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Could dark matter not exist?

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Re: Could dark matter not exist?

#51

Post by dkbrk »

InclinedPlane wrote: post #42

Sigh. Sure, it's possible, but very unlikely. These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot…

> These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't.

MOND consistently fits the rotational velocity profile of galaxies better than dark matter. This isn't too surprising as MOND originated as an ad-hoc formula derived from observations of said rotational profiles, however MOND works very well over a very wide range of galactic profiles. The mass deficit observed in tidal dwarf galaxies is also quite strong evidence against just dark matter. I don't think anybody is seriously claiming that MOND is a complete theory, though of course there have been attempts by theorists at creating complete models, but they've failed to explain large-scale structural observation and the cosmic microwave background. This just means these attempts at creating a complete theory are probably wrong, not that there isn't something funky going on with galaxies that isn't completely accounted for by dark matter.

> They explain some things and leave gaping holes elsewhere.

This is a false dichotomy. MOND is not incompatible with dark matter. This "us vs them" mentality is really quite harmful. A combination of MOND and dark matter matches most observations very well, far better than dark matter in isolation.

> Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.

While I'm sure there are quite a few people for whom this is the case, you shouldn't dismiss the possiblity there are people who take MOND seriously because it legitimately fits the data better in some circumstances.

Re: Could dark matter not exist?

#52

Post by pavel_lishin »

InclinedPlane wrote: post #45
mixmastamyk wrote: post #7

Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way. This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dar…

The current best explanation for all of the observational evidence is a "cold dark matter" (CDM) theory, where the dark matter is composed of "WIMPs" (weakly interacting massive particles). Neutrinos are WIMPs that we know exist, and compose some small fraction of dark matter, but neutrinos are "hot", they travel at relativistic speeds, rather than much slower speeds. There have been many theories of what the "unseen…

Nitpick: I don't believe neutrinos are classified as WIMPs, since they are either massless, or have an incredibly small mass.

Re: Could dark matter not exist?

#53

Post by Tycho »

zardo wrote: post #44
AnimalMuppet wrote: post #22

Earlier quoted context omitted.

Maxwell had 20 equations for electromagnetism, because he didn't use vector notation, so he had to do the equations component by component. But that had nothing to do with having multiple theories. The current "Maxwell's equations" are exactly the same as Maxwell's 20 equations, merely expressed in a more convenient form.

In geometric algebra it's a single equation. ∇E=∂B/∂t

what is B?

Re: Could dark matter not exist?

#54

Post by ci5er »

InclinedPlane wrote: post #42

Sigh. Sure, it's possible, but very unlikely. These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot…

Who are you arguing with? The author of the article agrees with you. And has some neat pictures. You should check it out.

Re: Could dark matter not exist?

#55

Post by bashinator »

mixmastamyk wrote: post #7

Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way. This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dar…

It's "dark", in that it doesn't interact with electromagnetic forces (photons and electrons). Neutrinos are one example of this kind of matter, so it seems reasonable that there are other kinds that are even more difficult to observe.

Re: Could dark matter not exist?

#56

Post by InclinedPlane »

pavel_lishin wrote: post #52
InclinedPlane wrote: post #45

Earlier quoted context omitted.

The current best explanation for all of the observational evidence is a "cold dark matter" (CDM) theory, where the dark matter is composed of "WIMPs" (weakly interacting massive particles). Neutrinos are WIMPs that we know exist, and compose some small fraction of dark matter, but neutrinos are "hot", they travel at relativistic speeds, rather than much slower speeds. There have been many theories of what the "unseen…

Nitpick: I don't believe neutrinos are classified as WIMPs, since they are either massless, or have an incredibly small mass.

Neutrinos conclusively have mass since they experience flavor oscillations (if they were massless that would be impossible). Current estimates are that the total mass of all flavors of neutrinos is a fraction of 1 eV (less than a billionth the mass of a proton).

Anyway, neutrinos aren't generally called "WIMPs" because they already have a name, the term "WIMP" is generally reserved for new particles that have yet to be directly observed.

Re: Could dark matter not exist?

#57

Post by sdenton4 »

dkbrk wrote: post #51
InclinedPlane wrote: post #42

Sigh. Sure, it's possible, but very unlikely. These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot…

> These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. MOND consistently fits the rotational velocity profile of galaxies better than dark matter. This isn't too surprising as MOND originated as an ad-hoc formula derived from observations of said rotational profiles, however MOND works very well over a very wide range of galactic profiles. The mass defic…

I imagine that in a combined mond+dark matter regime, one would need a lot less of the dark matter flying around... Any sense of how the ratio of dark matter to regular matter plays out amongst people working on hybrid theories?

Re: Could dark matter not exist?

#58

Post by darkmighty »

kijin wrote: post #49
jfoutz wrote: post #39

Earlier quoted context omitted.

I'll defer to your expert opinion of course, but it seems like faraday discovered some odd behavior in 1821, maxwell codified it in 1861. By 1884 Heaviside reformulated them into the modern form. Perhaps 100 years is an exaggeration, but the point is it takes time to make sense of what the heck is going on. Really, pick anything. someone notices something odd, tries to explain it. there are many many experiments. Som…

100 years is not an exaggeration. It took approximately 100 years after Darwin for all relevant fields of science to come to a more or less consistent understanding of the history of this planet. That's how long it took for (1) paleontologists to dig up enough fossils, sort them, categorize them, and determine the relationships between them; (2) geologists to figure out how plate tectonics have shaped Earth over long…

*Worked slowly.

Of course science is much faster nowawadays; although there are no more facts about the Earth to be discovered as basic as the outline of it's geologic history. Even getting a book across the globe was a major issue I suspect; you'd have to slowly accrue interest from the community around you and divulge your work a lot for it to start propagating and eventually start having effects. Nowadays cutting edge papers are often put in the arXiv even before hitting the journals (which have instant online access), and I believe even speculative theories spread much faster by email, etc. And every researcher from every field has ready access to what happens on other fields.

Although I think it's not nearly as fast as it could be, and a large problem in our way are the big journal publishers. We need to restructure publishing urgently.

Re: Could dark matter not exist?

#59

Post by themartorana »

bdamm wrote: post #48
BurningFrog wrote: post #16

Earlier quoted context omitted.

The name isn't great. It sounds like matter that doesn't produce light, when it's really "matter" that's invisible. But not matter you can touch. Which makes it not really matter in normal English. You probably can't see it either. So "Invisible Untouchable Gravitation Sources" would be a more descriptive name. I don't think it'll catch on.

Perhaps it could be some form of friction we don't yet understand? I mean a form of gravitational friction? The nice part of being an armchair physicist is spouting off nonsense - I suppose if I really wanted to find out, I could make up a mathematical model of my theory and then find out if the evidence fits. Sadly, I really have no idea how to do that!

Ditto. Out of curiosity, is dark matter spread unevenly throughout the universe, or is it directly inverse to matter?

Like, when we expect dark matter causes an effect, is it because we expect there to be a massive dark-matter "object" acting upon matter, or can we attribute it to denser dark matter as space between matter increases?

Edit: would viewing dark matter provide a negative image of the universe?

I'm asking mostly because I'm curious if there is dark matter, "light" matter, and empty space, or only one or the other? I love imagining this stuff, as if two different universes in a multiverse - ours and one we can't see - are acting upon each other and causing what we strive to explain. As if they were passing through each other.

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