#71
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WIMPS?
In article ,
Nicolaas Vroom writes: When you remove all the stars above 0.4 mass the total mass left is 9,46 and the density is 1E-12. This are Red Dwarfs and Brown Dwarfs and can be considered invisble baryonic matter. Fair enough, except that at least the higher end of that mass range would be detectable via lensing. One fly in the lensing ointment is that measurements have been made only towards the LMC and the Galactic center (unless I've missed other observations, which is quite possible). If you can arrange for red dwarfs to occupy regions other than these lines of sight and still explain the rotation curves, you might get somewhere. Regardless of lensing, if you want to explain rotation curves with red dwarfs or similar objects, you have to postulate that they have a different distribution than that of the visible stars. In particular, you need more low-mass stars at large radii than expected from the light distribution. That's possible, of course, but there's no evidence for it. Galaxy colors don't change much with radius, for example. Actually they become a little bluer at large radii because of lower metallicity. As I wrote earlier, I'm not sure all the parameter space is ruled out, but it is shrinking. Some comments on subsequent posts: 1) aside from rotation curves, there's a problem with spiral disk instability. Putting mass in a halo rather than a disk stabilizes the disk as well as solving the rotation curve problem. That doesn't prove the halo explanation, but it makes it more attractive. 2) gravitational lensing can easily detect objects of half a solar mass or larger. Smaller objects are more difficult, but observations have improved. I'm not sure just where things stand now, but objects more massive than some tenths of a solar mass cannot easily explain the dark matter inferred for the Milky Way. (But see above about "fly in the lensing ointment.") -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#72
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WIMPS?
In article ,
"Richard D. Saam" writes: Deceleration (aP) as well as radiation power half_life should then be on the order of 87.7/4 or 21.9 years You've lost me there. Radiative deceleration should be proportional to power (according to the equation you quoted earlier), which decreases exponentially according to the half-life. I think you've confused yourself by thinking about temperature. These half_lives should be considered very accurate Yes, half life is an intrinsic property of the Pu-238. The point of the recent analysis was that the radiative model is consistent with all known data, and there is no need for "new physics" to explain the so-called Pioneer Anomaly. That doesn't prove that no new physics exists, but it removes one piece of evidence. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#73
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WIMPS?
In article , Steve Willner
writes: One fly in the lensing ointment is that measurements have been made only towards the LMC and the Galactic center (unless I've missed other observations, which is quite possible). If you can arrange for red dwarfs to occupy regions other than these lines of sight and still explain the rotation curves, you might get somewhere. I'm pretty sure that there were observations of M31 looking for microlensing effects. For larger masses, one can rule them out because they would be visible in QSO light curves. (Hawkins claims such a detection of dark matter, but it doesn't stand up to a quantitative analysis.) |
#74
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WIMPS?
On 8/13/13 4:20 AM, Steve Willner wrote:
In article , "Richard D. Saam" writes: Deceleration (aP) as well as radiation power half_life should then be on the order of 87.7/4 or 21.9 years You've lost me there. Radiative deceleration should be proportional to power (according to the equation you quoted earlier), which decreases exponentially according to the half-life. but which half life are you indicating: the RTG plutonium half life of 87.7 years reflected in power equation http://arxiv.org/abs/1204.2507v1 page 2 Qrtg(t) = 2^(-(t-t0)/87.74)*Qrtg(t0) or the electrical half life of 87.7/4 or 21.9 years http://arxiv.org/abs/1204.2507v1 table 1 data Qelect(t) = 2^(-(t-t0)/21.9)*Qelect(t0) The authors choose a subset aP = n*Q/(pioneer