Why is it that Mercury rotates at a faster rate than say Pluto

[Sean]

If dark matter is responsible for the inner stars within a galaxy rotating at the same rate as the outer stars, then why is it that murcury rotates at a faster rate than say pluto.This posses the question why are we searching for evidence of dark matter deep in the Earth when there seems to be no evidence of it present in our solar system.

 

[phinds]

If dark matter is responsible for the inner stars within a galaxy rotating at the same rate as the outer stars, then why is it that murcury rotates at a faster rate than say pluto.This posses the question why are we searching for evidence of dark matter deep in the Earth when there seems to be no evidence of it present in our solar system.

The density of dark matter is such that its total effect on the solar system is that of a modest sized asteroid. In other words, basically zero. The galaxy is MANY orders of magnitude larger.

 

[Bandersnatch]

This posses the question why are we searching for evidence of dark matter deep in the earth

Do you mean the underground detectors? They're placed underground not because dark matter is expected to be more abundant there, but because Earth provides good shielding. You want your detectors to be free from interference from radiation, while being able to catch the hypothetical dark matter particles. The latter don't interact with baryonic matter, so the Earth is transparent to them.

 

[Sean]

Yes I realize why the detectors are underground to filter out the unwanted particles down from hundreds an hour to just a few a week but Dark matter makes up 85% of our universe,carrys mass and is present through out our galaxy and beyond locking stars in fixed orbits but not so with our solar system.

 

[Sean]

Ahhhh just read cdms Berkceley.

 

[Drakkith]

Yes I realize why the detectors are underground to filter out the unwanted particles down from hundreds an hour to just a few a week but Dark matter makes up 85% of our universe,carrys mass and is present through out our galaxy and beyond locking stars in fixed orbits but not so with our solar system.

The distribution of dark matter is such that it forms a spherical "halo" around the galaxy. This halo is made up of dark matter which is simply on the outermost portion of its orbit, the portion where it is traveling the slowest (just like how Pluto's orbital velocity is much smaller than Mercury's because of its increased distance from the Sun). From there it falls inwards, gaining speed and making a pass through the galaxy before slowing down again on the other side. Because the speed of its orbit is so much slower on the outer portions, dark matter spends most of its time out there. So there is relatively little dark matter present in the inner portions of the galaxy (including within our solar system) and its effect on the orbits of the planets around the Sun is negligible.

 

[Orodruin]

The distribution of dark matter is such that it forms a spherical "halo" around the galaxy. This halo is made up of dark matter which is simply on the outermost portion of its orbit, the portion where it is traveling the slowest (just like how Pluto's orbital velocity is much smaller than Mercury's because of its increased distance from the Sun). From there it falls inwards, gaining speed and making a pass through the galaxy before slowing down again on the other side. Because the speed of its orbit is so much slower on the outer portions, dark matter spends most of its time out there. So there is relatively little dark matter present in the inner portions of the galaxy (including within our solar system) and its effect on the orbits of the planets around the Sun is negligible.

This is not correct. Dark matter has a distribution that is typically much denser towards the center of the halo. It would be true if all dark matter particles had the same energy, but they do not. Still, at solar system scale, luninous matter is dense enough to dominate completely.

 

[Drakkith]

This is not correct. Dark matter has a distribution that is typically much denser towards the center of the halo.

I'm not quite seeing how that contradicts my post. While the density is higher, isn't the amount of dark matter near the center only a small percentage of the whole halo?

 

[Orodruin]

I'm not quite seeing how that contradicts my post. While the density is higher, isn't the amount of dark matter near the center only a small percentage of the whole halo?

I think your post makes it sound as if there is essentially a spherical shell of dark matter outside Earth's position and that dark matter will be more abundant in the outer regions. That the halo is spherical just means that it is spherically symmetric. That the dark matter in the center is a minor part of the whole halo does not mean that it is not at its densest there and the number you get depends on how you define "near the center".

 

[Drakkith]

I think your post makes it sound as if there is essentially a spherical shell of dark matter outside Earth's position and that dark matter will be more abundant in the outer regions.

