Dark matter galaxies, stars and planetoids

[Loren Booda]

Can and do they exist primarily separate from ordinary matter, and how might they be detected (e. g., gravitational lensing, Newtonian mechanics)?

 

[Chronos]

Dark matter - specifically the CDM model - does not gravitationally collapse to form such odd objects as stars and galaxies. Even in a cloud, the DM stuff just yoyo's between the halo and center of mass. DM particles are very unsociable and rarely interact even with one another. Detection methods are still works in progress.

 

[SpaceTiger]

Can and do they exist primarily separate from ordinary matter, and how might they be detected (e. g., gravitational lensing, Newtonian mechanics)?

Galaxies composed entirely of dark matter might exist, but they have yet to be detected. I don't think there's any easy way to detect them dynamically, but lensing would certainly do the job. Stayed on that subject, as gravitational lensing is in its prime.

Stars, by definition, are visible matter, so it wouldn't make sense for them to be dark. In the current model, as Chronos says, we're leaning towards weakly-interacting particles as the dark matter, so the "weakly-interacting" part would prevent them from collapsing into pressure-supported objects like stars.

As for planets, they are dark matter, whatever they're composed of, because they don't emit enough light for us to see them beyond the solar system. If there were 10^17 Earth's floating around inside of our galaxy, we wouldn't necessarily know about it. We have good reasons to think, however, that normal planets can't be the dominant form of dark matter.

 

[ohwilleke]

As for planets, they are dark matter, whatever they're composed of, because they don't emit enough light for us to see them beyond the solar system. If there were 10^17 Earth's floating around inside of our galaxy, we wouldn't necessarily know about it. We have good reasons to think, however, that normal planets can't be the dominant form of dark matter.

The term "dark matter" is usually used as a term of art to mean non-bayronic matter that is not visible which accounts for phenomena not explained by GR and visible matter and "ordinary" matter which is not visible.

Certainly everyone agrees that there is some ordinary matter which is "dark", like planets and hydrogen gas and even MACHOS. But, as these don't fit the bill in sufficient quantities they are often not thought of a true "dark matter".

 

[SpaceTiger]

The term "dark matter" is usually used as a term of art to mean non-bayronic matter that is not visible which accounts for phenomena not explained by GR and visible matter and "ordinary" matter which is not visible.

I realize that people often speak that way, but it's poor terminology and technically incorrect. If they mean that, they should say WIMPs or non-baryonic dark matter (depending on which they mean).

 

[selfAdjoint]

I realize that people often speak that way, but it's poor terminology and technically incorrect. If they mean that, they should say WIMPs or non-baryonic dark matter (depending on which they mean).

And what if they don't want to specify either one? BTW how is "non-baryonic dark matter" better than "dark matter"?

 

[Garth]

BTW how is "non-baryonic dark matter" better than "dark matter"?

Because it is non-baryonic!

DM has an average cosmological density of 23% closure density, whereas the standard model BBN only allows max 4% closure density, and that is pushing it, so according to the standard cosmological model DM has to be non-baryonic.

However the standard model may not be correct, for example the “Freely Coasting” Cosmology model produces about 20% baryonic closure density and so in that case DM, or most of it, might be baryonic.

Also there is a lot of dark baryonic density out there, in the form of BHs (MACHO's), possible free Jupiters, and the Lyman alpha forest IGM gas that could be the tip of an invisible (because it is dark!) iceberg. So if you mean the standard model it is more precise to specify "non-baryonic dark matter".

Garth

 

[SpaceTiger]

And what if they don't want to specify either one?

I'm not sure what you mean. Which possibility is being excluded?

BTW how is "non-baryonic dark matter" better than "dark matter"?

Garth covered the answer to this pretty well. My main objection is that it presupposes part of the solution to the dark matter problem (i.e. that it's non-baryonic WIMPs) which, although the current dogma, is not yet a sure thing. That said, my co-workers make the same mistake from time to time and I'm sure it's done in some papers, so it's certainly understandable that people would use "dark matter" that way.

 

[Chronos]

I admit it's naughty, but when I see "dark matter" I automatically assume it means non-baryonic. The other kind, while still extremely useful, is not that terribly interesting.