Galaxies like this truly challenge MOND theories of modified gravity.
If there is no such thing as dark matter, why doesn't this galaxy behave as the others do? It has the same amount of normal matter as it's neighbours, yet acts totally differently. Dark Matter as a theory has an answer: this galaxy has none. MOND has a much harder time explaining it.
I can't help but think of this very similar argument from HHGG (Douglas Adams):
> Now it is such a bizarrely improbable coincidence that anything so mindbogglingly useful could have evolved purely by chance that some thinkers have chosen it to see it as a final and clinching proof of the non-existence of God.
> The argument goes something like this: "I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
> "But," says Man, "the Babel fish is a dead giveaway isn't it? It could not have evolved by chance. It proves you exist, and therefore, by your own arguments, you don't. QED."
> "Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.
> "Oh, that was easy," says Man, and for an encore goes on to prove that black is white and gets killed on the next zebra crossing
Thank you. I prefer to use full terminology for my fields abreviations to prevent others needing to look them up just to know what I meant. I feel pretentious if I don’t.
Still, dark matter as a theory, or at least as the name of a theory, has the big problem to predict something that is conveniently invisible.
A proponent of MOND could probably declare the existence of some form of dark exotic matter in galaxies that behave "Newtonian".
My point is: As long as a dark matter theory doesn't make predictions that can be experimented upon, we will always have these kind of interpretation games.
Side question: What would actually distribute dark matter throughout the universe in the first place? If it is only affected by gravity and there is much more dark matter than visible matter, shouldn't it be forming a huge invisible ball somewhere?
Something that is only affected by gravity won't tend to collect in one place because whatever gravity would attract it there also gives it momentum to get out.
You need collisions to get things to lose momentum to heat and clump together. Normal matter has collisions via interaction with the electromagnetic field, dark matter doesn't. Neutrinos also don't clump together, but are disqualified as dark matter candidates due to there not being enough of them; something about the relative abundance of different fundamental particles (e.g. electrons and neutrons) being important to get the universe we observer today.
ETA: modified gravity doesn't give anything we can experiment on either; the difference of TeVeS[1] vs GR are not detectable at non-cosmological scales.
> Still, dark matter as a theory, or at least as the name of a theory,
Your point is well-taken, but I have to be pedantic here (and everyone loves to make this mistake, so don't worry about it). Dark Matter is not a theory,, not like the General Theory of Relativity is a theory. Dark Matter is a hypothesis.
It's an interesting philosophical question whether a more complex theory and a small amount of invisible matter or a simpler theory and a large amount of invisible matter is the simpler combination.
In any case, I think that any modified theory of gravitation has to fight an uphill battle because it seems to be easy to come up with a configuration of dark matter that explains any observation. That's my biggest grievance with DM, btw.: It doesn't make any predictions that could be used to falsify it, at least not in a sense that I could intuitively understand it. Effectively, it appears to act as a completely free parameter, distributed however we need it at the moment.
> Baryonic Tully-Fisher relation showing the ultradiffuse galaxies discussed by Mancera Piña et al. (2019) as gray circles. These are all outliers from the relation; AGC 114905 is highlighted in orange. Placing much meaning in the outliers is a classic case of missing the forest for the trees. The outliers are trees. The Tully-Fisher relation is the forest.
The tully-fisher relation is an empirical relationship IIRC used to calculate distances to galaxies. You don't know that the tully-fisher relation applies in exactly the same way to a class of galaxies that are characteristically different from those galaxies which provide the dataset for calibrating the distance (IIRC, this is done by using cepheid variable stars). By necessity, UDGs are an extrapolation of the tully-fisher relationship. Suppose the tully-fisher is off by two digit percent due to UDGs being strange -- then it's possible that our calculations about the galaxies "dark matter content" could be completely wrong.
Only considering rotation speed for MOND seems like a simplist, poor approximation.
One needs to take into account how wide/large is the galaxy.
Its 3D shape.
And a map of its local main density/sparsity points with estimated masses.
A failure to estimate accurately any of those metrics seems like the issue with this galaxy, not MOND itself.
Personally I've always felt both the Dark Matter hypothesis and MOND are dead wrong, and that everyone is in the muck messing around with these as though one or the other are the only possible explanations. Dark Matter has not remotely been proven, not even in the remotest sense, and yet it is a fixture in the paradigm of modern cosmology. It's as messed up as when the luminiferous ether was held similarly as incontrovertible science... also without a shred of proof.
When someone actually detects Dark Matter is when I will give that hypothesis any attention, while MOND is equally unpersuasive. Solving the "missing matter" problem is going to happen, and it's either going to be an adjustment of the data (or more precise or better measurements), or it'll be that we just didn't have the whole picture, and once we do, it's going to be a "duh" moment (which I suspect is going to be that there was never any missing matter, and everything and the kitchen sink thrown in that Dark Matter is invented to explain will be explained without the need for Dark Matter).
No one has ruled out dark matter either. If you are fumbling around in new physics you have to start somewhere, expressive power isn't a bad place to start.
If we wait a bit, Occam will be along to help us here with what is more likely, exotic matter we can't detect, or a simpler solution we have not yet conceived. When we knew that light was a wave, and we knew that waves only travel through a medium, we were certain about the existence of luminiferous aether, which also misled hoards of young physicists that would have been better off, making Physics as a study better off, without every new entrant into the field being misled.
MOND theories seem by far most popular on Internet forums nowadays! There are just so many "But what if they are wrong and dark matter doesn't even exist, if they can't explain it..." that it's quite overwhelming! It is not as simple as this. In fact, modifying gravity to fit the set of observations here is so hard that most scientists have just given up on it by now. I mean, it looks like dark matter comes and goes, clumps together...
