4 ms·

Do we expect to observe it in specific places and we do not, or is it just "not illuminated" or "not radiating" in the same way that other matter is?

by trevyn 7y ago

SiempreViernes 7y ago
It explicitly (almost) doesn't interact with normal matter, so it cannot be illuminated: it's dark because the light just goes right trough it, the same way space is dark because it doesn't scatter light.
- Sharlin 7y ago
  However, for a long time the main competing theory for WIMPs was MACHOs (yeah, really), basically "familiar" dim, compact objects such as brown dwarfs and black holes, scattered around in the galactic halo. However, gravitational microlensing surveys have found that the observed density of MACHOs is far too low to make up a significant fraction of dark matter.
Filligree 7y ago
Dark matter is technically anything that doesn't radiate, but often it's meant to refer to WIMPs -- weakly interacting massive particles. Now WIMPs are basically what it says on tin. We know they don't interact much, with themselves or anything else, because the rotation curves of the galaxies implies there are large halos. This means there's little to no friction, as otherwise the majority would fall down to the galactic cores, like normal matter does. We assume they're a separate type of particle from anything we know, as otherwise that shouldn't be possible. We know they're massive, as otherwise we'd see them in particle accelerators... well, we'd see the missing mass at least. Then, what are they? Well... Probably a lot of things. The "dark matter sector" outweighs visible matter by a lot, and there's no reason that should be just a single particle. But anyway, think of graph theory. You can map particles to nodes on a graph, and the fundamental forces to edges. Not all particles are connected by way of all forces. Electrons lack color charge; neutrinos lack basically everything. (Even when there's a connection, connections can be of varying strengths.) Dark matter is matter that lacks any connection to the other particles, except gravity. (Could there be particles that are unconnected by gravity? That gets speculative, but probably not. Gravity is special.) Ah, but-- there might be more than one dark matter particle, and forces connecting them that won't touch our type. For the most part we know this isn't true, since we know they don't interact with themselves either, but this is only to say that most dark matter can't be doing that. There absolutely could be dark-matter stars, or stranger things. ... This is purely speculative, though. Good for fiction, but the only kind of dark matter we know exists is the WIMPs.
- qubex 7y ago
  If WIMPs had forces between them, they’d form some kind of structure that would have been detected by other surveys for “dark” objects. Dark Matter is a bit of a misnomer... it’s invisible, transparent.
  - Filligree 7y ago
    They'd also have fallen to the center of the galaxy, probably following roughly the same mass distribution as visible matter. What I'm suggesting is just that there might be multiple dark matter particles, some of which might have structure. At present that is basically untestable, of course.
- perl4ever 7y ago
  To what extent are people confident dark matter WIMPs are not some sort of neutrino?
  - Filligree 7y ago
    Not very much. https://www.forbes.com/sites/startswithabang/2019/03/07/how-much-of-the-dark-matter-could-neutrinos-be/ https://www.forbes.com/sites/startswithabang/2019/03/07/how-... seems to cover the discussion fairly well.

Do we expect to observe it in specific places and we do not, or is it just "not illuminated" or "not radiating" in the same way that other matter is?

by trevyn 7y ago

Do we expect to observe it in specific places and we do not, or is it just "not illuminated" or "not radiating" in the same way that other matter is?

SiempreViernes 7y ago

It explicitly (almost) doesn't interact with normal matter, so it cannot be illuminated: it's dark because the light just goes right trough it, the same way space is dark because it doesn't scatter light.

Sharlin 7y ago
However, for a long time the main competing theory for WIMPs was MACHOs (yeah, really), basically "familiar" dim, compact objects such as brown dwarfs and black holes, scattered around in the galactic halo. However, gravitational microlensing surveys have found that the observed density of MACHOs is far too low to make up a significant fraction of dark matter.

Filligree 7y ago

Dark matter is technically anything that doesn't radiate, but often it's meant to refer to WIMPs -- weakly interacting massive particles. Now WIMPs are basically what it says on tin. We know they don't interact much, with themselves or anything else, because the rotation curves of the galaxies implies there are large halos. This means there's little to no friction, as otherwise the majority would fall down to the galactic cores, like normal matter does. We assume they're a separate type of particle from anything we know, as otherwise that shouldn't be possible. We know they're massive, as otherwise we'd see them in particle accelerators... well, we'd see the missing mass at least. Then, what are they? Well... Probably a lot of things. The "dark matter sector" outweighs visible matter by a lot, and there's no reason that should be just a single particle. But anyway, think of graph theory. You can map particles to nodes on a graph, and the fundamental forces to edges. Not all particles are connected by way of all forces. Electrons lack color charge; neutrinos lack basically everything. (Even when there's a connection, connections can be of varying strengths.) Dark matter is matter that lacks any connection to the other particles, except gravity. (Could there be particles that are unconnected by gravity? That gets speculative, but probably not. Gravity is special.) Ah, but-- there might be more than one dark matter particle, and forces connecting them that won't touch our type. For the most part we know this isn't true, since we know they don't interact with themselves either, but this is only to say that most dark matter can't be doing that. There absolutely could be dark-matter stars, or stranger things. ... This is purely speculative, though. Good for fiction, but the only kind of dark matter we know exists is the WIMPs.

qubex 7y ago
If WIMPs had forces between them, they’d form some kind of structure that would have been detected by other surveys for “dark” objects. Dark Matter is a bit of a misnomer... it’s invisible, transparent.
- Filligree 7y ago
  They'd also have fallen to the center of the galaxy, probably following roughly the same mass distribution as visible matter. What I'm suggesting is just that there might be multiple dark matter particles, some of which might have structure. At present that is basically untestable, of course.
perl4ever 7y ago
To what extent are people confident dark matter WIMPs are not some sort of neutrino?
- Filligree 7y ago
  Not very much. https://www.forbes.com/sites/startswithabang/2019/03/07/how-much-of-the-dark-matter-could-neutrinos-be/ https://www.forbes.com/sites/startswithabang/2019/03/07/how-... seems to cover the discussion fairly well.