This week on the Astronomy Cast podcast the topic was galaxy formation. Apparently, before the luminous (baryonic) galaxies coalesced under gravity, the non-baryonic dark matter had already begun collecting into large structures that eventually formed the scaffolding for the galaxies. As I understand it, so-called dark matter is affected by gravity but does not interact with light (except to alter its trajectory by bending spacetime).

Here is my question: If dark matter is affected by gravity (and exerts its own gravitational pull, or bends spacetime in the same way and to the same extent as baryonic matter of equal mass), then shouldn't we expect to find dark matter in the middle of large gravity wells of baryonic matter? Shouldn't the dark matter and the regular matter "fall down the same holes"?

Two follow-ups: (1) If so, then won't a certain amount of the mass of the sun actually be dark matter? And if that's so, then wouldn't the actual amount of elemental fuel for the successive types of fusion be less than expected? (2) If not, then what keeps the dark matter out?

--Ken Sibley, Greentown, IN

There's an important difference between ordinary matter and the dark matter, which is a consequence of the fact you already mentioned. Since dark matter doesn't interact with light, it cannot radiate away energy easily. In a cloud of ordinary gas, the individual atoms are flying every which way. When two atoms pass close to one another, they interact via (mainly) electromagnetic forces. One atom may bump into the other, shoving it and increasing its speed; or the first may slow the second down. The collision may excite one atom into a higher energy state, and then, after some period of time, that excited atom may emit a photon and return to its original energy state.

These interactions between atoms can dissipate both energy and angular momentum. Over a long period of time, a big cloud of gas may collapse into a rotating disk under the influence of its own gravity; after more time, the inner regions of a disk may collapse into a star.

Dark matter particles, on the other hand, do not interact with each other -- or with ordinary atoms -- in the same way. Their only (or main) type of interaction is gravitational. As a result, dark matter particles cannot lose energy or angular momentum via numerous little collisions and near collisions ... and so a big cloud of dark matter will not collapse and shrink to a much smaller size.

Here's an analogy: imagine a smooth road which dips down into a streambed and then rises out of it. That road will represent a gravitational potential well. Place an ordinary tennis ball on the road at the top of the dip. The ball will roll down the slope, across the bottom, then start to roll up the other side. However, because it loses a little bit of its energy to friction as it rolls, it won't make it all the way back up the other side; instead, it will roll back down, across the bottom, and back towards you. But it won't make it up your side, either. It will oscillate back and forth across the streambed, losing some energy each time, until it comes to rest in the middle of the streambed. That tennis ball is like ordinary matter: it eventually settles down into the middle of a gravitational well.

Dark matter, on the other hand, might be represented by a levitating object -- sort of like an air-hockey puck -- which can slide above the surface of the road without any friction at all. If you release it from the top of your side of the streambed, it will slide down the slope, across the streambed, and up the other side ... all the way to the top. It will _not_ lose any energy, and it will _not_ eventually settle in the middle of the streambed.

And that Non-Baryonic Dark Matter is going directly through the sun.

SO, Non-Baryonic Dark Matter is collisionless with itself and ANY baryonic matter, which means that it has a 'straight-line' path (Unless it is completely motionless?) that cannot be altered by baryonic matter or any collisions with itself.

SO, Please explain HOW Dark Matter 'spherical' Halos can form around galaxies?
How is that DM halo moving, ie; is it rotating spherically with the galaxies rotation?
What speed is it moving at?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

The path is altered by gravity. It won't be a "straight line" unless the transverse component of its velocity is zero. Its path will be a conic section. just like any other mass.

They don't! The dark matter was there first and the baryonic matter was attracted to it.
The dark matter in the halo moves the same way that stars in globular clusters move. Each particle is own its own conic-section orbit.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Hum,
Very slow moving Darkmatter could possibly have accumulated in the interior of the Sun.
This would result in the Suns density being higher than it should be, and there is a possibility (though unlikely) that it would have a cooling effect on the interior solar temperature and associated effect on the fusion processes.
The higher density may lead to perturbation effects on the orbits of the planets.

I have seen computer simulations that require some cold dark matter to explain the origin of the large scale structures seen in the universe today. But, most of this slow moving dark matter would now reside in galactic blackholes, imho.

The darkmatter halo may consist of slow/relativistic moving particles (at least 9 km/s) .
This is moving slow enough to be gravitationally bound to a cluster of galaxies, (or the matter bound to the DM) etc

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`Irony` actually does mean `metal like`...

I agree, BUT you have to be very careful here!!!

BUT, since there can be no transverse component to its velocity it MUST be a straight line.

What NO ONE is getting is that when Einstein said "the speed of light in 'empty' space is "c" AND "space is curved by ponderable matter", that these were both talking about baryonic matter NOT being present for 'empty space' and baryonic matter (clumped) curving space.

SO, neither of these was taking Planck length Non-Baryonic collisionless matter into account.

So this simply isn't true of Non-Baryonic Dark Matter.

SO those spheres (If they really exist) MUST form somehow...How?

So, are you saying that the spherical halo is rotating with the influence of the SMBH in the center of the galaxy as its main source of gravitational influence?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

[The darkmatter halo may consist of slow/relativistic moving particles (at least 9 km/s) .]

I see you saw that article on this. This is complete nonsense IMHO. When they talk about the difference between 'hot' and 'cold' Non-Baryonic DM, they are determining the hot/cold and therefore 'speed' based on the environment in which they are considering it!

