Dark Matter

Introduction

The further an object is from a central mass the slower it moves in its orbit. This is a consequence of Kepler's Laws and is certainly true of the planets that orbit the Sun. Pluto moves a lot slower than does the Earth.

However, the stars in the Milky Way (and other galaxies) do not obey this rule: as we move away from the center of the Milky Way the orbital speeds of stars do indeed fall, as expected, however, beyond about 8000 light years or so from the center the orbital speeds of stars remain roughly constant. The orbital speed as a function of the radial distance from the galactic center is called the galactic rotation curve.

Some scientists have suggested that this observation implies that Newton's law of gravity is incorrect on large distance scales. Other scientists have suggested that galaxies are cloaked in a large amount of unseen matter, called dark matter, that causes additional gravitational effects. This is the currently favored hypothesis. See article on dark matter.

We do not know the nature of the dark matter. However, several hypotheses are being considered, which fall into two classes: a) dark matter is made up of particles that do not interact readily with matter. They could be, for example, neutrinos or some new particles, as yet unknown to science. Generically, these particles are referred to as Weakly Interacting Massive Particles (or WIMPS); b) dark matter is made up of sub-stellar objects, like brown dwarfs or Jupiter-like objects. Or perhaps, dark matter is a combination of a) and b)!

WIMPS - These particles are assumed to pervade all of space. Indeed, if the WIMP hypothesis is correct, WIMPS are traversing your body right now. But because they interact very weakly with matter it is unlikely that any will actually interact with the particles in your body! Experiments are now being conducted worldwide to find evidence of WIMPS.

Sub-stellar objects - These can be thought of as ``failed stars'' - would-be stars that did not draw together enough mass to trigger thermonuclear reactions in their core. These are objects with masses less than about 0.01 solar masses. They shine so feebly that they cannot be seen from Earth. The microlensing experiments are looking for sub-stellar objects in the halo of our galaxy. Sub-stellar objects in our halo are referred to as Massive Compact Halo Objects, or
MACHOs.

Some Dark Matter Links

Last updated March 3, 1999, Harrison B. Prosper