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."
[0]: https://academic.oup.com/mnras/advance-article/doi/10.1093/m...
McGaugh says the same here https://www.scientificamerican.com/article/dark-matter-may-b....
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.
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.
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.
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?
- 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.
[1] https://twitter.com/dudedarkmatter/status/109452639162072268... [2] https://twitter.com/DudeDarkmatter/status/109452792999493222...
certainly, these are still open questions in cosmology
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.
Perhaps the "dark matter" of a galaxy is the rest of the galaxies of the universe.
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.
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?
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.
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".
I'm aware the jury is out on it.
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.
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.
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.
- It's a debate, not a settled fact.
- 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 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.
You -opened- 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.
And ignoring the other side, presenting it as settled.
Read your opening statement. It is not conducive to discourse. It just flames CDM without advancing any argument of its own.
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.
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.
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.
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.
This tends to not invite productive discourse.
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 did the math; still just one in seven billion.
Building websites and collating data isn’t that novel these days. It’s not like you alone collected it or defined any of the first principles.
Welcome to being a normie.
Like what is it? Something that has no activity on HackerNews for 15 months, then first thing it says is rude and false.
fine if you want to call me an "amateur cosmologist with a few credentials"
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.
Maybe try reading before calling others stupid
"lost" is what I would say the people pumping it are.
EDIT: Nvm, parent answered similar question here https://news.ycombinator.com/item?id=29914380
Following links it appears that proponents of MOND believe that running the calculations will show that MOND predicts this.
Despite popular belief physicists are actually quite clever and have evaluated theories such as this.
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
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.