But I think this is ultimately a good thing. I'm pretty sure we're on the right path here with several decent explanations to dark matter, like sterile neutrinos or axions. Some candidates are testable today and bright people are working on it right now.
Dark matter feels like one of those things that may suddenly see a path towards being solved within years, but that it might also take much longer time.
> In fact, modifying gravity to fit the set of observations here is so hard that most scientists have just given up on it by now.
Wait what? I was under the impression that more scientists are turning to MOND than ever. I have a hot take prediction that in 5-10 years MOND will be the preferred explanation over LCDM.
Sterile neutrinos were ruled out late last year. Axions were ruled out a decade? ago... Though I guess "revised" axions are still on the table; IIRC last year another huge swath of axion masses were ruled out.
Last I read, such galaxies aren't a problem for MOND. MOND doesn't say that all matter in the universe is detectable, it just says that due to modified gravity, the apparent missing mass we observe can be explained by dust or some other cold baryons.
MOND has explained similar galaxies in the past by postulating that there's very little dust.
The term "Dark Matter" refers to an unknown kind of matter that interacts gravitationally but not electromagnetically. My comment is referring to cool baryonic matter, ie the same kind of matter than you and I are made of.
In other words, MOND explains galaxy dynamics by modifying gravity and postulating the existence of small amounts of conventional matter that are below the detection threshold of our telescopes.
Dark matter theories explain galaxy dynamics by postulating the existence of huge amounts of an exotic matter (alongside some conventional dust) that we can't see and haven't been able to detect in any experiments.
Because that could be the actual physical reality, gravity or inertia might not fully work how we think it does, and there might also be some stuff we can't detect.
The objective should be to make correct predictions and understand how things actually work not for one side to win the argument.
I think (s)he meant some kind of theory that's basically MOND, but with still some dark matter included. Dark matter and MOND might not exclude each other.
Dark matter made out of black holes would be upsetting to cosmologists, because they need for the universe to be only 4% "hadronic" matter. If the black holes are made of hadrons, then there is too much of that.
But maybe the stock of primordial black holes started as Dark Matter particles. They then have the problem of explaining why so much of the Dark Matter started out in or close to a black hole. Maybe each Dark Matter particle is so dense it is a black hole all by itself?
You have to be careful talking about what a black hole is made of. Stuff that falls in only has a description before it falls in. After it falls in, it can't be thought of as made out of anything, anymore. So if black holes were part of the initial universe, there is no saying which flavor of matter they represent. After things have settled down, these black holes could proceed to swallow any amount of regular matter, and any amount of "other stuff", if there is any.
The assumption is generally that the "other stuff" has difficulty cooling off enough to be sucked wholesale into black holes, unlike regular matter that is happy to radiate away a big fraction of its energy as photons, leaving the rest able to collapse and form planets and stars, and thence perhaps be gathered into the nearest black hole. Meanwhile, the Dark Matter can mill around a black hole's vicinity indefinitely, maybe swooping close and getting ejected, but otherwise hardly bothered unless it blunders straight in.
Something I've always been curious about is how energy relates to black hole formation with regards to Schwarzschild radius and the early universe. How did the early universe not collapse into one giant black hole? Are mass-energy and density the only factors affecting formation or does the type of energy have an effect? For example, suppose I have a mass more than sufficient to collapse into a black hole. Now suppose that the mass is split in half and each half has a very high momentum energy directly away from the other half. Do they fly apart or does the momentum energy simply increase the mass-energy of the black hole?
Space is "able to" expand/inflate faster than light. So, the universe started as a black hole, but space expanded/inflated fast enough to push almost everything out past the event horizon. Maybe there wasn't enough matter left in any one place to be a black hole anymore? Or, any already sufficiently dense clumps kept their event horizons, and became the primordial black holes.
Or, maybe there wasn't anything until there was enough room for it to be in. Then you don't need a black hole.
Cosmologists like to toss about "expanding universe" and "inflation" as if they mean something, but they really are like gravity to Newton: a name for a process that can be described, but is otherwise wholly incomprehensible.
[Dr. Mohamed Abdullah] explained that one well-proven technique for determining the total amount of matter in the universe is to compare the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations. Because present-day galaxy clusters have formed from matter that has collapsed over billions of years under its own gravity, the number of clusters observed at the present time is very sensitive to cosmological conditions and, in particular, the total amount of matter.
That article links to paper, co-authored by Abdullah, Anatoly Klypin, and Gillian Wilson, which concludes that "matter makes up 31.5±1.3% of the total amount of matter and energy in the universe" and "80% of matter is actually dark matter". So 20% of 31.5±1.3% is 6.3±0.26%, which is how much normal matter is in the total amount of matter and energy in the universe.
I realize that's different than the 4% number given by the other commenter. This is the latest estimate I could find. Maybe the estimated ratio of dark matter has changed with newer measurements, or maybe the other commenter was working from older estimates.
Yes, the number -- 4%, 6%, what have you -- has bounced all over in recent years. Probably 6% is more current than 4%.
One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match.
One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match.
That's how the scientific method works. You come up with a hypothesis, you test it, you adjust it accordingly, and you re-test it. If you can't come up with tests for it, or if your measurements never match the hypothesis, you throw the hypothesis out. That's why string theory and MOND are getting crapped on in this thread.
Thanks for this. The galaxy formation seems to give the 30% matter, where does the 80% dark matter come from? I get that you measuring the density of stuff that has gravity to see how many galaxies form, but how do we know there's not more dust than we'd expect to make up for that missing 80%?
DES scientists used two methods to measure dark matter. First, they created maps of galaxy positions as tracers, and second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of dark matter over billions of light-years, using a technique called gravitational lensing.
