Dark Matter Halo & Newton's Shell Theorem

In summary, the conversation discusses the concept of a huge spherical dark matter halo surrounding the Milky Way. Despite Newton's shell theorem stating that there is no net gravitational pull within a shell, the halo is not a shell and can be as large as desired. The idea of a large halo was conceived through observations of other galaxies rather than the Milky Way itself. This is because the effect is more easily observed in other galaxies. The density profile of the halo is inferred to be cuspy towards the galactic center, similar to the non-uniform density of the Earth. The existence of extremely large halos was inferred from observing satellite galaxies and globular clusters.
  • #1
greswd
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I've read the postulate that there could be a huge spherical dark matter halo extending far beyond the edges of the Milky Way.

However, according to Newton's shell theorem, there is no net gravitational pull within a shell.

How do they arrive at the conclusion of a halo so huge?
 
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  • #2
It is not a shell.
 
  • #3
Orodruin said:
It is not a shell.
I was referring to Newton's shell theorem, and applying it to a spherical halo.
 
  • #4
Yes, the halo is not a shell. It is just spherical. Besides, there are objects at those distances that you can look at as well.

It is easier to do this for other galaxies though.
 
  • #5
Orodruin said:
Yes, the halo is not a shell. It is just spherical. Besides, there are objects at those distances that you can look at as well.

It is easier to do this for other galaxies though.

The parts of the halo beyond the milky way can be thought of as a really thick shell right?

darkmatter.jpe


So I was thinking that the net gravity would be zero within the shell, you can make the halo as big as you want but you won't make any difference.

How did they conceive the idea of a such a large halo? Was it through observations of the Milky Way or observing the objects within the 'gray zone' as seen in the picture?
 
  • #6
greswd said:
So I was thinking that the net gravity would be zero within the shell, you can make the halo as big as you want but you won't make any difference.
It will not make a difference for the objects within, but it will make a difference to objects at large distances. Just because most luminous matter is concentrated in the middle does not mean that there is nothing to see further out.

greswd said:
How did they conceive the idea of a such a large halo? Was it through observations of the Milky Way or observing the objects within the 'gray zone' as seen in the picture?
Neither. As I already said, the effect is much easier to see in other galaxies.
 
  • #7
Orodruin said:
Neither. As I already said, the effect is much easier to see in other galaxies.
I see. So when observing other galaxies, we see that gravitationally they behave like they possesses halos?
 
  • #8
Shell theorem requires a uniform density in addition to spherical symmetry for cancellation of gravity. Dark matter haloes are not assumed to be of uniform density, the density profile is inferred to be cuspy towards the galactic center. Think of it like the earth, which is also not of uniform density. When you dig a hole and toss something in it falls to the bottom of the hole, it does not however near the surface as you might expect if the Earth were uniformly dense and spherically symmetric...
 
  • #9
Chronos said:
When you dig a hole and toss something in it falls to the bottom of the hole, it does not hover near the surface as you might expect if the Earth were uniformly dense and spherically symmetric...
That's... not a feature of uniform density, but of a hollow shell of matter. The difference between uniform and nonuniform density in spherical distribution is that the latter does not have a straight-line dependence of force on radius as you dig down.
I'm sure you know that.
 
  • #10
You are correct. I should refrain from posting after 3 am.
 
  • #11
Bandersnatch said:
That's... not a feature of uniform density, but of a hollow shell of matter. The difference between uniform and nonuniform density in spherical distribution is that the latter does not have a straight-line dependence of force on radius as you dig down.
I'm sure you know that.
Did we infer the existence of extremely large halos from observing other galaxies?
 
  • #12
Did we infer the existence of extremely large halos from observing other galaxies?

Yes, satellite galaxies of the Milky Way, along with globular clusters. Similarly looking at stars in the disk of the Milky Way allows computing the amount of dark matter at that radius or inside it.
 
  • #13
eachus said:
Did we infer the existence of extremely large halos from observing other galaxies?

Yes, satellite galaxies of the Milky Way, along with globular clusters. Similarly looking at stars in the disk of the Milky Way allows computing the amount of dark matter at that radius or inside it.

I see. Thanks.
 

1. What is dark matter halo?

Dark matter halo is a theoretical concept in astrophysics that refers to a massive, invisible region surrounding a galaxy or galaxy cluster. It is believed to be composed of dark matter, a type of matter that does not emit, absorb, or reflect light and cannot be detected through traditional means.

2. How does dark matter halo relate to Newton's Shell Theorem?

According to Newton's Shell Theorem, the gravitational force exerted by a spherically symmetric mass distribution on a point located outside of the distribution is the same as if all the mass were concentrated at the center of the distribution. This theorem can be applied to dark matter halos, which are also spherically symmetric, to help explain their gravitational effects on surrounding objects.

3. How is dark matter halo detected?

Dark matter halo cannot be directly observed or detected through traditional means because it does not interact with light. Instead, its presence is inferred through its gravitational effects on visible matter, such as the rotation of galaxies or the bending of light from distant objects.

4. What is the current understanding of dark matter halo?

The concept of dark matter halo is still a subject of ongoing research and debate in the scientific community. While its existence is widely accepted, the exact nature and composition of dark matter remains unknown. Various theories and models have been proposed, but further observations and experiments are needed to fully understand this mysterious component of the universe.

5. Can dark matter halo be explained by other theories besides Newton's Shell Theorem?

Yes, there are alternative theories that attempt to explain the existence and behavior of dark matter halo, such as Modified Newtonian Dynamics (MOND) and the theory of Modified Gravity. However, these theories have not been widely accepted and do not have as much evidence supporting them as Newton's Shell Theorem and the concept of dark matter.

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