4 ms·
Except that the orbital period around the galaxy is ~230M years.
by indigobunting 3y ago
Except that the orbital period around the galaxy is ~230M years.
- __MatrixMan__ 3y agoMaybe dark matter bodies orbit faster.
- exe34 3y agoOrbital speed is a fixed function of central mass and distance from that mass.
- thfuran 3y agoIn theory. On the other hand, dark matter is pretty much just a name we give to a discrepancy in current theory.
- 4ad 3y agoNo, that is dark energy. From the pov. of gravity dark matter is simply regular matter that we can't see.
- exe34 3y agoNot quite. Dark matter is the hypothesis that the discrepancy between theory and observation is due to a form of matter that interacts gravitationally but not electromagnetically. So we can't see it, and thus "dark". There are other competing ideas including a family of modified Newtonian dynamics models, but nothing comes as close as explaining the observations as dark matter does. There was a paper recently that showed that the discrepancy may be the higher order terms from general relativity that is often neglected because they are believed to be small - but that idea still needs to be proven to work for a large variety of cases. The observations in question for this Dark matter hypothesis include the rotation velocity of stars in galaxies and a few other things like gravitational lensing. Dark energy is a different discrepancy with theory. It's a term that we have to add to Einstein's field equations to account for the observation that the universal expansion is accelerating instead of slowing down. Again there are competing hypotheses, like non uniform density on the largest scales, but nothing quite explains everything as dark energy.
- thfuran 3y agoI don't think that's really an accurate description. There's a discrepancy between observations relating to gravitation and general relativity's predictions. The discrepancy could be accounted for by significant extra mass, but no non-gravitational observations seem to confirm the presence of that extra mass. So "dark matter" is the supposition that there's a significant amount of extra mass that interacts only with gravity but not, for example, light, making it categorically different from ordinary matter. And I don't think there's any evidence that this dark matter follows the same gravitational constant as ordinary matter.
- adrian_b 3y agoIt does not matter whether it "follows the same gravitational constant as ordinary matter" or not. For any kind of matter, normal or "dark", which is observed only through gravitational effects, you cannot determine separately its mass and the gravitational constant that applies to it. You can determine only the product between mass and gravitational constant (which is the cause of measurable forces). Therefore for many astronomical objects the product between their mass and the gravitational constant is known with a much greater precision than their mass (because the gravitational constant is known with very poor precision even for ordinary matter). The same applies for "dark matter". You cannot compute the distribution in space of the mass of the dark matter, but only the distribution in space of the product between its mass and whatever gravitational constant is applicable to it. So even if a different gravitational constant were applicable to "dark matter" that fact would be irrelevant for any mathematical model that is fitted to the observations.
- thfuran 3y agoNevertheless, mass is a separate quantity, which means that the original claim that orbital speed is determined solely by mass and orbital radius is not supportible in the context of dark matter.
- exe34 3y agoYou could oscillate up and down across the orbital plane many times during one orbit. Not saying that's what's happening here, but it's a possibility.
- ckcheng 3y agoThat’s the argument I think. Per that Wikipedia link: > Randall hypothesizes a plane of dark matter exists roughly on the plane of the Milky Way galaxy. As the Sun oscillates in its orbit around the center of the galaxy, it passes through the dark matter.
- exe34 3y agoAh that's probably where I learnt it from then!
- jagged-chisel 3y agoWhat would cause the oscillation?
- roywiggins 3y agoIf you're not smack bang in the middle of the galactic plane, you're going to be accelerated towards it by gravity, so you'll tend to bob up and down, like a pendulum.
- cwillu 3y agoI'm not sure I buy that the oscillation around the plane can have a period other than the orbital period.
- partitioned 3y agodo you think motion in X is always dependent on motion in Y?
- tejtm 3y ago
- deleted 3y ago[deleted]