The DM distribution itself is effectively a huge number of parameters. You have to have just the right amount of dark matter distributed differently in each case to explain observations and get those best fits. For example, see: https://en.m.wikipedia.org/wiki/Cuspy_halo_problem
Yes, the cuspy halo problem wouldn't be a problem at all if you were to simply adjust the infinite parameters that you suggest that DM has. The fact that there are statistical discrepancies proves my point that no one is adjusting a huge number of parameters.
The point is that all kinds of dark matter distribution models are proposed and evaluated to see which one fits best. Again, for example:
And they still fail to explain other observations which require additional arguments as to why the dark matter is once again distributed in just the right way to give the rotation curve... that is already successfully predicted by MOND in most cases with just one universal parameter.
That doesn't make any sense at all unless they simultaneously believe that DM is actually the right answer.
I’m sorry, I can see where I was ambiguous. I meant I can see why dark matter is appealing to more researchers in general and why MOND is in the minority. If dark matter is right, we can find it. If MOND is right, we’ve just been doing incomplete math this whole time. One of those is way more exciting.
The last person who figured out we were doing incomplete math is the single most famous scientist in the history of the world. Being the next one isn't more exciting than getting some particle named after yourself?
Whenever I read about Planet Nine search, I have a very naive (as a non-physicist) childish question: if we detect anomaly in our Solar System just less than a decade ago and can’t find anything visual which explains it, assuming there is giant planet/piece of ice floating somewhere at the edge of Solar System, how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is? Simply, what is the probability that let’s say space between us and Alpha Centaurs is not filled with objects like this? Invisible and leaving a tiny gravitational trace at the edge of our ability to detect it?
If I understand correctly, MOND is suggesting that gravity behaves differently at sufficiently large scale (galactic), similar to how we observe different physics at a quantum level?
MOND and DM try to explain the same phenomena, and the debate can probably be settled on way or the other by looking at enough interesting galaxies, or by finding dark matter or definitely ruling out that it exists.
GR and Quantum Mechanics are not even really talking about the same thing and use incompatible mathematical frameworks. Another problem is also that there are actually not so many phenomena that we would need Quantum Gravity for. Particle physicists are yearning to find experimental evidence that would force us to retire the Standard Model to the history books. And their event horizon makes it kinda hard to study what is going on inside black holes.
Whether or not they're right in their answer, it's interesting to me that this debate is a modern repeat of the discussion about the oddities of Mercury's orbit that some attempted to explain with a hypothetical planet Vulcan [0]. One of the early evidences for Einstein's general relativity was that it accounted for those oddities with only the planetary bodies that had already been observed. [0] https://en.m.wikiped…
And since readers may find this interesting: The various writeups all say that relativity explained the precession of Mercury's orbit, but they never say exactly what factor was accounted for by relativity. The answer: Mercury is heavier when it is at perihelion in its elliptical orbit, because it's moving faster. The increased relativistic mass makes for increased momentum at perihelion, which carries the planet a l…
This might sound true if you have a superficial understanding of relativity (and I would not be surprised to see people repeating this stuff on the internet), but it's completely false. The correction to Mercury's orbit precession from General Relativity has nothing to do with Mercury itself (its mass, speed or otherwise). It is purely based on the sun's gravitational field. If you want to interpret it as a potential field in the classical Newtonian sense, it basically picks up an additional 1/r^3 term. That's what disturbs the orbit and it's also the reason why this effect is negligible for other planets apart from Mercury, because it drops off much faster than the normal 1/r potential. Any object very close to a large mass like the sun would feel this distortion, irrespective of how fast it travels.
If I understand correctly, MOND is suggesting that gravity behaves differently at sufficiently large scale (galactic), similar to how we observe different physics at a quantum level?
I wouldn't have put it that way, since the quantum/classical transition is an emergent phenomenon without additional parameters, while MOND proposes a new term being a added to the physics.
Still, at a broad level there are similarities, yes. In QM scale practically has a life of its own -- made explicit in the Copenhagen interpretation.
I've always thought that a good model or theory predicts observations (like a new particle, or an unseen planet) and is confirmed to be accurate by those observations. A new model that is designed to fit existing observations that don't match our expectations based on the old model, without it correctly predicting new unexpected observations, seems like a rather weak proposition to me.
> A new model that is designed to fit existing observations that don't match our expectations based on the old model, without it correctly predicting new unexpected observations, seems like a rather weak proposition to me.
This description roughly applies to General Relativity; there are clearly merits to both approaches.
Whenever I read about Planet Nine search, I have a very naive (as a non-physicist) childish question: if we detect anomaly in our Solar System just less than a decade ago and can’t find anything visual which explains it, assuming there is giant planet/piece of ice floating somewhere at the edge of Solar System, how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is…
Very good question. As a non-astronomer, my guess is that we don’t observe a lot of transient black outs for example of the milky way. Which would happen if there’s a lot of these around.
That gives you an upper bound of how many objects like that exist.
I’m sorry, I can see where I was ambiguous. I meant I can see why dark matter is appealing to more researchers in general and why MOND is in the minority. If dark matter is right, we can find it. If MOND is right, we’ve just been doing incomplete math this whole time. One of those is way more exciting.
The last person who figured out we were doing incomplete math is the single most famous scientist in the history of the world. Being the next one isn't more exciting than getting some particle named after yourself?
Yeah I interpreted that person’s post as saying “we’ve been doing incomplete math this whole time” is way more exciting, which I completely agree with.
Whenever I read about Planet Nine search, I have a very naive (as a non-physicist) childish question: if we detect anomaly in our Solar System just less than a decade ago and can’t find anything visual which explains it, assuming there is giant planet/piece of ice floating somewhere at the edge of Solar System, how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is…
The simple answer is that there are actually very large numbers of objects in the space between stars that aren't detectable. Including, certainly, planet-sized objects that formed in stellar environments, escaped, and now fly freely in between.