Hacker News Reader

Comment by zardo | Hacker News Reader

What's so illogical about it? Galaxies are spinning as though there's matter that we don't see. Maybe there's matter that we don't see.

View on HN

The fun question to ask is why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them. That dark matter is organising itself to produce the same outcome over many different circumstances!

> why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them

They don't. You can read exactly this article for 3 examples.

Modified Newtonian theories are anything but simple.

If you read the rebuttal by Professor McGaugh, you can include those 3 as perfectly good examples of MOND too.

You do make a good point though - astronomy and astrophysics is hard. There are often many ways of interpreting the observations we have. They can be misleading and take careful interpretation, particularly when they seem unusual in some way.

Do all galaxies spin in the same way? Do we predict an equal proportion of dark matter in all galaxies based on observations? Or is there some variability?

Yep, pretty much. They all have a flat rotation curve (which makes no sense unless gravity works differently or there is unseen mass), and that rotation correlates to the visible matter in the galaxy (the Tully Fischer relation).

Wouldn't we expect some galaxies to have more dark matter than others? Although, I guess if anything is big enough to converge to an average, a galaxy is.

Apparently they all have just the right amount arranged in the right way to give the same result in each case.

Isn't that weird?

Visually galaxies are quite varied, which implies, to me, they have a lot of variability in visible matter. Why wouldn't their supposed dark matter vary too?

They do! In particular, there are three of them out there that match expectations and thus have little/no dark matter! NGC 1052-DF2 (2018) NGC 1052-DF4 (2019) and AGC 114905 (2022)! Apparently there's another being discussed in a 2026 paper!

Nope, there are galaxies that spin exactly as our current model predicts they should, while others are way off, as if there was something unevenly distributed across them. There's no simple modification to our theories that can account for these discrepancies.

There are always exceptions that need additional explanation. The majority do spin exactly as the Tully Ficsher relation suggests. What specific examples are you thinking of?

I have yet to see a model of galaxy rotation that doesn't use the shell theorem, which does not apply to disks. I've seen it in very advanced (over my head) math stuff on Wikipedia.

Im half convinced its gonna be a stupid mistake like that. Whoever finds the correct "advanced" math that allows the mistake to be brushed under the rug could get a Nobel prize.

It feels unlikely that the pros have not just tried numerical integration over a disk. Do they really not ever do that?

Maybe there is, maybe there isn't. Maybe it's an un-acknowledged aspect of physics that we haven't properly discovered yet.

Assuming it's definitely matter, and operating on that basis for nearly 50 years, is not good science, period.

Reply on news.ycombinator.com