Ah, I see your point.

That the halo is spherical just means that it is spherically symmetric. That the dark matter in the center is a minor part of the whole halo does not mean that it is not at its densest there and the number you get depends on how you define "near the center".

Alright, so it would be better to describe it as spherically symmetric instead of a spherical halo, and that the density increases as you move inwards.

 

[phinds]

This is not correct. Dark matter has a distribution that is typically much denser towards the center of the halo. It would be true if all dark matter particles had the same energy, but they do not. Still, at solar system scale, luninous matter is dense enough to dominate completely.

@Orodruin:
First, I don't understand "center of the halo" ... sounds like a contradiction of terms.
Second, I was under the impression that the density of dark matter was higher IN the halo than in the center of the galaxy because of what Drakkith pointed out in post #6. Subsequent posts have not clarified for me why that is not the case. You even say that there is less DM at the center but "does not mean that it is not at its densest there". Why is it more dense in the center? I'm just not getting it. Can you clarify?

EDIT: the link that Sean provides in the post directly below says the same thing you are saying but offers no justification so I'm no closer to understanding WHY it should be more dense in the center.

 

[Sean]

Thank you for your replies http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html
I found this it explains a lot.

 

[Sean]

There is even theories that black holes are made of dark matter, this throws doubt on my personal view that black holes contain universes. Or does it .

 

[phinds]

There is even theories that black holes are made of dark matter, this throws doubt on my personal view that black holes contain universes. Or does it .

It is irrelevant whether BHs are made of dark matter although to think that most of their contribution was dark matter has been debunked here many times and the thought that BH's contain universes is just silly.

 

[Sean]

Throughout the universe most things work out dense towards the centre I surpose.

 

[phinds]

Throughout the universe most things work out dense towards the centre I surpose.

You are missing the point Drakkith made in post #6, which has been made on PF many times before and explains why there is a lot more DM in the HALO.

Now that I think about it though, I do see that the argument does not directly imply that it is more DENSE in the halo. I'll wait to see what Orodruin has to say.

LATER: Thinking about it more, I think I'm beginning to see how/why it can be (probably is) more dense in the center even though there's much more in the halo. First, obviously there is WAY more volume in the halo but that just supports the "more in the halo", not "more dense in the halo". Second, even though the particles are traveling very fast in the center and very slow in the halo, still, they ALL go though the center whereas they are enormously spread out in the halo. @Orodruin, have I got this right? @Drakkith what do you think?

 

[Sean]

I think the calculations and observations show that the main force in our solar system is our sun and that all the dark matter in our solar system does not constitute a mass greater than our sun where as dark matter is more dense towards the centre of our galaxy creating a force upon the stars.This is how I am seeing it let me know if that's wrong.And if it is wrong then my question still stands.Or is it that we just live in an area where dark matter is scarce.

 

[phinds]

I think the calculations and observations show that the main force in our solar system is our sun and that all the dark matter in our solar system does not constitute a mass greater than our sun

see post #2. That has nothing to do with the discussion of the last several posts which are about the density of DM throughout the galaxy.

 

[Orodruin]

You are missing the point Drakkith made in post #6, which has been made on PF many times before and explains why there is a lot more DM in the HALO.

The dark matter halo is not something surrounding a galaxy, it is a background that extends inside and beyond the galaxy and has the highest density in the center. Typical models for the dark matter density include the NFW and Einasto profiles. The NFW profile is essentially a broken power law diverging as 1/r towards the center. This is essentially what is expected from cold dark matter, but not very well in line with observations that suggest a more cored profile (density going towards some constant towards the center, the Einasto profile essentially describes such a situation.
See: https://en.wikipedia.org/wiki/Navarro–Frenk–White_profile
and: https://en.wikipedia.org/wiki/Einasto_profile
That halos seem to be cored more than cusped is one of the small scale structure problems (called the core vs cusp problem - very imaginative).

This image is taken from http://inspirehep.net/record/1332709/

and is essentially what you would expect from the halo of a dwarf galaxy in the CDM case. (Red represents higher density.)

