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PWiz
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Can you please explain this effect in some detail?mfb said:Neutrons do interact with photons of sufficient energy - they can scatter at the quarks inside the neutron.
Can you please explain this effect in some detail?mfb said:Neutrons do interact with photons of sufficient energy - they can scatter at the quarks inside the neutron.
wabbit said:AFAIK neutron stars are not counted as dark matter, but as ordinary matter
PWiz said:I just can't understand why a photon would interact with a neutron when according to the standard model it is a gauge boson whose interactions are exclusively limited to charged particles
PWiz said:I don't think that decay occurs in a neutron star
PWiz said:I thought that such invisible, yet massive objects would qualify as dark matter.
wabbit said:The argument I've seen simply states that neutron stars are hot so they emit blackbody radiation in the x ray range
Similar to deep inelastic scattering, just more direct. I don't know if it has a special name as the photons need such a high energy that it is impractical to study it in a lab. It was relevant in the very early universe, and above ~200 MeV the energy is sufficient to create new hadrons.PWiz said:Can you please explain this effect in some detail?
Yes, but they all have much shorter lifetimes.Jimster41 said:Would "uncharged Hadrons held together by g..." be little clumps of neutrons?
I just read they have @ 600s lifetime. Are there other kinds of uncharged hadrons?
It narrows the available parameter space a little bit.Jimster41 said:I'd appreciate a clarification of just how likely it is, given this new upper constraint on self interaction, that DM is truly non self interacting. I mean is this number truly small/large or just bounding off the normal end. If this stuff is at once non self interacting, though differentiated in space time (identifiably somewhere and not somewhere) then one implication is that it is everywhere in space time only itself - entangled, non-local.
That proves to be a very important distinguishing factor in the definition. It considerably alters my view of dark matter. I guess this means that the only possible candidates for non-baryonic dark matter are some hypothetical particles. Thanks.PeterDonis said:the key characteristic of dark matter is not just that it's not visible now, but that it never was visible; i.e., it has always been dark since the early universe
Jimster41 said:Are there any other major experiments or studies going on in the DM effort?
Beautiful. (I hope they wouldn't feel insulted if I said it looks a bit like a huge virus?)Lord Crc said:As an aside, the detector itself is a piece of art in my humble opinion, see the image on the Wikipedia page :)
wabbit said:Beautiful. (I hope they wouldn't feel insulted if I said it looks a bit like a huge virus?)
Lord Crc said:As an aside, the detector itself is a piece of art in my humble opinion, see the image on the Wikipedia page :)
To me that would be surprising: gravitational waves are very hard to detect with sophisticated purpose-built instruments, so for them to have a detectable macroscopic effect there (at the same distance to the source as we are) seems difficult to imagine.Jimster41 said:things like Saturn's Rings, whether or not a system like those, might display (...) or g-waves.
wabbit said:To me that would be very surprising: gravitational waves are very hard to detect with sophisticated purpose-built instruments, so for them to have a detectable macroscopic effect there (at the same distance to the source as we are) seems difficult to imagine.
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 said:One comment though... don't we know that Newton's mechanics, didn't do it?
Jimster41 said: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?
wabbit said:why is the QG scale so small / the QG density so high ?
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.Lord Crc said:How does their cross-section limit compare to that of neutrinos?
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.Jimster41 said: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?
That has nothing to do with quantum mechanics.Jimster41 said: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?
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.Jimster41 said: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.Jimster41 said:Not sure what you mean by QG density? I get the small scale.
Thanks! This result then does not rule out sterile neutrinos as candidates?mfb said: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.
mfb said:That has nothing to do with quantum mechanics.
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*.Jimster41 said: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.
mfb said: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.
(I formatted the formula for readability)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.