For the very first time, we have a complete and plausible accounting of matter and energy in the universe. The avalanche of data now and expected over the next few years will go a long way toward explaining the basic features of the universe with a theory rooted in fundamental physics, Turner concludes. What I want to argue is that in 1998, we had the first key evidence for a theory that takes us well beyond the hot Big Bang cosmology. The amount of mass per unit volume declines as the universe expands, but the energy density, at least in its simplest form, remains the same. Indeed, it is sometimes referred to as the cosmological constant.
Observations suggest that right now, the densities of matter and funny energy are roughly equal.
The energy density is just beginning to take over from matter density as the factor controlling cosmic expansion. At an earlier time, when the mass density was higher, the global effect of gravity would have been attractive, and we would not have observed the universe to be accelerating. At the end of the debate between Turner and Peebles, moderator Margaret J. Geller of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., took a straw poll. She asked the listeners if they thought that a century from now, the basic concepts exciting astronomers this year would be cornerstones of understanding. Most of the audience thought the concepts would be substantially different.