IIRC the only way we have to detect Dark Matter is through gravity; it does NOT interact with normal matter as DM has no electrons to provide a “solid feel”…and apparently, is invisible.
If DM also cannot interact with itself, would it be safe to say that DM won’t form clumps of solid, like asteroids or whatever…DM would just fall together and coalesce into a geometric point, much like a Quantum Singularity?
It will just fall, but that doesn’t mean it’ll collapse. If a particle of dark matter has any orbital angular momentum (which it almost certainly does), then it’ll just orbit, same as any normal matter particle will. In practice, this leads to clouds of dark matter surrounding any concentration of ordinary matter, like a galaxy.
No, you’ve got it backwards, it’s interactions that let matter lose speed and clump together. Say you have a star-mass cloud of normal matter gas. Gravity starts to pull the individual pieces together, converting gravitational potential energy to velocity. At first if they interact they just bounce off of each other, but as they start to fall towards the center the density increases and they start bouncing off of each other routinely instead of continuing in a straight line. This effectively turns their velocity into heat, and over time they stick into a hot, dense mass instead of just flying around through space.
With dark matter, there’s no ‘bounce’. Particles attract and fly towards and through each other, but there’s nothing for them to bounce off of, so they just keep flying past each other with no friction. There’s no method for them to cluster together or easy way to lose velocity, so the individual particles just keep shooting around through space even if it means going through ‘solid’ objects. Instead of ‘falling together’ the particles just keep ‘falling past’ each other.
Also, as a minor point, whether dark matter interacts with itself through other force(s) than gravity is still unknown.
Some observations imply that actually it does: I recall in the news last year dark matter was found to be trailing a galaxy, which for some reason implies dark matter self-interacting.
But it seems, yes, strong interactions as would be required for dark matter solid objects I think has been ruled out.
Here’s an article in Scientific American reporting on the evidence that Mijin alluded to, that dark matter may feel a “dark force”. Theories of “self interacting dark matter” are still considered unorthodox though.
–Mark
Dark matter possibly interacting with itself is more orthodox these days. Also, dark matter can interact with regular matter, just not very strongly. This is the basis of “direct detection” searches for dark matter, in which physical detectors (made of regular matter, naturally) are used to look for rare dark-matter-on-regular-matter collisions. There could also be multiple species of dark matter particles. The idea of a single new particle that doesn’t interact with anything except through gravity is the minimalist thing to try, but nothing about the Standard Model (which describes regular matter and its interactions) suggests that the dark matter part of nature ought to be minimalist. As available data improves, the ideas of additional interactions and additional species of dark matter start to become attractive due to potential issues with the minimalist version.
Eh, I dunno. It seems to me that assuming a bunch of different dark matter particles, each one about as common as “normal” particles, is a more minimalist assumption than assuming that there’s one single particle that makes up three times more than the other 20 or so combined.
A single particle with no interactions is clearly the minimal thing (in terms of what you write down on paper), but that doesn’t mean it’s the most “natural”. I think that’s what you’re saying, just with a different choice of definition for “minimal”.
For comparison, consider a massive neutrino. It doesn’t participate in the electromagnetic or strong interactions, and its role in the weak interaction is mostly to balance momentum in decay processes. Combined with its very small mass, it’s basically sterile. On the other hand, they’re extremely common yet hard to detect directly. Despite that, they don’t clump into any sort of singularity; for one thing, their masses are so small that the effect of gravity on them is practically nil, and their production processes impart some (usually high) momentum to them.