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Is dark matter’s “nightmare scenario” true?

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

#91

Post by naasking »

YawningAngel wrote: post #82
choeger wrote: post #56

Earlier quoted context omitted.

> 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

> 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?

Be more bold in exploring alternative gravity theories and conceiving of tests we can perform to check how gravity behaves in regimes that are not yet well tested.

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

#92

Post by naasking »

mr_mitm wrote: post #37
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…

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 has been unusually successful in cases where it had no right to be if gravity acts as it does under GR.

For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority?

[1] 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

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

#93

Post by DiogenesKynikos »

naasking wrote: post #90
sega_sai wrote: post #84

Earlier quoted context omitted.

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…

> the amount of DM and it's specific distribution is tuned to match each observation

This is simply not true.

No cosmological theory tries to explain why each star is in its exact observed location. They try to predict the statistics of galaxies, star formation, etc.

If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years, you get a universe that statistically looks almost identical to our own. You get the same mix of spiral and elliptical galaxies as observed, clustered together in the same way, with the same star formation history, etc. You don't get exactly our universe, with the Milky Way exactly here and Andromeda exactly there, because you begin with random starting conditions. However, you get a universe that is statistically identical in most ways.

Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. Alternative theories generally get one or a few things right, at the cost of getting a whole host of other things wrong.

It's not that astrophysicists a priori like dark matter. It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.

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

#94

Post by naasking »

DiogenesKynikos wrote: post #93
naasking wrote: post #90

Earlier quoted context omitted.

> 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…

> the amount of DM and it's specific distribution is tuned to match each observation This is simply not true. No cosmological theory tries to explain why each star is in its exact observed location. They try to predict the statistics of galaxies, star formation, etc. If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years, you g…

> No cosmological theory tries to explain why each star is in its exact observed location.

Nice strawman.

> Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this.

LCDM also fails at this (see Dwarf galaxy problem, for one example), so this is not a point in your favour.

> It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.

LCDM doesn't work that great either. See the review I linked.

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

#95

Post by AnIdiotOnTheNet »

naasking wrote: post #90
sega_sai wrote: post #84

Earlier quoted context omitted.

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…

> Specifically, the amount of DM and it's specific distribution is tuned to match each observation

...and? If we look at the way a star wobbles, then tune the amount and distribution of planets around it to fit, isn't that just science?

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

#96

Post by pydry »

onli wrote: post #67
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…

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 fiel…

As far as I understood it if somebody could craft an experiment that would demonstrate, quite clearly that it was something other than a special kind of matter causing this effect then the theory would be falsified.

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

#98

Post by AnIdiotOnTheNet »

namaria wrote: post #34
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…

Unless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.

Right? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.

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

#99

Post by enkid »

naasking wrote: post #90
sega_sai wrote: post #84

Earlier quoted context omitted.

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…

A single variable being different for each galaxy isn't a problem per se. If a bunch of other factors work out with just that one adjustment, that's pretty good validation that that one adjustment is meaningful. If, on the other hand, your theory could be adjusted to allow for any possible combination of observations, that's when there's a problem. From what I've read, DM theories do a pretty good job of correlating things like rotational speed, luminosity, and galaxy formation. If you had to change one number for luminosity, one for rotational speed, and another to get the right make up of galaxies for the universe, that's when you start having a "tuning" problem.

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

#100

Post by naasking »

AnIdiotOnTheNet wrote: post #95
naasking wrote: post #90

Earlier quoted context omitted.

> 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…

> Specifically, the amount of DM and it's specific distribution is tuned to match each observation ...and? If we look at the way a star wobbles, then tune the amount and distribution of planets around it to fit, isn't that just science?

> ...and? If we look at the way a star wobbles, then tune the amount and distribution of planets around it to fit, isn't that just science?

That is science because gravity has been extensively tested and confirmed in that regime.

Gravity on the scales where dark matter is relevant has not been tested to this extent, so we're just assuming our theory is correct in this new regime and thus inferring the DM distribution based on that assumption and then saying, "this must be the case because we understand gravity". Except as I said, our understanding in that regime has yet to be confirmed, so that's ultimately a circular argument.

By contrast, modified gravity theories can match many of the predictions of DM theories using only direct observations of visible matter. It actually made some successful predictions before observations were made, where those observations required adjusting DM models to fit. Successful prediction with fewer parameters is exactly what you want from a scientific theory, and that's what MOND has done. See the review I linked for more information.

At this time, neither theory is fully satisfactory as discussed in that review. Superfluid dark matter models were compelling for awhile because they combined MOND and dark matter in a compelling way, but the early models were also inconsistent with some other observations. This is far from a solved problem and the LCDM model gets too much credit and attention given its problems.

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