Observational evidence from a variety of sources currently points to a universe which is (at least approximately) spatially flat, with (M, ) (0.3, 0.7). The nucleosynthesis constraint implies that B ~ 0.04, so the majority of the matter content must be in an unknown non-baryonic form.
Figure 11. as a function of the scale factor a, for a universe in which M0 = 0.3, 0 = 0.7. Indicated are the scale factors corresponding to the Planck era, the electroweak phase transition, and Big Bang Nucleosynthesis.
Nobody would have guessed that we live in such a universe. Figure (11) is a plot of as a function of the scale factor a for this cosmology. At early times, the cosmological constant would have been negligible, while at later times the density of matter will be essentially zero and the universe will be empty. We happen to live in that brief era, cosmologically speaking, when both matter and vacuum are of comparable magnitude. Within the matter component, there are apparently contributions from baryons and from a non-baryonic source, both of which are also comparable (although at least their ratio is independent of time). This scenario staggers under the burden of its unnaturalness, but nevertheless crosses the finish line well ahead of any competitors by agreeing so well with the data.
Apart from confirming (or disproving) this picture, a major challenge to cosmologists and physicists in the years to come will be to understand whether these apparently distasteful aspects of our universe are simply surprising coincidences, or actually reflect a beautiful underlying structure we do not as yet comprehend. If we are fortunate, what appears unnatural at present will serve as a clue to a deeper understanding of fundamental physics.
I wish to thank Greg Anderson, Tom Banks, Robert Caldwell, Gordon Chalmers, Michael Dine, George Field, Peter Garnavich, Jeff Harvey, Gordy Kane, Manoj Kaplinghat, Bob Kirshner, Lloyd Knox, Finn Larsen, Laura Mersini, Ue-Li Pen, Saul Perlmutter, Joe Polchinski, Ted Pyne, Brian Schmidt, and Michael Turner for numerous useful conversations, Patrick Brady, Deryn Fogg and Clifford Johnson for rhetorical encouragement, and Bill Press and Ed Turner for insinuating me into this formerly-disreputable subject.