By chance the Hubble Space Telescope had observed a supernova in late 1997 and early 1998 that proved to be 11 billion light-years away -- the most distant yet seen. On Dr. Riess's Hubble diagram it appeared twice as bright as it should.
''Extraordinary claims require extraordinary evidence -- I hope the I.R.S. doesn't say that to you,'' Dr. Riess told his audience, but, he concluded, ''the cosmological constant looks good for this supernova.''
Dr. Livio said, ''A year ago probably a large fraction of the people in this room would not have believed it.''
But there were more complicated explanations, forms of dark energy other than the cosmological constant on physicists' drawing boards, as well as the possibility that astronomers were still being fooled. To explicate the nature of the dark energy, astronomers need to observe more supernovae as far back as 11 billion years ago, to cover the time when the universe began to accelerate.
''How fast did it go from deceleration to acceleration?'' asked Dr. Riess. Answering such questions could help astronomers determine how hard the dark energy is pushing on the universe compared with the predictions for the cosmological constant. A fast turnaround, he said, ''begins to tell you there is a lot of oomph for a given amount of 'it.' ''
''The cosmological constant is the benchmark oomph,'' he said.
To find those supernovae so far out cosmologists will have to go to space, said Dr. Saul Perlmutter, a physicist at the University of California's Lawrence Berkeley National Laboratory and a veteran dark energy hunter.
On the ground, the supernova researchers have to employ a wide network of people and telescopes to detect the explosions, diagnose their type and then to watch them fade. Dr. Perlmutter described an orbiting telescope that would perform all three functions. The Supernova/Acceleration Probe, or SNAP, would combine an 80-inch diameter mirror (only about 16 percent smaller than the Hubble), a giant electronic camera with a billion pixels and a special spectroscope.
If all goes well, Dr. Perlmutter said, the telescope could be launched in 2008. In three years of operation, he estimated, SNAP could harvest about 2,000 supernovae. To distinguish the different ideas about dark energy, observations would have to be refined to the level of 1 or 2 percent uncertainty .
''We're all excited,'' he said.
So are the physicists. Their list of suspects begins with Einstein's cosmological constant, but therein lies a problem. About the time that Einstein was abandoning it, quantum mechanics -- the set of rules that govern the subatomic realm -- was establishing a theoretical foundation for the cosmological constant. According to quantum theory, empty space should be foaming with temporary particles and their cumulative energy would outweigh the matter in the universe, including dark matter, by 120 orders of magnitude -- that is, a factor of 10 followed by 119 zeros. At that level, the force of the vacuum would either have crumpled the universe or blown it apart before even an atom had the chance to form.
The fact that the universe is in fact puttering along rather gently suggests that there is something fundamental about physics and the universe that physicists still don't know. Dr. Steven Weinberg, the Nobel Prize-winning particle theorist at the University of Texas, has called the cosmological constant ''the bone in our throat.'' If the dark energy really is Einstein's cosmological constant, then physicists have to answer questions like why it is so small -- roughly comparable, in fact, to the density of matter in our own epoch.
Lacking an answer so far, even from string theory, the mathematically daunting candidate theory of everything, some theorists have resorted to a controversial and somewhat philosophical notion called the anthropic principle, which, in effect, says that physicists need to factor in their own existence when they think about the universe. Out of all the possible universes that can be imagined, this line of thinking goes, the only ones in which humans can find themselves is one that is conducive to human life.
That means, Dr. Weinberg has pointed out, that the cosmological constant has to be small enough to allow time for galaxies and stars to condense from the primordial fog before it takes over and starts blowing the universe apart.
Dr. Alex Vilenkin of Tufts University in Massachusetts pointed out to the Dark Universe audience that the universe was at its peak in making stars about five billion or six billion years ago, just about the time that dark energy and the matter density would have been equal. Our own sun, some 4.5 billion years old, was on the tale end of that wave, and now here we are. ''Observers are where the galaxies are,'' said Dr. Vilenkin. ''A typical observer will see a small cosmological constant.''
Many physicists are uncomfortable with this line of reasoning, and they are seeking the answer in different class of theories known as quintessence, after the Greek word for the fifth element. Modern physics, noted Dr. Paul Steinhardt, a theorist at Princeton, is replete with mysterious energy fields that would exhibit negative gravity. The trick, Dr. Steinhardt explained, is finding a field that would act like the dark energy without a lot of fudging on the part of theorists.
''The observations are forcing us to do this,'' he said. ''Dark energy is an interesting problem. Any solution is quite interesting.''