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> Due to an effect called the Doppler shift, astronomers expect galaxies rotating opposite to the Milky Way’s motion to appear brighter How does this work? The

> Due to an effect called the Doppler shift, astronomers expect galaxies rotating opposite to the Milky Way’s motion to appear brighter How does this work? The page it links to doesn't explain why rotation would matter. Edit: To clarify - one side of the galaxy would be moving towards us, and one away from us, no matter which direction it spins in, so this should not affect the average brightness of the entire galaxy. The original paper (https://academic.oup.com/mnras/article/538/1/76/8019798?login=false https://academic.oup.com/mnras/article/538/1/76/8019798?logi...) links to a few papers discussing this, among them https://www.mdpi.com/2073-8994/15/6/1190 https://www.mdpi.com/2073-8994/15/6/1190 It doesn't answer my question (or if it does, I didn't understand it), but it gives a magnitude for the expected effect on brightness - 0.6%. I do not think that would explain the 1:2 ratio of observed spin directions.

ceejayoz 2y ago
One side is moving away from us, the other towards. This means their relative speeds are different, detectably so.
- perihelions 2y ago
  How are you linking that effect to the spin orientation of the Milky Way galaxy? Where is the all-sky anisotropy coming from?
- anomaloustho 2y ago
  I went to give a similar answer and realized that - in the instance that the two galaxies are sideways facing each other and with the text saying “it gets brighter” not “one side gets brighter” - I’m not sure if this is the actual answer. I can’t figure out why the Doppler Shift would make the entire galaxy brighter. I assumed it would make the rotation side spinning towards us brighter. But also redshift the side spinning away.
tomr_stargazer 2y ago
I think this may be a subtle reference to what is usually called "relativistic beaming" or "Doppler beaming" [0], though I normally associate this effect with matter that is moving quite close to the speed of light. [0]: https://en.wikipedia.org/wiki/Relativistic_beaming https://en.wikipedia.org/wiki/Relativistic_beaming
- like_any_other 2y ago
  perihelions [1] phrased it best - a galaxy spinning clockwise is just the mirror image of one spinning counter-clockwise. So why should mirroring change brightness? I don't really understand why Doppler beaming would cause this (though one of the papers did also mention it). The other question is - what does the rotation of our own galaxy have to do with it? Let's keep the Solar system as is, and mirror the rest of the Milky Way around it, so that it is now spinning the opposite direction. Why should this affect the apparent brightness of other galaxies? Especially since the Solar system is effectively moving in a straight line, on the scale of a human lifetime. [1] https://news.ycombinator.com/item?id=43535285 https://news.ycombinator.com/item?id=43535285
  - lostmsu 2y ago
    This is only obvious when you yourself are not moving on a curved trajectory. In GR curved trajectories lead to "weird" observations. For instance, I'd expect the side of the remote galaxy that is closer (visually, for you as an observer) to the center of your galaxy to be gravilensed slightly more than the other side. Because the effect is non-linear, it does not just compensate when sides are reversed. P.S. Not claiming this has any significant effect on the described phenomenon, just that mirror symmetry does not apply.
ordu 2y ago
Well, I don't know physics, but from I heard of it, there are two kinds of phenomena that are sensitive to a rotation. One is quantum particles, they have fixed spin, but won't delve into that topic, because I suspect that their "spin" is like the charm of the charm quark, probably physicists just liked the sound of a word and used it. But there is one other thing I heard about: electromagnetism. If you run charged particles in circles, they create magnetic field that is directed perpendicularly to the circles, and the magnetic field feels different depending on the side of the circles being observed. I see no way it influences the light emitted, but maybe I heard just too little of physics? BTW, does the direction of magnetic field of galaxies correlate with the direction of their rotation? edit: Ah, maybe magnetic field can polarize the light? And when you have two magentic fields they polarize in one direction or in two different, and maybe it influences observed brightness of the light? Or maybe it is just an uninformed guess?

ceejayoz 2y ago

One side is moving away from us, the other towards. This means their relative speeds are different, detectably so.

perihelions 2y ago
How are you linking that effect to the spin orientation of the Milky Way galaxy? Where is the all-sky anisotropy coming from?
anomaloustho 2y ago
I went to give a similar answer and realized that - in the instance that the two galaxies are sideways facing each other and with the text saying “it gets brighter” not “one side gets brighter” - I’m not sure if this is the actual answer. I can’t figure out why the Doppler Shift would make the entire galaxy brighter. I assumed it would make the rotation side spinning towards us brighter. But also redshift the side spinning away.

tomr_stargazer 2y ago

I think this may be a subtle reference to what is usually called "relativistic beaming" or "Doppler beaming" [0], though I normally associate this effect with matter that is moving quite close to the speed of light. [0]: https://en.wikipedia.org/wiki/Relativistic_beaming https://en.wikipedia.org/wiki/Relativistic_beaming

like_any_other 2y ago
perihelions [1] phrased it best - a galaxy spinning clockwise is just the mirror image of one spinning counter-clockwise. So why should mirroring change brightness? I don't really understand why Doppler beaming would cause this (though one of the papers did also mention it). The other question is - what does the rotation of our own galaxy have to do with it? Let's keep the Solar system as is, and mirror the rest of the Milky Way around it, so that it is now spinning the opposite direction. Why should this affect the apparent brightness of other galaxies? Especially since the Solar system is effectively moving in a straight line, on the scale of a human lifetime. [1] https://news.ycombinator.com/item?id=43535285 https://news.ycombinator.com/item?id=43535285
- lostmsu 2y ago
  This is only obvious when you yourself are not moving on a curved trajectory. In GR curved trajectories lead to "weird" observations. For instance, I'd expect the side of the remote galaxy that is closer (visually, for you as an observer) to the center of your galaxy to be gravilensed slightly more than the other side. Because the effect is non-linear, it does not just compensate when sides are reversed. P.S. Not claiming this has any significant effect on the described phenomenon, just that mirror symmetry does not apply.

ordu 2y ago

Well, I don't know physics, but from I heard of it, there are two kinds of phenomena that are sensitive to a rotation. One is quantum particles, they have fixed spin, but won't delve into that topic, because I suspect that their "spin" is like the charm of the charm quark, probably physicists just liked the sound of a word and used it. But there is one other thing I heard about: electromagnetism. If you run charged particles in circles, they create magnetic field that is directed perpendicularly to the circles, and the magnetic field feels different depending on the side of the circles being observed. I see no way it influences the light emitted, but maybe I heard just too little of physics? BTW, does the direction of magnetic field of galaxies correlate with the direction of their rotation? edit: Ah, maybe magnetic field can polarize the light? And when you have two magentic fields they polarize in one direction or in two different, and maybe it influences observed brightness of the light? Or maybe it is just an uninformed guess?