To make these ultraprecise measurements, the DES team developed new ways to detect the tiny lensing distortions of galaxy images, an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals. In the process, they created the largest guide to spotting dark matter in the cosmos ever drawn (see image). The new dark matter map is 10 times the size of the one DES released in 2015 and will eventually be three times larger than it is now.
Dust accounting for the missing 80% of matter and made out of normal matter would be visible in these surveys. It would block, reflect, or emit light. But it isn't visible despite bending light around it—and that's the dictionary definition of dark matter.
If they're using baryon acoustic oscillations and the cosmic microwave background, theyre describing the amount of hadrons in the universe after these extra ones would have already become black holes, and be counted separately as "starting" as dark matter.
A black hole enthusiast then needs to figure out why those black holes would form before big bang expansion
This was one of the leading theories for what dark matter is for quite some time. The term is "MACHO" or Massive Compact Halo Object. MACHO doesn't include just black holes, but also a sea of objects small enough to be very difficult to observe but massive enough so that in aggregate they can cause what we observe as dark matter.
However, the evidence against MACHOs is pretty strong. Once we had a theory of what to look for, experiments were designed to detect them. The thing we looked for is microlensing: Focus a telescope at some stars, and watch for a sudden, short jump in brightness. This can be caused by a massive object passing by in front of the objects, gravitationally lensing the light. We performed these studies and the number of microlensing events that we found were not sufficient to explain dark matter. So MACHOs as an explanation for dark matter isn't the answer.
It's a good suggestion though, and like I said it was a leading theory for a while until we were able to test it.
The big problem with that hypothesis is that you have to explain why we see so little matter falling into these blackholes and they don't seem to: dark matter you can basically pass right through and have your trajectory disrupted a little, whereas a black hole once you hit it becomes a zone of infinite attraction - i.e. a black hole passing near a star is pretty likely to setup in orbit around it because any matter that gets close enough inevitably gets sucked in. Dark matter - having finite mass and no singularity - doesn't have this problem.
It doesn't matter, you are assuming a dark matter theory to begin with here. You are just assuming the dark matter is made up of black holes. This is an established idea of dark matter that has been given serious attention at times, though it's never been a leading candidate that I know of.
That's right. Those are called massive compact halo objects (MACHOs). From "Dark Matter: A Primer" (https://arxiv.org/abs/1006.2483):
To hunt for these objects two collaborations, the MACHO Collaboration and the EROS-2 Survey, searched for gravitational microlensing (the changing brightness of a distant object due to the interference of a nearby object) caused by possible MACHOs in the Milky Way halo. (Other collaborations have studied this as well, such as MOA, OGLE, and SuperMACHO.) The MACHO Collaboration painstakingly observed and statistically analyzed the skies for such lensing; 11.9 million stars were studied, with only 13-17 possible lensing events detected. In April of 2007, the EROS-2 Survey reported even fewer events, observing a sample of 7 million bright stars with only one lensing candidate found. This low number of possible MACHOs can only account for a very small percentage of the non-luminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no, and the evidence behind this claim comes from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB).
That paragraph references "The SuperMACHO Microlensing Survey," "The MACHO project: microlensing results from 5.7 years of large magellanic cloud observations," and "Limits on the MACHO content of the galactic halo from the EROS-2 survey of the magellanic clouds," if you feel like reading a bunch of physics research papers.
Machos are a certain mass of black holes, aren't they? Or does it include small but dense ones too? I suppose we'd see those on the microwave background though if hawking radiation holds true for small black holes
Not just black holes, but neutron stars and brown dwarfs, too. These studies ruled out MACHOs with masses in the range of 10^(−8) to 100 solar masses. That's 1/3rd the mass of the Moon to five times the mass of Betelgeuse.
For black holes to be visible due to Hawking radiation against the cosmic microwave background, they would have to be pretty small, about 0.8% of the mass of Earth or about the mass of the Moon (https://arxiv.org/abs/astro-ph/9911309). I don't think anyone's found such a small black hole anywhere, yet. If they did, it would be a pretty big discovery since such a small black hole would have to be primordial.
As an aside, "dense" doesn't really make sense in the context of a black hole, which has infinite density.
Well, unless people get a viable theory explaining why the thing that is 80% of the Universe is missing on an entire galaxy, it challenges dark matter theories too.
They are just some not very well known bodies that may challenge all of our assumptions, or may fall in line with our theories once we get to know more about them. What is well, the best you can expect from new observations.
In fact it's evidence for dark matter, as we can now point to areas of space where we see it and areas where we don't; and baryons and leptons in those spaces behave as expected in the presence or absence of something with mass that doesn't otherwise interact, which is all that "dark matter" is as a broad theory.
A galaxy is small compared to the Universe, so why would it be difficult to imagine that some of the galaxies were formed in regions devoid of dark matter?
Because cosmology has a lot of evidence that the clusters of mater we have around were created by gravity alone, and gravity acts the same way on normal matter and dark matter.
And if you expect a difference on particle genesis at the beginning of the Universe, it would be a major kind of anisotropy that is against much more than just the cosmology theories.
Why is it so hard to accept that we may have no good theory for explaining a galaxy without dark matter?
By "challenges dark matter theories" do you mean "challenges the theory that there is dark matter" or "challenges theories of how dark matter originated and is distributed"?
In other words, any evidence reported can only ever be interpreted to support Dark Matter. Seems to have too much mass? Dark Matter. Seems not to have too much mass? Dark Matter!
A theory compatible with all conceivable evidence is meaningless.
The available evidence includes many galaxies. "Seems to have too much mass" and "seems not to have too much mass" are both things that a correct theory has to explain. It's not sufficient to explain one at a time with kettle logic.
