Major Announcement! Options

[DEChengst]

Press release on the web:

http://www.esa.int/esaCP/SEMZ6GSVYVE_index_0.html

The video:

http://esatv-movies.e-vision.nl/videos/mph...-07_wmphigh.wmv (18 MB)

And the same in high-res MPEG2:

http://esatv-movies.e-vision.nl/videos/mpe...-07_mpeg2ps.mpg (621 MB)

[The Messenger]

....
This announcement is (in my opinion) yet another example of assuming that dark matter exists, and then using that assumption as an unchallenged given while presenting 'proof' of dark matter's existence.

Bill

So it is a major announcement about the observation of nothing...

[karolp]

I still find the 3d graphic quite sexy

[Jeff7]

Okaaaay.....

I probably should not be making editorial comments on the existence (or lack of same) of dark matter, but I cannot help but point out that dark matter is still just a hypothesis -- it has never been DIRECTLY detected (or indeed explained by standard physics). The anomalous motions that the dark matter hypothesis was invented to explain are also explainable by other hypotheses; in particular, by Milgrom's MOND hypothesis -- or possibly by full General Relativity, which is not normally used in these models, as it is very difficult computationally, but which in its first use in a galactic-scale model a few months ago, reproduced the effects of dark matter while utilising only the known baryonic matter.

This announcement is (in my opinion) yet another example of assuming that dark matter exists, and then using that assumption as an unchallenged given while presenting 'proof' of dark matter's existence.

Bill

I too would really love to see more models done with GR taken into account. I would have thought of this as being a "Duh" kind of thing - according to general relativity, as objects move faster, their mass increases. Some stuff in space moves pretty quickly, and while it's not exactly close to the speed of light, we're still talking about very massive objects moving fast, so a small percentage increase in mass due to velocity could in fact be quite significant.
Do the models also factor in the speed of gravity? As I understand it, the effects of gravity also move at the speed of light. For example, if the sun were to suddenly vanish from existence, we wouldn't know about it for about 8.3 minutes. This applies to both the light coming from it, and its gravitational influence. So as objects are moving past each other at great distances, their gravitational pull versus position won't be exactly synchronized. Think of a jet flying overhead - if you look to where you hear the sound, the jet isn't there, but ahead of the sound. Same with a star moving past. Its actual position is different from where the observing star "sees" both its light and gravity. If the simulation doesn't account for this, the results will be flawed.

[Bob Shaw]

Do the models also factor in the speed of gravity?

Jeff:

I have the impression that gravity doesn't have a speed as such, being a distortion in space-time rather than a 'force'. Unlike gravy, which obviously has a speed, texture and aroma!


Bob Shaw

[nprev]

I've always been a bit curious about the cumulative effects of virtual particle/antiparticle production/destruction on a cosmological scale. That's potentially a LOT of mass flickering in & out of existence, in all probability many orders of magnitude more than the Universe's permanent matter.

Maybe that's what dark matter is; it's "dark" because it doesn't exist long enough to be observed!

[Mongo]

I too would really love to see more models done with GR taken into account. I would have thought of this as being a "Duh" kind of thing - according to general relativity, as objects move faster, their mass increases.

Here are some papers that describe a full GR galaxy model:

General Relativity Resolves Galactic Rotation Without Exotic Dark Matter

http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0507/0507619.pdf

A galaxy is modeled as a stationary axially symmetric pressure-free fluid in general relativity. For the weak gravitational fields under consideration, the field equations and the equations of motion ultimately lead to one linear and one nonlinear equation relating the angular velocity to the fluid density. It is shown that the rotation curves for the Milky Way, NGC 3031, NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter concentrated in flattened disks. Thus the need for a massive halo of exotic dark matter is removed. For these galaxies we determine the mass density for the luminous threshold as 10^{-21.75} kg.m$^{-3}.


