# I Dark Matter Halo & Newton's Shell Theorem

1. Feb 15, 2016

### greswd

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?

2. Feb 15, 2016

### Orodruin

Staff Emeritus
It is not a shell.

3. Feb 15, 2016

### greswd

I was referring to Newton's shell theorem, and applying it to a spherical halo.

4. Feb 15, 2016

### Orodruin

Staff Emeritus
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. Feb 15, 2016

### greswd

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

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. Feb 15, 2016

### Orodruin

Staff Emeritus
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.

Neither. As I already said, the effect is much easier to see in other galaxies.

7. Feb 15, 2016

### greswd

I see. So when observing other galaxies, we see that gravitationally they behave like they possess halos?

8. Feb 15, 2016

### Chronos

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. Feb 15, 2016

### Bandersnatch

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. Feb 15, 2016

### Chronos

You are correct. I should refrain from posting after 3 am.

11. Feb 24, 2016

### greswd

Did we infer the existence of extremely large halos from observing other galaxies?

12. Feb 24, 2016

### eachus

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. Mar 2, 2016

### greswd

I see. Thanks.