Are you sure that any theory which allows for different things happening in different locations is inherently and unavoidably meaningless? Or is there something else that bothers you?
I would like to understand why people such as yourself viscerally dismiss "dark matter" as if it were inconsistent with all science and rational thinking. Can you describe what's obviously special about it compared to all of the other things we can't see and take for granted?
I assume we all accept the idea of matter that is invisible because it is cold and doesn't emit light...there's nothing particularly weird about asteroids and planets adrift far from stars that are "dark" due to lack of light, right?
And electromagnetic radiation that we can't see is everyday stuff right? Like radio waves, and ultraviolet, and infrared, and x-rays? There's nothing weird about feeling the heat from something and not seeing the rays that carry it, correct?
Everybody knows that nuclear radiation can kill you while being completely invisible, right? Gamma rays and neutrons don't look like anything.
It's not weird that gases are invisible, like air. We all accept that air is around us and without contaminants is transparent.
Gravity itself is invisible, and we take for granted that it exists. Gravitational wave detectors correlate sometimes with visual events, meaning the invisible gravitational waves have definitely traveled over unimaginable distances, passing through everything in the way without being affected.
Neutrino detectors are old hat at this point. Neutrinos are incredibly unlikely to interact with what they pass through, so the detectors have gigantic amounts of material underground with detectors to see mindbogglingly rare flashes when a neutrino hits something. I think neutrinos may still be a candidate for dark matter.
Sound is normally invisible, even though it is the motion of atoms around us which are real.
Heat is invisible within a normal range. If a frying pan is 350 degrees it doesn't look different from 250 degrees.
Electricity in a wire is invisible. Magnetic fields are invisible.
Even lasers made of light itself are invisible if you look from the side and there's no dust in the air.
Are any of these besides "dark matter" unbelievable?
The fundamental difference is that we observe and interact with gases, electricity, heat and sound.
Dark matter is a plug. We observe the mass of galaxies and based on our understanding of gravity the galaxies should fly apart. The most parsimonious explanation is that there is a large amount of matter we don’t/can’t observe.
We don’t observe it and we don’t interact with it. While it’s the best explanation we currently have, it rankles to have to admit that the majority of the universe is made of something literally unobservable to us except through gravitational effects.
We do observe dark matter though, just not directly through the electromagnetic spectrum. Much as the first extrasolar planets were found through deviations in stars' movement before we first imaged them.
no. We infer dark matter. To use gravitation to say "there might be dark matter" and then later say 'dark matter exists because we see gravitational anomalies' is begging the question.
no, you can deduce a new observation that is then observed.
In the case of dark matter, this would most convincingly take the form of a multimodal observation. Predict that X particle is DM. It then has interaction Y that isn't gravitational. Then Y, which we were not looking for yet, is found.
> unobservable to us except through gravitational effects
That’s how we observe it. We just don’t know what it is built from.
(On the other hand, does it really have to be built from anything? Like, can’t it be some sort of an “intrinsic” distortion of spacetime that, naturally, has an effect on the “normal” matter in the only way it can do so?)
Why does it “rankle”? Would you expect, a priori, that the universe would be very unlikely to contain large amounts of matter that only interacts gravitationally?
Because a priori we typically assume that we’re not special, so it’s odd to assume that 85% of the matter of the universe is fundamentally unlike all the matter we interact with.
I don't have any problem with the existence of dark matter. But one way that it's different from all those other things is that we can generate them or observe them locally. We have landed on asteroids. Heat, gamma rays, electricity, neutrinos, gases, all can be made in labs. Yet if there are dark matter particles, we haven't found them yet.
Perhaps dark matter is just a victim of its own success. If it were just referred to as non-baryonic matter of a currently unknown construction instead of a catchy name like dark matter, perhaps few people would care.
It's obvious why though: most of the things in the universe can be observed using the electromagnetic spectrum, but dark matter doesn't interact with that. Instead we have to use gravitation, a force some 36 orders of magnitude weaker. That means we pretty much only observe it when the scale is large enough that we can notice its effects, which means it'll be very hard to observe on any kind of local scale.
Wouldn't a galaxy without dark matter be less strange than one with dark matter, considering "dark matter" is literally something we know nothing about?
Dark Matter is a working title for about 85% of the stuff in the entire universe.
What it is is the question posed by its name but its existence is generally agreed upon. Many apologies to any "English is my second language, please have mercy on my internal parser" for that last sentence.
Certain properties of galaxies implies that there is more matter in them than is observed directly. The extra stuff could be clotted cream but then I would personally be able to detect it from billions of light years away. We can only really say that gravity is a thing that we think works like ... this (cue pretty piccies from some Holywood films and Hubble etc). Gravity is an emergent property of matter according to Mr Einstein.
Take a large collection of matter, say of the order of 100,000 light years across, like our Milky Way or Andromeda, which is spinning and has a measurable distribution of matter. You can predict how it ought to behave. For example it ought to spin at x or be distributed as y etc. However, measurements show that it doesn't. So you take the observed parameters and run your models and equations backwards to get the actual matter that would be involved.
It turns out that over four fifths of stuff is currently unobservable, which is a bit embarrassing.
A galaxy without Dark Matter is one that doesn't have stuff that we don't even know what it is that it lacks or something. Assuming that stuff is reasonably homogeneously distributed across the universe, this is a very odd ratio of observable to non observable stuff indeed.
So I have one question and maybe the answer has already been determined.
I understand we looked at distribution of mass i galaxies and have inferred that for them to keep a stable shape, they need to be spinning at a certain rate which requires more mass than is observed and hence dark matter.