Perspectives on Galactic Dynamics via General Relativity

http://lanl.arxiv.org/PS_cache/astro-ph/pdf/0512/0512048.pdf

Responses to questions, comments and criticism of our recent paper "General Relativity Resolves.." are provided. It is emphasized that our model is entirely natural to describe the dynamics of an axially symmetric galaxy and that our solution, albeit idealized, contains the essence of the problem. The discontinuity of the metric derivative on the symmetry plane is necessarily interpreted as the effect of the mathematically idealized discontinuity of the gradient of the density and is shown to be naturally connected to the distributed volume density via the Gauss divergence theorem. We present arguments to the effect that for our approximate weak field model, we can choose the physically satisfactory mass distribution without an accompanying singular mass surface layer. To support this contention, we modify our solution slightly by removing the discontinuity with a region of continuous density gradient overlapping the $z=0$ plane. The alternative of invoking a surface layer leads to the presence of a negative mass surface layer approaching the numerical value of the positive mass continuous region. This is in contradiction with the assumed stationarity of the model. We find that a test particle behaves normally as it approaches the $z=0$ plane, the acceleration being towards the direction of this plane. This is in contradiction to the negative mass layer hypothesis as negative mass would repel the test particle. Thus, further support is added to the integrity of our original model.


Galactic Dynamics via General Relativity: A Compilation and New Developments

http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0610/0610370.pdf

We consider the consequences of applying general relativity to the description of the dynamics of a galaxy, given the observed flattened rotation curves. The galaxy is modeled as a stationary axially symmetric pressure-free fluid. In spite of the weak gravitational field and the non-relativistic source velocities, the mathematical system is still seen to be non-linear. It is shown that the rotation curves for various galaxies as examples are consistent with the mass density distributions of the visible matter within essentially flattened disks. This obviates the need for a massive halo of exotic dark matter. We determine that the mass density for the luminous threshold as tracked in the radial direction is $10^{-21.75}$ kg$\cdot$m$^{-3}$ for these galaxies and conjecture that this will be the case for other galaxies yet to be analyzed. We present a velocity dispersion test to determine the extent, if of any significance, of matter that may lie beyond the visible/HI region. Various comments and criticisms from colleagues are addressed.

Unfortunately, the scientific community is just as prone to the 'bandwagon effect' as any other. Dark matter is the current 'Hot New Thing', and anybody in the field publicly disputing the hypothesis will certainly find it more difficult to advance in their career. This is nothing new, of course -- one recent example is Carl Woese, who discovered the Archaea as a separate domain of life (joining the Bacteria and the Eucaryotes) -- but before the idea was accepted, he suffered decades of hostility and attempts to block his career. He is retired now, and apparently still quite bitter about his treatment in the years before the domain 'Archaea' was accepted.

Bill

[nprev]

"Nature is merciful in providing one linear equation..." indeed! Very good paper; thanks Mongo!

My only question lies in the dependency on the value of G for the validity of some of the core equations, and how much variance is permissible. I'm not enough of a mathematician to flesh out the constraints and consequences within the range of possible values of G (using the COSPAR datum as reference); anybody more suitably equipped to respond?

(Let me reveal my hidden agenda: I wonder if this research could be used to constrain G, which of course would prove beneficial across a wide variety of disciplines. We need a sensitivity analysis. One potentially interesting side effect might be a good model of the amount of non-luminous baryonic matter in galaxies, with many more implications.)

[ustrax]

A step leads to the next...
"Now that astronomers have begun to map out where dark matter is, their next objective is to determine what it is, and its relationship to normal matter."

[Guest_PhilCo126_*]

MACHOs and WIMPs ... physicists really have a sense of humor
I was thinking they were convinced that the dark matter problem would be solved by detecting "Neutrinos" ?

[Guest_Myran_*]

PhilCo126 wrote: I was thinking they were convinced that the dark matter problem would be solved by detecting "Neutrinos" ?

Neutrinos were detected the first time in one nuclear reactor more than 50 years ago in the Hanford Experiment.
You might have been thinking of finding the neutrino mass at rest, its smaller that 30eV and so are thought to be one unlikely candidate to contrinute much to the dark matter problem.

[nprev]

If this stuff exists (and I'm by no means convinced that it does) it's probably more lepton-like than hadron-like. Hadrons have a way of accreting to others & producing potentially observable artifacts, or generating very pronounced secondary effects when they encounter other matter at relativistic velocities. The whole term "dark matter" is problematic in this view; if it was at all analogous to matter as we encounter it in our direct experience, there should be a lot of otherwise unexplainable fireworks going on.

Bottom line is that the papers cited by Mongo seemed pretty convincing. GR effects absolutely have to be considered when observing either very massive or very distant objects, and the dark-matter theorists may not be fully considering alternative hypotheses within accepted physical frameworks.