The Distribution of Dark Matter in Spirals

[Garth]

The Distribution of Dark Matter in Spirals

In the past years a wealth of observations allowed to unravel the structural properties of the Dark Matter Halos around spirals. First, their rotation curves follow an Universal profile (URC) that can be described in terms of an exponential thin stellar disk and a dark halo with a constant density core, whose relative importance increases with galaxy luminosity. Careful studies of individual objects reveal that dark halos have a core, whose size r₀ correlates with the central density rho₀. These properties are in serious discrepancy with the cuspy density distribution predicted by N-body simulations in collisionless LambdaCDM Cosmology.

Perhaps another 'epicycle' is required?

Or is the Halo baryonic after all?

Just a thought.

Garth

 

[turbo]

The Distribution of Dark Matter in Spirals


Perhaps another 'epicycle' is required?

Or is the Halo baryonic after all?

Just a thought.

Garth

Perhaps no Halo is required, baryonic or otherwise. Why should we believe that matter or "no-matter" (non-baryonic DM) follows the complex distribution rules required to keep GR gravity somewhat predictive on galactic scales? Besides, once you have massaged the distribution of invisible matter to fix galactic rotation curves, you still have to explain how "no-matter" manages to distribute itself "just so" to produce the too-strong lensing and excess binding energy exhibited by clusters. Occam's Razor is getting awfully rusty.

 

[SpaceTiger]

This problem has been known for a long time, but there is no consensus on the solution as of yet. There have been some pretty crazy (but not crackpotty) things proposed, like in the paper discussed here. Another solution that's a bit simpler is just self-interaction of the dark matter, but this is still being debated.

 

[turbo]

This problem has been known for a long time, but there is no consensus on the solution as of yet. There have been some pretty crazy (but not crackpotty) things proposed, like in the paper discussed here. Another solution that's a bit simpler is just self-interaction of the dark matter, but this is still being debated.

If you will consider substituting "quantum vacuum" for "dark matter", I think we can reach common ground very quickly.

If your library has a copy of "The Philosophy of Vacuum", edited by Saunders and Brown, grab it and read pages 13-20. The article is "On the Ether" by Einstein. It was written in 1924, only 4 years after his Leyden address. In this article, Einstein explains that a dynamic polarizable ether is required for GR and that the ether must have physical properties that are "conditioned" by the matter embedded in it and that the ether in turn confers gravitation and inertia upon the embedded matter. He also says that logically, the gravitational ether and the EM ether are the same. He explains that the apparent separation of these ethers in GR is attributable more to an "imperfection in our theoretical edifice than to a complex structure of reality itself."

Einstein was uncomfortable with the strangeness of quantum theory, and by 1952 when Dirac proposed a quantum-compatible ether, he may have been past the point of flexibility and openness on the subject. The fact of the matter is that the quantum vacuum has the required properties to perform as Einstein's GR ether (gravitational, inertial, with EM properties) not the least of which is that it is invisible (dark). All we need is a polarizing mechanism. My vote goes for a failure of the Weak Equivalence Principle, scheduled to be tested in CERN's Athena project. Other people studying the vacuum think the polarizing mechanism will turn out to have an EM basis, analogous to the phase-synchronization action of the Van der Waals dispersive force.

 

[Chronos]

I think not. Modern observational evidence appears to trump 50 year old observations and the weird science theories they spawned.

 

[Hans de Vries]

I was reading this in the latest CERN Courier:

http://www.cerncourier.com/main/article/45/8/8

(About a claim that proper application of GR instead of Newtonian gravity
produces the right speed distributions without dark matter)

General relativity versus exotic dark matter

Determinations of the rotation speed of stars in galaxies (galactic rotation curves) based on the assumption that Newtonian gravity is a good approximation have led to the inference that a large amount of dark matter must be present - more than can be accounted for by non-luminous baryonic matter. While there are plenty of attractive theoretical candidates for the additional dark matter, such as a lightest supersymmetric particle (LSP), it is also interesting to look into the details of the calculations that suggest the need for such exotica. Now F I Cooperstock and S Tieu of the University of Victoria have reworked the problem using general relativity in place of Newtonian gravity, and they find no need to assume the existence of a halo of exotic dark matter to fit the observed rotation curves.

This is because even for weak fields and slow speeds, well-known nonlinearities change the character of the solution dramatically. The success of Newtonian mechanics in situations like our solar system can be traced to the fact that in this case the planets are basically "test particles", which do not contribute significantly to the overall field. However, in a galaxy this approximation is not a good one - all the rotating matter is also the source of the gravitational field in which everything rotates.

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


Regards, Hans

 

[Garth]

Thank you Hans for that important link. If their analysis stands up then this would radically change ideas of DM galactic halos.

However DM is still required in IGM to virialise galactic clusters and produce the observed lensing events.

Garth

 

[Chronos]

Thanks for the link, Hans. But, I see serious problems with that model. Specifically, what happens to cluster dynamics? That appears to pretty much blow all the fairly decent N-body simulations right out of the water.

 

[Nereid]

So, what do the observers have to say? How well do the high quality rotation curve results match various models?

Stacy McGragh's recent paper is, IMHO, worth a read; the abstract gives a hint of what's inside:

The rotation curves of spiral galaxies obey strong scaling relations. These include the Tully-Fisher and baryonic Tully-Fisher relations, and the mass discrepancy--acceleration relation. These relations can be used to place constraints on the mass-to-light ratios of stars. Once the stellar mass is constrained, the distribution of dark matter follows. The shape of the dark matter distribution is consistent with the expectations of NFW halos exterior to 1 kpc, but the amplitude is wrong. This is presumably related to the long-standing problem of the normalization of the Tully-Fisher relation and may imply a downturn in the amplitude of the power spectrum at small scales. More fundamentally, the persistent success of MOND remains a troubling fact.

Enjoy!

 

[SpaceTiger]

Do you mean Stacy McGaugh?

Although I respect her opinion, I think this observer will only give you one part of the story, given that she's at one of the homes of MOND (UMD) and has been pushing it for quite some time.

Personally, I'm not all that surprised that MOND fits rotation curves well -- the theory was created to solve that problem, even inventing new physics to do so. Dark matter theorists face the tougher challenge of both fitting the curves and explaining how dark matter came to be distributed in the way that it is.