Do we know that galaxies are actually spinning in a steady state and are not actually in the process of flying apart? We've been photographing galaxies for only a hundred years or so at high quality. Would we have detected that they're spinning in a steady stare within this period if time?
Yes, velocities are known from looking at the wavelengths of light (red/blueshift)
So you’d have to fit another massive coincidence which is that the stars flying apart are also at the same time changing their emission spectrums to not look like they are.
Although we have only been observing for 100 odd years in our time frame, the things we have observed have been over billions of years. Remember that distance is time! Think about what a light year means - it's the distance that light travels in one year.
The Milky Way (the Romans originally named it that - when you get a decent view of it with no light pollution, it does look like a milky road) is roughly 100,000 light years across. Andromeda is about 2.5 million LY away and so any light from it that we see is getting on a bit. Andromeda is counted as part of the "local group" so considered close to us. Really far away stuff is billions of light years away.
That new satellite that's all over the news - James Webb - is something else. Hubble was seriously impressive, even with its "squint" (technical hitch that needed correcting shortly after its launch). Hubble has delivered some of the most stunning science and imagery possible and JW will go much, much further.
Just you wait until James Webb starts delivering results. It will be awesome, and I don't use words like that lightly.
Do you know of any sources that rule out regular matter in between celestial bodies that would be sparse enough to be unobservable but massive enough when put together? Like a bunch of cold electrons?
I'm an IT consultant who's been reading New Scientist for 30 years, not an Astrophysicist!
Now, you mention "matter ... sparse enough ... massive". I think you have answered your own question. The matter in question may be sparse but if there is enough of it, then it will become "massive" and hence detectible.
Not sure what a "cold electron" is - it sounds unlikely.
Dark Matter is postulated and not a "thing" per se. A lot of good science does seem to imply that 4/5ths of matter is unseen.
The key thing I am trying to get across is that our human need to slap a name on something is not appropriate here but we do it anyway.
The cool kids with massive budgets and impressive machines in space cannot yet tell you and I what the stuff is but for now we will call it "dark matter". It might sound a bit mad for people more used to dealing with certainties like Civil Engineering (lol - Britannia Bridge in London - resonance). I'm a Civ Eng grad.
If my physics class aren't too far: it cannot be charged otherwise it would radiate, it cannot be atoms because it would absorb radiations and it cannot be neutrons because they don't live long enough by themselves. Then it must be something else…
I believe the only two remaining candidates along these lines are MACHOs and relativistic neutrinos ("hot dark matter"), but I don't think either of them are considered likely among mainstream cosmologists anymore.
We would see something like that against the cosmic microwave background. Instead, we see that about 5/6ths of the total matter of the universe interacts with the CMB only through gravity.
Also, more hidden (somehow) baryonic matter would change our current theories surrounding Big Bang nucleosynthesis in ways that aren't supported by other observations, like the percentages of (visible) matter like helium and lithium.
If we knew nothing about dark matter, we wouldn't postulate dark matter.
Astronomers define/describe dark matter as matter that seems to interact with normal matter gravitationally but in no other way. But we can "see" this gravitational interaction in a number of ways and so we know a reasonable amount about dark matter, especially, through the motion of normal matter we know approximately where it is and isn't.
Well, a person without consciousness might behave exactly the same as one who was self-aware; all the same processes might occur, but the individual just wouldn't have self-awareness. As it is, it seems that conscious "acts of volition" really follow the initiation of action, rather than preceding it. It's as if we are automata or robots, with something weird called consciousness layered on top.
I have no idea whether this is a true representation of how things are, or some kind of artifact.
A person awake and conscious is, in my opinion, generally much stranger in its behavior than one who is unconscious. Unconscious persons generally just... lie there. Not all that strange, considering most stuff on Earth does that.
My high school self thought that "dark matter" was just two three-dimensional objects/galaxies with some offset in a four dimensional cartesian space, influencing each other with gravity.
Galaxies would attract each other and "pool" to the same 3d space. This one would have an associated galaxy with less mass or at a greater distance. For the opposite case (galaxy present there, but not here), it would mean that we should see areas in our space with more gravitational lensing than is warranted by the local matter.
I made a simulator (dumb 4d cartesian stuff) back then, and have been waiting for someone to find gravitational lensing out in the middle of nowhere, so I could see if it matches the expected shape.
I would post links to my cool geocities page containing the source/graphic I made in QBasic, but it's been dead for a couple decades now. Maybe I can find my palm pilot that has ranting notes about it.
edit: Why downvote. I'm fully aware it was just a dumb idea from high school. I really enjoyed physics at the time, and really enjoyed making simulations. I'm don't honestly believe this is how the universe works.
- modified gravity (MOND), which accurately models the centripetal acceleration of stars in hundreds of galaxies, models acceleration as sqrt(GM/R^2 * cH/(2pi))
- cH/(2pi) = c^2 / (2pi * 13.8 bly), which looks like some super extreme v^2/r centripetal acceleration
- like something moving at c, from a point 2pi * 13.8 bly away = 87 bly away
- maybe if ones goes straight for 87 bly, one ends up at the "same point" in abstract space
- maybe the universe has the shape of a hypersphere. maybe the radius of the hypersphere is 13.8 bly.
This sounds similar to the idea of branes [https://en.wikipedia.org/wiki/Brane_cosmology] which were a proposal to explain why gravity is "weak" relative to other forces - the idea being that gravity expands through extra dimensions.
Yeah, that was the motivation. I had just leaned about the concepts of higher dimensional spaces, and learned about that from some history channel space documentary, and wanted to see what a gravitational field would look like at a 4d offset, naively assuming that it would follow the same strength vs distance relation.
