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Hubble makes 3D dark matter map
Hans Aberg wrote:
And would the planetary friction be equally negligible at this low density? Well, there shouldn't be any "planetary friction", as usually described, dark matter's _only_ interaction with normal matter is via gravity: http://www.nasa.gov/home/hqnews/2006...rk_Matter.html Can the dark matter within the solar system be observed somehow? Observation of dark matter is "challenging": There are also several experiments claiming positive evidence for dark matter detection, such as DAMA/NaI, PVLAS, and EGRET, but these are so far unconfirmed and difficult to reconcile with the negative results of other experiments. Other experiments searching for dark matter include the Cryogenic Dark Matter Search in the Soudan mine or the ArDM experiment. http://en.wikipedia.org/wiki/Dark_matter HTH xanthian. |
#12
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Hubble makes 3D dark matter map
In article ,
Hans Aberg wrote: So, if I got it right, the large distance between the stars admits the dark matter to exist at such a low density that it does no affect the planetary orbits*within the star systems. Yes, that's right. And would the planetary friction be equally*negligible at this low density? Yes. Even assuming that the dark matter were made of strongly-interacting particles that bounced off the planets' surfaces whenever they hit them, I think that the time scale for a drag force to change the planets' orbits significantly is much longer than the age of the solar system. I just did that calculation pretty quickly and roughly, so I could have messed it up. If anyone thinks I have, let me know. But if the dark matter were composed of particles that interacted relatively easily with ordinary matter, we'd have detected them by now, by noticing scattering events with particles in the atmosphere or something. That's one of the reasons that the leading hypothesis these days is that the dark matter consists of weakly interacting particles. Such particles would pass right through solid matter most of the time with only a low probability of interacting. In that case, any friction-like force due to planets passing through these clouds of dark matter would be even smaller. Can the dark matter within the solar system be observed somehow? We certainly hope so, but it hasn't been done yet. There are several experiments underway to search for dark matter particles passing through our neighborhood. They're similar in concept to experiments to detect solar neutrinos: they involve looking for the effects of particles bouncing off of atoms in detectors in deep underground labs. There was one controversial claim of a detection a few years ago, but it's generally believed that there was something wrong with that experiment. -Ted -- [E-mail me at , as opposed to .] |
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Hubble makes 3D dark matter map
In article ,
Richard Saam wrote: An interesting calculation. Here is another for Pioneer Spacecraft deceleration 'a' a ~ 2*Area*rho*c2 / M ~ 2*(58,965 cm2)*(6.38E-30 g/cm3)*(3E10 cm/sec)^2 /(241,000 g) ~ 2.8E10-9 cm/sec2 for Pioneer spacecraft I haven't paid attention to the Pioneer anomaly stuff lately, so I apologize if I'm just being stupid, but I don't see the physics behind this relation. In particular, I don't see how c gets involved. If you assume that the spacecraft is moving at velocity v through a cloud of stationary particles that bounce elastically off of it whenever they hit it, then you get a relation something like the above, but with v instead of c. That makes a big difference, of course. In addition, the dark matter particles are thought to be weakly interacting, so most of them pass right through Pioneer without exerting any force on it at all. Your average calculated local density of 7 x 10^{-25} g/cm^3 is quite a bit higher than 30 x 6.38E-30 g/cm^3 or 2E-28 g/cm^3 but there still remains a conceptual mechanism on how dark matter influences solar objects according to their area/mass (Pioneer some and planets negligible). Perhaps the local dark matter density is on the order of 30 x 6.38E-30 g/cm3 or 2E-28 g/cm^3. The calculation I did was based on well-measured Galactic dynamics. If you want to get a different answer, you'll have to come up with a different explanation for the motion of stars in the Galaxy. -Ted -- [E-mail me at , as opposed to .] |
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Hubble makes 3D dark matter map
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Hubble makes 3D dark matter map
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Hubble makes 3D dark matter map
In article , "Kent Paul
Dolan" wrote: And would the planetary friction be equally negligible at this low density? Well, there shouldn't be any "planetary friction", as usually described, dark matter's _only_ interaction with normal matter is via gravity: http://www.nasa.gov/home/hqnews/2006...rk_Matter.html Not really, that is how it is discovered, not a description of all its properties. See the post by . -- Hans Aberg |
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Hubble makes 3D dark matter map
