Important Findings on Dark Matter

[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).

 

[SpaceTiger]

This was already discussed https://www.physicsforums.com/showthread.php?t=93486". It has been twice refuted and nobody in the community is taking it seriously, AFAIK.

 

[Garth]

This was already discussed https://www.physicsforums.com/showthread.php?t=93486". It has been twice refuted and nobody in the community is taking it seriously, AFAIK.

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

 

[Chronos]

Bah, those studies prove nothing, Garth.

 

[Spin_Network]

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).

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.

 

[Wishbone]

I got this off space.com today

Newton's laws of physics explain why our solar system stays together. But the planets are negligible in the overall gravitational scheme, with the Sun being the total ruler and containing 99.86 percent of all the mass.

The same Newtonian physics were long ago applied to galaxies, and the rotation of stars couldn't be explained, so dark matter was invented to make theory work.

But a galaxy is much different than the solar system, Cooperstock explains. The conglomeration of all the matter -- stars, black holes, gas, and dust -- is collectively the source of the galactic gravity. Even a black hole at a galaxy's center typically packs less than 1 percent of the galaxy's overall mass.

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...

 

[Garth]

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?

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

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...

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 .

Garth

 

[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.

 

[franznietzsche]

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.

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.

 

[jhe1984]

Cool site link, Spin.

 

[ubavontuba]

Hello,

If this http://www.newscientist.com/article.ns?id=dn7056" 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?

 

[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 × 10^0.47 % = 12.6%
and their lower WHIM limit is:
$\Omega_b$^WHIM > 1.3 × 10^0.32 % = 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

 

[SpaceTiger]

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

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

https://www.physicsforums.com/showthread.php?t=103450"