So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies.
Link: https://www.space.com/19-galaxies-missing-dark-matter.html
Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.
Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter?
For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
They escape.
yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.
Dark matter looks/sounds hacky and broken, but in general matches real-world data better than the competing theories.
But the "dark matter" problem occurs for basically all galaxies, including things like elliptical and dwarf spheroidal galaxies that have no bulk rotation at all. (And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.) The same applies to groups and clusters of galaxies.
So? Have you calculated the relativistic effects of those incredibly complex mass currents and come to the conclusion that GR is not a sufficient explanation for the motion?
I’m sure there’s a high risk that GR can’t explain everything, but it sure is frustrating that people tend to grasp for these adhoc hypotheses without exhausting GR first.
This doesn't mean that you're wrong that the paper is wrong, but Dr. Hanson should absolutely not be our referent for certitude on the matter.
This reminds me of what happened with Sir Atiyah and his 'proof' on Riemann hypothesis few years ago. Sir Atiyah, despite of being a well-known mathematician (but in a different discipline), got absolute silence from researching community and experts when he proposed his proof on Riemann hypothesis and connection of fine structure constant of physics to mathematics.
If experts stay silent, if your work gets no follow-up, it's likely that what you said is wrong or has no value to people.
https://inspirehep.net/literature/690135
> Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.
Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters.
I couldn't find any non proof of concept papers that tested relativistic extensions of MOND for bullet-cluster-esq situations. Those proof of concepts tended to disagree greatly with reality, but thanks to the weird effects of (for instance) mass current models you'd expect to need a pretty accurate simulation to avoid compounding error. It certainly hasn't been ruled out.
That being said, while we haven't ruled out that some super weird interacting effect doesn't cause the apparent mass, we know that under a broad range of DM models the bullet cluster is normal. If you assume that apparent mass shows where actual mass is, pretty much all of the weird behavior goes away. It does suggest that so-called "hot" dark matter is unlikely, but that was indicated by cosmological surveys as well.
Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).