Dark Matter Fun
tbray.org
tbray.org
The fact that there seems to be galaxies without dark matter formed after high velocity collisions is clear indicator that dark matter is weakly interacting matter.
When two galaxies collide in high velocity, gases in these galaxies collide and slow down while most stars and dark matter continue with higher speed. Gases from new galaxies without dark matter, remaining galaxies have stars and dark matter but no gases.
A trail of dark-matter-free galaxies from a bullet-dwarf collision https://www.nature.com/articles/s41586-022-04665-6
The difficulty is in detecting. Weakly interacting massive particles are by definition hard to detect directly.
MaCHOs, modified gravity etc. match some, never all the evidence.
There has been no direct observation of Dark Matter, so saying "it matches all the evidence" may be redundant, since by definition, we said "Let's create something that matches all the evidence and look for it."
This is just wrong. This is not question of parameter tuning.
There is class of theories that include weakly interacting particles, you may tune or select model among these. But the most evidence (even newly discovered) points toward dark matter being particles because they behave like particles.
Sure, absence of evidence isn't evidence of absence, but you can search for star spangled swan only so much, before looking into alternatives.
Tell me one.
LZ detector. Do you need more?
With lensing we can "see" that dark matter (or whatever) passed through each other on a collision, while some "normal" matter actually collided and stopped in the middle (or slowed down).
Now, maybe "modified gravity" still makes sense somehow, but how come this modified gravity is following the previous path of a cluster in space like that?
This is as opposed to "The galaxies rotate too fast" argument, where the obvious problem is "Maybe it's rotating exactly as it should and you just don't know how fast it should rotate?"
This is a comment entirely about what's inside my head, not about the actual astronomical merit of any theory.
Now that I thought I finally got the point....
> 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.
What's wrong with that view?
6:50 or so for relevant piece.
For the folks who have spent their entire careers searching for dark matter, what would it take to convince them that dark matter may not exist? The search for ether ended with Michelson-Morley experiment. Is there no way of falsifying dark matter claims?
Just a better alternative that fits all the evidence. The problem is not scientists stubbornly hanging on to their pet theories and refusing to accept alternatives (though obviously they do have their pet theories), it's just a downright weird area of science.
Dark Matter is preposterous and unintuitive. It affects gravity just like a lot of mass, but it's totally invisible. We have no idea what it is or how it formed, but the majority of mass in the universe is somehow dark matter! Obviously there must be a simpler, more elegant explanation.
And yet, through numerous experiments, other theories get ruled out while dark matter survives.
This is the problem. There is a real possibility that what is "really" going on only fits some of the evidence and there are other phenomena at play. The explanatory power of "unseeable distribution of arbitrary matter" is dangerously high.
Forgive me, but shouldn’t it be, “the majority of gravity” we observe?
> through numerous experiments, other theories get ruled out while dark matter survives
We keep experimentally ruling out exotic modifications to gravity and relativity that don’t require extra mass, and dark matter theory continues because it makes the math work while it continues to fail experiment after experiment.
We’re like a kid in school being asked to show their work on a math problem, knowing the answer, and tacking “+3” at the end because it came up short. The gravity we get at the end is correct (we can see it) but we have no idea where that “3” actually comes from.
Yes you're right, or maybe more precisely, "if we assume dark matter is a form of matter that behaves like normal mass in all ways except that it doesn't interact with light, then it must form the majority of mass in the universe."
> and dark matter theory continues because it makes the math work while it continues to fail experiment after experiment.
The point is that it doesn't fail experiment after experiment. If it had, it would no longer be a valid theory. It's annoyingly robust.
> We’re like a kid in school being asked to show their work on a math problem, knowing the answer, and tacking “+3” at the end because it came up short.
Sort of, but you make it sound like scientists are making things up rather than just doing experimental science.
We can explain dark matter if we modify the laws of gravity by some factor. What is that factor? OK, so how would modification of those fundamental laws affect other things in the universe? Can we see evidence of this happening anywhere, or not happening where we think it should? OK, we observed something that disproves that factor within a certain range of values. Does that give us more information, how can we test it more, does it give us insight into how the theory can be tweaked? Etc.
No, I have a ton of respect for the process and experiments done so far.
I’m reminded of early attempts to understand Mercury’s orbit - there must be some massive body in the glare of the sun perturbing its orbit! (Is the extra body orbiting inside the sun, what is going on??) The possible explanations became increasingly out-there, until Einstein extended our theory of gravity and introduced spacetime.
I appreciate that we’ve tried and failed every which-way to modify gravity on large scales to make this work, but how close are we to saying we’ve tried every which-way to detect this supposedly normal mass?
