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Comment by bambax | Hacker News Reader

> Physicists do not like magic; when other cosmological observations seemed far easier to explain with dark matter than with MOND, they left the approach for dead.

But how is "dark matter" not magic?

To my untrained and pretty ignorant eye, dark matter looks like filler: the observation does not match the theory, and so, instead of looking for a better theory, we decided the observation was wrong and couldn't see what was there (and not just a little bit, but the greatest part of it).

Isn't that the definition of superstition? Invisible forces that explain events, in mysterious ways?

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"Invisible forces that explain events, in mysterious ways"

The cold dark matter component of the standard cosmology is not the first time anomalous momentum was attributed to invisible (as in, does not experience electromagnetism) matter.

Twenty-six or so years from proposal (which incidentally was about two years before the discovery of the neutron) to direct detection:

https://www.wikiwand.com/en/Neutrino#/History

Note that neutrinos are hot dark matter; hot because they move quickly compared to the speed of light.

While neutrinos are definitely part of the dark matter sector, they cannot be the sole component because their hotness would smear out galaxies. A colder neutrino would work very well to match all the observations associated with observed anomalous momentum that strongly suggests dark matter, and hunts are on for direct detections of a cold (implying high rest-mass) neutrino-like particle.

There are other options, of course, but the four key things are: there is non-ignorable anomalous momentum; matter generates momentum (in this case, through gravitation); there is no visible matter associated with the momentum in question (that's why it's anomalous); and we have discovered non-visible matter in the past while trying to explain other cases of anomalous momentum.

You really helped me understand the concept of dark matter in a new way. I appreciate that, thanks! Specifically: hot/cold dark matter and rest mass.

> But how is "dark matter" not magic?

Because the theory made predictions that were then checked.

> instead of looking for a better theory, we decided the observation was wrong

Except they didn't. They looked for a better theory. In fact, they considered multiple different theories. And the dark matter theory was the one that fit the observations best.

"Because the theory made predictions that were then checked."

? What predictions and made and then tested by 'Dark Matter'?

Dark Matter was conceived of the other way around - it helps confirm predictions that don't actually work properly.

If Dark Matter theory made testable predictions, it would be accepted no? Where are these experiments?

It's widely accepted. And I'm also a sceptic. More recently scientists have started actually mapping out regions where dark matter is concentrated than - from gravitic perturbations - and have discovered galaxies which contain more dark matter than others, up to 99% as well as the mentioned globular clusters which contain very little dark matter.

"More recently scientists have started actually mapping out regions where dark matter is concentrated than - from gravitic perturbations "

This is not evidence of Dark Matter - it's really just quantifying and classifying the perturbations and anomalies derivations in gravity that we see.

Basically, more detail to the observation that 'galaxies are not rotating the way we think they should'.

That doesn't validate Dark Matter, it just more deeply quantifies the unknown.

The flogiston theory also made predictions that checked.

But the motivation behind dark matter is deeper than its gravitational observations. For instance, almost all particles physicists believe the standard model of particles is incomplete. No one is really sure exactly what we need to add to the standard model, but it turns out that if you add things to it in any sensible way, you almost always get things that look like they could explain dark matter.

So, it's more: particle physicists all believe the real theory has this aspect; when we look at gravity it looks like it had this aspect; therefore we should probably find a way to modify our theory so it has this aspect.

The major reason we haven't been able to go further than this is because we haven't been able to pin down exactly what these new particles are. Particle experiments are /hard/ (see:LHC), and it might take a long time.

Dark matter is pretty well defined within a particular model, not magic. You can take lambda-CDM, a very simple model, run a simulation, and compare it to observations.

I can't really comment on why MOND is magic. That seems a bit unfair, it's just a phenomenological modification of F=ma, and can similarly be simulated and compared to observations.

Did the model predict dark matter, or was dark matter introduced to get the model closer to observed reality?

AFAIUI (layman, at best) it's a bit of both. There are multiple independent[1] lines of evidence, some of those led to the formation of the Dark Matter model, others were since discovered and just so happen to also conform to that model.

[1] Maybe that's really the big question here.

It's not as "magic" as you believe it to be.

