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nemo136 19 hours ago [-]
Idea for a science fiction short story: dark matter interacts with normal matter only in specific conditions requiring advanced science, with extremely rare occurrences naturally.
Humanity, after migrating from biological carbon based substrate, finds ordinary matter not practical to live in. Dark matter has much better properties.
All humanity transfers to dark matter where it finds all the other "ascended" species historically hosted by our universe. Solves Fermi's paradox and keeps the "normal matter" nursery clean for new species to appear.
M95D 5 hours ago [-]
From the article:
> That all three gossamer galaxies existed along a line, as neighbors, didn’t escape the attention of astrophysicists, who began to coalesce around an origin story.
How about this:
What we see is an advanced galaxy-killer alien weapon being fired at one target, and then continuing to travel beyond that target and making 2 more (unintended?) victims.
munchler 18 hours ago [-]
This is a cool idea.
I was tempted to comment that there’s no evidence that you can make anything complex out of dark matter, but it turns out that some physicists hypothesize that “self-interacting” dark matter could bind into something like atoms and molecules, so carry on!
cwillu 15 hours ago [-]
It's worth noting in passing that “some physicists hypothesize” is entirely compatible with “there’s no evidence”; this should not be disqualifying for science fiction.
sonicvroooom 15 hours ago [-]
Curious indeed. But that "Humanity" part is a bit stale. I'd make it the not too far end after the current or next speciation event ... then "time travel" can get it's more or less natural and scientific manifestation when that ascended gene pool somehow suddenly notices that genomes from their archives are born to non-descendant parents .. [ spontaneous genetic recurrence, probabilistic replication, or a wild-type convergence? *had to use AI to pin down these terms, I'm tired ]
2 hours ago [-]
blauditore 20 hours ago [-]
For someone like me who doesn't know much about modern physics, dark matter has the vibe of a band-aid to me. Like a programmer who discovered that the algorithm was always off by 1 in the results, and just ended up returning r + 1 without thinking twice.
I'd bet a large chocolate cake that one day, physicists will discover a much more logical explanation for this unexpected behavior of matter.
nwallin 17 hours ago [-]
So--first thing's first. Scientists know Dark Matter is not complete. Consider the progress we've made from the Copernican model of the solar system, to the Kepler model, to Newtonian gravity, to Einstein's theory of gravity. Right now, our understanding of Dark Matter is at the Kepler stage. We know what it's doing. We do not know why it is doing it.
But we have plenty of observations to lead us to what dark might be and what it definitely isn't.
Any theory of dark matter needs to explain all of the following:
1. The Virial Theorem, galaxy clusters, and the speeds of galaxies in clusters. (both MOND and CDM both do a good job of explaining this)
2. Galactic rotation curves. (MOND and CDM both do a good job of explaining this)
3. Accoustic Baryon Oscillations. (ABO) Immediately after the big bang, everything was ludicrously hot. The most powerful force in the universe was the wash of gamma rays everywhere. These gamma rays carried electrons around. The electric fields of the electrons carried the protons around. (the light dominated phase) As the universe cooled, this flipped; the protons were carried around by momentum, which brought the electrons with them, which mediated the light. (the matter dominated phase, where we are today) During the light dominated phase, sound waves pulsed through the universe, causing dense regions and underdense regions. When the universe flipped from being light dominated to matter dominated, these regions were frozen in time, and we can see them directly in the CMB. (cosmic microwave background) We can analyze the properties of these pressure waves to determine what the properties of the matter and light in the universe were. These waves demand some sort of dark matter. CDM explains these observations, MOND does not.
4. The Bullet Cluster. The Bullet Cluster is a collision of two galaxy clusters. We can measure gravitation lensing and light to determine where the dark matter is and where the regular matter is. The bullet cluster demonstrates that these two properties are not the same. There is matter which has gravity and gravitational lensing, but that we can't see. CDM explains these observations, MOND does not.
5. Ultra diffuse galaxies with very little dark matter. Not all galaxies have galactic rotation curves which require some extra sauce to explain the movement of their stars. Some galaxies are explained perfectly by the matter that we can see in them. So we need dark matter for most galaxies, but some galaxies need to be dark matter free. These galaxies falsify MOND.
6. Leading spiral arms in barred spiral galaxies. The large majority of barred spiral galaxies have their arms trailing from their bars, but a very small number of them have arms which lead in front of the bars. Computer simulations show this happens naturally with CDM, but MOND or dark matter free spiral galaxies don't have a mechanism to cause this to happen.
