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Still, dark matter as a theory, or at least as the name of a theory, has the big problem to predict something that is conveniently invisible. A proponent of MO

Still, dark matter as a theory, or at least as the name of a theory, has the big problem to predict something that is conveniently invisible. A proponent of MOND could probably declare the existence of some form of dark exotic matter in galaxies that behave "Newtonian". My point is: As long as a dark matter theory doesn't make predictions that can be experimented upon, we will always have these kind of interpretation games. Side question: What would actually distribute dark matter throughout the universe in the first place? If it is only affected by gravity and there is much more dark matter than visible matter, shouldn't it be forming a huge invisible ball somewhere?

gliptic 5y ago
So they would propose dark matter+MOND? I will just throw out MOND then because it's superfluous.
- choeger 5y ago
  It's an interesting philosophical question whether a more complex theory and a small amount of invisible matter or a simpler theory and a large amount of invisible matter is the simpler combination. In any case, I think that any modified theory of gravitation has to fight an uphill battle because it seems to be easy to come up with a configuration of dark matter that explains any observation. That's my biggest grievance with DM, btw.: It doesn't make any predictions that could be used to falsify it, at least not in a sense that I could intuitively understand it. Effectively, it appears to act as a completely free parameter, distributed however we need it at the moment.
andreareina 5y ago
Something that is only affected by gravity won't tend to collect in one place because whatever gravity would attract it there also gives it momentum to get out. You need collisions to get things to lose momentum to heat and clump together. Normal matter has collisions via interaction with the electromagnetic field, dark matter doesn't. Neutrinos also don't clump together, but are disqualified as dark matter candidates due to there not being enough of them; something about the relative abundance of different fundamental particles (e.g. electrons and neutrons) being important to get the universe we observer today. ETA: modified gravity doesn't give anything we can experiment on either; the difference of TeVeS[1] vs GR are not detectable at non-cosmological scales. [1] https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
Maursault 5y ago
> Still, dark matter as a theory, or at least as the name of a theory, Your point is well-taken, but I have to be pedantic here (and everyone loves to make this mistake, so don't worry about it). Dark Matter is not a theory,, not like the General Theory of Relativity is a theory. Dark Matter is a hypothesis.
mhh__ 5y ago
Invisible to our intuition perhaps.

gliptic 5y ago

So they would propose dark matter+MOND? I will just throw out MOND then because it's superfluous.

choeger 5y ago
It's an interesting philosophical question whether a more complex theory and a small amount of invisible matter or a simpler theory and a large amount of invisible matter is the simpler combination. In any case, I think that any modified theory of gravitation has to fight an uphill battle because it seems to be easy to come up with a configuration of dark matter that explains any observation. That's my biggest grievance with DM, btw.: It doesn't make any predictions that could be used to falsify it, at least not in a sense that I could intuitively understand it. Effectively, it appears to act as a completely free parameter, distributed however we need it at the moment.

andreareina 5y ago

Something that is only affected by gravity won't tend to collect in one place because whatever gravity would attract it there also gives it momentum to get out. You need collisions to get things to lose momentum to heat and clump together. Normal matter has collisions via interaction with the electromagnetic field, dark matter doesn't. Neutrinos also don't clump together, but are disqualified as dark matter candidates due to there not being enough of them; something about the relative abundance of different fundamental particles (e.g. electrons and neutrons) being important to get the universe we observer today. ETA: modified gravity doesn't give anything we can experiment on either; the difference of TeVeS[1] vs GR are not detectable at non-cosmological scales. [1] https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...

Maursault 5y ago

> Still, dark matter as a theory, or at least as the name of a theory, Your point is well-taken, but I have to be pedantic here (and everyone loves to make this mistake, so don't worry about it). Dark Matter is not a theory,, not like the General Theory of Relativity is a theory. Dark Matter is a hypothesis.

mhh__ 5y ago

Invisible to our intuition perhaps.