8
$\begingroup$

This question is relevant but it was speaking about a specific discovery reported by pieter van Dokkum, and the answer then mentioned it was disputed whether the paper's conclusion is even correct.

I want to know if there is a galaxy that has been discovered that is completely lacking in dark matter, or as close to lacking in dark matter to such a degree that it it is not certain that it even contains any dark matter at all. because that specific discovery was disputed and I am not sure how to navigate that dispute.

I don't know how scientists "detect dark matter", but based on my googling I think what they do is:

(1) make a mathematical prediction of the mechanics of some of the outer orbiting bodies in a galaxy using the mathematical apparatus of general relativity, and these calcs are based on the observable non-dark matter in the galaxy

(2) instrumentally measuring and recording the positions of those same orbiting bodies and then deducing their mechanics this way.

then comparing the mechanics deduced by the two methods, and depending on the difference in velocities concluded by both methods I guess they would be able to determine the "missing mass" that causes it?

So this question is also "Is there a galaxy in which the mechanics of the outer orbiting bodies' determined by both measurement and general relativity agree with each other perfectly, or almost perfectly?"

$\endgroup$

1 Answer 1

8
$\begingroup$

I don't work in this field, but it looks like the answer is "yes". The paper in question is here. See also 2018 preliminary results (linked to in your OP) and 2022 follow-up.

My impression after investigating the question for 10 minutes (i.e., a very short time) is that these galaxies without dark matter exist, but they're rare, and explainable with standard GR + Cold Dark Matter.

PS: One of the papers mentions "very low velocity dispersions" as a reason to believe there is no dark matter. You can find other ways we infer the existence of dark matter on Wikipedia.

$\endgroup$
3
  • $\begingroup$ The fact such a galaxy has been found proves that dark matter exists doesn't it?, in both cases GR was applied on galactic distances and masses and it worked for one while it didn't for the other, and whenever it doesn't work it always produces the basis to deduce the outer galactic bodies' orbital velocities as lower than they really are, both perfectly explained by dark matter. (this might be worth posting as a question) $\endgroup$
    – Hisham
    Apr 6, 2023 at 1:17
  • 1
    $\begingroup$ @Maalik it's a problem for both theories. Standard GR/CDM needs to be able to show that it can form galaxies without dark matter (because dark matter is needed to form structure, see en.wikipedia.org/wiki/Dark_matter#Structure_formation). Modified gravity needs to show they can explain the rotation curves, velocity dispersions, etc. of these galaxies too. $\endgroup$
    – Allure
    Apr 6, 2023 at 1:29
  • $\begingroup$ +1 Note though that, in these kinematical studies gravity is weak enough that GR isn't used; Newtonian gravity is fine. But when inferring mass from gravitational lensing studies, you need GR to (re)construct a map of the mass distribution. $\endgroup$
    – pela
    Apr 6, 2023 at 11:34

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .