I list the overall constraints on cosmological parameters discussed in this review in the table below. It is worth stressing how completely remarkable the present situation is. After 20 years, we now have the first direct evidence that the Universe might be flat, but we also have definitive evidence that there is not enough matter, including dark matter, to make it so. We seem to be forced to accept the possibility that some weird form of dark energy is the dominant stuff in the Universe. It is fair to say that this situation is more mysterious, and thus more exciting, than anyone had a right to expect it to be.
|Parameter||Allowed range||Formal Conf. Level (where approp.)|
|B h2||.019±.0018 or .032±.009|
The new situation changes everything about the way we think about cosmology. In the first place, it demonstrates that Geometry and Destiny are no longer linked. Previously, the holy grail of cosmology involved determining the density parameter , because this was tantamount to determining the ultimate future of our universe. Now, once we accept the possibility of a non-zero cosmological constant, we must also accept the fact that any universe, open, closed, or flat, can either expand forever, or reverse the present expansion and end in a big crunch . But wait, it gets worse, as my colleague Michael Turner and I have also demonstrated, there is no set of cosmological measurements, no matter how precise, that will allow us to determine the ultimate future of the Universe. In order to do so, we would require a theory of everything.
On the other hand, if our universe is in fact dominated by a cosmological constant, the future for life is rather bleak . Distant galaxies will soon blink out of sight, and the Universe will become cold and dark, and uninhabitable....
This bleak picture may seem depressing, but the flip side of all the above is that we live in exciting times now, when mysteries abound. And these mysteries, or new ones, are likely to remain with us for some time.
I thank my collaborators involved in various aspects of my own work described here, including Michael Turner, Brian Chaboyer, Craig Copi, and Glenn Starkman, and also the observers whose results have helped make cosmology so exciting in the past decade. I also thank the the organizers for a very stimulating and enjoyable meeting.