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Self-Interacting Dark Matter
We have no confirmed non-gravitational detection of dark matter
either astrophysically or in the laboratory. But that does not mean that we cannot learn anything about dark matter. Recently, Massey et al. a claimed possible detection of dark matter interacting with itself in a galaxy in the cluster Abell 3827. http://arstechnica.com/science/2015/...f-interacting/ http://www.eso.org/public/usa/news/eso1514/ The authors use gravitational lensing to map out the dark matter distribution in the galaxy cluster. They highlight a galaxy falling into the (dark matter) cluster core, in which the dark matter halo of the galaxy appears to lag behind its visible contents. They argue that the dark matter halo is slowed down by its interaction with the dark matter cluster core (whereas the baryonic contents experience no interaction) I have two questions about this work. 1. How likely is it that this tentative detection will be confirmed (or disproved)? 2. How much does a measurement of self-interaction actually tell us about dark matter. For the first question, I am astonished by the rapid progress in the use of gravitational lensing to map out the distribution of non-baryonic dark matter. (I was previously impressed with the rapid progress in microlensing to rule out significant baryonic dark matter.) But Massey et al. point out the fortuitous circumstances that made their observation possible. In particular they require a nearby cluster (so that small lag can be measured) and a complex background galaxy (so the the mass distribution of the lensing galaxy can be measured). I should think we need measurements of three or four such systems giving consistent results before we can fully believe we have detected self-interacting dark matter. As for what this measurement can tell us about dark matter, the fact that we *don't* see self-interaction at the galaxy cluster level (e.g. the bullet cluster) but do see at the galaxy level suggests that it might solve one of the very few problems with Lambda CDM. Lambda CDM works extremely well with large-scale structure but less well at the galaxy level, perhaps because because current models neglect dark matter self-interaction. But the Massey et al measurements only return a single interesting parameter-- the self-interaction rate -- that doesn't satisfy the question of what dark matter is. In the July 2015 issue of Scientific American (quarantined) Dobrescu and Lincoln suggest that there are a variety of dark matter particles and "dark forces" analogous to electromagnetism which mediate only between dark particles. If true, then we may have a new tool for studying dark matter but it is an extremely crude one at best. --Wayne |
#2
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Self-Interacting Dark Matter
In article ,
wlandsman writes: I have two questions about this work. 1. How likely is it that this tentative detection will be confirmed (or disproved)? Pretty much impossible to say. As you note, it is a new result, a tentative detection. 2. How much does a measurement of self-interaction actually tell us about dark matter. We don't know what dark matter is. Some ideas for it are self-interacting, others are not. For the first question, I am astonished by the rapid progress in the use of gravitational lensing to map out the distribution of non-baryonic dark matter. (I was previously impressed with the rapid progress in microlensing to rule out significant baryonic dark matter.) But Massey et al. point out the fortuitous circumstances that made their observation possible. In particular they require a nearby cluster (so that small lag can be measured) and a complex background galaxy (so the the mass distribution of the lensing galaxy can be measured). I should think we need measurements of three or four such systems giving consistent results before we can fully believe we have detected self-interacting dark matter. Yes, they should definitely be confirmed. As you note, it needs a rather unlikely system, but now with big telescopes surveying the whole sky on a regular basis (or, if not now, soon), even unlikely systems can be found. As for what this measurement can tell us about dark matter, the fact that we *don't* see self-interaction at the galaxy cluster level (e.g. the bullet cluster) but do see at the galaxy level suggests that it might solve one of the very few problems with Lambda CDM. Lambda CDM works extremely well with large-scale structure but less well at the galaxy level, perhaps because because current models neglect dark matter self-interaction. Perhaps. This would be a refinement to an existing idea, which is the way science almost always progresses. But the Massey et al measurements only return a single interesting parameter-- the self-interaction rate -- that doesn't satisfy the question of what dark matter is. Sure, but one can't expect more. In the July 2015 issue of Scientific American (quarantined) Dobrescu and Lincoln suggest that there are a variety of dark matter particles and "dark forces" analogous to electromagnetism which mediate only between dark particles. Could be. It would be strange if the only unknown matter which exists is that for which we now have evidence. |
#3
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Self-Interacting Dark Matter
