New study shows Dark Matter isn't needed? Relativty explains it?

[Garth]

If it turns out to be repulsive, this would explain the finite thickness, but then one wonders why the solution models galactic rotation which requires more (not less) matter.

Hi pervect - is that the rebuttal correction compared to C&T or C&Tcompared to Newtonian?

The C&T solution requires less matter (no DM halo) than the Newtonian.

Garth

 

[Garth]

My admittedly crude intuition insists a rotating, roughly spherical mass will naturally flatten out into a disc-like structure. Deriving the observed features of galaxies appears almost incomprehensively difficult. I see all kinds of complications - classical physics, turbulence, tidal forces, electromagnetism, backreactions and relativistic corrections. Perhaps dark matter represents an approximation of these combined effects.

Thanks Chronos obviously a detailed model able to explain the spiral arms, the central bulge, the warp in the disk and the contribution of the globular clusters and anything else out there in the form of a DM halo is going to be horribly complicated - not a 'back of the envelope' type of calculation!

However it would be good to get the basic flat rotation profile sorted.

Garth

 

[Chronos]

Agreed, Garth. Would you concede that even the 'simple' model is anything but simple? I think if we could get that much right, the details would be mostly easier.

 

[pervect]

Hi pervect - is that the rebuttal correction compared to C&T or C&Tcompared to Newtonian?
The C&T solution requires less matter (no DM halo) than the Newtonian.
Garth

The article in question is http://arxiv.org/abs/astro-ph/0508377

They first point out that the second derivative of N with respect to z is undefined at z=0 because the first derivative changes sign. Their argument that the disk must contains a "shell" of matter is based on the Komar integral - this is equivalent to the Komar mass, which is mass defined in terms of a Killing vector (this concept of mass is valid in any static space-times). They take the limit of the Komar integral (which gives the enclosed mass) for a cylinder which approaches zero volume (by shrinking the height 'a' of the cyliner to zero), and find that the resulting limit as the height a->0 is non-zero

I don't know offhand whether non-zero means positive, or negative. I see a minus sign in (20), but it's easy to lose track of signs.

BTW the concept of Komar mass is the one found on pg 298 of Wald - I've posted about it before, but never under that name - I didn't realize it had a name until just now (it's handy to know it's name).

 

[Garth]

Thank you, yes I have read the rebuttal paper, its good to have it explained so clearly.

Garth

 

[SpaceTiger]

Interesting sidenote. The speaker at my seminar today used this paper as an example of "How not to do GR."

 

[Garth]

Concur - what is needed is an equivalent to the Kerr metric.

Garth

 

[Garth]

And now it seems spiral galaxy rotation curves are not even flat - as C&L were assuming! Comment on "Scalar-tensor gravity coupled to a global monopole and flat rotation curves" by Lee and Lee

Garth