Steinhardt has proposed that the dark energy is a hitherto-unknown substance, called “quintessence.” Its name hearkens back to the ancient notion of four natural elements—earth, air, water, and fire. Quintessence would be the fifth. Instead of a steady cosmological constant, it would be a field that kicked in during a particular epoch of the universe, causing a far milder version of an inflationary burst. Using a variable field offers greater flexibility in modeling different cosmic phases. However, current observations have not been able to distinguish between variable and constant forms of dark energy.

To resolve these and other vital issues, astronomers have pressed on with further testing. The supernova teams have continued their endeavors, accumulating a bevy of examples to enhance their data. The LBL group has proposed a space-based mission, called the Supernova Acceleration Probe, to improve their capability by 20-fold. Meanwhile, spectacular new results from the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, have uncovered a treasure trove of critical information about the young cosmos.

The beginning of the 21st century has witnessed cosmology becoming an exacting enterprise, with ample tools to elucidate the state of the observable universe. It has also ushered in considerable confusion as to the future direction of the field. A snapshot of the early