On 7/4/15 3:44 PM, wlandsman wrote:
We have no confirmed non-gravitational detection of dark matter either astrophysically or in the laboratory. But that does not mean that we cannot learn anything about dark matter. Recently, Massey et al. a claimed possible detection of dark matter interacting with itself in a galaxy in the cluster Abell 3827. 1. http://arstechnica.com/science/2015/...f-interacting/ 2. http://www.eso.org/public/usa/news/eso1514/ 3. http://arxiv.org/abs/1504.06576 4. http://arxiv.org/abs/1503.07675 The ref 3 complements Massey et al. work ref 1,2 "Further studies of such systems are imperative to establish if the indication from A3827 for a non-zero self-interaction cross section (of order 1 cm^2/g) is indeed correct." "It should therefore be clear that A3827 is no more sensitive to DM self-interactions than other systems considered in this context and can certainly not be used to probe cross-sections as small as 10^(-4) cm^2 g^(-1)" This conclusion from ref 3 would indicated that the 0.47 cm^2/g cross-section ref 4 may be weighted more towards .47 cm^2/g Ref 1 indicates "it 'on the order 1 cm^2/g' would be just right to solve the observational issues with the leading dark matter model, which calls for a cross-section between 0.1 and one square centimeter per gram My question- What is the leading dark matter model that predicts DM cross-section on the order of 1 cm^2/g ? The authors use gravitational lensing to map out the dark matter distribution in the galaxy cluster. They highlight a galaxy falling into the (dark matter) cluster core, in which the dark matter halo of the galaxy appears to lag behind its visible contents. They argue that the dark matter halo is slowed down by its interaction with the dark matter cluster core (whereas the baryonic contents experience no interaction) Dark matter slowing down at a rate other than observed baryonic contents does not in itself exclude a baryonic DM character. Give this evidence alone, DM could be a different baryonic size/mass distribution providing a different lag character. I have two questions about this work. 1. How likely is it that this tentative detection will be confirmed (or disproved)? see ref 4 "Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both `major' and `minor' mergers" for concluded aggregate 0.47 cm^2/g cross-section 2. How much does a measurement of self-interaction actually tell us about dark matter. Ref1 provides the DM lag mechanism "The more they collide (or the higher the cross-section), the more a blob of dark matter should lag, due to the friction created by the colliding particles" Another mechanism may be DM drag or lag due to vacuum (momentum transfer) viscosity Vacuum viscosity is physically discussed in arXiv:0806.3165v3 [hep-th] 14 Nov 2008 Hydrodynamics of spacetime and vacuum viscosity For the first question, I am astonished by the rapid progress in the use of gravitational lensing to map out the distribution of non-baryonic dark matter. (I was previously impressed with the rapid progress in microlensing to rule out significant baryonic dark matter.) But Massey et al. point out the fortuitous circumstances that made their observation possible. In particular they require a nearby cluster (so that small lag can be measured) and a complex background galaxy (so the the mass distribution of the lensing galaxy can be measured). I should think we need measurements of three or four such systems giving consistent results before we can fully believe we have detected self-interacting dark matter. see ref 4 for Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both `major' and `minor' mergers As for what this measurement can tell us about dark matter, the fact that we *don't* see self-interaction at the galaxy cluster level (e.g. the bullet cluster) Ref 1:"In the Bullet Cluster observations, no lag was found, but the study was not precise enough to rule it out entirely. But the study did constrain it; if dark matter self-interactions were taking place, they weren't doing so that often (the cross-section had to be 1.25 square centimeters per gram or less)." Also pointing to other work The Bullet Cluster provides further evidence of DM decceleration http://arxiv.org/abs/astro-ph/0608408 http://en.wikipedia.org/wiki/Bullet_Cluster Bullet Cluster E0657-56 length 7.714E+23 cm 250 kpc Bullet Cluster E0657-56 time after collision 4.730E+15 sec 1.50E+08 years Bullet Cluster E0657-56 acceleration 2*length/time^2 = 6.9E-08 cm/sec^2 This deceleration is of the same order as the Pioneer deceleration. A Pioneer 10 and 11 cross_section value 0.24 cm^2/g (within range of Ref 1,3,4) may be computed from the NASA published Pioneer dimensions (pi/4)*pioneer_diameter^2/pioneer_mass = (pi/4)*274^2/241,000 = 0.24 cm^2/g with universe critical density (3/8pi)*H^2/G ~ 1x10^-29 g/cm^3 6.9E-08 = Cd * .24 * 1x10^-29 * c^2 = 2x10^-9 cm/sec^2 where a reasonable unitless hydrodynamic drag coefficient Cd = 32 The case for Pioneers' deceleration predominately due to thermal affects http://arxiv.org/pdf/1204.2507v1 may have been premature considering reported decelerations did not include the residual constant values as these spacecraft entered intergalactic space with concurrent diminishing thermal affects. https://groups.google.com/forum/#!se...E/FJy5LOLrSOEJ Further data