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Hubble makes 3D dark matter map
In article ,
Hans Aberg wrote: What would happen if the matter is very thin infancy matter, of the kind known to form very young stars? Would that be easily detectable, or possible to rule out? I don't know the term "infancy matter," but it sounds like it means stuff that includes a lot of hydrogen gas. If you try to put a large amount of diffuse gas in the Sun's neighborhood of our Galaxy, I think it'd be pretty easy to spot. In particular, it'd produce whopping great absorption lines in the light from nearby stars. I think you're right that it'd be blown out of the inner solar system by the solar wind, by the way. That's why I think that the main way to look for it would be on slightly larger scales. So if you want to put enough hydrogen- and helium-rich stuff in our neighborhood to make a significant contribution to the dark matter, you can't make it diffuse. You might try to stick it in gravitationally-bound (Jupiter-ish) lumps. That gets around the absorption line problem. You can detect such lumps in other ways, especially by the technique known as gravitational microlensing. The limits set by this technique show that some such objects exist, but that there can't be enough of these lumps to make up all of the dark matter. -Ted -- [E-mail me at , as opposed to .] |
#19
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Hubble makes 3D dark matter map
In article ,
Richard Saam wrote: wrote: In article , Richard Saam wrote: An interesting calculation. Here is another for Pioneer Spacecraft deceleration 'a' a ~ 2*Area*rho*c2 / M ~ 2*(58,965 cm2)*(6.38E-30 g/cm3)*(3E10 cm/sec)^2 /(241,000 g) ~ 2.8E10-9 cm/sec2 for Pioneer spacecraft I haven't paid attention to the Pioneer anomaly stuff lately, so I apologize if I'm just being stupid, but I don't see the physics behind this relation. In particular, I don't see how c gets involved. If you assume that the spacecraft is moving at velocity v through a cloud of stationary particles that bounce elastically off of it whenever they hit it, then you get a relation something like the above, but with v instead of c. That makes a big difference, of course. The conceptual physics is very simple. Assume that space is a continuous medium with mass critical density of 6.38E-30 g/cm^3 . Then think of an object passing through it defined something like a ram jet engine (analogous to atmospheric oxygen intake) which takes in this space medium with neglible effect (m v^2) but which expells it as m c^2 with resultant thrust in the same direction as intake resulting in object deceleration. It's always embarrassing when something is described as "very simple" and I still don't get it! I don't understand the model you're describing in about half a dozen different ways. What does "expels it as m c^2" mean? Why doesn't the final answer depend on the speed of the object through the medium? Surely the faster the object is moving, the more of this stuff it'll sweep up. We're hypothesizing something like the Pioneer moving through a cloud of particles, right? When a single particle of mass m strikes the Pioneer, the *largest* impulse that can be imparted is surely 2mv, where v is the relative speed of the particle and Pioneer. The total mass of all such particles striking per second is something like rho A v, so the total force would be 2 rho A v^2. So I get the same formula as you have above, but with v instead of c. And that's bound to be an overestimate, because not every particle imparts the maximum possible impulse. Your average calculated local density of 7 x 10^{-25} g/cm^3 is quite a bit higher than 30 x 6.38E-30 g/cm^3 or 2E-28 g/cm^3 but there still remains a conceptual mechanism on how dark matter influences solar objects according to their area/mass (Pioneer some and planets negligible). Perhaps the local dark matter density is on the order of 30 x 6.38E-30 g/cm3 or 2E-28 g/cm^3. The calculation I did was based on well-measured Galactic dynamics. If you want to get a different answer, you'll have to come up with a different explanation for the motion of stars in the Galaxy. I have replicated your calculations and they are right. I think you will agree there are potentially many components to Galactic dynamics. I'm sorry; I'm missing your point yet again. The question I thought we were talking about was, more or less, "what's the dark matter?" If we're now talking about some component whose density is several orders of magnitude less than the density implied from Galactic dynamics, then more or less by definition we're not talking about the dark matter. I'm not sure I see the point of postulating that the solar system contains some component of some unknown material with a density equal to the critical density. We know that the density in the solar neighborhood is orders of magnitude larger than that, so what problem are we trying to solve by assuming such a component? -Ted -- [E-mail me at , as opposed to .] |
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Hubble makes 3D dark matter map
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