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elite4koga t1_iu973f3 wrote

Reply to comment by forte2718 in Is dark matter orbiting galaxies with the same speed as normal matter? by taracus

I wasn't attempting to refute dark matter as a particle or come off as a MOND supporter, I just wanted the poster to provide more info about the knowns vs unknowns when responding to the question.

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forte2718 t1_iu995o2 wrote

That's fine. I was just responding to try and correct some of the misconceptions in what you said — about dark matter never being directly observed (it has), about us not having strong evidence about its location (which we do), about the certainty of statements about dark matter's other properties, etc.

A lot of this info is outlined in encyclopedia articles like Wikipedia's, so it's pretty accessible. We get a lot of boneheads who won't even do that tiny amount of research before insisting that dark matter is some kind of hoax or fudge factor, so apologies if I stomped on your foot there, that wasn't my intention. I was just trying to respond to the things you said and point out that they aren't correct, lest those misconceptions propagate and get even further out of control. :(

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Throwaway_97534 t1_iu9gtyo wrote

If we were in space at relative rest and there was an earth-mass chunk of dark matter that wandered into our path, does that mean we would suddenly find ourselves falling toward it for seemingly no reason?

I'm assuming we haven't observed dark matter at smaller than galactic scales, but I'm wondering if the current theories and observations allow for smaller amounts as well.

Can we run into planet-sized bits of dark matter just like we can run into planet-sized primordial black holes? One of the theories is that we haven't observed a 9th planet in the solar system that's shepherding trans-neptunian objects because it may actually be a primordial black hole... Could it also be a small bit of dark matter?

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forte2718 t1_iu9iwcn wrote

>If we were in space at relative rest ...

At relative rest with respect to what? Remember ... all speeds are relative to other things! :)

>... and there was an earth-mass chunk of dark matter that wandered into our path, ...

Dark matter doesn't really clump up the way ordinary matter does. The only way you would get this is if dark matter happens to be in the form of "MACHOs" (massive compact halo objects) such as black holes, but that hypothesis has almost been completely ruled out except for a range of masses that excludes things anywhere near the Earth's mass. So, this just isn't an actual possibility, I'm afraid!

>... does that mean we would suddenly find ourselves falling toward it for seemingly no reason?

We would feel its gravitational effects, yes. If you had something like an Earth-mass MACHO, it could do things like disrupt orbits, to the extent that something of Earth mass can do so (naturally something like the Sun or Jupiter wouldn't be significantly affected, but smaller planets would).

>I'm assuming we haven't observed dark matter at smaller than galactic scales, but I'm wondering if the current theories and observations allow for smaller amounts as well.

Unfortunately, not really ... at least, not also in grouped clumps that are within a few orders of magnitude of the Earth's mass. You can have smaller amounts if it is very diffuse (like axions or neutrinos or some other particulate form that doesn't interact electromagnetically) but not if it clumps up due to an interaction of some kind.

>Can we run into planet-sized bits of dark matter just like we can run into planet-sized primordial black holes?

That's a hard no, unfortunately. The observational evidence (at least that which I am aware of) excludes as a form of dark matter MACHOs including primordial black holes in the mass range of 10^(-8) Earth masses or higher. [1]

>One of the theories is that we haven't observed a 9th planet in the solar system that's shepherding things because it may actually be a primordial black hole... Could it also be a small bit of dark matter?

It and things like it couldn't be a significant form of dark matter, no. I don't see why it couldn't be a black hole though, whether primordial or otherwise.

Hope that helps,

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Victra_au_Julii t1_iuaj791 wrote

>about dark matter never being directly observed (it has),

Do you mean a type of dark matter has been observed? That doesn't mean this particular observation is the dark matter that is responsible for the differences in our calculations - measurements for galaxies.

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forte2718 t1_iuao3ig wrote

>Do you mean a type of dark matter has been observed?

Yes as I explained two posts prior, we have observed dark matter directly — three types of dark matter, in fact: the electron neutrino, muon neutrino, and tau neutrino.

>That doesn't mean this particular observation is the dark matter that is responsible for the differences in our calculations - measurements for galaxies.

Yes, I explained that too, also in the second to previous post. Perhaps you should go back and read it!

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Victra_au_Julii t1_iubkfor wrote

I was being tongue in cheek. No we have not observed dark matter, as in, the dark matter that makes the massive difference in mass observed in the universe. Those things you mentioned are not what makes up most Dark Matter.

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