[CraigS]

Originally Posted by renormalised

I won't add anymore, but there are some 388 papers that are written about MOND (Modified Newtonian Dynamics). Whilst MOND is an interesting and possible alternative to the CDM theory there are still problems with it...especially when MOND is derived in terms of quantum gravity. So far attempts to derive MOND in this way have been unsuccessful. There is also some observational evidence with clusters of galaxies and a few other situations which MOND has not been able to model well, as yet. In fact, just like most of the others theories it's a work in progress. Whether it stands the test of time is a matter of more research and observation.

Yep. From Ned Wright's site .. (Mr CMBR):

"Aguirre, Schaye & Quataert (2001) find that while MOND works well on galaxy scales, it fails for Lyman alpha clouds and clusters of galaxies. The basic problem with MOND on cluster scales is that a point mass in MOND gives the observed isothermal profile in clusters, but most of the mass in clusters is due to a diffuse cloud of hot gas. Thus the observed gas density profile generates a gravity field under MOND that is not consistent with the observed gas temperature and density profile. Sanders (2002), written by a long time supporter of MOND, recognizes this problem and finds that dark matter is needed in clusters even under MOND.

A long standing problem with MOND was that it did not have a relativistic version. Bekenstein & Sanders (2005) have proposed a solution to this problem. But Zhao et al. (2005) find that this model requires different values for the universal constant ao in different gravitationally lensing clusters of galaxies, so the problem of not having a consistent relativistic version of MOND remains."

As for the first of the links I posted previously...I wouldn't take what it says as a given until it's been tested.

No worries, there !

Also remember that it has to explain the observational evidence better than current theory and have credible alternative processes and mechanisms. For instance, the Sun's oscillations about the central plane of the galaxy and mass extinction timings. Whilst there maybe some circumstantial evidence for a possible correlation between spiral arm crossings and large scale mass extinctions, the general background pattern and timing for extinctions does not fit the proposed timing for any dominant extraterrestrial cause. There are probably several factors both terrestrial and extraterrestrial which may induce extinction events...extreme climate changes, large scale volcanic activity (La Garita/Lake Toba/Yellowstone type events, as well as massive basalt province type of eruptions such as Siberian/Deccan Traps), asteroid impacts, close by gamma ray bursts (hypernovae explosions), disease pandemic outbreaks etc etc.

Yep. I go with disease outbreaks .. but who knows ? (or cares, for that matter .. don't know why they threw that into a good paper !).

Coming back to the paper though, what they've done is use everyday plain old vanilla physics to explain lots of astro observations with no need for anything else. They also makes the point that upon deeper consideration of the complex mechanics that's going on in the arms, (which they believe most scientists have skipped over), there are rational explanations for what we see. I like the approach. A 'middle-of-the-road' approach using unsensational conventionality.

Very worthwhile reading, but never worthwhile holding it up as "The Truth".

Cheers