 

[phinds]

The dark matter halo is not something surrounding a galaxy, it is a background that extends inside and beyond the galaxy and has the highest density in the center.

Ah ha. So at least part of my problem is that I've been using a mistaken definition of "halo". Thanks.

 

[Janus]

I think the calculations and observations show that the main force in our solar system is our sun and that all the dark matter in our solar system does not constitute a mass greater than our sun where as dark matter is more dense towards the centre of our galaxy creating a force upon the stars.This is how I am seeing it let me know if that's wrong.And if it is wrong then my question still stands.Or is it that we just live in an area where dark matter is scarce.

No, The increasing density towards the center is not relevant here.
What determines how an object orbits is the total mass inside the spherical volume which has a radius equal to the distance of the object from the center. For a planet in the Solar system, the vast majority of the mass inside this "interior sphere" resides in the Sun, so as you move outward from planet to planet, it stays almost the same. This is why the planets orbit the Sun with the velocities they have.
If we look at a galaxy, its is a bit different. At the center you are in the central bulge, and as you move away from the center, the mass inside your interior sphere grows rapidly, so fast that the its increase overrides the effect of your increasing distance and your orbital speed increases as you move outward.
Once you leave the bulge and enter the region of the disk, the mass increase with distance should drop off sharply (as long as you only consider the visible matter), and your orbital velocity should begin to decrease as you move outward.( similar to how it does with the solar system). It doesn't, and that is where dark matter comes in.
Dark matter is not distributed in the same was as the stars making up the visible galaxy are. It continues to spread out spherically from the center even once you have left the central bulge. So as you move outward the mass it contributes to determining your orbit continues to increase rapidly. This means that when you get out into the disk, Your interior sphere encompasses a large volume which is taken up in part by visible matter, but mostly consists of space filled with just dark matter. Since this is a very large volume, the dark matter doesn't have to be very dense to add up to a considerable amount of mass.
To give you an idea of what I mean, consider this: Even if you assumed that dark matter was evenly distributed at the same density as it is found in the Solar system, the total amount of the mass of dark matter, spread out in the spherical volume with a radius equal to the distance of our Solar system from the center of the galaxy, would add up to a large fraction of the estimated total mass of the visible matter in the galaxy.
In other words, Dark matter doesn't be of a great density to have a profound effect because of the huge volume that it occupies.

 

[PeterDonis]

There is even theories that black holes are made of dark matter

Can you give a reference?

 

[Chronos]

I too am puzzled by the assertion of more dark matter in a galactic core than halo. Leaving aside the DM density issue, which was addressed by Janus, this quote from https://briankoberlein.com/2016/09/29/galactic-motion-challenges-dark-matter/ seems to conflict "According to the dark matter model, a galaxy’s mass isn’t concentrated in its center. Most of the visible matter is, but a galaxy is surrounded by a halo of dark matter. Most of a galax’s mass is dark matter, and most of it is in the halo." and appears to be affirmed by this source: http://www.astro.caltech.edu/~george/ay20/eaa-darkmatter-obs.pdf.

 

[Drakkith]

I too am puzzled by the assertion of more dark matter in a galactic core than halo.

Who's asserted that more dark matter exists in the core than in the halo?

 

[phinds]

Who's asserted that more dark matter exists in the core than in the halo?

Yes, I also have not seen anyone assert that. The discussion was about the DENSITY in the core being greater than the density in the outer part of the halo.

 

[Chronos]

The short explanation is I whiffed on the pitch.

 

[Drakkith]

The short explanation is I whiffed on the pitch.

No worries. You've still got plenty of innings left.

 

[Vanadium 50]

Is this thread mistitled or do we misunderstand? Dark matter - or for that matter, the attraction to the entral mass in a system - has nothing to do with the rotational speed of planets. And besides, Pluto rotates faster than Mercury.

 

[Drakkith]

Is this thread mistitled or do we misunderstand? Dark matter - or for that matter, the attraction to the entral mass in a system - has nothing to do with the rotational speed of planets.

Given the context of the question and subsequent replies I assume the OP meant that Mercury revolves around the Sun at a faster rate than Pluto.