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Stacy McGaugh's blog post of 7-Apr-2023: A Few Words About the Milky Way left me gobsmacked. A "few" words? Ha! Here follows my "short" version...
Stars in a galaxy don't have just a flat orbit around the galaxy centre. During their orbit they also do small oscillatory motions up and down in the z direction (think: cylindrical coordinates ##r,\phi,z##). This means their z-acceleration is nonzero.
The stars' orbital motion involves their ##r \dot\phi^2## centripetal acceleration, which involves their tangential velocities. By red/blue-shift experimental analysis we discover that their centripetal acceleration is too high to be accounted for by Newtonian gravity and the galaxy's baryonic mass distribution. Note (for later) that this ##r \dot\phi## orbital motion is perpendicular to the gravitational force they experience, which is overwhelmingly in the -##r## direction.
In contrast, the ##z## oscillatory motion is parallel to the ##z##-component of the gravitation force generated by a disk-like galaxy.
Here's the kicker: MOND accounts well for the orbital motion discrepancies, but seems totally irrelevant to the z motion. In the z direction, Newtonian gravity seems to do just fine by itself.![Jaw Drop :)) :))](/styles/physicsforums/xenforo/smilies/jawdrop.png)
McGaugh is (rightfully) very cautious about this, emphasizing that lots more work needs to be done, analyzing vast amounts of Gaia data, before this puzzle could be considered solid. (Indeed, I was hesitant whether to even mention it here in the BTSM forum, since it's probably pushing the current boundaries.)
Stars in a galaxy don't have just a flat orbit around the galaxy centre. During their orbit they also do small oscillatory motions up and down in the z direction (think: cylindrical coordinates ##r,\phi,z##). This means their z-acceleration is nonzero.
The stars' orbital motion involves their ##r \dot\phi^2## centripetal acceleration, which involves their tangential velocities. By red/blue-shift experimental analysis we discover that their centripetal acceleration is too high to be accounted for by Newtonian gravity and the galaxy's baryonic mass distribution. Note (for later) that this ##r \dot\phi## orbital motion is perpendicular to the gravitational force they experience, which is overwhelmingly in the -##r## direction.
In contrast, the ##z## oscillatory motion is parallel to the ##z##-component of the gravitation force generated by a disk-like galaxy.
Here's the kicker: MOND accounts well for the orbital motion discrepancies, but seems totally irrelevant to the z motion. In the z direction, Newtonian gravity seems to do just fine by itself.
![Jaw Drop :)) :))](/styles/physicsforums/xenforo/smilies/jawdrop.png)
McGaugh is (rightfully) very cautious about this, emphasizing that lots more work needs to be done, analyzing vast amounts of Gaia data, before this puzzle could be considered solid. (Indeed, I was hesitant whether to even mention it here in the BTSM forum, since it's probably pushing the current boundaries.)
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