# Dark Matter Question

Discussion in 'Astronomy, Exobiology, & Cosmology' started by siphra, Jul 30, 2012.

1. ### siphraRegistered Senior Member

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Let me start by saying I haven't searched hard enough.

If Dark Matter is is 84% of the mass in the universe, and has only a gravity interaction (for the purpose of this question all other interactions are irrelevant) it seems to me that it should clump mixed in with the regular matter. If this is the case why then wouldn't our own sun, and planets be 84% dark matter?

I realize there is a reason out there, but can someone link me a good, detailed reason, or even better, explain it here?

Thank you.

3. ### OnlyMeValued Senior Member

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Some may try, but I doubt they will succeed. Dark Matter is a place holder to explain observations that are not explained otherwise. Read "Dark Matter" as "Unknown Matter".

You are coorect though it would seem that it should clump together with ordinary matter, but through observations of gravitational lensing there are areas it does not appear to do so.

5. ### FTLinmediumRegistered Senior Member

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Right, it's just unexplained mass; it does not seem to clump as we would expect of massive particles. That's pretty much all we know- we don't know that it's a material at all.

It's almost like it exerts gravity, but doesn't obey it.

If it is some kind of legitimate material, rather than just a burp in our calculations:

This could be possible if it exerts of repulsive force against itself (something that could be possible with exotic materials, but hasn't been demonstrated on a large scale).

Perhaps if the matter has an enormous wavelength and so some kind of very low density; this might be possible if it is very cold and can't interact with light (but then I would expect the particles to behave as a condensate and occupy each-other's space rather than behaving as strict fermions)

Maybe it's something with a very small mass moving very quickly, so the velocity is sufficient to keep it from clumping (like a cloud of neutrinos)

Or something else, like space-time is just bumpy.

Lots of possibilities (you can find some lists somewhere, though I don't have a link off hand)

7. ### mathmanValued Senior Member

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Partial explanation of why it doesn't clump. Ordinary matter is held together by electromagnetic force (atoms and molecules) as well as nuclear forces. Since dark matter is unaffected by these forces there is nothing to make it clump, since gravity by itself won't do unless you get a lot of it to clump in the first place.

8. ### FTLinmediumRegistered Senior Member

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It seems that wouldn't stop it being pulled in and clumping around normal material (like stars and planets) though.

9. ### Janus58Valued Senior Member

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Yes it would. Imagine two atoms of ordinary matter being drawn together by gravity. The closer they get, the faster they move towards each other. when they get close enough, their electromagnetic fields begin to interact. It is this interaction not only allow the particles to collide, they can cause them to stick together. Not only that, but this interaction causes an acceleration of the particles. When you accelerate charged particles, they radiate electromagnetic energy. This energy comes at the cost of their kinetic energy. IOW, the atoms will be moving slower after the collision than before. Even if they bounce apart after collision, they are moving slower and are more likely to be pulled back together by. A large number of these collisions by a large number of atoms bleeds off enough energy to cause the atoms to settle into a clump.

Now envision to particles of Dark matter being drawn together. They also get faster as they approach. However, they do not have electromagnetic fields nor do they interact with them. Thus thye have no mechanism that allows them to "collide". They would pass through each other as if the other wasn't even there. (This would be also true for a DM particle meeting a ordinary atom, it cannot react to the atom's field and behaves as if it isn't there.)

The DM particles will interact gravitationally which will alter their trajectories, however since they cannot emit electromagnetic radiation, this acceleration does not cause them to radiate energy away and slow down like ordinary atoms do. The DM particles will come together and separate again leaving each other at the same speed at which they met. Since they can't bleed off kinetic energy, a large collection of DM has no tendency to clump up into a smaller more compact mass (With one caveat which I'll bring up in the next paragraph.)

The caveat, that I mentioned is that since DM does interact gravitationally, it is subject to emitting gravitational energy. So when it interacts it will radiate some energy away in the form of gravity waves. However, compared to electromagnetic radiation, gravitational radiation is extremely weak, which means this process is really slow and takes a long time to cause any appreciable clumping. As a result we do see some clumping when it comes to DM, but is only on really large scales, Galactic halos ,etc. This is because it just hasn't had enough time to clump any tighter.

10. ### RealityCheckBannedBanned

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Hi Janus58.

I was just on my way out when I caught your post.

Hmmm. How would that work for the colossal Black hole features at the centre of galaxies?

Wouldn't their massive gravity not only attract any dark matter doing by but also trap it like any other matter falling below its event horizon?

Or is it possible that DM can also 'tunnel through' the Black Hole 'domain' just as it is implied it does through ordinary matter features?

Any thoughts on that possibility/necessity if we are expected to assume that DM does not get trapped by gravity to the extent that all galaxies should be 'harvesting' DM from their surroundings and concentrating it at their black hole centre?

Gotta go. Back in a couple days. Bye!

.

11. ### FTLinmediumRegistered Senior Member

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RealityCheck, dark matter, as Janus58 suggested, would likely be trapped by a black hole. The singularity involves time dilation- so it would "slow down" due to that, and never reach the other side (at least, not for a very, very long time if it did- it might have to wait for the black hole to decay) even if it didn't interact with anything inside the black hole.

12. ### Janus58Valued Senior Member

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Black holes can capture DM, however, you would not expect them to contain large percentages of it. There are two reasons for this:

The first is that since ordinary matter tends to clump more readily than DM, forming such structures as planets, Sun and Galaxies, and black holes form from concentrations of matter, they will form where there is more ordinary matter. To give you an idea of what I mean: DM is more or less evenly distributed in a sphere that is much larger than the visible galactic disk which contain the majority of the ordinary matter. So, IOW even though there is much more DM, it is spread out in a much much much larger volume. So, in the regions where black holes are likely to form, there will be much more Ordinary matter. (For example, the estimated total amount of DM expected in the Solar System is about equal to the mass of a large asteroid.) At the center of the Galaxy, the density of Ordinary matter is even higher.