7. Gravitational waves travel at the speed of light. Most MOND formulations predict gravitational waves travel slower than c; all of those formulations are wrong.
There is a genuinely absurd amount of time, effort, and money that has gone into explaining dark matter. It is not a band aid, we haven't just added 1 to moved on. Everybody is pouring over every step of the algorithm trying to figure out why it's giving us the wrong answer. CDM is the leading hypothesis right now, because it explains all of the observations, and is not falsified by any of the observations. MACHO is/was another theory; it has gone out of fashion because it does not explain all of the observations and makes predictions we do not observe when we check for them. MOND is out of fashion because it is falsified by the data. MOND is still in fashion among a particular brand of people who just want to have their names in the history books; if someone did prove MOND, the history books would all have lists that go "Ptolemy, Copernicus, Kepler, Newton, Einstein, <your name here>."
But for Pete's sake it's not as if people have their fingers in their ears.
explanation of acronyms/terms:
1. Dark Matter. Dark Matter is the observation that the universe acts as if there's a lot more matter in it other than the stuff we can see. Basically no one says that Dark Matter isn't real; that would imply that the observations are wrong.
2. MOND. Modified Newtonian Gravity. MOND is a family of models which aim to explain the dark matter observation. Instead of using F=Gm1m2/r^2 to determine the force of gravity, we have better math, that gives a different equation, that accurately explains Dark Matter. When you say "physicists discover a much more logical explanation" that's what you're talking about.
3. CDM. Cold Dark Matter. CDM is a family of models which aim to explain the dark matter observation. CDM implies that Dark Matter is explained by particles. CDM implies there is some sort of particle of family of particles which interact with normal matter via gravity as normal, via the electromagnetic or strong nuclear force not at all, or via the weak nuclear force super weakly or not at all. CDM interacts with other CDM particles via gravity as normal, but only weakly via either the weak nuclear force or some other as-yet undiscovered force. People often use the terms Dark Matter (referring to the observation) and Cold Dark Matter interchangeably.
4. CMB. Cosmic Microwave Background radiation. After the Big Bang, the universe was opaque. Light would be absorbed by the stuff in the universe and then re-emitted. During recombination, the universe became transparent. The last moment of the various bits and parts of the universe switching from being opaque to transparent has the entire universe glowing in microwaves. This moment is captured in a snapshot, known as the cosmic microwave background. As we understand the CMB better, we learn a lot--a lot a lot--about the early universe.
schiffern 7 hours ago [-]
My pet (wrong but fun!) theory is some sort of lensing effect could effectively "confine" gravity within the galactic plane. This changes the large-scale propagation law from 1/r^2 to 1/r,[0] exactly what we observe in galactic rotation curves, while avoiding the cushy halo problem.[1] So far so good.
Globular clusters should spherically self-lens too, explaining apparent CDM.
However it does neatly explain these ultra-diffuse galaxies. They're not sufficiently dense to self-lens so they can't "get the flywheel going" to ramp up their stellar density and self-gravity-lensing, to shape the large scale structure.
In effect, these ultra-diffuse galaxies would be right on the edge of "failed galaxies," in the same way Jupiter-size objects are sometimes considered as failed stars.
--
EDIT I admit CDM irks me because it has (effectively) unlimited tuning parameters, and what's more, you can tune them differently per object! As the history of science shows us, Here There Be Dragons...
Thank you for this dark-matter-theories-in a nutshell comment! I learned a lot!
MattPalmer1086 17 hours ago [-]
TeVes (one relatistic formulation of MOND) predicts gravity propagation at different speeds than light. This is not a general result of MOND theories.
sonicvroooom 15 hours ago [-]
> Immediately after the big bang, everything was ludicrously hot. The most powerful force in the universe was the wash of gamma rays everywhere.
Classic obfuscation method. That remaining static noise gave it away. Somewhere out there is a Savant who reads it like Tank and Link read the Matrix' machine code--blonde, brunette, redhead.
KingMob 8 hours ago [-]
Can you expand more on #5?
It seems that having some galaxies without dark matter is also pretty bad for CDM. Why would CDM be absent in those cases? Without an explanation, isn't it bad for CDM (as well as MOND)?
Nobody wants a theory where exceptions can be hand-waved away as "Oh, that galaxy must not have DM" or "That galaxy has too much/too little/different balance/etc".
schiffern 7 hours ago [-]
Right. MOND has one tuning parameter, but effectively CDM has infinite tuning parameters, and (worse) they can be tuned on a case-by-case basis.
potamic 7 hours ago [-]
What the article above posits, that dark matter could be stripped out from a galaxy after a collision.
