say CERN does for the Standard Model.

But other than that, is there a fundamental reason ? I really don't know at all, perhaps the experts here would. The QG scale is somewhat naturally expected to be of the same order of magnitude as a combination of the gravitational constant and Planck constant etc.., but that doesn't really answer anything : )

 

[Jimster41]

why is the QG scale so small / the QG density so high ?

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

 

[mfb]

How does their cross-section limit compare to that of neutrinos?

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.

Is Dark Matter thought to be a component of the dynamics of a gravitational system like a planetary disk or ring system, or is it just way way too weak and diffuse for even a hope of detection at those scales?

The local density of dark matter is too low. There are upper limits on invisible mass in the solar system, but those measurements are not sensitive to the small expected amount yet.
I don't see how eLISA would contribute to dark matter research in any way. For gravitational waves you need massive amounts of stuff accelerated quickly - dark matter does not do that.

I think what I'm confused about, are any of the features of something like Saturn interesting in a different way when viewed as manifestations of a QM gravitation process (not a Newtonian one), like statistical periodicity in space-time structure?

That has nothing to do with quantum mechanics.

 

[Lord Crc]

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

If you just have a buch of particles in a small volume, then their gravitational attraction is negligible. This means the spacetime curvature can be accurately approximated as being static, and regular QM can be used.

For QG to be relevant you need enough particles in a small enough volume for their gravitational attraction to be significant, in other words that they affect the spacetime curvature. Then neither GR or QM is sufficient on their own.

 

[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.