Comment by MattPalmer1086
4 hours ago
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.
2 replies →
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?