Dark Matter

[IM Egdall]

So, based on the motion of stars in galaxies and galaxy cluster motion etc., there is roughly 5 times more "dark matter" than ordinary matter in the universe. No one knows what this dark matter is. But, presumedly, it is in our solar system. So how does it affect the orbits of our planets around the Sun and moons around the planets, not to mention comets etc. ?

 

How is it that we can explain these orbits and motions to such great accuracy using general relativity and only the masses of the Sun, planets, moons, etc? These do not include the effects of dark matter. If there is 5 times as much dark matter as the ordinary matter which makes up our stellar objects, why does it not affect these motions more?

 

Edited to fix my dumb typos.

Edited March 26, 2012 by IM Egdall

[mathematic]

So, based on the motion of stars in galaxies and galaxy cluster motion etc., there is roughly 5 times more "dark matter" than ordinary matter in the universe. No one knows what this dark matter is. But, presumedly, it is in our solar system. So how does it affect the orbits of our planets around the Sun and moons around the planets, not to mention comets etc. ?

 

How is it that we can explain these orbits and motions to such great accuracy using general relativity and only the masses of the Sun, planets, moons, etc? These do not include the effects of dark matter. If there is 5 times as much dark matter as the ordinary matter which makes up our stellar objects, why does it not affect these motions more?

 

Edited to fix my dumb typos.

 

Although there is a lot more dark matter than ordinary matter, the dark matter is much more spread out, so that within the solar system it has a much lower density.

 

http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/

 

The above may help.

[IM Egdall]

Although there is a lot more dark matter than ordinary matter, the dark matter is much more spread out, so that within the solar system it has a much lower density.

 

http://www.universet...e-solar-system/

 

The above may help.

 

Ya I thought it was something like that. There is so much space between stellar objects. The link you provided is very helpful . Thanks.

 

I still have a question though. Why is it dark matter doesn't clump due to gravity like ordinary matter into dark matter stellar objects?

Edited March 26, 2012 by IM Egdall

[Janus]

So, based on the motion of stars in galaxies and galaxy cluster motion etc., there is roughly 5 times more "dark matter" than ordinary matter in the universe. No one knows what this dark matter is. But, presumedly, it is in our solar system. So how does it affect the orbits of our planets around the Sun and moons around the planets, not to mention comets etc. ?

 

How is it that we can explain these orbits and motions to such great accuracy using general relativity and only the masses of the Sun, planets, moons, etc? These do not include the effects of dark matter. If there is 5 times as much dark matter as the ordinary matter which makes up our stellar objects, why does it not affect these motions more?

 

Edited to fix my dumb typos.

 

 

Okay, consider this:

If you take all of the mass of the sun and Planets and distributed evenly in a sphere with a radius equal to the orbit of Neptune, you get a density of ~5 kg per cubic Kilometer.Consider this the "density of the Solar system" Now, there are 135 stars with 20 light years of the Earth. Some are larger than the Sun but most are smaller. If we are generous we will assume they average out to being the mass of the Sun then we have ~2.7e32 kg within a sphere 20 light years in diameter. This works out to an average density of ~10e-12 kg/km³ This means that that density of the Solar system is some 5e11 times denser than the average density of the Local Galactic neighborhood.

 

So even if the density of DM in the local galactic neighborhood was 5 times that of "normal" matter, you would only expect to find 4e18 kg of DM in the Solar system. In addition, the Visible matter in the galaxy is mostly within the disk of the galaxy. DM exists in a sphere that is larger than this disk and thus has a much larger volume to be spread out into. There may be 5 times as much, but it is spread out into a much larger volume. This added with what we noted above means that we would expect even less DM to be found in the Solar system. The only reason that we would find more is covered in the article given. we have slowly been collecting over time. Even with that there is still less of it in the Solar system than the mass of the Dwarf Planet Ceres.

[imatfaal]

Ya I thought it was something like that. There is so much space between stellar objects. The link you provided is very helpful . Thanks.

 

I still have a question though. Why is it dark matter doesn't clump due to gravity like ordinary matter into dark matter stellar objects?

 

the clumping via gravity also needs a way to dissipate the energy of the in-falling matter - normal matter generates heat; dark matter does not have the necessary interactions to do this and thus no overall concentration of matter occurs. that said dark matter does seem to occur in filaments and strands on a very large scale

[IM Egdall]

the clumping via gravity also needs a way to dissipate the energy of the in-falling matter - normal matter generates heat; dark matter does not have the necessary interactions to do this and thus no overall concentration of matter occurs. that said dark matter does seem to occur in filaments and strands on a very large scale

 

Hmmm. Very interesting! Thanks. Do you know of a link where I could get more info on this effect where dark matter does not tend to form into stellar objects?