The dark matter is at the moment is not only the simpler explanation (that is, you have formulas that can produce the results) of the observations (Occam razor etc) but the only one that explains different effects that are still fully unexplained (in the sense, there are no formulas that can even match anything) by MOND.

And since 2006 there are the actual photos of "dark mater effects":

http://www.forbes.com/sites/startswithabang/2016/09/19/dark-...

Now, how the MOND handles it?

"Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine."

Note "thinks" not that anybody managed.

The dark matter approach could be declared really problematic even before anybody manages if some observations simply can't be explained with the dark matter. But at the moment it's still not so.

And all the observations will remain:

https://medium.com/starts-with-a-bang/10-facts-everyone-shou...

"It’s not going away

You will not wake up one day and hear physicists declare it’s not there at all. (Well, you will, but those claims are rare, and those physicists are wrong.) The evidence is overwhelming: Weak gravitational lensing demonstrates that galaxies have a larger gravitational pull than visible matter can produce. Additional matter in galaxies is also necessary to explain why stars in the outer arms of galaxies orbit so quickly around the center. The observed temperature fluctuations in the cosmic microwave background can’t be explained without dark matter, and the structures formed by galaxies wouldn’t come out right without dark matter either. Even if all of this was explained by a modification of gravity rather than an unknown type of matter, it would still have to be possible to formulate this modification of gravity in a way that makes it look pretty much like a new type of matter. And we’d still call it dark matter."

Then, not as the underlying principle, but as a simpler handling for all the observations up to now that look like "the invisible-matter-like effects there."

As long as there is no theory on dark matter it is not an explanation for anything. It's more a placeholder for a mechanism we don't understand. The best hypothesis so far is, that there is matter that doesn't interact electromagnetically. But this explanation shouldn't be confused with a theory that actually explains what is going on.

No, it's not a placeholder. There are the exact formulas that match what's observed.

https://medium.com/starts-with-a-bang/7-independent-pieces-o...

No other approaches (MOND included) are near, still.

MOND can at the moment match just one of the 7 different kinds of observations listed above. That's why the physicists aren't impressed.

And all the observations are not going away, even if MOND manages once to produce enough formulas to cover them or to be a better basis to explain the observation, see my other comment.

Where did you get this "no theory" claim?

"no theory" because we don't know what this matter really is. Up to now, explanations on the nature of the matter have problems in one way or another. High Energy Neutrinos are too light and not abundat enough. Axions were once a promising particle, that could explain dark matter phenomenology, but if axions were produced in stars, variable stars would oscillate in other frequencies. Supersymmetric partners are not ruled out but become increasingly difficult to explain, because the lightest susy-particle has not been found in the LHC experiments.

Yes, "no theory" is maybe not the best choice of words, lets call it "no verified theory".

By the way, I don't think MONDs are a good explanation either.

Massive sterile neutrinos, which are are part of a theory of explaining the very low neutrino masses, are also dark matter candidates.

It is already "verified," (as in, observations are there) it's that there are extensions of particle physics like WIMP for which these researchers would have liked to be able to fit them to the observations of the dark matter, and the conclusion is:

http://www.forbes.com/sites/startswithabang/2016/07/21/dark-...

"The null detection is incredible, with a fantastic slew of implications:

- Dark matter is most likely not made up, 100%, of the most commonly thought-of WIMP candidates.

- It is highly unlikely that whatever dark matter is, in light of the LUX results, will be produced at the LHC.

- And it is quite likely that dark matter lies outside of the standard mass range, either much lower (as with axions or sterile neutrinos) or much higher (as with WIMPzillas)."

(I'm not quoting it for you, but for possible others to get the idea what the topic is).

So it's that these popular supersymmetry models turned out that they don't fit to the experiments.

It's just problematic for those that expected too much of the supersymmetry approach.

It's good that some researchers try with MOND too -- sometimes there are some new results exactly there were they are less expected. But they have quite a lot to do to match what's easier to do otherwise.

I've always had trouble seeing "dark matter" as anything more substantial than modern-day epicycles.

The quip I'm fond of is:

   Dark Matter is the Phlogiston
   of Contemporary Cosmology

That certainly is a common perspective from people who get their knowledge of dark matter exclusively from pop-science articles!

Dark matter is the oxygen of contemporary cosmology.

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