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Is dark matter’s “nightmare scenario” true?

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Re: Is dark matter’s “nightmare scenario” true?

#81

Post by kryptiskt »

scotty79 wrote: post #38
doliveira wrote: post #35

Why 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.

Do you know of any good derivation of existence of dark matter a layman could follow through and be convinced?

"The Road to Precision Cosmology" is a good summary of how we ended up with the current theories: https://arxiv.org/abs/2201.04741

Re: Is dark matter’s “nightmare scenario” true?

#82

Post by YawningAngel »

choeger wrote: post #56
antonvs wrote: post #47

Earlier quoted context omitted.

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…

> You’d have to explain what should be done about that. Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless. > That’s living in the 19th century, basically - which admittedly, many people still seem to want to do. I thin…

Indulge the counterfactual for a second. Suppose that dark matter really does exist, and we can't ascertain that fact with any certainty. What do you want Physics as a whole to do about that? I think everybody agrees a simple answer would be great, we're just unable to provide one in this context

Re: Is dark matter’s “nightmare scenario” true?

#84

Post by sega_sai »

choeger wrote: post #30

My 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…

You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations.

* The right primordial abundance of different elements

* The amount of structure in the cosmic microwave background.

* The rotation speeds/velocity dispersions in large number of galaxies

* Large scale structure/The amount of clustering of galaxies.

* Gravitational lensing observations.

And much much more.

It is certainly true that there are individual cases that look challenging for Lambda CDM paradigm (CDM stands for Cold Dark Matter), but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. (there are currently some tensions with the Hubble constant measurements and some other cosmological parameters between different techniques, and it remains to to be seen whether those are systematic errors or point towards some modification in the theory).

Comment on the article itself. I think at some point I would be fine with dark matter being essentially a computing trick, i.e. if the behaviour of the universe is well reproduced when we add gravitationally interacting but not interacting otherwise matter. It would be up to theorists to see if somehow that can be written as some sort of extra-term in Einsten's Equations and show we don't really need an actual particle. I think there is a possibility of that, but personally I still think it's more likely there is a particle responsible for dark matter.

Re: Is dark matter’s “nightmare scenario” true?

#85

Post by mytailorisrich »

donatj wrote: post #74
antonvs wrote: post #47

Earlier quoted context omitted.

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…

> Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do. To a layman like myself this implies both that the things being studied then are both unprovable and not potentially of…

The comment you replied to does not suggest that modern physics is unprovable... no simple experiment to prove something does not mean no experiment at all.

The other, related point is that modern physics' frontier is in the quantum world and thus way outside layman's intuition, common sense, or reasoning. Even experts are struggling.

Re: Is dark matter’s “nightmare scenario” true?

#86

Post by _a_a_a_ »

choeger wrote: post #30

My 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…

Where does "infinite amount of additional parameters" come from? It's just one parameter - something's (we call dark matter) is either there or it isn't.

> I fail to see how dark matter could ever be falsified

I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.

Re: Is dark matter’s “nightmare scenario” true?

#87

Post by pantalaimon »

MrScruff wrote: post #70
mr_mitm wrote: post #43

Earlier 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.

Well there is a connection. Often I read news articles about science where I think "Wait, that doesn’t make sense." or "Hey, this assumption is not explained at all!" and without the original paper to read (or the knowledge to understand it) the only place to take this confusion is the comments section, with the hope that someone might explain it.

Re: Is dark matter’s “nightmare scenario” true?

#88

Post by cauch »

choeger wrote: post #56
antonvs wrote: post #47

Earlier quoted context omitted.

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…

> You’d have to explain what should be done about that. Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless. > That’s living in the 19th century, basically - which admittedly, many people still seem to want to do. I thin…

> I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.

Well, first, LHC was "unlucky": they could have observed a lot of new particles if they would have been there. It feels unfair to say "this experiment that looked at X and had 50% chance of seeing something was a waste of money because they haven't found anything, this experiment that looked at Y and had 50% chance of seeing something was not because they saw something": both experiments are as worth a try. LHC still have observed the Higgs boson, which is still quite a big thing. LHC has also constrained a lot of theories (seeing that a particle predicted by a given theory is not there is very useful scientifically).

But more importantly, I have the feeling that the notion of "bang for the buck" is pretty different from a scientific point of view and a layman point of view.

For example, you mention exoplanets. Scientifically, those are really not a big deal. We already know that planets can exist, there are 8 around the Sun, so finding others around other stars is really not a big deal: everybody was expecting them to exist and knew the only reason they were not seen yet was the limits of the technology. One could even argue that using money to detect things that are pretty trivial is in fact the worst delivery of bang for the buck. Don't get me wrong, this is still nice to see them, but it's the same level as pentaquark (discovered at LHC).

It is sometimes surprising to see the public/media reactions to discoveries. I'm still not sure why layman people were so excited about gravitational waves but not much about the tau neutrino.

Re: Is dark matter’s “nightmare scenario” true?

#90

Post by naasking »

sega_sai wrote: post #84
choeger wrote: post #30

My 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…

You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations. * The right primordial abu…

> You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy.

No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.

> but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM.

I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity:

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

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