5 ms·

If we were on the other side of those galaxies, wouldn't they look like they were spinning counter-clockwise? Or are they measuring spin some other way?

by snitty 2y ago

scribu 2y ago
The point is that you’d expect a roughly even distribution of clockwise and counterclockwise spins, not all of them to rotate in the same direction.
- throwawaymaths 2y ago
  wouldn't it be the case that you would see almost exactly 50/50 if all galaxies had parallel axes and rotated in the same absolute direction?
- 1oooqooq 2y ago
  why? if you subscribe to big bang then all matter got the same "initial kick". would be easier to assume same spin?
  - aurareturn 2y ago
    So what caused the "initial kick" to favor one side?
    - Aardwolf 2y ago
      What causes a perfectly symmetric ball on top of a perfectly symmetric hill to roll down via one side? (Probably quantum randomness if everything else is perfectly symmetric)
    - pixl97 2y ago
      What caused this universe to favor matter over anti-matter? So many unanswered questions.
  - mnky9800n 2y ago
    From my understanding, the big bang requires that the proto-universe was in a completely homogenous state that was then pushed out of that equilibrium for some reason. But that reason doesn't require non-zero angular momentum. It only requires that a the proto-universe was homogenous and now the universe isn't. And that is what separates pre and post big bang. I could be wrong, I am not a cosmologist. Would be happy to hear from one though.
iainmerrick 2y ago
I was wondering the same thing -- "direction of spin" is ambiguous on its own, you also need to pick which direction is up. But if objective spin directions are roughly evenly split because the universe is isotropic, the spins from our viewpoint ought to be evenly split as well. If they're not evenly split, the universe must have a preferred axis, which would be an amazing discovery. I guess if the preferred axis just happens to align with our own galaxy, that would support the alternative theory that it's due to an observation effect such as doppler shift. Either way, it's incredibly cool to have such a simple but totally unexpected observation pop up out of nowhere.
RotationPedant 2y ago
That is correct, "clockwise" only makes sense relative to a single observer: on Earth we set up out coordinate system so that the Milky Way's directed axis of rotation points one way, and most galaxies have it pointing the other way. "Clockwise / counterclockwise" makes sense for images coming from telescopes but it's not cosmologically meaningful. Note that this is not that easy to determine: When done manually, the determination of the direction of rotation of a galaxy can be a subjective task, as different annotators might have different opinions regarding the direction towards a galaxy rotates. A simple example is the crowdsourcing annotation through Galaxy Zoo 1 (Land et al. 2008), where in the vast majority of the galaxies different annotators provided conflicting annotations. Therefore, the annotations shown in Fig. 1 were made by a computer analysis that followed a defined symmetric model (Shamir 2024e). The point is that we would typically assume a 50-50 ratio regardless of where you are in the universe.
Patient0 2y ago
The actual paper makes more sense: "the number of galaxies in that field that rotate in the opposite direction relative to the Milky Way galaxy is ∼50 per cent higher than the number of galaxies that rotate in the same direction relative to the Milky Way."

If we were on the other side of those galaxies, wouldn't they look like they were spinning counter-clockwise? Or are they measuring spin some other way?

by snitty 2y ago

If we were on the other side of those galaxies, wouldn't they look like they were spinning counter-clockwise? Or are they measuring spin some other way?

scribu 2y ago

The point is that you’d expect a roughly even distribution of clockwise and counterclockwise spins, not all of them to rotate in the same direction.

throwawaymaths 2y ago
wouldn't it be the case that you would see almost exactly 50/50 if all galaxies had parallel axes and rotated in the same absolute direction?
1oooqooq 2y ago
why? if you subscribe to big bang then all matter got the same "initial kick". would be easier to assume same spin?
- aurareturn 2y ago
  So what caused the "initial kick" to favor one side?
  - Aardwolf 2y ago
    What causes a perfectly symmetric ball on top of a perfectly symmetric hill to roll down via one side? (Probably quantum randomness if everything else is perfectly symmetric)
  - pixl97 2y ago
    What caused this universe to favor matter over anti-matter? So many unanswered questions.
- mnky9800n 2y ago
  From my understanding, the big bang requires that the proto-universe was in a completely homogenous state that was then pushed out of that equilibrium for some reason. But that reason doesn't require non-zero angular momentum. It only requires that a the proto-universe was homogenous and now the universe isn't. And that is what separates pre and post big bang. I could be wrong, I am not a cosmologist. Would be happy to hear from one though.

iainmerrick 2y ago

I was wondering the same thing -- "direction of spin" is ambiguous on its own, you also need to pick which direction is up. But if objective spin directions are roughly evenly split because the universe is isotropic, the spins from our viewpoint ought to be evenly split as well. If they're not evenly split, the universe must have a preferred axis, which would be an amazing discovery. I guess if the preferred axis just happens to align with our own galaxy, that would support the alternative theory that it's due to an observation effect such as doppler shift. Either way, it's incredibly cool to have such a simple but totally unexpected observation pop up out of nowhere.

RotationPedant 2y ago

That is correct, "clockwise" only makes sense relative to a single observer: on Earth we set up out coordinate system so that the Milky Way's directed axis of rotation points one way, and most galaxies have it pointing the other way. "Clockwise / counterclockwise" makes sense for images coming from telescopes but it's not cosmologically meaningful. Note that this is not that easy to determine: When done manually, the determination of the direction of rotation of a galaxy can be a subjective task, as different annotators might have different opinions regarding the direction towards a galaxy rotates. A simple example is the crowdsourcing annotation through Galaxy Zoo 1 (Land et al. 2008), where in the vast majority of the galaxies different annotators provided conflicting annotations. Therefore, the annotations shown in Fig. 1 were made by a computer analysis that followed a defined symmetric model (Shamir 2024e). The point is that we would typically assume a 50-50 ratio regardless of where you are in the universe.

Patient0 2y ago

The actual paper makes more sense: "the number of galaxies in that field that rotate in the opposite direction relative to the Milky Way galaxy is ∼50 per cent higher than the number of galaxies that rotate in the same direction relative to the Milky Way."