Submitted by **Sabre-Tooth-Monkey** t3_zyesvt
in **askscience**

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**Aseyhe**
t1_j26oxu4 wrote

Reply to comment by **swampshark19** in **How fast does the Milky Way spin? How far does Earth move through space in a year?** by **Sabre-Tooth-Monkey**

You might have misheard or been misinformed -- the impact of the extended galactic mass distribution (including dark matter) is that the orbital *velocity* remains approximately uniform over a wide range of radii (see again figure 16 of this review article). The orbital period does not.

Orbital periods are only uniform near the very centers of some galaxies (not ours, and mostly dwarf galaxies). That's actually a challenge to the standard dark matter picture (the core-cusp problem) because it requires that the system's density be uniform in the relevant region, which is not what dark matter simulations predict. But there are lots of proposed solutions to this.

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**swampshark19**
t1_j26t33z wrote

Thank you for educating me.

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**canineraytube**
t1_j277ik4 wrote

To what extent could it be said that the distribution of dark matter changes the effective dimensionality of our galaxy? I ask this because, in contrast to typical circular orbits in our 3+1 dimensional spacetime, which slow with increasing radius, all circular orbits around a given mass in 2+1 dimensional gravity (attenuating at 1/d^1) share the same velocity. Is this coincidental?

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**Aseyhe**
t1_j284skm wrote

That's correct that the orbits within the extended galactic mass distribution resemble orbits about a point mass in two dimensions (or an infinite line mass in 3D). In both cases the gravitational potential is logarithmic with respect to distance. That's coincidental, and I'm not familiar with any mathematical tricks that take advantage of the correspondence.

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