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Is dark matter’s “nightmare scenario” true?

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

#51

Post by newaccount2021 »

mr_mitm wrote: post #41
TEP_Kim_Il_Sung wrote: post #39

Earlier quoted context omitted.

*hypothesis Theory implies they found out what it is.

In physics, we have laws, rules, models, equations, theories, theorems, postulates, and maybe more. I can't think of one instance in which something has been "promoted" from one to the other after some data came in and couldn't even rank these in terms of certainty. We still call it Newton's law even though we know it's wrong. We still call it Born rule even though it's one of the most tested things we have. We still…

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

#52

Post by febusravenga »

akomtu wrote: post #24

Call it stellar matter at least, for its influence on stars. Calling it dark only adds confusion.

But the very definition of this phenomenon is something interacting with gravity (like matter) and not with EM... And EM in layman terms equals light, thus dark... It totally makes sense to call it "dark matter"

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

#53

Post by mr_mitm »

LatteLazy wrote: post #48
scotty79 wrote: post #40

How small a black hole would have to be to be completely undetectable by us even when it flies through the solar system?

The minimum mass for a black hole is about 3 times the size of the sun. So we would definitely detect such an object. In fact it would have pretty catastrophic consequences if it entered the solar system...

You mean stellar black holes, right? Primordial black holes could be much smaller, if they exist.

I just looked up what would happen in that case, and it sounds really bizarre:

> Nevertheless, a PBH would have a notable local impact, Rahvar notes. “The passage of a black hole could melt a cylinder along the interior of the Earth with a radius of almost 10 cm. After a short time, this tunnel would freeze,” he explains. In theory, this would produce a distinct metamorphic fingerprint of the passage in the rock record. But given that we might expect only eight of these to have been produced in the Earth’s lifetime (one on each side of the planet per collision episode), “I would expect that such a geological trace is difficult to detect,” Rahvar concedes.

https://physicsworld.com/a/concerning-primordial-black-holes...

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

#54

Post by atemerev »

“Gravitationally only interacting dark matter” is sort of synonymous with “non-homogeneous spacetime”, or am I missing something?

Ah, yes, we also need to have some sort of bound energy, to balance Einstein’s equations. So, geons! https://en.wikipedia.org/wiki/Geon_(physics)

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

#55

Post by nonrandomstring »

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…

"Dark Matter" defies parsimony. Occam's Razor suggests rejecting the assumption that spacetime is uniform. The search for "Laws" may be a parochialism built into human epistemology. Advanced aliens may be comfortable with the idea of "space weather" and local variations. :)

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

#56

Post by choeger »

antonvs wrote: post #47
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…

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 think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? 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.

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

#57

Post by sebzim4500 »

dotancohen wrote: post #13
miika wrote: post #7

Nightmare 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?

One funeral at a time, as the saying goes.

Anecdotally it seems like, inside the scientific community, the dark matter skeptics are all in the older generation.

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

#58

Post by pja »

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?

You could read through the relevant Wikipedia pages for an overview: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...

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

#59

Post by Ygg2 »

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.

Because dark matter is too convenient, and it's not like LCDM prediction are 100% on the money.

http://backreaction.blogspot.com/2017/01/the-bullet-cluster-...

Another big problem is that all LCDM predictions (MACHO/WIMP) were disproven but it still remains a theory. Now we need energy too high to use in universe.

It's eerily similar to Phlogiston theory that at one point was considered to have negative mass.

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

#60

Post by rocqua »

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…

I very much agree with you, and have raised these questions before. I got some interesting responses that half-mollified my worries.

Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice.

This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing.

I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else.

Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science.

As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that.

Edit: also see this: https://medium.com/starts-with-a-bang/7-independent-pieces-o... from a different reply in this HN discussion.

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