KingMob 6 hours ago [-]
Right, I read that as a possible explanation, but that's not the root explanation.
Roughly, my question is "Don't we see other post-collision galaxies that still have dark matter, and if so, why?"
If we have two identical scenarios with different outcomes under a theory, that doesn't bode well for the theory.
stinkbeetle 9 hours ago [-]
Great post. Very interesting though the physics is well above my head.
> if someone did prove MOND
How could MOND be proven if it is disproven? Is there a chance it is not really disproven, or you mean the equations for forces might still be modified in a way that fits observations? Or that it could be MOND plus something else?
> When you say "physicists discover a much more logical explanation" that's what you're talking about.
I wonder what is illogical or band-aid feeling about matter that interacts with gravity but not other forces though. Why would all types of matter interacting with electromagnetism and strong nuclear forces be logical
metalman 5 hours ago [-]
you left out 'other', which is important because our capacity to observe what is happening and test theories as to why, when measured against the scale of discrepancies, isn't that great, and I think it is just good policy to point out just how maddening it is to have missed most of the universe in our observations of it, and how by default this gap places ALL of the rest of our theorising on the shakiest of footings, hence as you point out, the frantic efforts to figure it out from observable evidence, as there is zero possibility of selling the idea of another collider that is yet one order a magnitude bigger than the last.
Though I think that there is solid support for very large space based telescopes where this kind of science could ride beside other more popular missions, as photons are photons, and no one is going to argue about how precisly they are measured or defined as long as there are pretty pictures of things no one has seen before.And if the optical platform needs to be stabilised to prevent the shaking caused by gravitational waves, so be it;)
auntienomen 18 hours ago [-]
Dark matter is what we'd get if we had a lot of stable particles that didn't interact via electromagnetism, but only by gravity and short range forces.
The standard model has lots of this kind of thing. Leptons don't feel the strong force. Neutrinos are electrically neutral. The weak force only couples to left-handed particles. It seems completely natural to expect some dark matter. The only question is how much.
samirillian 17 hours ago [-]
the Milky Way is pretty ghostly have we considered that we may lack dark matter
Shitty-kitty 4 hours ago [-]
It is notoriously difficult to observe the galaxy from the inside. We know much more about other galaxies then our own.
zardo 20 hours ago [-]
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.
uoaei 17 hours ago [-]
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.
MattPalmer1086 19 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!
marcosdumay 18 hours ago [-]
> 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.
MattPalmer1086 17 hours ago [-]
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.
17 hours ago [-]
Buttons840 19 hours ago [-]
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?
MattPalmer1086 19 hours ago [-]
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).
Buttons840 17 hours ago [-]
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.
MattPalmer1086 17 hours ago [-]
Apparently they all have just the right amount arranged in the right way to give the same result in each case.
Buttons840 15 hours ago [-]
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?
solyin 13 hours ago [-]
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!
thrance 17 hours ago [-]
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.
MattPalmer1086 16 hours ago [-]
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?
phkahler 19 hours ago [-]
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.
nyeah 17 hours ago [-]
It feels unlikely that the pros have not just tried numerical integration over a disk. Do they really not ever do that?
analog31 20 hours ago [-]
I'm not a cosmologist, but the Wikipedia article on dark matter has a pleasantly readable summary of the main sources of evidence for dark matter at this time.
whatshisface 18 hours ago [-]
Another way to look at it is that the rules of gravity already have a rule for "stuff falls down," and adding stuff is much less extraordinary of a band-aid than changing "falls" or "down."
srdjanr 20 hours ago [-]
It is just a band-aid.
We noticed that some equations only make sense if mass is greater than what we'd expect from what we can see. So for now we pretend that there's this dark matter that we can't see, and with it all math so far checks out.
But we don't know if that missing mass exists at all, or if some formulas are wrong. So it's definitely a "hack", but it works for now and we don't have anything better. And we do expect to find an explanation at some point.
Silfen 19 hours ago [-]
I think its standing is a bit better than that. We have several independent lines of evidence like patterns in the CMB that aren't just "hmm, we need m to be bigger in our model of a galaxy." And we have a couple of energy scales compatible with standard particle physics where the dark matter might be hiding.
nyeah 19 hours ago [-]
TFA mentions a possible speculative explanation, that perhaps we can see some galaxies that are very low on dark matter.
Starlevel004 20 hours ago [-]
Well, any other theory would have to explain all of the following better than cold dark matter does:
1. Galactic rotation curves (galaxies spin too fast)
2. Galactic gravitional lensing (galaxies bend too much light)
3. The shape of the cosmic microwave background (it only matches if you have dark matter)
4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)
Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.
