<p>I was wondering what we would observe if the universe had a small rotation. My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than polar galaxies. Has anyone ever looked for a directional anisotropic redshift effect? If there was such an effect, sky surveys of Hubble redshift would be a bit messed up if they contained data from random directions that did not take this effect into account.</p>

CC BY-SA 4.0
2024-06-05T16:59:22.860Z
"My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than polar galaxies. " Why?
ProfRob
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2024-06-05T17:43:58.133Z
I was thinking of something analogous to an observer standing at the centre of a rotating carousel. They would see sources on the perimeter of the carousel as redshifted. I might be wrong, but I imagine an observer located above the axis of rotation of a galaxy would see see light from stars on the perimeter of the galaxy as redshifted due to the time dilation of the tangential motion as well as the normal transverse Doppler effect.. The effect would of course require the equatorial galaxies to be rotating relative to a non rotating spacetime background...
KDP
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2024-06-05T17:44:12.110Z
..  Centrifugal force would also cause peculiar motion outwards over and above the outward Hubble flow. Both factors cause time dilation that in turn contributes to the magnitude of the redshift.
KDP
CC BY-SA 4.0
2024-06-05T18:13:44.780Z
We definitely do see the CMB as blueshifted in one direction and redshifted in the other ("CMB dipole anisotropy") due to the movement of Earth, the movement of the Sun, the movement of the Milky Way, and so on.  It doesn't directly address your question but we are certainly aware of anisotropies in redshift and have to account for them. https://astronomy.stackexchange.com/questions/20880/true-motionlessness-red-shifts-and-cmb
Darth Pseudonym
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2024-06-06T19:23:26.997Z
A rotating universe is definitely not isotropic and hence in violation of the cosmological principle (which is not a physical theorem).
Walter
<astrophysics><cosmology><observable-universe><hubble-constant><sky-survey>
Would we know if the universe was rotating?
KDP
# Would we know if the universe was rotating?

(Asked by: username_0 on 2024-06-05)

I was wondering what we would observe if the universe had a small rotation. My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than polar galaxies. Has anyone ever looked for a directional anisotropic redshift effect? If there was such an effect, sky surveys of Hubble redshift would be a bit messed up if they contained data from random directions that did not take this effect into account.

**Tags:** astrophysics, cosmology, observable-universe, hubble-constant, sky-survey

> **Comments:**
> **username_1**: "My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than polar galaxies. " Why?  
> **username_0**: I was thinking of something analogous to an observer standing at the centre of a rotating carousel. They would see sources on the perimeter of the carousel as redshifted. I might be wrong, but I imagine an observer located above the axis of rotation of a galaxy would see see light from stars on the perimeter of the galaxy as redshifted due to the time dilation of the tangential motion as well as the normal transverse Doppler effect.. The effect would of course require the equatorial galaxies to be rotating relative to a non rotating spacetime background...  
> **username_0**: ..  Centrifugal force would also cause peculiar motion outwards over and above the outward Hubble flow. Both factors cause time dilation that in turn contributes to the magnitude of the redshift.  
> **username_2**: We definitely do see the CMB as blueshifted in one direction and redshifted in the other ("CMB dipole anisotropy") due to the movement of Earth, the movement of the Sun, the movement of the Milky Way, and so on.  It doesn't directly address your question but we are certainly aware of anisotropies in redshift and have to account for them. https://astronomy.stackexchange.com/questions/20880/true-motionlessness-red-shifts-and-cmb  
> **username_3**: A rotating universe is definitely not isotropic and hence in violation of the cosmological principle (which is not a physical theorem).  

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