analysis is required particularly with Pioneers' spin deceleration as a vacuum viscosity (momentum transfer) probe that was entirely excluded from http://arxiv.org/pdf/1204.2507v1. but do see at the galaxy level suggests that it might solve one of the very few problems with Lambda CDM. Lambda CDM works extremely well with large-scale structure but less well at the galaxy level, perhaps because because current models neglect dark matter self-interaction. and the viscosity of space arXiv:0806.3165v3 [hep-th] 14 Nov 2008 Hydrodynamics of spacetime and vacuum viscosity Lambda CDM appears to be too smooth & too perfect But the Massey et al measurements only return a single interesting parameter-- the self-interaction rate -- that doesn't satisfy the question of what dark matter is. In the July 2015 issue of Scientific American (quarantined) Dobrescu and Lincoln suggest that there are a variety of dark matter particles and "dark forces" analogous to electromagnetism which mediate only between dark particles. Seems too complicated. If true, then we may have a new tool for studying dark matter but it is an extremely crude one at best. Given the logarithmic nature of the universe, having a value within one order of magnitude should be very useful. Richard D Saam for comments --Wayne |
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Self-Interacting Dark Matter
On 7/5/2015 7:15 PM, Phillip Helbig (undress to reply) wrote:
In article , wlandsman writes: ... In the July 2015 issue of Scientific American (quarantined) Dobrescu and Lincoln suggest that there are a variety of dark matter particles and "dark forces" analogous to electromagnetism which mediate only between dark particles. Could be. It would be strange if the only unknown matter which exists is that for which we now have evidence. Perhaps not. Seen from the standard model of particle physics it could mean (for instance) that the only unknown particle to be added is the axion. That would just mean that the standard model of particle physics is basically correct (i.e. that it is complete at ordinary energy levels, while of course some additions might still be found if you go way up to the Planck scale. So, would it be "strange" if we only need the axion to complete our working set of particles? -- Jos |
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Self-Interacting Dark Matter
In article , "Richard D.
Saam" writes: Dark matter slowing down at a rate other than observed baryonic contents does not in itself exclude a baryonic DM character. True. Optical depth alone does not exclude that dark matter is made of bricks. Or back issues of the ApJ. But other arguments do. Ditto for a significant fraction of dark matter being baryonic. Give this evidence alone, DM could be a different baryonic size/mass distribution providing a different lag character. Sure, but "this evidence alone" is not a good choice when there is other evidence available. |
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Self-Interacting Dark Matter
On 7/9/15 12:11 AM, Phillip Helbig (undress to reply) wrote:
In article , "Richard D. Saam" writes: Dark matter slowing down at a rate other than observed baryonic contents does not in itself exclude a baryonic DM character. True. Optical depth alone does not exclude that dark matter is made of bricks. Or back issues of the ApJ. But other arguments do. Ditto for a significant fraction of dark matter being baryonic. Give this evidence alone, DM could be a different baryonic size/mass distribution providing a different lag character. Sure, but "this evidence alone" is not a good choice when there is other evidence available. Other evidence 1. Transparency as per above optical depth concept does not rule out small bricks composing galactic halos in aggregate ~1x10-24 g/cm^3. 2. Calculation of nucleosynthetic H, He abundances correlate with observed values based on their gaseous EM absorptivity and emissions, but all H, He may not be in the gaseous state. Present nucleosynthetic calculations do not take into account plasma colligative properties such as may be observed by high atomic Z (gold) work performed at RHIC Brookhaven National Laboratory influencing H nucleosynthetic phase magnitude relationships. 3. DM Bricks not heating up due to CMBR over the billions of years appears to rule out their baryonic character, but DM bricks may be in thermal equilibrium with another much colder phase associated with universe density presently at ~1x10-29 g/cm^3 ~H^2/G rather than CMBR(at ~1x10-34 g/cm^3) rendering the DM bricks electromagnetically unobserved. 4.CMBR fluctuations characterizing DM and observed ~4% universe baryonic matter correlated to BAO are limited due to finite CMBR Black Body WMAP and Planck observation at circa 160 GHz of universe with near infinitely available EM spectra . 5. Previous discussion on this newsgroup (jacob et al) reference early galactic formation that may be fueled by DM baryonic mass action. Logical conclusion: DM bricks could be extremely cold nucleosynthetic hydrogen. How cold? Probably ~1x10^-15 K Richard D. Saam [[Mod. note -- It seems unlikely to me that such "bricks" would survive for many Gigayears at a temperature far BELOW that of the CMBR. -- jt]] |
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Self-Interacting Dark Matter
In article , "Richard D.