XorNot 18 hours ago [-]
It also has to do so without destroying a bunch of other exceptionally well tested theories - it's very easy to explain something if you ignore other requirements, like being compatible with relativity - which has been a big problem for MOND but is no problem at all for dark matter (because it's just extra matter being normal out in space).
daveguy 19 hours ago [-]
I don't understand why it has to be non-electromagnetically interacting. Can we honestly say we should be able to resolve all the matter in a galaxy with our current sensing technologies? Why can't it just be the equivalent of non-light-emitting asteroids and planets. We can barely detect whether a star has planets in our own galaxy. And according to Wikipedia the amount of matter we would be missing in a sphere with radius from the sun to Neptune is 10^17 kg or approximately a large asteroid. Which seems like a rounding error. But my knowledge of astronomy is definitely minimal.
Starlevel004 19 hours ago [-]
> I don't understand why it has to be non-electromagnetically interacting
Because we have zero evidence of it electromagnetically interacting. We have evidence of other particles like this, like neutrinos.
> Why can't it just be the equivalent of non-light-emitting asteroids and planets
The thing is, we've looked for them. If they were small (asteroid sized), the amount required would be visible as dark clouds obscuring the objects behind them. If they were large (brown dwarf or bigger), we'd see gravitational lensing from them. We don't really see either of those on the scale required to explain the gravitational effect.
daveguy 19 hours ago [-]
shit man. I was asking a question. So much for curiosity at HN.
Shitty-kitty 3 hours ago [-]
Unfortunately it is very common for people to spend 10 minutes using their "common sense" to find some simple explanation that seems to have defied the Scientist, rather then spend that time to read a quick article that explains the foundamentals.
auntienomen 18 hours ago [-]
I think it's the "can we honestly say" phrasing that got you flamed here. It reads as "those stupid scientists don't know what they're doing".
daveguy 13 hours ago [-]
Oof. Like I said. Just fyi, for any of the HN'ers left "can we honestly say" is just a colloquialism for "is it reasonable to say, given..." Pretty sure my "given" wasn't nutso anti-science. Especially considering I am a scientist. HN was fun while it lasted. Peace out.
13 hours ago [-]
andrewflnr 19 hours ago [-]
Curiosity is one thing, but in practice "just asking questions about dark matter" is uncomfortably close to pseudoscience and a lot of us are sick of it. It's similar to if you had accidentally said something similar to a young-earth creationist.
EdwardDiego 8 hours ago [-]
Did you just compare CDM-Lambda to the age of the of earth?
That implies far more evidence than actually exists. The science on the age of the Earth is far more grounded in experimental results simply because, well, we live on the Earth.
This doesn't mean CDM-L is incorrect in any way, but saying that questioning it is similar to being a YEC is ridiculous.
There's confident and there's cocky, I think you veered too much in one direction there.
z3c0 18 hours ago [-]
HN is less hackers than it is pedantic VC lackeys training for their aspirational role as a wealthy-and-opinionated Twitter personality.
19 hours ago [-]
uoaei 17 hours ago [-]
It is a band-aid: dark matter maps are literally nothing more than "evidence minus theory equals this distribution of anomaly".
MattPalmer1086 1 days ago [-]
Also see [1], a follow up post by Stacy McGaugh, a MOND proponent, who is quoted in the Quanta article. He does not agree with the evidence as presented in that article.
According to MOND theorists, a dwarf galaxy like DF9 doesn't prove dark matter, since it's sitting in the gravitational field of a big neighboring galaxy (NGC 1052) and thus its boost is suppressed. They call this the "external field effect".
“We don’t know what dark matter is or how to directly measure it but this distant galaxy has none”
Ok then
cwmma 21 hours ago [-]
> DF2 has consistently been portrayed as fatal for MOND despite consistently being shown not to be a problem. I’ve given up writing formal papers showing this, as nobody cares. They just hear what they want to hear, which is that MOND is wrong. Indeed, after the 2018& incident I swore off doing so.
This should set off crank alarms when you read it.
MattPalmer1086 20 hours ago [-]
Professor McGaugh has commented many times on the sociology of science and how the dominant paradigm tends to be hostile or indifferent to alternate views. He is also quite frank about the limitations of MOND as well, although he is definitely in the MOND camp overall.
He is hardly a crank - but he is definitely not in the mainstream camp.
Shitty-kitty 3 hours ago [-]
The biggest problem with MOND is that it was hand-crafted with additional parameters simply to fit the data. What's missing is a reason or a mechanism by witch gravity stops following the inverse square.
