Dark Matter Stars

[CaptainToonces]

If dark matter makes up a large portion of the mass in the galaxy, and exerts gravity on itself, why doesn't it squeeze together to form stars?

[trinitree88]

Cap'n. It does. Unlike regular baryonic matter, with protons and neutrons and electrons, heating up and emitting visible photons, with some neutrinos....dark matter stars emit neutrinos only and your eye can't see them. They are really shiny.:shifty:

[ngc3314]

Or for a more completely mainstream response - dark matter does not seem to have any way to dissipate energy as baryonic matter does ("cooling"), so it can collapse only via the very slow gravitational exchange of energy between particles. This would take vastly longer than the age of the Universe to get much action out of. The strongest evidence behind this is that we don't see small-scale lumpiness in dark matter as seen from gravitational lensing (and in fact it seems to be rather smoother than some theoretical treatments expect).

[CaptainToonces]

Cap'n. It does. Unlike regular baryonic matter, with protons and neutrons and electrons, heating up and emitting visible photons, with some neutrinos....dark matter stars emit neutrinos only and your eye can't see them. They are really shiny.:shifty:

So does the creation of the neutrinos from these stars hold the stars up against gravity like nuclear fusion does for baryonic stars? Do the black matter stars eventually collapse into black holes or something else?

[trinitree88]

So does the creation of the neutrinos from these stars hold the stars up against gravity like nuclear fusion does for baryonic stars? Do the black matter stars eventually collapse into black holes or something else?

Cap'n. Nope. I was being a wise guy here. Nothing produces only neutrinos, and their cross-sections for interacting are prohibitively low.NGC's answer is more mainstream. The reason for me is I have a hard time buying into lots of exotic things that are never seen in either cosmic ray studies or in accelerator labs. There is sufficient energy in CR studies to see anything we can dream of, yet nothing unusual appears there. Accelerator labs with exquisite control over energy ranges of interactions at a lower level,also see nothing beyond the Standard Model. There have been far too many coffee table books spouting gibberish over the last twenty years, leading people to believe in lots of stuff that has no experimental basis in fact.
It remains a mystery why the rotation curves of galaxies can get very non-Keplerian, but the Stephan's Quintet thread gives a real possibility in ATM, using real baryons, knowable physics. The issue is sensitivity....will we be able to separate signal from noise carefully enough? catch a falling star. pete

[CaptainToonces]

Why doesn't dark matter attract itself closer and closer until it forms a black hole?

[Strange]

Why doesn't dark matter attract itself closer and closer until it forms a black hole?

see post #3

[Hornblower]

Why doesn't dark matter attract itself closer and closer until it forms a black hole?

Perhaps for the same reasons that most of the visible components of a galaxy do not fall in on themselves to form or add to a black hole.

[mikeg64]

Pauli exclusion principle might help! Doesn't explain spin 1 particles though!

[WayneFrancis]

Very basic answer is that dark matter does not seem to interact with itself. The bullet cluster shows a signature of 2 galaxies colliding. All the barionic mater interacting and the "dark matter" signatures seemly have passed through each other with little effect.

Not sure what the implications of this are for all other merging/merged galaxy. I'd expect the answer is along the lines of #3 that it does interact with itself just as a much lesser scale.

[CaptainToonces]

I'm still having trouble understanding. Is dark matter attracted to itself via the gravitational force? Does it spin with the galaxy or just sit there? Does it attract itself but pass right through itself so as to avoid accretion?

[Jeff Root]

All your guesses-- but especially your last guess-- are what the
mainstream view appears to be.

Ordinary matter clumps because gravity attracts gas particles
together, then electric forces sometimes hold them together when
they bump into each other. Without the electric forces, the matter
would not be able to clump, and no stars or planets could form.
All matter would be gas. That's what dark matter seems to be.

But there's a pretty good chance it will turn out to be something
completely different from what it seems to be.

It is thought that dark matter responds to and causes gravity the
same as ordinary matter. To the extent that it *does* clump, due
to "virialization", in which less-energetic particles go into low
orbits and more-energetic particles go into higher orbits, the dark
matter should also orbit the galaxy. It would not have to orbit in
the same direction as the visible matter, though, and it would not
be confined to a disk like most of the visible matter is.

-- Jeff, in Minneapolis

[WayneFrancis]

Like Jeff says we just don't know enough about it right now to give good answers ... not sure what to say... check back in a few years maybe.

[Baryonic Matter]

This thread answered a number of questions of mine! Thanks all! I plan on posting here a lot!

[Ken G]

Perhaps the answer in post #3 can be expounded on to make sure its message is getting across. All matter attracts itself via gravity, but this is not enough to make the high densities of a star, because attracting matter has a tendency to "fall through" itself and "come out the other side" to the same distance as where it originally fell from. Even if it collides with itself, it still "bounces off" like a super ball, unless it has some way to dissipate energy (which is the "cooling" business). A star is very hot, but actually the energy is much less than the giant molecular cloud that made the star, because a huge amount of gravitational potential energy was lost in making the star. About half that energy went into making the star hot, but the rest had to be able to escape or else you can't make the star-- it just re-expands back to a giant molecular cloud, in effect. The best way to lose energy is to emit light. That's what stars do, even as they are still forming, and that's just what dark matter does not do. Since it is not good at losing energy, dark matter is also not good at collapsing into stars, though it can make galaxy-size overdensities (and that's what sucks in the baryonic matter that does make stars).

[transreality]

It seems dark matter can't interact with photons, neither emitting or absorbing them, and these are processes that almost characterise a star.