Modified Newtonian Dynamics

The review article Modified Newtonian Dynamics: A Falsification of Cold Dark Matter was written by R.H. Sanders of the Kapteyn Astronomical Institute at the University of Gronigen. This article was obtained by the EBSCO Academic Search complete and was originally published in Advances in Astronomy Volume 2009. I have the original article and would be happy to email it to anyone interested in it.

This review article is written by someone who is clearly a proponent of MOND (Modified Newtonian Dynamics) instead of CDM (Cold Dark Matter). The author begins the article by laying out the problem with galactic rotation curve and the problem with Keplerian motion accounting for observed evidence. The author mentions the lack of evidence for dark matter, excluding the DAMA project which he labels as controversial, in the standard model. The author then mentions the origin of MOND as a result of rotation curves being asymptotically flat and the Tully-Fisher law of mass and luminosity relationship.

The author states that changing the the gravitational acceleration in "low gravity" areas can be modeled to fit the observational data. The gravitational acceleration is modified the term Mu(g/a0) where a0 is a new parameter and Mu is an assymptotic function which has yet to be finalized but there are three main candidates (1/x^2, 1/x, e^-x). In areas of low acceleration this allows the velocity relationship to be:

v^4 = GMa0; a0 is currently valued at 1.2 X 10^-10 m/s^2

The asymptotic nature of Mu allows the modification of Newtonian gravity to approach zero the stronger the gravitational field and therefore match the observational evidence for the inner part of galactic rotation curves. The MOND theory even accounts for pressure supported systems showing that they are finite with density falling rapidly as a function of 1/r^4

The impressive part about the MOND theory is its ability to predict the galactic rotation curve based solely on observable evidence in the near infrared region of the spectrum. This is not just the general averaged rotational velocity but very specific values based on the clumping of observable matter. The MOND theory even accounts for variation in the M/L relationship of the T-F relation based on the color of galaxies.

The most important and the most emphasized point in the paper is that every variation of surface brightness of the galaxy has a corresponding variation in the MOND curve and the observed rotational curve.

The author then lays out the current problems associated with the MOND theory. Clusters of galaxies do not match the observed rotational values with the MOND curves with MOND curves predicting 2-3 times the mass that is observed in clusters. It is suggested that we may be unable to view all of the matter and we recently discovered hot gas emitting xrays that were previously neglected. The missing mass is also theorized to be "soft bosons", unaccounted for neutrinos, and even "dark matter."

MOND began as a simple theory that has progressed over the years. The initial relationship was shown to violate conservation of angular momentum and conservation linear momentum and was modified accordingly with no loss in fitting the model. The MOND theory has also been shown to work with general relativity and is free of anomalies, the first attempt had wave propagation faster than light. MOND says there can be "dark matter" but nowhere near the amounts that are used in the current model of 20% observable matter and 80% dark matter.

After reading this paper I find it difficult to argue with the author. The MOND theory has less unknown variables, is easily modifiable to match observations, the flaws with observation can be explained, uses only one new variable (as opposed to a min of 3 in CDM) and it does not require the creation of a new form of matter. The MOND theory is also similar in structure to Maxwell's equations demonstrating that it can match existing laws of the universe. Above all the MOND theory is more scientific at this time. It is clearly falsifiable, if the mass discrepancy of the galactic clusters is unaccounted for then the theory must be modified or thrown out. CDM is impossible to test because you can not prove that something does NOT exist. From my perspective the CDM proponents seem similar to the ether proponents of the late nineteenth century (although my knowledge is limited). In the absence of other evidence I believe that the simplest explanation is the best and in my opinion that is the MOND theory.