Scientists generally take a dim view of such theories.
"With four parameters I can fit an elephant, and with five I can make him wiggle his trunk." John von Neumann
MattPalmer1086 1 hours ago [-]
MOND has one constant parameter, a0. This is hardly a good example of a theory tweaking many parameters to fit the data.
Yes, it is empirically determined and provides no deeper insight into why it works.
cwmma 20 hours ago [-]
perusing his blog posts, basically if a calculation is at all within expected values, this is evidence of MOND, but for something to be evidence of dark matter it has to jump through quite a few hoops in his opinion.
He also seems to have the same problem that many MOND proponents have which is arguing for MOND in blog posts instead of in papers, though he doesn't seem to be as bad as others in this regard.
MattPalmer1086 19 hours ago [-]
He is a professional scientist with a blog - he has written a lot of papers as well [1]. It is worth taking his views seriously, even if you have reason to dispute them.
> Professor McGaugh has commented many times on the sociology of science and how the dominant paradigm tends to be hostile or indifferent to alternate views
This should also set off crank alarms.
tikhonj 17 hours ago [-]
I remember object-oriented programming being on the same pedestal when I was younger and first learning programming—a lot of OOP folks and mainstream programmers were definitely hostile to other schools of thought like functional or procedural programming.
Hell, I've worked with programmers who, entirely honestly, treated "object oriented programming" as a synecdoche for good programming...
MattPalmer1086 19 hours ago [-]
Maybe it should - but it does not prove that the subject is a crank. Thomas Khun's theory of paradigms shows that science is not an entirely objective undertaking.
Having read Prof McGaugh's blog and some of his published papers for quite some time, I do not place him in the crank bucket. Make your own mind up I guess. My take is he is a legitimate scientist who holds views outside the mainstream.
janalsncm 20 hours ago [-]
I don’t really understand. Dark matter is in most galaxies interacting gravitationally, but not these three. Their idea is that somehow there was a collision event which stripped the galaxies of their dark matter, because the regular matter got tangled up but dark matter somehow didn’t.
But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up? And is that something we have seen in other collisions ever?
Rumudiez 20 hours ago [-]
the Bullet Cluster makes for a good comparison, there's a good image of it in the article. the bulk of the 2 galaxy clusters continued on but left behind 2 clouds of inter-cluster gas
my first impression was this could actually be a counterexample, because there we can see the heavy, dark matter-full clusters continuing on their trajectories at full speed whereas the gas slowed down and is being left behind. compared to these 3 lightweight galaxies which seem to be alone, rather than trailing behind their dark matter-hosting parents
but perhaps these 3 _are_ going slower than their full-scale origin galaxies. I would stake my view as a layman on whether anyone can find the dark matter galaxies that spawned these farther along their expected trajectories
Starlevel004 20 hours ago [-]
> But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up?
Friction between regular matter causes it to lose energy and thus velocity. Dark matter doesn't have self-interaction outside of gravity, so it just keeps going.
janalsncm 18 hours ago [-]
Ah, interesting. Maybe it should be called “slippery matter” then.
Sniffnoy 20 hours ago [-]
It bugs me that articles about dark matter so frequently frame things as dark matter vs. MOND, and don't consider any of the other theories of modified gravity. I think MOND is just not a great theory and it has a lot of evidence against it by this point; but other theories of modified gravity have not, to my knowledge, been tested to the same extent, nor do they get anywhere near the publicity. Dark matter still seems more likely than modified gravity, but I wish they wouldn't emphasize MOND so much as the representative of modified gravity, and I wish we'd hear more about tests of other modified gravity theories!
martinpw 18 hours ago [-]
The problem with any theory of modified gravity is that there is just no physical justification for it. It's only motivation seems to be to explain away dark matter by adding and tweaking free parameters until you fit the data, which you can always do given enough variables.
van Neumann's quote comes to mind - "With four parameters I can fit an elephant, and with five I can make him wiggle his trunk."
Sniffnoy 17 hours ago [-]
Hm, MOND is full of new parameters (an entire interpolation function!), but do other modified gravity theories really have so many? I guess actually I don't know the answer to that, maybe they do. Do you know how many others have?
KingMob 8 hours ago [-]
> explain away dark matter
This kind of presumes dark matter has been definitively proven already, though.
From the outside, CDM may be a better theory than MOND, but it doesn't seem settled to me. I haven't heard anyone describe it as a "Law" yet.
Couldn't someone equally characterize the creation of dark matter as "adding and tweaking mass until you fit the data"?
