Dark Matter Real?

Is the problem of dark matter a "preserving appearances" problem? In other words, if some other non negotiable factor in equations was treated differently, could the matter in the universe be squared up?
- Michael (age 37)
Singapore

Excellent question. I want to be careful to distinguish between dark matter and dark energy. Dark matter would be some sorts of particles which happen to have very little interaction with the particles of which we are made. It would show up mainly by gravitational effects, with the distribution of matter in galaxies showing more mass in a somewhat different spatial pattern than the visible mass. That's just what's seen. Some of the effects could be mimicked by changes in the laws of gravity at galactic distances. That would be very surprising. Furthermore, the dark matter shows up in the behavior of the cosmic microwave background in just the way expected if it's simply some collection of particles. So yes, if you look at any one effect of the dark matter, you could get something like it by changing other laws, but it's very unreasonable to make many changes in fundamental physics when you can get the whole collection of effects by a much simpler, less radical,  hypothesis.

Now the other unseen component is called 'dark energy'. So far, its main effects are on the pattern of expansion of the universe over time, measured from red-shifts, and on the form of the microwave background fluctuations. The standard view is that these effects are produced because our particular universe is stuck in a state where empty space itself has some small fixed energy density. There are said to be some serious competing hypotheses, and astrophysical tests are being planned to check them.

Mike W.

(published on 05/12/2008)