The author is implying that other solutions like Modified Gravity are dead in the water, I don't think they are.
The interested reader might find Roscoe's work on MOND worth a browse: https://www.researchgate.net/profile/David-Roscoe-3
Is dark matter’s “nightmare scenario” true?
61–70 of 183 posts
Re: Is dark matter’s “nightmare scenario” true?
#62Nightmare for who? How hard it is to admit that their equations are incorrect? Even amateur like myself can see that the dark matter is adhoc value that was calculated in order to patch the standard model. There are several better alternatives but do we need to wait until these stubborn believers have passed away until the world can move forward?
At the moment there are no better alternatives that satisfy all the observations. If you try to replace General Relativity with something like Modified Newtonian Dynamics (MOND) [1], then you can get good fits to galaxy rotation curves. However, there are many sources of astronomical/cosmological data that will not bend to alternative gravity models, e.g the precise fluctuations of the cosmic microwave background put…
I vaguely recall hearing that there are galaxies "without any dark-matter". Or, more accurately, galaxies whose rotational speed is consistent with general relativity without needing to presume dark matter exists in those galaxies.
If that is correct, does it not immediately invalidate MOND?
Re: Is dark matter’s “nightmare scenario” true?
#63My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated. I fail to see how dark matter could ever be falsified. The best po…
There is only a handful of parameters. They describe the probability distribution of finding one particular matter configuration. We are simply observing one realization of that probability distribution, in which we should be able to calculate the percentage of galaxies with an unusual amount of dark matter for any definition of "unusual". We don't consider the position and momentum of every baryon a parameter of the…
This seems like a very nicely detailed response to OPs question. Thanks! Your response brought up some questions and remarks from myself.
> DM can be falsified perfectly well.
I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct?
> If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia?
> Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.
Re: Is dark matter’s “nightmare scenario” true?
#64There's a chance dark matter is black holes created in the aftermath of the Big Bang. Some of these then merged into the supermassive black holes we see in the center of galaxies today. https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...
Yet black holes do interact with ordinary matter--they can consume it--and they emit Hawking radiation. Dark matter does not appear to be doing either of those things.
Re: Is dark matter’s “nightmare scenario” true?
#65Nightmare for who? How hard it is to admit that their equations are incorrect? Even amateur like myself can see that the dark matter is adhoc value that was calculated in order to patch the standard model. There are several better alternatives but do we need to wait until these stubborn believers have passed away until the world can move forward?
> dark matter is adhoc value that was calculated
Well the overall tone of your comment is pretty crummy but this is absolutely not true. You got this exactly backwards. Dark matter is just missing mass that is being observed. It doesn't patch any model, because physicists don't know what it is or what it is made of. But whatever that mass is, it is there, because all the standard ways that we observe mass, it exhibits. So it would instead be a leap to "patch the standard model"--whatever you mean by that--with these other "better alternatives" (I assume you mean MOND? No dice there, doesn't explain gravitational lensing).
Re: Is dark matter’s “nightmare scenario” true?
#66Earlier quoted context omitted.
Sort of, but gravitational lensing has been seen in places where there is no visible matter. If dark matter doesn't exist, then you'd have to come up with another explanation for the gravitation lensing: https://kipac.stanford.edu/highlights/using-gravitational-le...
You're absolutely right and this is the one thing about dark mayer that doesn't sound like "just keep sprinkling it on your problem till it works." I don't know enough about the found examples of gravitational lensing to know whether all possible arrangements of black holes that could have the same effect, have been considered and found lacking. But I presume so.
Well it definitely is one of the proposed solutions
https://en.wikipedia.org/wiki/Primordial_black_hole https://en.wikipedia.org/wiki/Massive_compact_halo_object
Re: Is dark matter’s “nightmare scenario” true?
#67My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated. I fail to see how dark matter could ever be falsified. The best po…
Dark matter is not a “recursive property of gravity” because it’s not consistent like that, as your own initial objection suggests. The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter…
If it is not falsifiable, it is not science nor physics. It is meta-physics, ersatz-religion. Which is exactly the core of the complaint against concepts like this.
Of course same thinking can't easily be applied to things that are not always true, like social behaviour. Part of the reason the replication crisis hit so hard in the social sciences.
And of course this is not really as absolute as I wrote above. No field works like this all the time. But it is a good guideline. If falsifiability is completely ignored, then a concept really is nothing more than a religious belief and should be considered as such.
BTW, Popper did not publish in the 19th century.
Re: Is dark matter’s “nightmare scenario” true?
#68Why do people in the comments always ignore that dark matter has way more evidence for it than just galaxy rotation curves (something very well explained in the article)? It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.
Because it is the only evidence that is presented, and lay-people can easily come up with objections. These people then express their frustration or confusion why some of these objections seem completely unaddressed.
These people are effectively asking about these objections. Perhaps many of them are phrasing it differently. But, with exceptions, I think people tend not to be angry or vitriolic. At best they are frustrated. Just treat these remarks as questions.
It might be annoying to get the same questions repeatedly, but that is a fault of wider communication about dark-matter.
Re: Is dark matter’s “nightmare scenario” true?
#69Earlier quoted context omitted.
Do you know of any good derivation of existence of dark matter a layman could follow through and be convinced?
You could read through the relevant Wikipedia pages for an overview: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
Wikipedia is rarely a good introduction for laymen. It tends to be a great reference for people already in the field.
Re: Is dark matter’s “nightmare scenario” true?
#70Why do people in the comments always ignore that dark matter has way more evidence for it than just galaxy rotation curves (something very well explained in the article)? It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.
Probably because it's the only piece of evidence you can understand with just high school physics. Most laypeople have trouble understanding that the universe is not expanding from a single point, so how would they be able to understand what the cosmic background radiation is, much less why its angular power spectrum has peaks at particular positions? It's a common pattern with crackpots as well, that they focus on "…
Yes exactly. We've all experienced reading an account on our particular domain of expertise by a journalist and realising they have absolutely no clue what they're talking about. I find it amazing how presumably otherwise intelligent people reading Hacker News don't see the issue with offering lay opinions on Cosmology of all topics.