mass *c) of this more general equation aP = F*stefan_constant*T^4*(4/(3*c))*pioneer area/pioneer mass where n*Q = F*stefan_constant*T^4*(4/3)*pioneer asymmetric area with two components aP = nrtg*Qrtg/(pioneer mass *c) + nelect*Qelect/(pioneer mass *c) with a well established finite element basis for nelect*Qelect/(pioneer mass *c) and Qelect(t)=2^(-(t-t0)/21.9)*Qelect(t0)=Qelect(t0)*exp(-ln(2)(t-t0)/21.9) but with no physical basis for Qrtg(t) = 2^(-(t-t0)/87.74)*Qrtg(t0)= Qrtg(t0)*exp(-ln(2)(t-t0)/87.74) The RTG component contribution to Pioneer deceleration (aPrtg) is nrtg*Qrtg = F*stefan_constant*T^4*(4/3)*RTG_asymmetric_area The RTG symmetrical design defines RTG_asymmetric_area = 0 and therefore aPrtg = 0 Any RTG_asymmetric_area would be outside manufacturing tolerances that perhaps or on the order of mm^2 These half_lives should be considered very accurate Yes, half life is an intrinsic property of the Pu-238. The point of the recent analysis was that the radiative model is consistent with all known data, and there is no need for "new physics" to explain the so-called Pioneer Anomaly. That doesn't prove that no new physics exists, but it removes one piece of evidence. In http://arxiv.org/abs/1204.2507v1 a detailed finite element radiative model was applied to electrical power with half life of 87.7/4 or 21.9 years assuming the model daP/dt = -ln(2)(t-t0)/21.9. The remaining aP 'constant with time' component was modeled as daP/dt = -ln(2)(t-t0)/87.7 a modeling that can not be physically justified due to Pioneer design. (A 87.7 year half life changes aP on the order of 20% over Pioneer life for all intensive purposes represents a constant) This 'constant with time' component on the order of 7E-8 cm/sec^2 cannot be linked to the internal physical nature of Pioneer and can truly be considered the Pioneer anomaly indicating a property of space through which the Pioneers travel. This recent analysis http://arxiv.org/abs/1204.2507v1 does not 'remove one piece of evidence'. but unnecessarily stops investigation in this astrophysical area at this important time when 95 percent of the unknown universe is under investigation as to identity and all research avenues are needed as tools. Richard D Saam |
#75
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WIMPS?
SW Radiative deceleration should be proportional to power
I was wrong about that. See below. In article , "Richard D. Saam" writes: but which half life are you indicating: the RTG plutonium half life of 87.7 years reflected in power equation http://arxiv.org/abs/1204.2507v1 page 2 Qrtg(t) = 2^(-(t-t0)/87.74)*Qrtg(t0) or the electrical half life of 87.7/4 or 21.9 years http://arxiv.org/abs/1204.2507v1 table 1 data Qelect(t) = 2^(-(t-t0)/21.9)*Qelect(t0) In http://arxiv.org/abs/1204.2507v1 a detailed finite element radiative model was applied to electrical power with half life of 87.7/4 or 21.9 years I don't see where you got that half life for electrical power, but it may be right. The electrical efficiency decreases as the plutonium decays and the RTGs cool off, so the electrical power generated drops off faster than the total power of the plutonium. As the OP wrote, the authors have a detaled thermal model, but in essence there are two components: electrical power used in the spacecraft itself and waste heat in the RTGs. The point -- that I completely missed earlier -- is that these have entirely different efficiencies (eta) for being turned into acceleration. The RTG waste heat is emitted nearly uniformly in all directions, and its efficiency for accelerating the spacecraft is only 1%. When powered components inside the spacecraft generate waste heat, that heat is directed predominantly away from the Sun (because of the geometry of the spacecraft) and has a 41% efficiency for conversion into electrical power. The net acceleration is the sum of these two. The bottom line is that the calculated thermal acceleration is 80% of the acceleration derived from the Doppler tracking data. However, the error bars are larger than the 20% difference, so there is currently no evidence of "new physics." (The RTG asymmetry is uncertain because of unknown changes in the coating properties. Making it just a bit bigger than 1% would give excellent agreement.) -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#76
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WIMPS?
