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.
> 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
Modified newtonian dynamics
If so, would it be strange to find a galaxy with less blackhole mass?
Each non-dark matter theory (i.e. MOND, primordial blackholes effort) usually has some pretty large glaring issue that's unsolvable.
I'm all game to refute dark-matter but still waiting for the contender that fits the observations[1].
[0] https://www.youtube.com/watch?v=qy8MdewY_TY
[1] https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
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?
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.
[1] https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
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.
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.
https://tritonstation.com/2022/01/10/the-curious-case-of-agc...
> 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.
I'm not clear why the distance errors wouldn't average themselves out over the rotation
A failure to estimate accurately any of those metrics seems like the issue with this galaxy, not MOND itself.
For people new to the Tully-Fisher relation, the essence of it is that at galactic scale, gravity is not like v^2/r = GM/r^2.
It's more like v^2/r ~ sqrt(GM/r^2), so v^4 ~ M
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).
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.
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.
MOND has explained similar galaxies in the past by postulating that there's very little dust.
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.
The objective should be to make correct predictions and understand how things actually work not for one side to win the argument.
https://www.sciencedaily.com/releases/2021/12/211220120813.h...
Maybe these disperse galaxies have little to no black holes?
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.
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.
My understanding is:
1. there needs to be somewhere to escape to, for the schwarzschild radius to make sense, and there's no "outside" to the universe
2. we don't know how big the universe is or what it looks like; if it's infinite and uniform then there won't be any central point for a singularity
3. as others have mentioned, inflation is a get-out-of-black-hole card
4. the physics of the early universe aren't necessarily the same as physics now
It’s more that we had infinite density. Then space itself spread out.
Oversimplified disclaimer.
[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.
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.
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.
There's more info here:
https://news.fnal.gov/2021/05/dark-energy-survey-releases-mo...
And here:
https://www.darkenergysurvey.org/
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.
A black hole enthusiast then needs to figure out why those black holes would form before big bang expansion
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.
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.
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.
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.
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?
It doesn't have to behave like visible matter to behave in a specified manner. It's defined by interacting with gravity and not electromagnetism.
A theory compatible with all conceivable evidence is meaningless.
But maybe you didn't mean to say what you said.
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?
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.
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.
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?)
I see what you did here.
What does "observe" mean to you? It's a sticky philosophical question, but I think that consistency in definitions is essential.
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.
The more variation we see, the more likely that we're missing something