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To speak of the "dark matter" problem is to beg one of the most important conundrums in present-day science; after all we have no direct evidence that dark matter actually exists in appreciable quantities. All we know is that the masses directly observed in galactic systems fall below what is calculated using standard dynamics. Stuffing galactic systems and the universe with putative dark matter is perhaps the least painful remedy for most people, but it is not the only one possible. Another avenue worthy of consideration builds on a possible failure of standard dynamics under the conditions that prevail in galactic systems. As you may know, the modified dynamics (MOND) has been put forth in just this vein [1]. It hinges on the accelerations in galactic systems being very small compared with what is encountered in the solar system, say. MOND asserts that non-relativistic dynamics involves the constant a0, with the dimensions of acceleration, so that in the formal limit a0 rightarrow 0 - i.e., when all quantities with the dimensions of acceleration are much larger than a0-standard dynamics obtains (in analogy with the appearance of hbar in quantum mechanics, and the classical limit for hbar rightarrow 0). In the opposite (MOND) limit of large a0 dynamics is marked by reduced inertia; one may roughly say that in this limit inertia at acceleration a is ma2 / a0, instead of the standard ma. This still allows for different specific formulations. Indeed we have nonrelativistic formulations of MOND, derivable from actions, based on either modified gravity [2], or on modified inertia [3]; these will be described below. A simple, if primitive, formulation that captures much of the content of MOND, and which gives the basic idea, is this: Imagine a test particle in the gravitational field of some mass distribution whose standard (Newtonian) gravitational acceleration field is gN. In standard dynamics the acceleration, g, of the particle is gN itself. MOND posits that this is so only in the limit gN >> a0. In the opposite limit gN << a0 we have roughly g ~ (gN a00)1/2. To interpolate between the limits we use a relation of the form µ(g / a00)g = gN, where µ(x) approx x for x << 1, and µ(x) approx 1 when x >> 1. This relation gives an approximate relation between the typical accelerations in a system (as embodied, say, in an exact virial relation derived from an exact theory). It also gives a very good approximation for the acceleration in circular motion relevant for rotation curves of disc galaxies [3] [4] (in modified inertia theories it give the exact rotation curve). In the more decent formulations of MOND, the actual acceleration of a test particle is not directly related to the local Newtonian acceleration as in the above relation (in particular, the two are not in the same direction, in general).

Some immediate, and unavoidable, predictions of even the basic tenets are [1] [5]:

1. The rotation curve for any isolated body becomes flat, asymptotically.

2. The asymptotic rotational velocity, Vinfty, depends only on the total mass of the body, M, via V4infty = MGa0. This predicts a Tully-Fisher relation between velocity and luminosity if the M / L values are narrowly distributed.

3. A similar approximate relation exists, for a body supported by random motions, between the mean velocity dispersion and the total mass. This is relevant to mass determinations of systems such as dwarf-spheroidal, and elliptical, galaxies, and of galaxy groups and clusters. It also predicts an approximate L propto sigma4 relation in such systems (with similar M/L values).

4. The smaller the typical acceleration of a gravitationally bound system, the larger the mass discrepancy it should evince. It had thus been predicted that all low-surface-brightness (LSB) systems should evince large mass discrepancies since, for a given M/L, surface brightness is proportional to acceleration (in the mean). This pertains, e.g., to dwarf-spheroidal satellites of the Milky Way, and to low-surface-brightness disc galaxies.

5. Above all, the full rotation curve of a disc galaxy should be obtained, using MOND, from the distribution of the observed mass alone.

Comparison with the data, as discussed later, yields a value of a0, determined in several, independent ways (using the different roles of a0 in the theory). Very interestingly, the value a0 turns out to be of the same order as c H0 - an acceleration parameter of cosmological significance [1]. Anticipating later discussion, I remark here that this might be a crucial clue as to the origin of MOND, and its possible origin in effects related to cosmology.

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