On 8/30/13 1:34 AM, Steve Willner wrote:
The bottom line is that the calculated thermal acceleration is 80% of the acceleration derived from the Doppler tracking data. However, the error bars are larger than the 20% difference, so there is currently no evidence of "new physics." (The RTG asymmetry is uncertain because of unknown changes in the coating properties. Making it just a bit bigger than 1% would give excellent agreement.) This presented logic is in a constant time frame. 'calculated thermal acceleration is 80%' The RTG contribution is 1% A more complete model would analyze these deceleration contributions with time for a constant residual. The much detailed finite element electrical contribution fades faster with time (21.9 year half life) (based on regression of http://arxiv.org/abs/1204.2507v1 table 1 data) leaving the RTG contribution (87 year half life) that can be considered constant in the measured Pioneer time frame. No detailed finite element thermal RTG deceleration analysis was done in http://arxiv.org/abs/1204.2507v1. A prior RTG analysis in http://arxiv.org/abs/gr-qc/0104064 page 32-33 reads in part: "So, even though a complete thermal/physical model of the spacecraft might be able to ascertain if there are any other unsuspected heat systematics, we conclude that this particular 'RTG' mechanism does not provide enough power to explain the Pioneer anomaly" That leaves a constant deceleration residual with time with no on board mechanistic origin. I won't say 'new physics' is required but verification of known universal physics (external to Pioneers) is warranted to explain this constant deceleration residual. Richard D Saam |
#77
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WIMPS?
"Richard D. Saam" writes:
No detailed finite element thermal RTG deceleration analysis was done in http://arxiv.org/abs/1204.2507v1. A prior RTG analysis in http://arxiv.org/abs/gr-qc/0104064 page 32-33 reads in part: "So, even though a complete thermal/physical model of the spacecraft might be able to ascertain if there are any other unsuspected heat systematics, we conclude that this particular 'RTG' mechanism does not provide enough power to explain the Pioneer anomaly" The "particular mechanism" of section VIII B is "anisotropic heat reflection off of the back of the spacecraft high-gain antennae", a mechanism that doesn't play any significant role in the 1204.2507 model--so that quote does not appear to be relevant. 1204.2507 accounts for the deceleration via approximately equal contributions from differential emissivity of the RTGs and non-isotropic radiative cooling from the electronics in the main body of the spacecraft, which are covered in VIII C and VIII D respectively. (I'll also note that, while the RTG portion of the 1204.2507 model isn't as detailed as it is for the main body of the spacecraft, it is still considerably more developed than the arguments in 0104064). That leaves a constant deceleration residual with time with no on board mechanistic origin. It leaves a residual consistent with the mechanisms of 1204.2507 at the one-sigma level. That doesn't exclude the possibility of an unaccounted-for residual, but it also doesn't require one. To show that something more is needed would require somehow reducing the statistical and systematic uncertainties in 1204.2507. -dan |
#78
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WIMPS?
On Sat, 31 Aug 13, "Richard D. Saam" wrote:
I won't say 'new physics' is required Recently I posted an analysis of the Pioneer anomaly showing that its scale is consistent with universal size, thusly: At 20AU Pioneer was travelling 12500m/s. The anomalous sunward acceleration was 9 x 10^-10 m/s^2. Therefore, per each second, the distance travelled was 12500m, and the anomolous distance shortfall was d=.5a = 4.5 x 10^-10m. Thus the ratio of the shortfall to distance travelled is 3.6 x 10^-14. Let's hypothesize that this anomaly is simply a function of distance. Thus, 20AU / ratio = 3 x 10^9 km / 3.6 x 10^-14 = 8.33 x 10^22 km = 8.8 x 10^9 LY, which approximates the Einstein radius usually written as 10^10 LY. If this hypothesis is right, then the Pioneer anomaly would be seen to be twice at 40AU as it was at 20AU. Eric Flesch |
#79
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WIMPS?
On 9/4/13 1:28 AM, Richard D. Saam wrote:
[Mod. note: entire quoted article snipped -- mjh] I have had to digitize the reported data. Surely the data is available in textual format for the scientific community. As a follow-up I have forwarded this Pioneer anomaly investigation logic to the authors and director of JPL No response. The reported JPL Pioneer anomaly investigation is a case of science by bureaucracy. The Pioneer anomaly is not supposed to exist therefore it does not. Richard D. Saam |
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