Modified Newtonian Dynamics - Consistency With The Observations

Consistency With The Observations

According to the Modified Newtonian Dynamics theory, every physical process that involves small accelerations due to gravity will have an outcome different from that predicted by the simple law F=ma. Therefore, astronomers need to look for all such processes and verify that MOND remains compatible with observations, that is, within the limit of the uncertainties on the data. There is, however, a complication overlooked up to this point but that strongly affects the compatibility between MOND and the observed world: in a system considered as isolated, for example a single satellite orbiting a planet, the effect of MOND results in an increased velocity beyond a given range (actually, below a given acceleration, but for circular orbits it is the same thing) that depends on the mass of both the planet and the satellite. However, if the same system is actually orbiting a star, the planet and the satellite will be accelerated in the star's gravitational field. For the satellite, the sum of the two fields could yield acceleration greater than a₀, and the orbit would not be the same as that in an isolated system.

For this reason, the typical acceleration of any physical process is not the only parameter astronomers must consider. Also critical is the process's environment, which is all external forces that are usually neglected. In his paper, Milgrom arranged the typical acceleration of various physical processes in a two-dimensional diagram. One parameter is the acceleration of the process itself, the other parameter is the acceleration induced by the environment.

This affects MOND's application to experimental observation and empirical data because all experiments done on Earth or its neighborhood are subject to the Sun's gravitational field, and this field is so strong that all objects in the Solar system undergo an acceleration greater than a₀. This explains why the flattening of galaxies' rotation curve, or the MOND effect, had not been detected until the early 1980s, when astronomers first gathered empirical data on the rotation of galaxies.

Therefore, only galaxies and other large systems are expected to exhibit the dynamics that will allow astronomers to verify that MOND agrees with observation. Since Milgrom's theory first appeared in 1983, the most accurate data has come from observations of distant galaxies and neighbors of the Milky Way. Within the uncertainties of the data, MOND has remained valid. The Milky Way itself is scattered with clouds of gas and interstellar dust, and until now it has not been possible to draw a rotation curve for the galaxy. Finally, the uncertainties on the velocity of galaxies within clusters and larger systems have been too large to conclude in favor of or against MOND. Indeed, conditions for conducting an experiment that could confirm or disprove MOND may only be possible outside the Solar system. A couple of near-to-Earth tests of MOND have been proposed though: one involves flying the LISA Pathfinder spacecraft through the Earth-Sun saddlepoint; another involves using a precisely controlled spinning disk to cancel out the acceleration effects of Earth's orbit around the Sun, and Sun's orbit around the galaxy; if either of these tests are carried out, and if MOND holds true, then they should feel a slight kick as they approach the very low acceleration levels required by MOND.

In search of observations that would validate his theory, Milgrom noticed that a special class of objects, the low surface brightness galaxies (LSB), is of particular interest: the radius of an LSB is large compared to its mass, and thus almost all stars are within the flat part of the rotation curve. Also, other theories predict that the velocity at the edge depends on the average surface brightness in addition to the LSB mass. Finally, no data on the rotation curve of these galaxies was available at the time. Milgrom thus could make the prediction that LSBs would have a rotation curve which is essentially flat, and with a relation between the flat velocity and the mass of the LSB identical to that of brighter galaxies.

Since then, the majority of LSBs observed has been consistent with the rotational curve predicted by MOND.

An exception to MOND other than LSB is prediction of the speeds of galaxies that gyrate around the center of a galaxy cluster. Our galaxy is part of the Virgo supercluster. MOND predicts a rate of rotation of these galaxies about their center, and temperature distributions, that are contrary to observation.

Computer simulations show that MOND is generally very precise at predicting individual galaxy rotation curves, of all kinds of galaxies: spirals, ellipticals, dwarfs, etc. However, MOND and MOND-like theories are not so good at predicting galactic cluster-scale, or cosmological scale structures.

A test that might disprove MOND would be to discover any of the theorized Dark Matter particles, such as the WIMPs.

A recent proposal is that MOND successfully predicts the local galactic escape speed of the Milky Way, a measure of the mass beyond the galactocentric radius of the Sun.

Lee Smolin and co-workers have tried unsuccessfully to obtain a theoretical basis for MOND from quantum gravity. His conclusion is "MOND is a tantalizing mystery, but not one that can be resolved now."

In 2011 University of Maryland Astronomy Professor, Stacy McGaugh, examined the rotation of gas rich galaxies, which have relatively fewer stars and a prevalence of mass in the form of interstellar gas. This allowed the mass of the galaxy to be more accurately determined since matter in the form of gas is easier to see and measure than matter in the form of stars or planets. McGaugh studied a sample of 47 galaxies and compared each one's mass and speed of rotation with the ratio expected from MOND predictions. All 47 galaxies fell on or very close to the MOND prediction. No dark matter model performed as well. On the other hand, another 2011 study observing the gravity-induced redshift of galactic clusters found results that strongly supported general relativity, but were inconsistent with MOND.. A recent work has found mistakes in the work by Wojtak, Hansen, and Hjorth, and confirmed that MOND can fit the determined redshifts only slightly worse than does general relativity with dark halos .