Saam" writes: 1. Transparency as per above optical depth concept does not rule out small bricks composing galactic halos in aggregate ~1x10-24 g/cm^3. Assuming that they are baryonic, then they are ruled out because BBN gives us good upper limits on the amount of baryons. 2. Calculation of nucleosynthetic H, He abundances correlate with observed values based on their gaseous EM absorptivity and emissions, but all H, He may not be in the gaseous state. Present nucleosynthetic calculations do not take into account plasma colligative properties such as may be observed by high atomic Z (gold) work performed at RHIC Brookhaven National Laboratory influencing H nucleosynthetic phase magnitude relationships. Is there any evidence that either of these could increase the allowed number of baryons by a factor of 10? 3. DM Bricks not heating up due to CMBR over the billions of years appears to rule out their baryonic character, but DM bricks may be in thermal equilibrium with another much colder phase associated with universe density presently at ~1x10-29 g/cm^3 ~H^2/G rather than CMBR(at ~1x10-34 g/cm^3) rendering the DM bricks electromagnetically unobserved. By definition, DM does not interact electromagnetically. It does interact gravitationally. Assuming DM bricks, as opposed to baryonic bricks (which is what I originally meant), then there must be self-interaction. In that case, however, it is unclear why there are not DM planets etc, which could be seen via microlensing. 4.CMBR fluctuations characterizing DM and observed ~4% universe baryonic matter correlated to BAO are limited due to finite CMBR Black Body WMAP and Planck observation at circa 160 GHz of universe with near infinitely available EM spectra . First, explain why this "limitation" affects the conclusions. Second, you are just as limited with respect to your alternative theory. Logical conclusion: DM bricks could be extremely cold nucleosynthetic hydrogen. How cold? Probably ~1x10^-15 K [[Mod. note -- It seems unlikely to me that such "bricks" would survive for many Gigayears at a temperature far BELOW that of the CMBR. -- jt]] Right. |
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Self-Interacting Dark Matter
On 7/13/2015 2:22 PM, Phillip Helbig (undress to reply) wrote:
[[Mod. note -- It seems unlikely to me that such "bricks" would survive for many Gigayears at a temperature far BELOW that of the CMBR. -- jt]] Right. If these bricks do not interact electromagnetically then there is no way the CMBR could affect their temperature, is there? |
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Self-Interacting Dark Matter
On 7/13/2015 9:22 PM, Phillip Helbig (undress to reply) wrote:
.. By definition, DM does not interact electromagnetically. I think you take the definition too rigorously. If DM consists of axions, they still have some interaction with the EM field, although more complicated than other particles (they are not a direct source term). http://depts.washington.edu/admx/home.html Or if DM consists of neutrinos, those can still have an anapole moment, which leaves some EM interaction, although again weaker than ordinary dipole or monopole interactions. It does interact gravitationally. I think "It does interact predominantly gravitationally on large scales" is covering the situation better. That would not rule out interactions of any other kind and even allow for them to become significant in dense regions on (sub-)galactic scales. Assuming DM bricks, as opposed to baryonic bricks (which is what I originally meant), then there must be self-interaction. In that case, however, it is unclear why there are not DM planets etc, which could be seen via microlensing. The "other interactions" might be such that those planets remain much smaller than usual planet sizes (instead they might not grow larger than the size of, well, bricks!) But of course then they wouldn't fit the definition of planet. At least that definition is made very rigorous. :^) [[Mod. note -- It seems unlikely to me that such "bricks" would survive for many Gigayears at a temperature far BELOW that of the CMBR. -- jt]] Right. Agreed (but it's only likelihood guessing!) -- Jos |
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Self-Interacting Dark Matter
In article , David Staup
writes: On 7/13/2015 2:22 PM, Phillip Helbig (undress to reply) wrote: [[Mod. note -- It seems unlikely to me that such "bricks" would survive for many Gigayears at a temperature far BELOW that of the CMBR. -- jt]] Right. If these bricks do not interact electromagnetically then there is no way the CMBR could affect their temperature, is there? The claim was: 3. DM Bricks not heating up due to CMBR over the billions of years appears to rule out their baryonic character, but DM bricks may be in thermal equilibrium with another much colder phase associated with universe density presently at ~1x10-29 g/cm^3 ~H^2/G rather than CMBR(at ~1x10-34 g/cm^3) rendering the DM bricks electromagnetically unobserved. So talking about baryonic, and hence electromagnetically interacting, matter, but with another "explanation" for the cold temperature, which to me seems a bit speculative and ad-hoc. |
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