After almost ten years of diligent observations of the earliest light in the universe, the Wilkinson Microwave Anisotropy Probe (WMAP) fired its thrusters on September 8, and entered the world's longest and most tedious holding pattern, circling the sun in a so-called graveyard orbit.
WMAP has been a triumph of observational cosmology. It has strengthened the case for cosmic acceleration, one of nature's most mysterious phenomena; measured the composition of the universe, teaching us about dark matter and dark energy; observed the polarization signal, telling us about reionization; provided results consistent with an inflationary origin for the universe, while constraining and even ruling out some of the simplest models; and has left us with some intriguing open questions of its own. To those of us in the field, it seems like only yesterday that we were eagerly awaiting the first 3-year data release from WMAP. Now, the final data, collected on August 20, will form part of the complete 9-year dataset, capping a remarkable decade of cosmic discovery. From a fundamental physics perspective, WMAP is a crucial component of increasingly accurate cosmological observations that challenge the standard model of particle physics. In accurately determining the dark matter abundance, it has specified even more precisely the requirements of the new particle physics required to account for that portion of the energy budget. If dark matter is made of Weakly Interacting Massive Particles (WIMPs), for example, the range of properties they might have is more tightly constrained. In supporting the case for cosmic acceleration, it has sharpened the need for a fundamental explanation for the size of the cosmological constant and possibly for entirely new physics, such as dark energy or a modification of General Relativity. In providing a measurement of the baryon content of the universe in agreement with that required for successful primordial nucleosynthesis, it has further underscored the need for an explanation for the matter-antimatter asymmetry of the universe, some proposals for which will be tested at the Large Hadron Collider (LHC). And in its precise measurements of the spectral index of the temperature fluctuations, it has constrained existing and newly-proposed models for the physics of the very early universe, requiring an almost scale-invariant spectrum, with specific small deviations. We're going to miss WMAP, but we're not standing still - cosmologists are a very forward looking bunch for people whose lives revolve around what happened so far in the past. A host of new projects are coming, and in the microwave field the big one is the Planck satellite, already taking eagerly awaited data. So goodbye WMAP, and thanks! Now, what's next? We're hungry for more information!