02 Oct 16
Modified Newtonian Dynamics has made successful predictions, there is observational evidence for this type of theory.
Science is very conservative and I think that's a good thing. But have there been any successful predictions producing observational evidence for the existence of dark matter?
03 Oct 16
Originally posted by KazetNagorraAnd even your favorite theory, black holes as dark matter:
https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence
http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter
He said the same thing I said, that more direct detections of black hole collisions may tell the story more fully, supporting or refuting that theory. Maybe in a few decades we will know.
03 Oct 16
Originally posted by KazetNagorra
https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence
The distribution of dark matter in galaxies required to explain the motion of the observed matter suggests the presence of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a central disc.
Since dark matter, should it exist, is thought to be influenced by gravity in the same way as baryonic matter, why should it be distributed in a spherically symmetric fashion about the galaxy whilst the baryonic matter is confined to a disc?
03 Oct 16
Originally posted by SoothfastI think because there is so little interaction of dark V our matter, there would be no motions that would force the mass into a disk. Dark matter could care less how our matter moves, it is just gravity V gravity and dark matter overwhelms our matter in terms of mass.The distribution of dark matter in galaxies required to explain the motion of the observed matter suggests the presence of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a central disc.
Since dark matter, should it exist, is thought to be influenced by gravity in the same ...[text shortened]... spherically symmetric fashion about the galaxy whilst the baryonic matter is confined to a disc?
It will be interesting to see if some kind of Mond wins out or is there actual stuff there we can't see but only deduce.
So far there has been zero in the way of direct detection of dark matter and the longer that goes on, the more traction Mond gets.
03 Oct 16
1 edit
Originally posted by SoothfastAs sonhouse says, discs are a result of physical interaction. Interaction removes all opposing motions leaving the total angular momentum intact.
Since dark matter, should it exist, is thought to be influenced by gravity in the same way as baryonic matter, why should it be distributed in a spherically symmetric fashion about the galaxy whilst the baryonic matter is confined to a disc?
In general gas or fine particles quickly form into discs, larger bodies do not. Planets and most of the stars of the galaxy formed form the gas of the discs.
04 Oct 16
Originally posted by SoothfastI had to think about this and do a little reading. In simulations, assuming aspherical starting conditions for the dark matter halo collapse, the Cold Dark Matter (CDM) model produces flattened discs [1]. We don't know what dark matter interactions there are. If dark matter consists of black holes, which need to be in the 30 to 43 solar mass range based on some observational bounds [1], then it only interacts gravitationally. This still should be enough for disc formation. If there is some sort of dark sector with its own spectrum of particles we might expect interactions between particles in the dark sector which one would imagine would tend to favour disc formation. As far as I can tell observations indicate that galactic rotation is consistent with spherical dark matter halos. So, a theory of dark matter has to account for the apparent absence of observed dark matter discs. I don't know if the observational data is necessarily inconsistent with an ellipsoidal halo. So it could be that the Milky Way's dark matter halo is not spherical. It's also possible that the mechanism for disc formation doesn't work in the dark sector for some reason. I think you put your finger on a problem for the theory.The distribution of dark matter in galaxies required to explain the motion of the observed matter suggests the presence of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a central disc.
Since dark matter, should it exist, is thought to be influenced by gravity in the same ...[text shortened]... spherically symmetric fashion about the galaxy whilst the baryonic matter is confined to a disc?
[1] https://en.wikipedia.org/wiki/Dark_matter_halo#Shape
04 Oct 16
Originally posted by DeepThoughtDo you have any references on this topic? In Newtonian physics, gravitational interaction does not lead to loss of velocities except in dense group interactions. This may be different in relativity, but is it still significant enough for disc formation? My understanding of our own solar system was that the star clusters that formed very early on prior to significant disc formation remain in orbits outside the disc ie they have not been gravitationally drawn into the disc.
If dark matter consists of black holes, which need to be in the 30 to 43 solar mass range based on some observational bounds [1], then it only interacts gravitationally. This still should be enough for disc formation.
https://en.wikipedia.org/wiki/Star_cluster
In our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the galactic centre, orbiting the centre in highly elliptical orbits.
04 Oct 16
Originally posted by twhiteheadhttps://en.wikipedia.org/wiki/Dynamical_friction
Do you have any references on this topic? In Newtonian physics, gravitational interaction does not lead to loss of velocities except in dense group interactions. This may be different in relativity, but is it still significant enough for disc formation? My understanding of our own solar system was that the star clusters that formed very early on prior to ...[text shortened]... actic halo, around the galactic centre, orbiting the centre in highly elliptical orbits.[/quote]
There are only about 150 globular clusters, it's not a large fraction of the mass of the Milky Way. Dark Matter is meant to make up 95% of the mass of the Galaxy, one needs some explanation as to why, if it does not form a disc, it does not tend to disrupt the formation of the visible matter disc. I think this is a definite problem for theories involving stellar mass black holes.
05 Oct 16
Originally posted by DeepThoughtInteresting reading and I was wrong about gravitational effects. However, it says nothing about disc formation via gravitational dynamical friction. If globular clusters are apparently unaffected, what makes you think stellar mass black holes would be?
https://en.wikipedia.org/wiki/Dynamical_friction
There are only about 150 globular clusters, it's not a large fraction of the mass of the Milky Way.
If anything, a sphere off black holes might explain why the globular clusters have not been captured by the disc.
Dark Matter is meant to make up 95% of the mass of the Galaxy, one needs some explanation as to why, if it does not form a disc, it does not tend to disrupt the formation of the visible matter disc. I think this is a definite problem for theories involving stellar mass black holes.
Do globular clusters disrupt the disc? We would want to find a globular cluster that passed through the disc and see if any obvious disruption can be seen.
Keep in mind that a typical black hole will only have passed through the disc about 100 times (wild estimate) since the birth of the galaxy.
https://en.wikipedia.org/wiki/Galactic_year
06 Oct 16
Originally posted by twhiteheadThere are of the order of 150 globular clusters in the Milky Way, I can't remember which galaxy but the record is of the order of thousands. This is no mass whatsoever. Dark matter needs to take up 95% of observed gravitational effects. If it consists of 30-45 solar mass black holes, for example, then the black hole halo ought to so disrupt the visible galactic disc in every galaxy that there would be no spiral galaxies. We live in one.
Interesting reading and I was wrong about gravitational effects. However, it says nothing about disc formation via gravitational dynamical friction. If globular clusters are apparently unaffected, what makes you think stellar mass black holes would be?
If anything, a sphere off black holes might explain why the globular clusters have not been captured by ...[text shortened]... es (wild estimate) since the birth of the galaxy.
https://en.wikipedia.org/wiki/Galactic_year
06 Oct 16
1 edit
Originally posted by DeepThoughtI would like to see actual reasoning that leads to this conclusion (rough figures, references, anything). Why would black holes disrupt the disc? Even if 95% of the mass of the milky way is black holes, it still would not constitute very much actual space taken up. There is a lot of space between stars so collisions would be rare.
There are of the order of 150 globular clusters in the Milky Way, I can't remember which galaxy but the record is of the order of thousands. This is no mass whatsoever. Dark matter needs to take up 95% of observed gravitational effects. If it consists of 30-45 solar mass black holes, for example, then the black hole halo ought to so disrupt the visible galactic disc in every galaxy that there would be no spiral galaxies. We live in one.
Would the gravitational effects be significantly different than WIMPs.
And why would astronomers be seriously considering it:
http://www.space.com/33122-dark-matter-black-hole-connection.html