Edited March 27, 2012 by IM Egdall

[Arch2008]

Four fundamental forces are present in the universe, gravity, electromagnetism and the strong and weak nuclear forces. Traditional matter is governed by all of these forces. The strong force binds quarks into atoms. The weak force causes certain atoms to decay. The electromagnetic force binds atoms into molecules. Gravity causes atoms and molecules to form larger structures like pebbles and super galaxy clusters. Dark matter particles are not bound by the strong force into atoms, do not decay radioactively by the weak force and do not have an electromagnetic charge. These particles only clump together weakly through gravity.

[IM Egdall]

Four fundamental forces are present in the universe, gravity, electromagnetism and the strong and weak nuclear forces. Traditional matter is governed by all of these forces. The strong force binds quarks into atoms. The weak force causes certain atoms to decay. The electromagnetic force binds atoms into molecules. Gravity causes atoms and molecules to form larger structures like pebbles and super galaxy clusters. Dark matter particles are not bound by the strong force into atoms, do not decay radioactively by the weak force and do not have an electromagnetic charge. These particles only clump together weakly through gravity.

 

Ya, I am aware of all this. My question was not about dark matter and strong force, weak force, or EM force. It was about dark matter and gravity. I was asking for a good link which explains the "weak" clumping of dark matter due to gravity.

[Janus]

Ya, I am aware of all this. My question was not about dark matter and strong force, weak force, or EM force. It was about dark matter and gravity. I was asking for a good link which explains the "weak" clumping of dark matter due to gravity.

 

One mechanism for this is energy loss by gravitational radiation.

 

Another is gravitational interaction between DM particles where during an interaction some particles gain energy at the expense of others. Something like when we steal a little orbital energy from a planet to speed up a space probe using a gravitational slingshot.

[Arch2008]

Gravity alone affects DM normally, not weakly. One might say that particles affected by the mechanics of all four fundamental forces strongly clump together forming the objects we know, however DM is only affected by gravity and weakly clumps together, forming diffuse clouds.

http://www.physorg.com/news176457990.html

http://arxiv.org/pdf/0903.0101.pdf

These links describe how in the early universe the density of DM was perhaps enough that “Dark Stars” (stellar objects) did form and that Weakly Interactive Massive Particles (a candidate for DM) can collide and annihilate creating energy that would produce light and heat like more common stars.

So the reason DM does not form stellar objects from gravity alone is due to its density at present.

[IM Egdall]

Gravity alone affects DM normally, not weakly. One might say that particles affected by the mechanics of all four fundamental forces strongly clump together forming the objects we know, however DM is only affected by gravity and weakly clumps together, forming diffuse clouds.

http://www.physorg.c...s176457990.html

http://arxiv.org/pdf/0903.0101.pdf

These links describe how in the early universe the density of DM was perhaps enough that "Dark Stars" (stellar objects) did form and that Weakly Interactive Massive Particles (a candidate for DM) can collide and annihilate creating energy that would produce light and heat like more common stars.

So the reason DM does not form stellar objects from gravity alone is due to its density at present.

 

Fascinating! Thanks for the clarification on "weak" clumping and the dark star links.

[questionposter]

If dark matter has gravity, shouldn't it culminate to also form clumps like planets?

[Janus]

If dark matter has gravity, shouldn't it culminate to also form clumps like planets?

 

Haven't you read anything in this thread from post #3 on?

[questionposter]

Haven't you read anything in this thread from post #3 on?

 

I get that it has gravity, but why don't we see planets of it going in front of the sun if it has such high gravity? How could light seamlessly pass through it if it's that dense? You would think light would at least get altered not just from the gravity but also form the substance itself...

Edited March 31, 2012 by questionposter

[Janus]

I get that it has gravity, but why don't we see planets of it going in front of the sun if it has such high gravity?

It has gravity, nothing was said about it having unusually high gravity

 

How could light seamlessly pass through it if it's that dense?

Who said anything about it being dense? Again, read the above posts, there may be 5 times as much of it in the Galaxy, but it is spread out in a volume much more than 5 times the volume of the visible disk of the Galaxy.

 

You would think light would at least get altered not just from the gravity but also form the substance itself...

 

In order for light to be altered by it, it has to interact with light. Light interacts with baryonic matter because this type of matter interacts electromagnetically.(Even a neutral particle like the neutron is made up of quarks with charges.) DM would not interact electromagnetically, so no matter how much of it you had, light would pass through it like it wasn't there. There is no way for the light to interact with it except via gravity.

