Computer models for galaxy rotational curves

[kk434]

A quick description how astronomers calculate the D.M. halo:

Firsth you create a computer model that simulates the orbital speed of the stars. You place all the visible mass and run the model, it shows that the outermost stars have a lower than observed speed. Then you place invisible mass at specific places in the galaxy and run the model again, if you placed the mass at the right places it maches the observed rorational curve.

Does anyone know what set of equations are used? How complicated the simplest model is ?

 

[kk434]

Oh no, 15 vievs, 0 replays and this is such a important topic. The reason for postulating D.M. was to explain the behavior of galaxies. If the models are wrong there is no need for D.M and we may start looking for other explanations.
(MOND and other like it)

 

[UFmbutler]

Just because you don't understand a model doesn't mean it is wrong. You can't just assume something you don't understand is wrong and go looking into other ideas not supported by observation (i.e. MOND). It sounds like you think dark matter evidence is purely theoretical, taken only from simulations/models. This is not true. It is an observational result. You observe the galaxy's rotation curve(if you want to know the various methods to measure this, read any basic galactic astronomy text such as binney and merrifield Galactic Astronomy), and based on Newtonian mechanics, you make a comparison. You know the amount of luminous matter, you know what mass would be required to produce such a curve, and you make the conclusion that there needs to be a large amount of non-luminous matter.

Here is a website that explains the equations: http://www.haystack.mit.edu/edu/undergr ... ation.html scroll down to the "Theory:" section.

 

[jaxtraw]

Er, dark matter isn't an "observational result". It's a postulated fix for observation not fitting prediction. That isn't the same thing. In such a situation you cannot use the original observation as supporting evidence for the proposed hypothesis. That is circular reasoning. For instance, an invisible substance with negative mass called phlogiston was once proposed to explain the phenomenon of burning. A phlogistion supporter cannot say, "the proof that phlogiston exists is that wood burns". He needs other supporting evidence- like, directly detecting the phlogiston.

There was a respectable paper published a while ago, sorry can't remember the specifics, proposing that the observed effect is simply relativistic, and it is erroneous to use Newtonian mechanics because relativistic effects predominate at galactic scales. That is, the virial theorem doesn't hold in this situation and relativistic effects predominate in the galactic disk.

It seems to me that Occam would prefer us to explore the subtleties of the well tested theories we already have before postulating vast quantities of undetectable exotica to make simplistic Newtonian models work. But cosmologists do like exotic speculations, it seems. It is disturbing how these days speculations like dark matter and dark energy get built into the hegemonic model and declared beyond doubt.

 

[kk434]

This is exactly what bothers me, DM is treated like something proven beyond doubt. After many searches there is almost no clue to what it is and every scientist has his favorite set of hypotetical DM particles.

 

[SpeedFreek]

Some observational evidence for dark matter - gravitational lensing. We see the amount of gravitational lensing for objects behind a cluster of galaxies that is predicted by the dark matter model, rather than the lensing being attributed to just the mass of the galaxies themselves.

http://www.astro.gla.ac.uk/users/martin/outreach/lensing.html