NOND and alternatives are less and less likely with more evidence.
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.
They are all faint galaxies, so any estimates on their baryonic mass are in the "challenge your assumptions" regime. And that "line" is pretty spurious, if I'm reading the paper correctly they are saying the chance of the line happening randomly is 0.6% -- not very low, the sky is a huge place. the bullet galaxy that has the "rest of the dark matter" hasn't been found.
So far there is nothing saying old models are wrong. Only the mainstream dark matter hypotheses that matches all the evidence is some type of particle that is not interacting.
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.
It matches all the evidence because we looked at the available evidence and highly tweaked the model until it matched.
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."
>It matches all the evidence because we looked at the available evidence and highly tweaked the model until it matched.
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.
What if the effect is dependent on the concentration of high mass objects like the stars themselves? Instead of the stars being embedded in dark matter they are in a field that modifies gravity.
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.
> 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.
That's funny. The parent's NASA link uses the high relative velocities as "direct evidence for dark matter and supports the view that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity." The lack of friction seems obvious with this view of limited interaction between dark matter particles.
Well, ok, but that means there's still _less_ friction with those particles because they don't interact electrically, right? So I still would expect the same result of the 4 blobs in different positions in space.
> For their part, the (large majority) who think the Dark Matter is out there and we just have to measure harder are serenely confident.
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?
> what would it take to convince them that dark matter may not exist?
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.
Exactly - often people forget the 'dark' in 'dark matter' just means 'we have no idea what it is'! All dark matter researchers would love for it to become '<x> matter'. It's not like dark matter is an ideal explanation, as another comment says it's more like a remainder term in an equation that we don't know the origin of.
> Just a better alternative that fits all the evidence.
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.
There's no danger it will be left at a vague "unseeable distribution of arbitrary matter", if that's what you mean. Every astrophysicist studying in the field of dark matter wants to know exactly what it is, even if it's an artefact caused by multiple different unknowns.
> but the majority of mass in the universe is somehow dark matter!
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.
> Forgive me, but shouldn’t it be, “the majority of gravity” we observe?
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.
> 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
> you make it sound like scientists are making things up rather than just doing experimental science
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.
My understanding is that it's still the most parsimonious theory that explains everything, including (especially?) the CMB power spectra[1]. Even TeVeS still requires some dark matter to fit all the phenomena, just not as much as LCDM.
Disappointing, yes. Except this is the norm in science if we look back historically. Theories that were accepted for hundreds of years are replaced with more accurate representations through the scientific method. For example, planetary epicycles.
> Except this is the norm in science if we look back historically.
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.
> The people who find MOND interesting are dubious.
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.
Dr. Becky has a good video on new evidence against dark matter [1] that I find super interesting, especially new evidence for the External Field Effect, 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.
I suppose when science is based on paradigms and paradigms propped up by bias, a decade old is really pretty new.[1]
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.
The paper I was referring to is from 2020. She brings up MOND because she's explaining the alternatives to dark matter, and EFE is (as far as I know) an extension to MOND that was postulated Milgrom back in 1983.
Then I may have incorrectly assumed the video I linked explains EFE, and I'm not sure if the theory described in the video has a name, but it uses what I would call tidal effect to explain galaxy rotation curves, which sounded to me a lot like:
> 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.
I don't know what that video is describing, relative to EFE. The paper [1] abstract describes EFE and their findings this way:
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. [...]
As I understand it, the real dubious thing about MOND, and what makes most physicists discount it, is that it looks a lot like an exercise in curve-fitting[0]. Maybe that'll actually lead to an explanation of reality, but in general physicists are like programmers: they like elegance. They want to start with a simple equation or modification and, after applying all the math, find that it suddenly explains a bunch of data that never fit the old model and makes testable predictions.
[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.
I remember reading about an interesting paper that could explain some weird astrophysics by just correctly applying general relativity. The explained weirdness came from mass currents. Strangely I never hear about this when dark matter is discussed. It’s always “Newton mechanics has to be extended somehow”.
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.)
Are you thinking of Timescape Cosmology [0]? I think it's a great explanation for dark energy that should be further explored. A summary is that due to the large difference in gravitational gradients between in-a-galaxy and between-galaxies, it appears that intergalactic space is expanding, but in reality it's just progressing at a faster rate of time due to general relativity, thus appearing to inflate from our perspective.
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!
> has a nice little introduction and the math in the overview isn’t too scary.
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.
I am wary of the rotation curves not as evidence of dark matter but pedagogically. Looking at those a long time ago I came to think that a galaxy has dark matter in it while really it's more like dark matter has a galaxy in it.
I understand that in the very early universe, the 4 fundamental forces (strong nuclear, weak nuclear, gravity, electromagnetism) weren’t all separated. At very high energies, the weak nuclear force and electromagnetism acted as a single electroweak force.
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.
No, obviously not. Previous theories without dark matter (or MOND) aren’t sufficient to explain what we can observe. So additional complication is necessary.
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