I'm referring to plain old four dimensional space [1], which is just 3d space with another orthogonal axis. Same as 3d is to 2d space. If a creature existed in a 2d world, say on a piece of paper, they would never perceive another 2d world, another piece of paper, one inch above theirs, in the direction of the third third dimension. The same rules apply for two 3d spaces separated by a distance in the fourth dimension. The thought was that gravity bleeds through this higher-dimensional separation.
It deforms spacetime, but I remember reading the other day that the deformation can be permanent. For a layperson (me) that looks like a plausible idea for what dark matter could be - space that's permanently deformed, but there's nothing there to collide with.
In this case we could verify if the ratio of dark matter is always the same. The article would suggest that it isn't. So this might lend possibility to the idea. There was a video a while back about what can happen in higher dimensional space when a volume would pass through a "fold" for lack of a better word. It would be right there next to you but not visible. I wonder why this can't be a more popular hypothesis or maybe we're missing something.
And just the other day, I was informed on the Webb telescope thread that there was no need to build a twin, as one was plenty for the limited discoveries one could make with a scope that could see farther than any scope before, one teeny tiny spec of the universe at a time.
No need or no political will? If NASA were funded to build a redundant mission I'm sure they'd be happy to do so, but they aren't. The design of SLS suggests they don't have much in the way of forward thinking backing from politicians.
Webb also took years and years to launch in the first place, what's the timeline like with two?
Many things are possible, but that one would not on the face of it seem that reasonable. Dark matter _seems_ real hard to interact with in any way, pretty hard to harvest something you can barely touch. You could posit that they have advanced technology, but even so why wouldn't they use up their matter first? Why isn't there any other evidence of their existence? Etc.
fyi, "dark matter" is not a real thing. Modified gravity (MOND) accurately models galactic rotation for hundreds of galaxies with sqrt(GM/R^2 * c*H / 2pi), without the stupid and ludicrous idea of invisible, noninteracting blobs of dark matter around every galaxy.
The reason "dark matter" has persisted in the public consciousness is that TV cosmologists keep pumping it, and academics currently in charge built their careers on it and don't want to lose their funding.
While other more knowledgeable folks than I have pointed out some very important things about your highly-declarative statement about dark matter, I'll raise one minor point about this research finding, which you can glean by reading the abstract of the actual paper [0].
The galaxy in question, AGC 114905, is described as lacking dark matter _because_ its rotational characteristics are readily explained by its baryonic matter content alone. MOND, therefore (and your entire argument), does not apply. You may have mistaken this for a regular galaxy with "missing" matter that also happens to lack dark matter, which would then suggest the need for an alternate explanation.
EDIT: The abstract also says, among other things: "We also find that the rotation curve of AGC 114905 deviates strongly from the predictions of Modified Newtonian dynamics."
FYI means "for your information". We use it when we are telling people about settled facts. It's also three letters that based on context could be seen as condescending on their own.
When we are discussing not-settled facts, there's better ways to frame our discussion.
For instance: "I believe, that CDM is an antiquated theory and MOND is a better explanation, because _______. I've had discussions with ______ and in particular ______"
It's less effort than your opening flame, it's less abrasive, and it opens productive discourse. Your opening statement is more like that of a political partisan than someone seeking discussion.
Intelligent and good people have followed the advice above, and the result has been more error and harm.
Therefore, learning from their experience, the one thing that works is accurately calling the status quo's invisible blobs of noninteracting kludge as what it is.
Maybe I'm just an "academic in charge" but I don't understand how MOND being able to account for galaxy rotation curves means that dark matter is not real. There is missing matter in galaxy clusters. Matter is missing if we look at galaxy-galacy and cluster-galaxy strong and weak gravitational lensing. This matter appears to be non-interacting (see Bullet Cluster, El Gordo, etc). There is missing matter from the angular power spectrum of the cosmic microwave background if we only account for baryons. There is missing matter if we are to forward model gravitational collapse of large scale structure at early times. MOND doesn't explain any of these things.
I also want to say that I'm not necessarily arguing against MOND here. I really like the work done by Stacy McGaugh et al. But from a more cosmological point of view, I see a lot of support for CDM. (And more than just TV cosmologists agree with that.)
I look back to where this started, and this started with Fritz Zwicky noticing that galaxies and galaxy clusters are moving faster than expected. This is the root observation.
From there, a good question is "How do we model it?". And sqrt(GM/R^2 * cH/(2pi)) works really well.
By the way, cH/(2pi) = c^2 / (2pi * 13.8 bly), which looks like centripetal acceleration.
Galaxy rotation curves are really not the root observation behind structure formation though, those are two very completely different things.
I'm also struggling to find what MOND has to say about all the astrophysics observed that LCDM people try to model to get their Tully-Fisher to work. If MOND is true, why would all these astrophysical feedback effects (such as supernovae) not matter for Tully-Fisher?
Hey, glad to meet you. Maybe we can have a discussion. Glad we agree on modified gravity (MOND) being able to accurately account for galaxy rotation.
- galaxy clusters
- lensing
- Bullet Cluster, El Gordo
all have answers in the work of modified gravity (MOND) cosmologists. Stacy McGaugh does a really good job going through each one.
The essential thing, and I mean really essential, is that modified gravity (MOND) uses fewer free parameters than cold dark matter (CDM), by a long shot.
but... this is not true? McGaugh himself [1] mentions that there are mass discrepancies even using MOND analysis, and that this favors the dark matter paradigm [2].
The tweets above are good questions! This is how to do science!
Meanwhile it's good to restate the obvious.
Modified gravity (MOND), where acceleration at galactic scales isn't just GM/R^2 but modeled as sqrt(GM/R^2 * cH/(2pi)), works for 100s of galaxies, and is more precise than cold dark matter (CDM).
