Isn't there a modified MOND which fits all the observation data?
I don't believe so. If it did exist, it would be more complex than LCDM.
Is dark matter’s “nightmare scenario” true?
111–120 of 183 posts
Re: Is dark matter’s “nightmare scenario” true?
#112Ugh..this writing style, so unreadable.
I think the first paragraph could be rewritten better as:
"Dark matter puzzles us. Einstein's theory of gravity, plus known matter and radiation in the Universe, including particles and antiparticles described by the Standard Model, cannot explain cosmic observations. We need an additional gravity source - dark matter."
Re: Is dark matter’s “nightmare scenario” true?
#113My 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…
>If DM interacts purely gravitationally
If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.
Re: Is dark matter’s “nightmare scenario” true?
#114Earlier quoted context omitted.
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…
>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.
Also, if there's 5x as much dark matter in the universe why is our solar system so devoid of it? We should be able to detect the equivalent of dark matter suns or planets around us.
Re: Is dark matter’s “nightmare scenario” true?
#115Earlier quoted context omitted.
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…
>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.
On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.
Re: Is dark matter’s “nightmare scenario” true?
#116Earlier quoted context omitted.
Why isn’t it good for physics? It has found DM and can observe it via gravitational discrepancies, gets finally “solved” in this part. Does physics really need infinite modifications?
It’s not good because it means there is something in nature that we cannot understand and study, because we cannot in fact observe it directly. The gravitational discrepancies we have already, but without an explanation or observation, they’re not very satisfactory.
"The eternal mystery of the world is its comprehensibility … The fact that it is comprehensible is a miracle."
If the nightmare scenario is true, there would finally be a counterexample to this.
Re: Is dark matter’s “nightmare scenario” true?
#117Earlier quoted context omitted.
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.
I think that's just a factor of the open nature of HN meaning there's always someone ready with a hot take, and plenty of social incentive to provide one. None of us is as dumb as all of us.
Re: Is dark matter’s “nightmare scenario” true?
#118Why 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 "…
Crackpots emerge when a conceptual model is weak. They don't care how strong a mathematical model is, which is why they ignore SM and other areas where "theory" literally means a mathematical model, and nothing else. Having a lot of crackpots proposing bad theories is a sign that your conceptual model is garbage, just a bad attempt to hand wave a bunch of math that happens to predict reality, and you should stop pretending.
There are infinitely many mathematical systems that could describe "reality" all with divergent behavior in the parts of the system we haven't observed yet. Trying to make authoritative claims by analogy to coincidentally correspondent mathematical systems is foolhardy and vain.
Re: Is dark matter’s “nightmare scenario” true?
#119Earlier quoted context omitted.
>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.
On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.
I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.
Re: Is dark matter’s “nightmare scenario” true?
#120Earlier quoted context omitted.
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…
> However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity…
>why did these falsifications fail to convince the majority?
Because they are cherry picked. In the big picture, for every area where particle DM struggles, you will find a dozen datapoints where MOND struggles. Physicists are also wary of approaches like MOND+particle DM, because it just introduces more parameters which you can arbitrarily tune to explain data. It feels like every week someone comes with a new model that fits certain edge cases better, but doesn't actually tell us anything new or could act as a reasonable guide for future work. You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science.