Dark energy and the destiny of the Universe

Only 0.5% of the material in the Universe exists in the form of stars. The rest exists as dark matter (about 1/3) and as dark energy (about 2/3). While we have now determined that our universe is nearly spatially flat, the presence of dark energy breaks the simple and familiar relationship between the geometry of the Universe and its destiny: a positively curved universe recollapses and a flat or negatively universe expands forever. Our ignorance of the nature of the dark energy allows for three futures: continued accelerated expansion and a darkening of the sky in 150 billion years; eventual slowing, with the number of visible galaxies increasing with time; or even recollapse. I summarize what we presently know about dark energy and the prospects for getting at its nature with future cosmological measurements.