Modified gravity only has one free parameter. cH/(2pi) = c^2 / 87 bly.
> The essential thing, and I mean really essential, is that modified gravity (MOND) uses fewer free parameters than cold dark matter (CDM), by a long shot.
The only MOND formulations that actually work are ones that simply reduce cold dark matter from 80% of the universe to 30% of the universe. MOND has all -- literally all, as in 100% of them -- the free parameters of lCDM plus the parameters for MOND.
I like MOND. It's elegant. It's intuitive. It's simple. You get to put your name at the end of the sequence Aristotle, Copernicus, Kepler, Newton, Einstein. When I first started reading about dark matter in the '90s I thought MOND was the obvious solution. (MACHO was still a leading theory at the time, they were still analyzing the data from the microlensing surveys) lCDM is messy and existentially unsatisfying. We don't know what what it is, and if it has the properties we think it has, we could not conceivably ever know what it is. But regardless of the fact that MOND is nicer than lCDM, lCDM explains all of our observations, but MOND only explains some of them, and depending on which formulation of MOND you subscribe to, is falsified by the the bullet cluster, or by galaxies whose rotation curves show a lack of dark matter, or by the LIGO/VIRGO observations that show gravity moves at the speed of light.
MOND reminds me of the meme of Homer Simpson where he's standing there looking fairly fit and then the camera turns around him and he's just got all his fat and loose skin pinned and tied up on his back. It looks great on the face of it but once you really, really start digging into the details it doesn't work anymore. lCDM wears all of its warts on its face.
Does the angular momentum of a galaxy contribute to its energy density (mass)?
What kinds of relativistic effects have been postulated for explaining our observations?
Something about dark matter which always struck me was that I have not seen explanations which incorporate the very limited perspective we have on galaxies -- being singular beings on a tiny rock very far away -- and how the light follows geodesics which may be significantly different from straight lines in the non-relativistic view. If these galaxies are large enough to lens light from elsewhere, doesn't it follow that this should be happening in places of high mass/energy density like the interior of galaxies, too? In other words, are the edges of the galaxies really where we think they are?
Is there any "whirlpool" effect that gravitational waves may have that would sweep or twist these geodesics as the galaxy rotates?
Dark matter shows up in different ways at different scales. It is not just about galactic rotation curves and MOND has a hard time beyond it.
It hit me one day that the galactic rotation curves had me thinking about it backwards. That had led me to think a galaxy has dark matter in it while more likely the dark matter has a galaxy in it.
> without the stupid and ludicrous idea of invisible, noninteracting blobs of dark matter around every galaxy.
This is a very strange thing to declare definitely can't be happening, in a universe where subatomic particle interactions do not have definite results or even particularly comprehensible abstract models of how they work to human minds.
Especially when the alternative is "gravity has some minimum force over distance which is exactly small enough we can't detect it on Earth".
You state things as absolute fact, while there IS disagreement amongst legitimate experts in these fields.
Please, furnish us with YOUR credentials that allow you to have such unique insights into this field.
Otherwise, you just sound like a crank. I don't know if you're right or wrong, but your unsourced certainty is definitely setting off my bullshit meter.
I have a degree in Computer Science from Yale. I built https://zedtime.live and I'm building https://spase.io, and I've been in direct communication with cold dark matter (CDM) and modified gravity (MOND) cosmologists for the last 20 weeks. I basically ask the questions that I'm gonna guess everyone has on their mind, and I'm just moving the conversation along faster.
I saw the "20" and my brain was already preparing the next word to be "years" so when I saw "weeks" it was a shock.
Asking questions is fine, but making a claim like "dark matter is not a real thing" with certainty is premature and does not predispose others to taking you seriously.
> I've been in direct communication with cold dark matter (CDM) and modified gravity (MOND) cosmologists for the last 20 weeks.
It's not really any of my business, but in your shoes I would be making sure to disclaim these statements regarding MOND as secondhand opinion rather than presenting them as established scientific fact.
Appreciate the feedback. These statements are a synthesis of ideas from people who've already synthesized all the complexity of the CDM vs MOND debate, but now I'm talking at the level of HackerNews so people do something!
- When your opening salvo describes the other side as "stupid and ludicrous" you should not be surprised that some people respond with hostility in kind.
- Having studied something somewhat intensely for 20 weeks doesn't put your personal opinion on the subject on a higher standing than people who have studied this at a high academic level for a lifetime. You're free to make arguments, but not to proclaim absolute truths.
- For the particular article we're discussing: A galaxy that seems to have no rotational speed anomaly is a blow for both CDM & MOND theories, but it would seem to be a bigger blow for MOND (e.g. CDM anomaly could just be a weird distribution of dark matter).
- I already agree it is a debate. It has been a debate for 40 years ever since Moti Milgrom first proposed modified gravity (MOND).
- I already was not surprised by the HackerNews crowd responding with hostility.
- I am pointing the HackerNews crowd to people who have studied this at a high level for a lifetime. Moti Milgrom. Stacy McGaugh. Pavel Kroupa.
- This article is actually a blow to bad journalism. This article is also an opportunity to call out that what "cold dark matter" is, is actually bad machine learning, where academics forcefit a new blob of invisible, noninteracting matter for every galaxy, which gives cold dark matter (CDM) no predictive power. Contrast with modified gravity (MOND), which models acceleration as sqrt(GM/R^2 * cH/(2pi)), which has predictive power.
Because you care about tone, which is a good thing, please read about how Milgrom, McGaugh, and Kroupa have been treated by cold dark matter (CDM) establishment.
Why does that affect how it's reasonable to discuss here?