19 hours ago [-]
analog31 20 hours ago [-]
I wonder if this means there are giant balls of dark matter hurtling around the universe.
> If two typical galaxies met in a head-on collision, the dark matter would experience very little friction, or drag, and would keep on rocketing by. But ordinary matter, which interacts using all four fundamental forces, would get all tangled up.
Isn't a galaxy mostly empty space? I thought I read before that when galaxies collide there's actually very little physical contact between the matter. Stars mostly pass through affected predominantly only by change in surrounding gravity. Is this not the case?
Calavar 22 hours ago [-]
The ordinary matter that gets tangled up is interstellar dust clouds, not stars and planets.
pfdietz 21 hours ago [-]
The mean free path of a hydrogen molecule in the "warm interstellar medium" is about 0.03 light years, much smaller than the diameter of a galaxy. So if gas clouds collide they will interact strongly.
sandworm101 21 hours ago [-]
They don't collide but they do get "tangled" in each other's gravity. Small things (stars) get tossed in random directions and big things (gas clouds) pass into each other and start condensing into smaller/brighter things.
tsimionescu 20 hours ago [-]
It can't be in each other's gravity, else the same phenomenon would affect dark matter. For something to affect regular matter but not dark matter, it has to be an electromagnetic interaction (at this scale - as the weak and strong forces only interact on very very small distances). So it has to be a significant amount of head-on collisions, or some kind of charged dust clouds projecting large EM fields.
bonzini 17 hours ago [-]
I interpreted that as gravity causing things from the two galaxies to mix up, after which normal matter can interact electromagnetically (in more head on collisions than usual) but dark matter won't.
sandworm101 2 hours ago [-]
There are direct and indirect collision. If two bits of diffuse gas "collide" the first interaction is not collisions between particles. The two diffuse gasses merge into each other without many particles actually touching. But local density does increase, triggering gravity interactions which then might lead to more collisions between particles. Collisions between particles turn momentum into heat/radiation, slowing the movement of the gas while increasing its temperatures. but dark matter particles don't collide. So the gasses don't slow down and may well pass through each other.
Starlevel004 20 hours ago [-]
Very big object x Very small chance x Very long time = normal probabilities
Humanity, after migrating from biological carbon based substrate, finds ordinary matter not practical to live in. Dark matter has much better properties.
All humanity transfers to dark matter where it finds all the other "ascended" species historically hosted by our universe. Solves Fermi's paradox and keeps the "normal matter" nursery clean for new species to appear.
> That all three gossamer galaxies existed along a line, as neighbors, didn’t escape the attention of astrophysicists, who began to coalesce around an origin story.
How about this:
What we see is an advanced galaxy-killer alien weapon being fired at one target, and then continuing to travel beyond that target and making 2 more (unintended?) victims.
I was tempted to comment that there’s no evidence that you can make anything complex out of dark matter, but it turns out that some physicists hypothesize that “self-interacting” dark matter could bind into something like atoms and molecules, so carry on!
I'd bet a large chocolate cake that one day, physicists will discover a much more logical explanation for this unexpected behavior of matter.
But we have plenty of observations to lead us to what dark might be and what it definitely isn't.
Any theory of dark matter needs to explain all of the following:
1. The Virial Theorem, galaxy clusters, and the speeds of galaxies in clusters. (both MOND and CDM both do a good job of explaining this)
2. Galactic rotation curves. (MOND and CDM both do a good job of explaining this)
3. Accoustic Baryon Oscillations. (ABO) Immediately after the big bang, everything was ludicrously hot. The most powerful force in the universe was the wash of gamma rays everywhere. These gamma rays carried electrons around. The electric fields of the electrons carried the protons around. (the light dominated phase) As the universe cooled, this flipped; the protons were carried around by momentum, which brought the electrons with them, which mediated the light. (the matter dominated phase, where we are today) During the light dominated phase, sound waves pulsed through the universe, causing dense regions and underdense regions. When the universe flipped from being light dominated to matter dominated, these regions were frozen in time, and we can see them directly in the CMB. (cosmic microwave background) We can analyze the properties of these pressure waves to determine what the properties of the matter and light in the universe were. These waves demand some sort of dark matter. CDM explains these observations, MOND does not.
4. The Bullet Cluster. The Bullet Cluster is a collision of two galaxy clusters. We can measure gravitation lensing and light to determine where the dark matter is and where the regular matter is. The bullet cluster demonstrates that these two properties are not the same. There is matter which has gravity and gravitational lensing, but that we can't see. CDM explains these observations, MOND does not.
