<p>According to accepted astronomical wisdom, dark matter represents a large fraction of the total mass of the Milky Way.  And thanks to Papa Einstein, we know that light rays can be bent when passing through spacetime distorted by a mass distribution. Thus, does the presence of dark matter in and around our galaxy significantly distort measurements of parallax and the distances calculated therefrom?</p>

CC BY-SA 4.0
2022-05-17T07:34:37.510Z
Interesting question. Yet I believe: If it was distorting parallax a lot, it would mean it's clumpy - and that would be well visible in the dynamics of the visible matter on small(er) scales.
planetmaker
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2022-05-17T12:39:34.677Z
@planetmaker Why does a large distortion imply clumpiness? It just implies a lot of mass which could be either clumpy or evenly distributed.
zephyr
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2022-05-17T12:50:55.407Z
@zephyr as the parallax implies the annual variation of the apparent position, a constant distortion of the true position doesn't matter. But the curvature difference must be large enough that it amplifies measurably the apparent position change due to a difference to the light path. Absolute position change due to large-scale masses but low gradients in curvature does not matter.
planetmaker
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2022-05-17T14:07:09.823Z
@zephyr uniformly distributed dark matter would lead to a net path 'bending' of zero.
Carl Witthoft
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2022-05-17T16:01:30.627Z
To put it another way, a point source would deflect a photon one way, another point source in the right place would bend it back.  So uniformly distributed mass would attempt to deflect it in every direction, resulting in no deflection.
Greg Miller
<general-relativity><dark-matter><parallax>
Does the presence of dark matter in the Milky Way significantly distort our parallax based measurements of distance?
MarkVonTexas
# Does the presence of dark matter in the Milky Way significantly distort our parallax based measurements of distance?

(Asked by: username_0 on 2022-05-17)

According to accepted astronomical wisdom, dark matter represents a large fraction of the total mass of the Milky Way. And thanks to Papa Einstein, we know that light rays can be bent when passing through spacetime distorted by a mass distribution. Thus, does the presence of dark matter in and around our galaxy significantly distort measurements of parallax and the distances calculated therefrom?

**Tags:** general-relativity, dark-matter, parallax

> **Comments:**
> **username_1**: Interesting question. Yet I believe: If it was distorting parallax a lot, it would mean it's clumpy - and that would be well visible in the dynamics of the visible matter on small(er) scales.  
> **username_2**: @username_1 Why does a large distortion imply clumpiness? It just implies a lot of mass which could be either clumpy or evenly distributed.  
> **username_1**: @username_2 as the parallax implies the annual variation of the apparent position, a constant distortion of the true position doesn't matter. But the curvature difference must be large enough that it amplifies measurably the apparent position change due to a difference to the light path. Absolute position change due to large-scale masses but low gradients in curvature does not matter.  
> **username_3**: @username_2 uniformly distributed dark matter would lead to a net path 'bending' of zero.  
> **username_4**: To put it another way, a point source would deflect a photon one way, another point source in the right place would bend it back.  So uniformly distributed mass would attempt to deflect it in every direction, resulting in no deflection.  

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## 0 answers