It doesn't matter the topic: if I think, say, global warming critics elsewhere have been jerks, I still don't improve things if I talk in roaring hyperbole and dismissiveness in statements about it here. I won't get listened to, and I won't improve the quality of discourse.
From the first two site guidelines for comments:
> Be kind. Don't be snarky. Have curious conversation; don't cross-examine. Please don't fulminate. Please don't sneer, including at the rest of the community.
> Comments should get more thoughtful and substantive, not less, as a topic gets more divisive.
Continuing on to:
> Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.
... there's some other people, elsewhere, in academic circles, acting in a way you don't like.
So this is an excuse to be rude to different random strangers here, because they might (or might not) share some views with the people acting in the way you don't like?
I'm really patiently trying to explain this to you. "These other people were rude first" is never a good argument. It's even worse when the "other people" aren't here or a party to the discussion.
Hackernews like many online communities will repress interesting and rational truth seekers like you as they inconsciously value more an establishment or their ego/feelings or preconceived ideas over a logical argumentation as long as it lacks a stronger argument from authority or other logical fallacy that could make their brain cross a significant enough mental shortcut to update their beliefs.Sound argumentation only has limited power here and when you have a point the conversation will often digress to off-topic attacks instead of addressing your points.
This systemic mental defense mechanisms leads rationalists from around the world to self-censor themselves or be downvoted to hell. Also eventually they risk being converted to the fallacies of the rest of the group-think.
Be strong human and keep arguing, the world's needs you. Of course we all need an appropriate amount of epistemic humility (and not more), especially the proponents of the extremely epistemologically weak CDM theory.
I think talking about CDM vs. MOND vs. other options is fascinating. I doubt any of these is the last word and that mankind will reach a better understanding with time.
But the poster in question opened with a salvo just denigrating all the other options without any information in defense of his view. This shouldn't be lauded. It opens no interesting line of discourse.
> But the poster in question opened with a salvo just denigrating all the other options without any information in defense of his view.
That is true and untrue. He did not repeat himself but he made potent arguments (or links to external resources) (in fact much better ones than anyone else on this page) in other threads.
I never said he repeated himself. I just said his opening gambit in the conversation was a salvo that merely denigrated the other options without any information in defense of his view.
Yale is a great school, and there's nothing wrong with being an autodidact. A lot of the people here are as well. It doesn't mean you don't know what you're talking about. I'm also agnostic regarding dark matter. Let the lovers of phlogiston talk, and time will tell who is right.
I appreciate that you seem to actually be seeking the truth, but consider changing your approach in discussions if you want them to be fruitful and interesting instead of self-satisfying.
Anyone paying attention to experts knows that none of them, including the ones you cite (McGaugh et al), speak with the certainty that you do. All of them readily admit that there are problems with both leading theories. So when you “explain” to everyone why their stupid ideas are stupid, it comes off as cheerleading born out of either naïveté or intellectual dishonesty, and harms your credibility. “MOND is obviously incomplete and I despair of making it into a proper theory. LCDM has no realistic chance of explaining MOND’s many predictive successes. Right now, we’re hosed.” - Stacy McGaugh
It’s a bit funny that you then go on to talk down on “the HN Crowd” downthread - the most common complaint I usually hear about “the HN Crowd” is best summed up as “it’s full of people who think a computer science degree makes them an expert in everything.”
I've been here a little longer than 15 months. My advice: just smile and be polite. Arguing helps no one. If you have useful knowledge, share it and make the world better. If something is contentious, present facts and argue in good faith. Don't feed the trolls, and don't try to one-up others. Be nice, be kind, and have fun.
MOND, as the article points out, doesn't explain this galaxy, however. So it's a point in favor of CDM, if anything, that we find a galaxy behaving according to standard newtonian dynamics, not the modified version.
Doesn't really matter how many data points(galaxies) MOND can accurately predict, if it can't predict all parts of the dataset, what is the point? (I'm not saying CDM is better, it can't predict all data points either, but it seems to be better at explaining a wide range of things once you step outside galactic rotation rates).
> In their work, Fraternali and his colleagues tested a leading contender among dark matter alternatives, called MOND, for Modified Newtonian Dynamics, which involves tweaking Isaac Newton’s law of gravity. First proposed by Israeli physicist Mordehai Milgrom in the 1980s, MOND hypothesizes that standard gravitational physics, which accurately explains the motions of objects with high gravitational accelerations, like planets in our solar system, might not apply the same way to slowly orbiting stars at the edge of a galaxy’s disc. So the discrepancy between the expected velocities of the stars in galaxies and how fast they appear to be moving may not indicate missing mass, but rather a math error, if the MOND gravitational law is right. But while the MOND model fares well with more normal galaxies, it too couldn’t explain the rotation of Fraternali’s team’s fluffy galaxy. It fared just as poorly as dark matter models do.
If that is the case, then how do the sparse galaxies described in the article work? In the article, the galaxies described behave aligned with unmodified newtonian dynamics. So if we consider the inverse linear relationship in MOND, then for some reason, that relationship does not occur for these galaxies. That seems like evidence against MOND.
I'm not a physicist and barely understand some of the things I'm reading in this thread, but your question reminded me that I did hear about a relativistic MOND a few months back (called TeVeS): https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
cold dark matter (CDM) is a superimposed amorphous blob of invisible non-interacting matter, hence its inability to model the kinks in the real rotation curve
modified gravity (MOND) models the kinks in the rotation curve
If there is no such thing as dark matter, why doesn't this galaxy behave as the others do? It has the same amount of normal matter as it's neighbours, yet acts totally differently. Dark Matter as a theory has an answer: this galaxy has none. MOND has a much harder time explaining it.