 

You are trying to extend everyday experience to a area where it does not apply. You are use to anything with "substance" interfering with light, but that is because your only experience is with stuff that interacts with light. So you equate "substance" with interfering with light and assume that this is just natural. But just because your experiences is limited to substances that interferes with light does not mean that everything with substance does.

[questionposter]

Why would the ability to form planets make it's gravity "unusually" high? The article said dark matter can could clump up.

Edited April 1, 2012 by questionposter

[Janus]

Why would the ability to form planets make it's gravity "unusually" high? The article said dark matter can could clump up.

 

you are the one who used the phrase "if it has such high gravity?"

 

I was just pointing out this was a misnomer.

 

No one has said that DM can't clump up, only that it doesn't clump as easily as visible matter. This because, the very characteristics that allow baryonic matter to interact with light also aids in its clumping up. Electromagnetic interaction allows it to collide with itself, stick together, and shed the energy of collision via EM radiation. DM does none of this, It passes right through itself as if there was nothing there, there is nothing to make it stick together, and it does not emit EM radiation.

 

While there are some mechanisms as mentioned above, that allow DM to clump, they are much less efficient than those available to visible matter, thus DM tends to form much, much looser clumps than visible matter.

[mathematic]

Clumping is a bad term for what happens to dark matter. All that we do know is that the density of dark matter will vary in different regions of space, but as mentioned already there is no mechanism for dark matter particles to stick together.

[questionposter]

you are the one who used the phrase "if it has such high gravity?"

 

I was just pointing out this was a misnomer.

 

No one has said that DM can't clump up, only that it doesn't clump as easily as visible matter. This because, the very characteristics that allow baryonic matter to interact with light also aids in its clumping up. Electromagnetic interaction allows it to collide with itself, stick together, and shed the energy of collision via EM radiation. DM does none of this, It passes right through itself as if there was nothing there, there is nothing to make it stick together, and it does not emit EM radiation.

 

While there are some mechanisms as mentioned above, that allow DM to clump, they are much less efficient than those available to visible matter, thus DM tends to form much, much looser clumps than visible matter.

 

Well why does there happen to be a concentration in a halo around the galaxy?

[Janus]

Well why does there happen to be a concentration in a halo around the galaxy?

 

That happen to be one of the looser clumps I mentioned. The average density of the DM halo is much less than the average density of the visible galactic disk, and much much less than that of the average density of the Solar system.

[questionposter]

That happen to be one of the looser clumps I mentioned. The average density of the DM halo is much less than the average density of the visible galactic disk, and much much less than that of the average density of the Solar system.

 

I guess that makes more sense, but what about in the case of something like this?

 

http://www.dailygalaxy.com/my_weblog/2009/12/image-of-the-day-a-dark-matter-halo-.html

 

Or is that purely because the galaxies are colliding?

[Arch2008]

Janus already mentioned that the only way DM can interact with light is through gravity. To elaborate, everything with mass bends space-time. As the article points out, the enormous DM mass around the center of this galaxy has bent space-time so much that light passing through is “focused” similar to the effect of a lens. However, the enormous mass is still spread out over a similarly enormous volume, certainly trillions of cubic light years. So its density is still really low as Janus described.

[juanrga]

So, based on the motion of stars in galaxies and galaxy cluster motion etc., there is roughly 5 times more "dark matter" than ordinary matter in the universe. No one knows what this dark matter is. But, presumedly, it is in our solar system. So how does it affect the orbits of our planets around the Sun and moons around the planets, not to mention comets etc. ?

 

How is it that we can explain these orbits and motions to such great accuracy using general relativity and only the masses of the Sun, planets, moons, etc? These do not include the effects of dark matter. If there is 5 times as much dark matter as the ordinary matter which makes up our stellar objects, why does it not affect these motions more?

 

Edited to fix my dumb typos.

 

It is possible to explain the same motions without using the dark matter hypothesis using theories as MOND, TeVeS...

 

In fact, it has been recently shown that dark matter model cannot explain recent observations

 

http://www.bbc.co.uk/news/science-environment-12571965

 

http://blogs.nature.com/news/2011/02/post_73.html

Edited April 7, 2012 by juanrga

[Arch2008]

WMAP confirmed predicted effects of DM for the entire universe. McGaugh has a few observations that he “contends” cannot be equally well explained with DM. Both of these links explain that MOND is not well accepted.

[Alberto]

It is possible to explain the same motions without using the dark matter hypothesis using theories as MOND, TeVeS...

 

The problem with MOND is that it is a phenomenological theory that uses arbitrary functions and some new constant to fit observations. That is why in some cases it goes wrong. This is not so with, say, Anisotropic Geometrodynamics which provides the same results starting from the first principles and using already known constants. And no dark matter is needed there as well.