There may be a deeper insight to the universe we’re missing, and it’ll take another general-relativity-spacetime-style revolution crashing into astrophysics before we figure it out.
[1] https://www.kicc.cam.ac.uk/research/cosmic-microwave-backgro...
Why, yes there is: http://www2.phys.canterbury.ac.nz/~dlw24/universe/wager.html
Euclid is scheduled to launch next year.
Science, for better or worse, is paradigmatic. The paradigm is strong and drives careers. Only when the paradigm is toppled and a new paradigm established does Science begin to advance in what will be ultimately seen historically as a leap or bound, metaphorically of course.
This sentence tripped me up. I’m basically only familiar with ‘dubious’ meaning ‘unreliable’ however it can also mean doubtful (as in describing someone with doubts) and I think that is the meaning here.
post video edit: Dr. Becky inexplicably spends more than half the video explaining MOND, and if the evidence is not exactly spanking new, the paper is. The video I link visually explains the evidence found in the paper. MOND does not explain it, so I'm not sure why Dr. Becky distracted us with it, except that it is a more well known theory than External Field Effect. I believe the EFE theory to be correct, and painfully obviously so, once the toy model in the video linked[1] is understood. Dark Matter loses a lot of ground when it is unnecessary to explain galaxy rotation curves, the initial reason for it's existence.
> where the anomaly in the rotation of galaxies (which is what dark matter is needed to explain) can be explained by the gravitational pull of other nearby galaxies.
Is it possible producers of the 2013 video I linked above were unaware of EFE Theory (esp. if it appeared in 1983)? I do not see MOND necessary anywhere in the video I linked, personally think MOND is bollocks, but the narrator seems to have self-referred to the theory described as External Gravitational Effect (I think) at least once. I have to watch it again, it's very boring so I may have missed reference to EFE, or it may be unrelated and just similar, or the same and described from different perspectives, but on the surface, they appear the same. I've just never heard of EFE, not that surprising, but I don't really know for sure if it is the same as what's in that video I harvested from a Slashdot comment about 6 years ago.
Theories of modified gravity generically violate the
strong equivalence principle, so that the internal
dynamics of a self-gravitating system in free fall
depends on the strength of the external gravitational
field (the external field effect). We fit rotation
curves (RCs) from the SPARC database with a model
inspired by Milgromian dynamics (MOND), which relates
the outer shape of a RC to the external Newtonian
field from the large-scale baryonic matter
distribution through a dimensionless parameter eN.
We obtain a >4σ statistical detection of the external
field effect (i.e. eN>0 on average), confirming
previous results. We then locate the SPARC galaxies
in the cosmic web of the nearby Universe and find a
striking contrast in the fitted eN {values} for
galaxies in underdense versus overdense regions.
Galaxies in an underdense region between 22 and 45
Mpc from the celestial axis in the northern sky have
RC fits consistent with eN≃0, while those in
overdense regions adjacent to the CfA2 great wall
and the Perseus-Pisces supercluster return eN that
are a factor of two larger than the median for SPARC
galaxies. [...]
[0] IIRC this wasn't really true of the initial form that only explained galactic rotation anomalies, but every other anomaly we credit Dark Matter for has required more and more tweaking to explain with MOND.
Yes, why not start with the widely accepted extension called general relativity? I understand GR most likely does not explain everything, but that doesn’t mean you can take the bits and pieces it does explain and use those to underpin your pet theory…
All this makes me a bit skeptical of the whole area to be honest.
Read the "Observational evidence" section; it has a great overview of the current mainstream arguments in dark matter cosmology. (I don't link this to be snarky—it's a very good Wiki article and covers almost all the basic questions.)
Typical cosmological models assume homogeneity, which IMO is a poor assumption with the lumpy universe we live in. I'm a fan of more exactly applying known models before inventing new terms!
https://link.springer.com/article/10.1140/epjc/s10052-021-08...
...not to be confused with the hypothesis that regular magnetism is the source of the unexplained measurements of the galactic rotation curves:
Is it possible for a regular person(regular meaning not a researcher i guess, someone with unrelated day job) to understand the math and this kind of stuff at a deeper level? How would you go about getting started?
I just read a book that gave a high level overview of some astronomy and physics topics like this one, but im unsure how to get a deeper understanding or if its even possible.
https://en.m.wikipedia.org/wiki/Electroweak_interaction
I wonder if gravity devolves into separate forces at very low energy. Maybe attractive gravity and repulsive gravity. Attractive gravity could draw things together, warping space into wells, like on the 2-D abstractions used to describe the gravity well if the sun versus a black hole. Repulsive gravity could create “hills” that cause things to “roll down the hill”, giving the appearance of things moving away from each other.