5. Ultra diffuse galaxies with very little dark matter. Not all galaxies have galactic rotation curves which require some extra sauce to explain the movement of their stars. Some galaxies are explained perfectly by the matter that we can see in them. So we need dark matter for most galaxies, but some galaxies need to be dark matter free. These galaxies falsify MOND.
6. Leading spiral arms in barred spiral galaxies. The large majority of barred spiral galaxies have their arms trailing from their bars, but a very small number of them have arms which lead in front of the bars. Computer simulations show this happens naturally with CDM, but MOND or dark matter free spiral galaxies don't have a mechanism to cause this to happen.
7. Gravitational waves travel at the speed of light. Most MOND formulations predict gravitational waves travel slower than c; all of those formulations are wrong.
There is a genuinely absurd amount of time, effort, and money that has gone into explaining dark matter. It is not a band aid, we haven't just added 1 to moved on. Everybody is pouring over every step of the algorithm trying to figure out why it's giving us the wrong answer. CDM is the leading hypothesis right now, because it explains all of the observations, and is not falsified by any of the observations. MACHO is/was another theory; it has gone out of fashion because it does not explain all of the observations and makes predictions we do not observe when we check for them. MOND is out of fashion because it is falsified by the data. MOND is still in fashion among a particular brand of people who just want to have their names in the history books; if someone did prove MOND, the history books would all have lists that go "Ptolemy, Copernicus, Kepler, Newton, Einstein, <your name here>."
But for Pete's sake it's not as if people have their fingers in their ears.
explanation of acronyms/terms:
1. Dark Matter. Dark Matter is the observation that the universe acts as if there's a lot more matter in it other than the stuff we can see. Basically no one says that Dark Matter isn't real; that would imply that the observations are wrong.
2. MOND. Modified Newtonian Gravity. MOND is a family of models which aim to explain the dark matter observation. Instead of using F=Gm1m2/r^2 to determine the force of gravity, we have better math, that gives a different equation, that accurately explains Dark Matter. When you say "physicists discover a much more logical explanation" that's what you're talking about.
3. CDM. Cold Dark Matter. CDM is a family of models which aim to explain the dark matter observation. CDM implies that Dark Matter is explained by particles. CDM implies there is some sort of particle of family of particles which interact with normal matter via gravity as normal, via the electromagnetic or strong nuclear force not at all, or via the weak nuclear force super weakly or not at all. CDM interacts with other CDM particles via gravity as normal, but only weakly via either the weak nuclear force or some other as-yet undiscovered force. People often use the terms Dark Matter (referring to the observation) and Cold Dark Matter interchangeably.
4. CMB. Cosmic Microwave Background radiation. After the Big Bang, the universe was opaque. Light would be absorbed by the stuff in the universe and then re-emitted. During recombination, the universe became transparent. The last moment of the various bits and parts of the universe switching from being opaque to transparent has the entire universe glowing in microwaves. This moment is captured in a snapshot, known as the cosmic microwave background. As we understand the CMB better, we learn a lot--a lot a lot--about the early universe.
Globular clusters should spherically self-lens too, explaining apparent CDM.
However it does neatly explain these ultra-diffuse galaxies. They're not sufficiently dense to self-lens so they can't "get the flywheel going" to ramp up their stellar density and self-gravity-lensing, to shape the large scale structure.
In effect, these ultra-diffuse galaxies would be right on the edge of "failed galaxies," in the same way Jupiter-size objects are sometimes considered as failed stars.
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EDIT I admit CDM irks me because it has (effectively) unlimited tuning parameters, and what's more, you can tune them differently per object! As the history of science shows us, Here There Be Dragons...
[0] classic example being the SOFAR channel https://en.wikipedia.org/wiki/SOFAR_channel
[1] https://en.wikipedia.org/wiki/Cuspy_halo_problem
Classic obfuscation method. That remaining static noise gave it away. Somewhere out there is a Savant who reads it like Tank and Link read the Matrix' machine code--blonde, brunette, redhead.
It seems that having some galaxies without dark matter is also pretty bad for CDM. Why would CDM be absent in those cases? Without an explanation, isn't it bad for CDM (as well as MOND)?
Nobody wants a theory where exceptions can be hand-waved away as "Oh, that galaxy must not have DM" or "That galaxy has too much/too little/different balance/etc".
Roughly, my question is "Don't we see other post-collision galaxies that still have dark matter, and if so, why?"
If we have two identical scenarios with different outcomes under a theory, that doesn't bode well for the theory.
> if someone did prove MOND
How could MOND be proven if it is disproven? Is there a chance it is not really disproven, or you mean the equations for forces might still be modified in a way that fits observations? Or that it could be MOND plus something else?