The second reason goes back to the previous discussion. Ordinary matter collides and radiates away energy. As a result, matter that swings in close to a Black hole tends to collide with other matter in the region, radiate away energy and fall in closer to the BH. In this way, matter that initially did not have a trajectory that would bring it across the event horizon, will find itself falling into the black hole.
DM doesn't collided or radiate to the degree that ordinary matter does. This means that for DM to be captured by a black hole, it must start with a trajectory that intersects the event Horizon. This effectively makes the black hole a smaller target to hit for DM than it is for ordinary matter.

13. ### FTLinmediumRegistered Senior Member

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Janus58, great explanation!

I like the point about the collision course; otherwise the DM would take a very, very long time to pass the event horizon if it were caught in orbit.

Would you expect to find a disproportionate amount of DM in orbit around a black hole vs. zipping around in space generally? Or would you expect that the black hole just deflects the trajectory of the DM and slings it away? Is its radiation of energy enough to get it caught on orbit in any significant way at all, even with a black hole?

14. ### Crunchy CatF-in' *meow* baby!!!Valued Senior Member

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You are correct and it in fact does; however, it has no means to bind to regular matter.For example, let's say there is a baseball on the ground and it has some dark matter clumped within it somewhere. If I pick up the baseball, the dark matter is left where the baseball used to be, simply because the baseball went right through it when I picked it up.

Nope, 84% is an estimate of the total mass in our universe. The notion that it is evenly distributed amongst all normal matter at exactly 84% is an error in your thinking.

15. ### wlminexBannedBanned

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Perhaps . . . . DM is actually "equivalent" mass expressed as energy (re: E = mc^2) and it is a driving force for universe expansion (cosmological constant?). It's (energy) characteristics may be such (>>>energy and frequency) that it simply does not significantly interact with matter. It may also be a manifestation at quantum (or subquantal) scales.

Last edited: Aug 1, 2012
16. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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It appears as if you wrote some scientific sounding terms on scraps of paper and put them in a hat, shook them up and then randomly pulled them back out and wrote them in order that you removed them for your post. Fine job, fine job indeed.

17. ### AlphaNumericFully ionizedRegistered Senior Member

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And perhaps gravity is invisible fairies pushing things around. Perhaps....

Dark energy and dark matter are different things. And since models of gravity already include $E^{2} = m^{2} + p^{2}$ but which have different galaxy rotation rates than observed, as well as perturbations in the microwave background, just invoking Einstein isn't enough.

wlminex, that's a line between informed hypothesising and random guessing. Unfortunately you're actually on the wrong side of the line separating random guessing from just plain incoherence. If you can't even make your posts coherent, instead just throwing out buzzwords in ways which are meaningless then don't post. Even if, by some random chance, the idea you're trying to get across is accurate you have utterly failed to explain what you're talking about and thus the post is just pointless. If you're going to post please make the effort to make yourself coherent.

18. ### wlminexBannedBanned

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Origin #13: " . . . . Fine job, fine job indeed." . . . . Thanks for the compliment, Origin! (humor here)
AN #14: . . . . . . . . . . And your point IS? (humor here)

Last edited: Aug 2, 2012
19. ### kaduseusmelencolia IRegistered Senior Member

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It could also be a burp in the observation methods, since the angular velocity of galaxies can be measured at a distance from the optical image, the apparatus needs further experimental testing.
Parabolic caustics would cause the shortest path light to be rotated in proportion to the distance from the center of the reflector, long slit methods and IFUs would simply register the rotated light as a spectrum shift.
Nobody wants to do the experiments though, I can see why, it would be very embarrassing to admit you'd spent 50 years and way too much money measuring the curvature of your mirror.

So to account for the observations.... dark matter must exist in the space between galaxies, it can't be in the galaxy at all, if you placed it in the galaxy it would contradict your observations, you'd have to add more dark matter outside the galaxies to account for the dark matter in the galaxies.
So no the planets and sun aren't made from 84% dark matter.
To account for the observations 84% of the universe consists of matter external to the galaxies, which we can't see or detect, which is why it's dark.

20. ### AlphaNumericFully ionizedRegistered Senior Member

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wlminex, I've told you before about this. Putting (joke!) and the like at the end of a post which is just noise doesn't add to the discussion. If you don't know any science pertaining to the discussion then don't say anything. That includes posting advertising for your pet theory. You have a thread on it in the Fringe forum and anyone who is interested in non-mainstream stuff will be looking in that forum anyway.

If you think just jamming together some words you have seen on Wikipedia but don't understand counts as giving a constructive response you're mistaken. Even if you understand the terminology you use your style of posting is extremely disjointed and often borders on incoherent. If you can't make the effort to construct posts properly don't expect people to read them.

That's a 'friendly' comment, please try not to make another comment necessary, here or elsewhere in this subforum.

21. ### wlminexBannedBanned

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AN: . . . SO . . .there's no room for humor in response to assnine comments posted in these discussions? My point in injecting (humor here) is so that readers, or argumentative respondents, do not mistakenly interpret the meaning of the post. . . . . BTW, the "advertising" (your verbage) Was originally started in 'Alternative Theories', which was appropriate . . . one of YOU, admin or moderators, apparently moved it to 'Fringe'.

Last edited: Aug 4, 2012
22. ### prometheusviva voce!Registered Senior Member

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Alternative theories is a board within the fringe category.

23. ### wlminexBannedBanned

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P: I stand corrected . . . thanks for the clarification