5. THE ALTERNATIVE

If the problems listed in Section 3 cannot be solved in the context of an acceptably simple DM model, the only logical alternative is to modify gravity in the weak-field regime. No satisfactory theory that achieves this has yet been proposed.

Remarkably, we already know the effective force-law for galaxies (McGaugh 1999a): MOND (Milgrom 1983) is a highly successful phenomenological description of the dynamical data on spiral galaxies which effortlessly resolves problems 1 through 4, 6, 10 & 11 above. Thus it disposes of those most intractable in DM models (although problem 3 is ``resolved'' only by introducing a new fundamental physical scale). Problem 2 was a notable prediction of MOND. It seems not unreasonable to imagine that the other problems might also be resolved once a theory of galaxy formation is formulated in some modified gravity theory.

But modifying the law of gravitation is not a panacea; in particular it faces serious challenges in reproducing the successes of standard cosmology. The following is a list of some more obvious difficulties.

1. A modified theory must be cast as a generally covariant theory to reproduce Einstein gravity in its well-tested regimes. So far, attempts to fit MOND into a natural metric framework have been largely unsuccessful (Sanders 1997).

2. It must establish a cosmology in which the scale factor evolution yields the first CMB acoustic peak at its observed angular scale.

3. Without DM, Silk damping (Silk 1968) at the time of recombination would eradicate primordial fluctuations on scales of galaxies and smaller. A new source of fluctuations to seed the growth of galaxies would therefore be required (Sanders 1998).

4. In the DM picture, the tiny fluctuations observed in the microwave background have grown to the observed current large scale structure only because the DM density variations started growing before recombination. A cosmology without DM requires other means to increase the growth factor since the time of recombination (Sanders 1998).

5. Observed gravitational lensing must arise solely from the baryonic matter distribution.

One other issue might be added to both this list of problems and that in Section 3. Current observational data strongly indicate a cosmological constant (e.g. Jaffe et al. 2000), while conventional physical theories offer no explanation for its tiny value (Weinberg 2000) and it requires us to live at a special time in the evolution of the universe. It is conceivable that a dramatic alteration of gravitation and cosmology might provide a different reason for accelerated expansion.

Future signatures of the absence of DM might include a still more strongly suppressed third acoustic peak in the CMB. In the absence of any driving force from dark matter, its amplitude should be further suppressed relative to the second peak due to Silk damping, while most conventional models predict an enhanced third peak from driven baryon compression. Another possible test is the imprint of these peaks on the large-scale structure power spectrum (Eisenstein & Hu 1998): without a dominant contribution of DM to the gravitational potential, substantial oscillations in the power spectrum of large-scale structure might survive.