Modified Newtonian Dynamics as an Alternative to Dark Matter

MODIFIED NEWTONIAN DYNAMICS AS AN
ALTERNATIVE TO DARK MATTER
Robert H. Sanders1 & Stacy S. McGaugh2

1Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
2Department of Astronomy, University of Maryland, College Park, MD, USA
KEYWORDS: dark matter, galaxy dynamics, gravitational theory, cosmology


MODIFIED NEWTONIAN DYNAMICS AS AN
ALTERNATIVE TO DARK MATTER
Robert H. Sanders1 & Stacy S. McGaugh2
1Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
2Department of Astronomy, University of Maryland, College Park, MD, USA

KEYWORDS: dark matter, galaxy dynamics, gravitational theory, cosmology

ABSTRACT:
Modified Newtonian dynamics (MOND) is an empirically motivated modification of Newtonian gravity or inertia suggested by Milgrom as an alternative to cosmic dark matter. The basic idea is that at accelerations below ao _ 10−8 cm/s2 _ cHo/6 the effective gravitational attraction approaches pgnao where gn is the usual Newtonian acceleration. This simple algorithm yields flat rotation curves for spiral galaxies and amass-rotation velocity relation of the form M / V 4 that forms the basis for the observed luminosity-rotation velocity relation– the Tully-Fisher law. We review the phenomenological success of MOND on scales ranging from dwarf spheroidal galaxies to superclusters, and demonstrate that the evidence for dark matter can be
equally well interpreted as evidence for MOND. We discuss the possible physical basis for an acceleration based modification of Newtonian dynamics as well as the extension of MOND to cosmology and structure
formation.

These two authors discuss and compare math solutions and their accuracy using observed data in our known universe. The incomplete MOND theory cannot be extended to problems of cosmology and structure formation. However by making reasonable assumptions one may speculate on the form of a MOND cosmology.

The authors show equations relating the force of gravity to the force of charge particles as:

F = (GM/ r^2 ) x f (r/ro)

Scientist Milgrom (1983a) realized that this length in kpc law was incompatible with observed TF law, L is proportional to v^4. Moreover any modification of a length scale would imply that larger galaxies should exhibit a larger discrepancy. (Sanders 1986) This is contrary to observed data.

A most interesting question for me concerning gravity and dark matter was investigated directly by “dark halos” with an acceleration scale can the phenomenology of MOND be reproduced by dark halos– specifically the kind of dark halos that emerge from cosmological N-body simulations with an initial fluctuation spectrum given by cold dark matter (CDM)? This is an important question because the phenomenological success of MOND may be telling us something about the universal distribution of dark matter rather than anything about the law of gravity or inertia at low accelerations.
This question was first considered by Begeman et al. (1991) who attempted to devise disk-halo coupling rules that could yield a one-parameter fit to rotation curves (M/L of the visible disk) similar to modified Newtonian dynamics (MOND). Without any physical justification the core radius and asymptotic circular velocity of an isothermal halo were adjusted to the scale length and maximum rotation velocity of the disk to yield a characteristic acceleration. With such coupling rules, the fits to galaxy rotation curves were of lower quality than the MOND fits (particularly for the dwarf systems), and there were numerous ambiguities (e.g., in gas dominated galaxies, what is the proper disk length scale?). Similar ad hoc coupling rules between visible and dark components have also been considered by Giraud (2000).

While this disagrement between scientist has been intense for over thirty years this is a short period for science history. The disagreement leads scientist to follow their own intuitive judgment about how to apply science money and methods. I do not know of any other group better qualified to make such decisions. ( Like maybe politicians, actors, or news anchors??)