It also makes it sound as if there are "DIFFERENT" particles that are capable of 'going through' baryonic matter. IMHO, again, this is where particle physics has become confused...there is baryonic matter (that cannot go through itself) and then there is a 'base particle'... Planck length Non-baryonic DM that makes up all of 'space'.

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

No, you are mistaken. As Kaptain K pointed out, dark matter is accelerated by gravity, just as ordinary matter is.

Gravitational interactions between clumps of dark matter can cause some clumps to lose angular momentum and energy. In addition, gravitational interactions between dark matter and _regular_ matter -- which can dissipate energy and angular momentum -- can lead the dark matter to coalesce, to some extent. It's not an efficient process, which is one of the reasons that dark matter halos are so extended compared to baryonic matter.

No, the influence of a SMBH at the center of a galaxy on the non-baryonic dark matter in that galaxy's halo will be minimal. The regular matter distributed throughout the galaxy will have a larger effect. The non-baryonic dark matter itself will have the largest effect. Halos need not be rotating, by the way; they may be triaxial.

Why don't you spend a few days reading some recent papers describing the formation of structure on galactic scales? There are plenty of papers on the topic freely available on astro-ph.

Huh? Every particle of dark matter in the universe is heading straight toward (or away from) the Sun? Aren't we special!

As far as gravity is concerned, matter is matter, whether baryonic or dark.

"Planck length Non-Baryonic matter" is definitely ATM. The consensus is that DM is (as yet) undiscovered supersymmetric elementary particles.

Gravity. See the post above by StupendousMan for details.

No! I'm saying that each individual particle of DM is orbiting the barycenter of the galaxy (baryonic and nonbaryonic). The (baryonic) mass of the galaxy is on theorder of 200 billion solar masses. Against that, the 2 or 4 million solar masses of the SMBH is negligible outside of the core.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Thanks! I'm grateful for all your thoughtful replies.

[Quote:RussT]
SO those spheres (If they really exist) MUST form somehow...How?[/quote]

Sorry, I wasn't clear enough. Since DM is collisionless with itself or baryonic matter how can it ever clump anywhere?

And since they have not even identified what the DM is, and since it is going right through stars (AT every concievable angle in a straight line) planets and our bodies in prolific amounts, what can make it lose energy, and how do we even know how its energy manefests itself. If anything, they appear to be absolutely 'inert'.

The non-baryonic DM totally permeates the galaxy from my understanding.
Do you think that the halo outside the galaxy has more DM than the galaxy does from the outer rim inwards? Also, all that DM is going right through all the stars and planets in the galaxy, so are you saying that the DM that is around each galaxy is gravitationally bound to that galaxy, or is it moving right through the whole galaxy being replaced by more DM coming into that galaxy?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Yes, at every conceivable angle. Dark Matter is omni-directional. How does that make us special? How do you think it moves?

I have heard this before, and Stupendous Man quoted you too.

The reason gravity is so weak everywhere and to give the best example I have so far for here on earth, since a considerable % of the earths mass/gravity (probably much more than is currently realized) is non-baryonic DM flowing through the earth, it is 'inertial gravity' traveling at high velocity.
Which is most likely why nature was able to evolve 'wings'. If the DM flowing through the earth was 'at rest', man would not be flying today because it is highly unlikely that insects or birds would have ever evolved.

Really? Okay, how about 'base particles' that give electrons/protrons their mass???

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

No, no, no! Don't you read the responses?
One...more...time...

Dark matter move in curved (conic section) orbits in response to gravity, just like every other mass in the universe.
Again, the DM was there first and is more massive than the baryonic matter. So, technically, the galaxy is gravitationally bound to the DM, not the other way around.
As StupendousMan suggested, "Why don't you spend a few days reading some recent papers describing the formation of structure on galactic scales?"

Or a recent textbook on the subject.

Or even some recent articles on the subject in periodicals such as Scientific American and Sky & Telescope.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

That would be the Higgs boson. One of the predicted, but (so far) undetected SUSY (supersymmetry) elementary particles.

StupendousMan did not quote me. This is consensus, mainstream physics.

Again, it moves in conic section orbits as defined by gravity. Just like baryonic matter.

Please educate your self. This is elementary material, taught in any university at the undergraduate level.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

RussT, here's a handy picture to help you understand why dark matter doesn't always follow a straight line, and why dark matter can have transverse velocity:

Now, imagine that that particle is a particle of dark matter. I've shown its velocity, and have broken that down into radial and transverse components. The radial component is simply the component of the velocity which follows a line radially inward or outward from the sun. The transverse component is whatever is left after the radial component is taken (and is always orthogonal to the radial component).
The green line in the picture is the path that that particle takes. The reason it takes this path is because it is gravitationally attracted to the Sun in the middle, and thus is accelerated toward it. But because it has some transverse component to its velocity, it doesn't follow a straight line into the sun. Instead, it follows the hyperbolic trajectory you see there.

Through Gravitational interactions, the same way a cloud of baryonic matter is formed. The only difference is that over time the cloud of baryonic matter will condense (usually into a star) because the electromagnetic interactions between particles cause the already-formed cloud to radiate away energy, thus lowering the velocities of individual particles and condensing the cloud. But the formation of the cloud in the first place is entirely due to gravity, so there is nothing stopping a cloud of dark matter from forming. It's just that the cloud of dark matter, once formed, won't condense the way normal matter does. So what you end up with a huge cloud of dark matter the size of a galaxy.