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On 11/11/2017 9:50 PM, jacobnavia wrote:
Le 10/11/2017 à 08:10, Jos Bergervoet a écrit : We know what the rotation curves are, so the acceleration of stars on average is known, and it is known that this does not fit with the gravity of known matter in the galaxies. So you do not need the observations as you describe here to get this information. What we do *not* know is: 1) Is there more matter than the known matter, so stronger gravity and therefore restoring agreement with the movement? That would be a nice solution. And if space between the stars wasn't empty but filled with some kind of very thin gas? Yes, but then this gas would have to be undiscovered until now for some reason. Which means it has to consist of things we can't easily see, e.g. particles like axions or sterile neutrinos. It can also be a collection of massive, as of yet undiscovered, black holes. Actually they are now starting to be discovered by LIGO. But you'd still need quite a lot of them to have enough extra mass.. 2) Is there another force that adds to the effect of gravity so together they give agreement with the motion? The first possibility leads to the hunt for dark matter, the second to the search for a "fifth force" Dark matter was supposed to be in some kind of "halo" outside the galaxy. No, dark matter is supposed to form a cloud with the galaxy immersed in it. Probably densest in the galaxy itself but also extending to some region (halo) outside the galaxy. The stars then, are pulled by the outside. No they are not! Whatever the distribution in the radial direction, gravity *always pulls inwards*, except if the distribution is completely confined to a shell, in which case in the empty inner region it gives gravity zero, but is *still* not pulling outwards! (Do your 1/r^2 exercises again, for a distributed mass with spherically symmetric distribution, only a function of r.) A fifth force would have a vector centered in the center of the milky way. Yes, if it is to explain the rotation curves. But a cloud of dark matter would also pull inwards. Is it possible then, to figure out this from the orbit of a known star, say, the sun? No, a fifth force or a cloud of unseen mass will both give more inward attraction (to the galactic center) for this star, you can't see the difference. The observations with telescopes as discussed above will not help with these questions at all, they will just reproduce the already observed disagreement between motion and the gravity of known matter. Which then leaves us again with the same two questions. A fifth force would perturb the orbit of the sun in a different way than matter in a halo. No, as proposed solutions for the motion of the stars, both these things of course are supposed to influence the orbits of the stars in the same way (giving them more inward acceleration). Besides, even if it is weak it has been there since eons. After all this time (age of the sun around 5GY) some perturbation should be observable. Of course. The whole structure of the universe (zooming in from the largest levels) would have to agree with any new force, or new kind of matter we introduce! It looks like this: http://www.atlasoftheuniverse.com/universe.html Shouldn't an analysis of the orbit of the sun give an answer to that? [[Mod. note -- Interstellar space is indeed filled with a "very thin gas": https://en.wikipedia.org/wiki/Interstellar_medium https://en.wikipedia.org/wiki/Intracluster_medium This is already included in counts of "known matter". -- jt]] And it should not only be in agreement with the orbit of the sun, but with all galaxies, clusters, filaments, voids, and anything else we observe in the universe. -- Jos |
#12
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Verifying
jacobnavia wrote:
[[Mod. note -- 19 excessively-quoted lines snipped here. -- jt]] Dark matter was supposed to be in some kind of "halo" outside the galaxy. The stars then, are pulled by the outside. A fifth force would have a vector centered in the center of the milky way. There would be no gravitational effect on bodies within such a halo shell. That was a first year physics problem. [[Mod. note -- 24 excessively-quoted lines snipped here. -- jt]] |
#13
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Verifying
In article ,
jacobnavia writes: Suppose some star S at 60 thousand light years from the center of the galaxy. A normal star whose mass can be accurately determined. Its speed can be measured, and its mass is known. You don't need the mass. At these scales, stars are effectively massless "test particles," and only accelerations are relevant. Then, we subtract gravity effects The "gravity effects" depend on the mass distribution, which is unknown. (This is what jt was getting at.) In other words, if you knew the accelerations and assumed a gravity law, you could derive the mass distribution of the Milky Way. Or if you knew the mass distribution from other data, you could derive the gravity law. But you can't derive both. What you could do is look for the _simplest_ set of assumptions that would explain all the accelerations. That would be a huge advance, of course. As jt also mentioned, the observables (in principle!) are the three spatial coordinates and the three components of velocity. With GAIA and ground-based radial velocity surveys, those will soon be measured for millions of stars! What you want, though, are the _accelerations_, which can't be directly measured. However, the statistical distribution of velocities should at least put constraints on _either_ the mass distribution _or_ the gravity law, and the task again will be to look for the simplest assumptions that explain the data. In article , jacobnavia writes: Mmm the galaxy has a plane of rotation. Th center of the galaxy, that star and we are rotating around the same central object, the galaxy, in a plane. Any three points define a plane, but neither the Sun nor an arbitrary star is in general moving in that plane. To put it another way, the Milky Way disk has a finite height, and the halo is roughly spherical, and neither stellar component has motions perpendicular to the vector towards the Galactic center. [Moderator's note: A pedantic note to avoid posts pointing this out: any three points define a plane if they are not colinear. -P.H.] "We don't know that is moving in a circular orbit"... wow, I always thought that they are doing so, I'm not sure why you thought that. The orbits are roughly elliptical, generally with modest eccentricities, but the ellipses are not closed as for solar system planetary orbits. That's because the mass distribution of the Galaxy is not spherically symmetric. and that the "arms" we see are density waves in the disc of stars circling the center. Indeed so. These density waves perturb the stellar orbits. The bar is also an important perturber. You might look up "Local Standard of Rest" and "Solar Motion". The stars must be doing "some" kind of circle around the center since the form of the galaxy (a rotating plane of stars ) indicates so. The flatness of the disk indicates that the orbital inclinations are small (not zero, though) but says nothing about eccentricities. In fact, you could even have large inclinations if the eccentricities were also large, but that's not what's observed. I even thought that the sun was rotating about 1 rotation per 250 million years, so it is around 20 galactic years old. I thought that the orbit was a circle. Is that not correct? The time scale is about right (I get 220 Myr), but "circle" is an approximation even rougher than "ellipse." Dark matter scenarios suppose some form of invisible matter outside Not only "outside." The morphology of the putative dark matter halo is unknown, but simulations say it ought to be concentrated toward the Milky Way center, roughly but not exactly spherically symmetric, and more extended than the stellar distribution. A second star we could use of course, is the nearest one, the sun. The sun's orbit could tell us about the force effects here. Here we have more data and less problems than with the star's far away. We know very precisely the distance to the center, and the mass between us and the center, so the effects of gravity could be calculated much more easily. The mass is derived from the solar motion; we don't know it independently. Unless I'm missing something. One standard reference on the subject is at http://iopscience.iop.org/article/10...ta#apj490685s4 but the authors don't derive masses from the rotation curve. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#14
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Verifying
I would like to thank all people (and the moderator) that took the time
to answer my (rather silly sometimes) questions. jacob |
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