> When you say "physicists discover a much more logical explanation" that's what you're talking about.
I wonder what is illogical or band-aid feeling about matter that interacts with gravity but not other forces though. Why would all types of matter interacting with electromagnetism and strong nuclear forces be logical
The standard model has lots of this kind of thing. Leptons don't feel the strong force. Neutrinos are electrically neutral. The weak force only couples to left-handed particles. It seems completely natural to expect some dark matter. The only question is how much.
Assuming it's definitely matter, and operating on that basis for nearly 50 years, is not good science, period.
They don't. You can read exactly this article for 3 examples.
Modified Newtonian theories are anything but simple.
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.
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?
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.
We noticed that some equations only make sense if mass is greater than what we'd expect from what we can see. So for now we pretend that there's this dark matter that we can't see, and with it all math so far checks out.
But we don't know if that missing mass exists at all, or if some formulas are wrong. So it's definitely a "hack", but it works for now and we don't have anything better. And we do expect to find an explanation at some point.
1. Galactic rotation curves (galaxies spin too fast)
2. Galactic gravitional lensing (galaxies bend too much light)
3. The shape of the cosmic microwave background (it only matches if you have dark matter)
4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)
Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.
Because we have zero evidence of it electromagnetically interacting. We have evidence of other particles like this, like neutrinos.
> Why can't it just be the equivalent of non-light-emitting asteroids and planets
Because they've been ruled out: https://en.wikipedia.org/wiki/Massive_compact_halo_object
You are not proposing anything new. Your ideas have been thought of before, and discarded because they lack evidence.
You are describing MACHOS, which was one hypothesis for dark matter. https://en.wikipedia.org/wiki/Massive_compact_halo_object
The thing is, we've looked for them. If they were small (asteroid sized), the amount required would be visible as dark clouds obscuring the objects behind them. If they were large (brown dwarf or bigger), we'd see gravitational lensing from them. We don't really see either of those on the scale required to explain the gravitational effect.
That implies far more evidence than actually exists. The science on the age of the Earth is far more grounded in experimental results simply because, well, we live on the Earth.
This doesn't mean CDM-L is incorrect in any way, but saying that questioning it is similar to being a YEC is ridiculous.
There's confident and there's cocky, I think you veered too much in one direction there.
[1] https://tritonstation.com/2026/10/15/ghostly-galaxies-in-the...
https://arxiv.org/abs/2104.13961
Ok then
This should set off crank alarms when you read it.
He is hardly a crank - but he is definitely not in the mainstream camp.
"With four parameters I can fit an elephant, and with five I can make him wiggle his trunk." John von Neumann
Yes, it is empirically determined and provides no deeper insight into why it works.
He also seems to have the same problem that many MOND proponents have which is arguing for MOND in blog posts instead of in papers, though he doesn't seem to be as bad as others in this regard.
[1] https://scholar.google.com/citations?user=lM8J8noAAAAJ&hl=en
This should also set off crank alarms.
Hell, I've worked with programmers who, entirely honestly, treated "object oriented programming" as a synecdoche for good programming...
Having read Prof McGaugh's blog and some of his published papers for quite some time, I do not place him in the crank bucket. Make your own mind up I guess. My take is he is a legitimate scientist who holds views outside the mainstream.
But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up? And is that something we have seen in other collisions ever?
my first impression was this could actually be a counterexample, because there we can see the heavy, dark matter-full clusters continuing on their trajectories at full speed whereas the gas slowed down and is being left behind. compared to these 3 lightweight galaxies which seem to be alone, rather than trailing behind their dark matter-hosting parents
but perhaps these 3 _are_ going slower than their full-scale origin galaxies. I would stake my view as a layman on whether anyone can find the dark matter galaxies that spawned these farther along their expected trajectories
Friction between regular matter causes it to lose energy and thus velocity. Dark matter doesn't have self-interaction outside of gravity, so it just keeps going.
van Neumann's quote comes to mind - "With four parameters I can fit an elephant, and with five I can make him wiggle his trunk."
This kind of presumes dark matter has been definitively proven already, though.
From the outside, CDM may be a better theory than MOND, but it doesn't seem settled to me. I haven't heard anyone describe it as a "Law" yet.
Couldn't someone equally characterize the creation of dark matter as "adding and tweaking mass until you fit the data"?
Isn't a galaxy mostly empty space? I thought I read before that when galaxies collide there's actually very little physical contact between the matter. Stars mostly pass through affected predominantly only by change in surrounding gravity. Is this not the case?