# Important Findings on Dark Matter

1. Nov 1, 2005

### franznietzsche

Just realized that this paper hadn't been posted here, and thought everyone might like to see this.

http://xxx.lanl.gov/abs/astro-ph/0507619

Very interesting findings, not too hard to read.

The ultimate point is that, traditional computer simulations of galactic rotation are done using newtonian physics, because they are simpler and easier to calculate. These simulations find that the rotation rates of galaxies cannot be explaines by the amount of visible mass requiring some amount of unseen dark matter to explain their observed motion. This paper reports that by using general relativity rather than newtonian gravity, the discrepancy disappears, even though the system appears to be non relativistic (very weak gravitational fields, very low velocities).

2. Nov 1, 2005

### SpaceTiger

Staff Emeritus
Last edited by a moderator: Apr 21, 2017
3. Nov 2, 2005

### Garth

But it does raise the interesting and unresolved question of whether GR non-linear effects are significant in determining spiral galactic halo DM. The subject has also been addressed before by others such as Vogt & Letelier in Relativistic Models of Galaxies and earlier authors cited therein.

Garth

Last edited by a moderator: Apr 21, 2017
4. Dec 4, 2005

### Chronos

Bah, those studies prove nothing, Garth.

5. Dec 4, 2005

### Spin_Network

Go here:http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
click ISSYP
then click on the talk:Nature of science2 by Lee Smolin, after a short introduction of aXarchive pre-print workings, Smolin gives a really good overview of the Tully Fischer Relation and its relation to DM.

Last edited by a moderator: Apr 21, 2017
6. Dec 13, 2005

### Wishbone

I got this off space.com today

Im sure they tried to make this article as easy to read as possible, but can this be accurate? Was dark matter really born out of the failure of newtonian mechanics on large systems? Was it not the failure of general relativity? I would have thought that newtonian mechanics would have failed on the galactic level for more reasons than simply not taking into account dark matter...

7. Dec 13, 2005

### Garth

Yes - Newtonian gravitation cannot explain either the rate of rotation or the flat rotation curve of spiral galaxies without there being a massive dark halo
This is Cooperstock & Tieu's point; according to their analysis GR does explain spiral galaxy rotation curves without dark matter. Unfortunately not many people agreed with them.

In fact as the galactic field {GM/rc2} and rotation velocities {(v/c)2} are about 10-6 it is generally thought that "newtonian mechanics would not have failed on the galactic level".

C&T say that because the gravitating mass is itself in orbit around the galactic centre then GR non-linear effects do become important in galactic dynamics.

C&T have recently responded to their critics in a second http://arxiv.org/abs/astro-ph/05120480 [Broken].

Garth

Last edited by a moderator: May 2, 2017
8. Dec 13, 2005

### Wishbone

How come it is up for debate? Since we know GR, can we simply try to use it to predict paths of stars and whatnot, then measure such movements, and then actually see if the theory agrees with our evidence? Or is it that we cannot accuratley take the measurements we would need to do such a thing.

9. Dec 13, 2005

### franznietzsche

1) Simulations using GR are much more complicated. Newtonian gravity is always used over GR in cases where it is considered sufficiently accurate (i.e. low-momentum, low gravitational fields).

2)They simulate overall galactic rotation, not the paths of individual stars.

3) To my knowledge, we can not reliably measure the paths of individual stars in other galaxies. Too small an angular position shift. But we can measure overall rotation rates of a galaxy.

I agree with Garth that C&T raise a worthwhile point of examining the nonlinear effects of GR on galactic rotation.

10. Dec 14, 2005

### jhe1984

11. Jan 21, 2006

### ubavontuba

Hello,

If this http://www.newscientist.com/article.ns?id=dn7056" [Broken] is true, wouldn't this put an end to this debate?

Is this proof of an underlying prevalence of dark matter?

Is it possible the results are misinterpreted, as stated in the article?

Last edited by a moderator: May 2, 2017
12. Jan 21, 2006

### Garth

In the standard model the inventory looks like this, where densities $\Omega$ are measured as a fraction of the critical density:

Visible matter: $\Omega_{vis}$ ~ 0.003
Baryonic matter: $\Omega_b$ ~ 0.04
Non baryonic exotic Dark Matter: $\Omega_{DM}$ ~ 0.23
Dark Energy: $\Omega_{DE}$ ~ 0.73
Total density $\Omega_T$ ~ 1.0 - 1.1

Thus even in the LCDM model there are two kinds of Dark Matter, the exotic DM and ordinary baryonic matter that is 'dark' (not observed), this baryonic DM is about 13 times more massive than all the normal stars and visible nebula.

About half these dark baryons can be attributed to cold neutral H/He, hot IGM plasma and local Lyman $\alpha$ forests, which leaves 46% unaccounted for. It is suggested that this component may be in the form of WHIM (Warm-Hot Intergalactic Medium). Measured Cosmological Mass Density in the WHIM: the Solution to the ’Missing Baryons’ Problem, their upper limit of WHIM is:
$\Omega_b$WHIM > 4.3 × 100.47 % = 12.6%
and their lower WHIM limit is:
$\Omega_b$WHIM > 1.3 × 100.32 % = 2.7%?

Which is only just consistent (by stretching a point) with the standard model of about $\Omega_b$ = 0.04 of which $\Omega_b$(missing) = 2.1 +0.5/−0.4) % (Nicastro et al., A Warm-Hot Intergalactic Medium Location for the Missing Cosmic Baryons, Nature, accepted for publication (2004):). That is, the lower limit on the WHIM observed is slightly more than the standard model can allow.

I also note this is consistent with a much higher $\Omega_{WHIM}$ allowed by the Freely Coasting Model BBN $\Omega_b$ ~ 0.2

So with that much baryonic dark matter about these dark galaxies may simply be baryonic ones in which stars have not formed for some reason.

Note the standard model also requires the presence of massive exotic DM halos to assist in structure formation and deliver the 'flat' galactic rotation profiles, there may therefore also be exotic DM in these dark galaxies as well.

Garth

Last edited: Jan 21, 2006
13. Jan 21, 2006

### SpaceTiger

Staff Emeritus
The validity of this interpretation is still being debated. See here for some links and discussion: